FACULTY OF HEALTH SCIENSES
UNIVERSITY OF COPENHAGEN
PHYSICAL FITNESS AND LOW BACK PAIN
Performance-based and self-assessed physical
fitness as risk indicator of low back pain among
health care workers and students
PhD thesis by Jesper Strøyer Andersen
NATIONAL RESEARCH CENTRE FOR THE WORKING ENVIRONMENT
Physical Fitness and Low Back Pain
PHYSICAL FITNESS AND LOW BACK PAIN
­ Performance­based and self­assessed physical fitness as risk indica­
tor of low back pain among health care workers and students
PhD thesis by Jesper Strøyer Andersen
National Research Centre for the Working Environment
Faculty of Health Sciences, University of Copenhagen, Denmark
November 2007
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Physical Fitness and Low Back Pain
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Physical Fitness and Low Back Pain
CONTENTS
Preface ............................................................................................................................... 7
Summary in English .......................................................................................................... 9
Dansk resumé (summary in Danish) ............................................................................ 11
List of papers .................................................................................................................... 13
1. Introduction ..................................................................................................................14
2. Materials and Methods................................................................................................20
2.1 Study populations and designs ...........................................................................20
2.2 Performance­based physical fitness ....................................................................20
2.2.1 Maximal Oxygen Uptake ............................................................................20
2.2.2 Back Muscle Strength ..................................................................................20
2.2.3 Back extension Endurance..........................................................................22
2.2.4 Back flexion Endurance...............................................................................22
2.2.5 Flexibility.......................................................................................................22
2.2.6 Balance ...........................................................................................................23
2.3 Self­assessed physical fitness ...............................................................................24
2.4 Low back pain ........................................................................................................24
2.5 Covariates ...............................................................................................................25
2.6 Statistics...................................................................................................................25
3. Results............................................................................................................................27
3.1 Basic characteristics of the study populations...................................................27
3.2 Associations between Performance­based physical fitness and LBP.............27
3.3 Associations between Self­assessed physical fitness and LBP........................29
3.4 Covariates with associations to LBP ...................................................................30
3.5 Characteristics of self­assessed physical fitness ................................................30
3.5.1 Distribution, sex differences and inter-item correlations ......................30
3.5.2 Convergence and divergence of self-assessed and
performance­based fitness ......................................................................... 31
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Physical Fitness and Low Back Pain
4. Discussion .....................................................................................................................33
4.1 Performance­based physical fitness as risk indicator of LBP..........................33
4.2 Self­assessed physical fitness as risk indicator of LBP .....................................35
4.3. Self­assessed compared with performance­based physical fitness ...............36
4.3.1 Associations between self-assessed and performance-based
physical fitness ............................................................................................. 36
4.3.2 Predictive validity........................................................................................38
4.4 Methodological considerations............................................................................39
4.4.1 Limitations and strengths of the thesis.....................................................39
4.4.2 Differences in test procedures and conditions ........................................40
4.4.3 Test­retest reliability of self­assessed physical fitness............................40
5. Conclusions ...................................................................................................................42
6. Perspectives ..................................................................................................................43
References .........................................................................................................................45
Paper 1 ............................................................................................................................. 53
Paper 2 ............................................................................................................................. 72
Paper 3 ............................................................................................................................. 89
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Physical Fitness and Low Back Pain
PREFACE
This PhD project was initiated at the National Research Centre for Working Environment (the former National Institute of Occupational Health), Copenhagen,
Denmark, at the department of physiology in September 2003. The thesis was
submitted to the faculty of Health Sciences, University of Copenhagen, November
2006 and defended November 2007. The studies were conducted in accordance
with the declaration of Helsinki. The project was financially supported by the Na­
tional Research Centre for Working Environment, Copenhagen, Denmark.
Academic advisors:
Associate Professor in Social Medicine Kirsten Schultz Larsen MD, PhD,
Institute of Public Health, University of Copenhagen.
Senior Researcher Ole Olsen MSc, the National Research Centre for Working
Environment
Associate Professor in Public Health, Kirsten Avlund PhD, Dr.Med.Sci.
at the University of Copenhagen, Institute of Public Health
Senior Researcher Bente Schibye PhD, National Institute of Occupational Health
Opponents:
Professor Clas-Håkan Nygaard, Tampere School of Public Health,
University of Tampere, Finland
Professor Jan Hartvigsen, Insitute of Sports Science and Clinical Biomechanics,
Odense, University of Southern Denmark.
Lektor Dan Meyrowitsch, Institute of Public Health, Department of Epidemiology,
University of Copenhagen
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Physical Fitness and Low Back Pain
ACKNOWLEDGEMENT
­ Lots of thanks and thoughts to my former superior and first mentor of the PhD
project, Bente Schibye, for inspiring and supporting me in the decision to do this
PhD project. As the result of a traffic accident a month after my matriculation as a
PhD student, she was unfortunately unable to return to work again. I have missed
not only her critical questioning and physiological knowledge, but also the inspiring and friendly atmosphere surrounding her.
- Thanks to Kirsten Avlund for being two mentors in one person when Bente was
prevented from working, for her optimistic ways, and for keeping me on track
when my children had kept me awake the half night. Very unfortunately, also
Kirsten was unable to remain my mentor due to sickness in October 2006 – lots of
thoughts to her.
- Thanks to Ole Olsen for stepping in halfway and joining as a mentor, and for his
critically statistical advice.
- Thanks to Kirsten Schultz Larsen for the stepping in at the last moment without
any knowledge about my project or me, and for all the hours she spent reading,
correcting and discussing my thesis.
­ The thesis is based on an almost incredible number of field measures. Thanks to
all my very good colleagues at AMI who spent so much time in the field with me:
Nis Hjortskov Jensen, Morten Essendrop, Christian Hye-Knudsen, Anne Faber,
Klaus Hansen, Dorte Ekner, Jørgen Skotte, Hanne Giver, Susan Warming, Jette
Nygaard Jensen, Kirsten Nabe Nielsen
- Thanks to Sisse Warming, Mette Øllgaard Jakobsen, Pernille Mikkelsen and Ute
Bültmann for the computerizing of VAS and test data.
- Thanks to Karl Bang Christensen for statistical advice and nice talks about soccer.
- Thanks to all other good colleagues in the SOSU group, the former AEA and ITA
departments, and at AMI for support and a convivial working atmosphere.
- Thanks to Annemarie Eskelund and Lone Donbæk Jensen for constructive and
inspiring teamwork.
- During my PhD period I have had the luck to become the father of three lovely
children. Thanks to Marie, Laurits and Emil and to you Lotte for putting my life
into perspective.
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Physical Fitness and Low Back Pain
SUMMARY IN ENGLISH
Background: The prevalence of Low Back Pain (LBP) is particularly high in the
health care sector. Various physical and psychological work-related aspects typical
of the sector have been identified as risk factors of LBP. Despite high physical
work demands being characteristic of the health care sector, to date, there is no
conclusive evidence that a high level of physical fitness can prevent LBP. Isome­
tric back extension endurance is one of the physical fitness parameters that most
studies has found to associate with LBP, but as many other studies do not find
that low endurance increases the risk of LBP, there is no consensus on its ability to
predict LBP.
Aim: The overall aim of this PhD thesis was to examine whether performancebased and self­assessed physical fitness is associated with the development and
prevalence of LBP among persons who work or are training to work in the health
care sector. An additional aim was to test convergence and divergence validity of
self­assessed dimensions of physical fitness against performance­based physical
fitness parameters and the influence of LBP on this relationship.
Methods: The association between physical fitness parameters (performance­
based and self-assessed) and the increase in LBP intensity (>2) after 30 months
(follow-up) was examined in a cohort of persons working with physically and
mentally disabled persons (n=327). Convergence and divergence validity was
tested among social and healthcare helpers and assistants in training (healthcare
students), persons working with physically and mentally disabled persons, and
hospital staff. Associations between 12­month prevalence of LBP and physical fit­
ness parameters and the influence of LBP on the association between self­assessed
and performance­based physical fitness were examined in a cross­sectional study
of healthcare students (in the first week of their training) (n=612). In all studies
(the prospective study, the validity study and the cross-sectional study), back
endurance, flexibility and balance were tested. In addition to the aforementioned
tests, the level of aerobic fitness, back extension strength, and back flexion strength
were measured in the validity study. Self­assessed physical fitness was measured
identical in all study populations using illustrated visual analogue scales (VAS),
(a newly-developed and untested tool for self-assessment). The reproducibility of
this new tool (self­assessed physical fitness (week by week)) was tested in a sepa­
rate group of healthcare students (n=159). Logistical regression analysis was used
in the prospective and cross-sectional study. In the validity study linear regression
analyses were performed (adjusted for age and sex) to examine the convergence
and divergence between related and non-related self-assessed and performancebased parameters.
Results: Isometric back extension endurance measured using a modified version
of Biering­Sørensen’s procedure was the only physical fitness parameter that
showed any kind of prospective association with increased LBP intensity. The
overall association (likelihood ratio test) was only close to significance (p=0.067),
whereas those with a medium level of back extension endurance were at a signifi­
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Physical Fitness and Low Back Pain
cantly higher risk of increased LBP intensity during follow-up (OR=2.7, p=0.034)
in relation to the group with the highest endurance. Those with the lowest score in
endurance came only close to having a significantly higher risk (OR=2.37, p=0.076)
compared with those with high level endurance. Self-assessed level of aerobic
fitness showed a reverse association with LBP. Those with a moderate level of self­
assessed aerobic fitness had a reduced risk of increased LBP intensity compared to
those with a high level of self­assessed physical fitness (OR=0.37, p=0.02), whereas
those with the lowest level did not have a significantly reduced risk (OR=0.58,
p=0.23). Self­assessed muscular strength, endurance and flexibility all showed the
same tendency in relation to LBP as self­assessed level of fitness; however, the
associations were not significant. In the cross­sectional study an experience of LBP
during the previous year was significantly associated with lower score of self­as­
sessed aerobic fitness, flexibility and balance, and performance­based flexibility.
In the validity analysis, we found that level of self­assessed aerobic fitness, muscle
strength and flexibility had a small to moderate association with the correspon­
ding performance-based parameters. An association between the self-assessed
flexibility score and the result of the performance­based flexibility test result was
also confirmed in the cross­sectional study. Additionally, self­assessed endurance
and balance scores were also associated with the corresponding performance-based test result. LBP did not confound any of the associations between self-assessed
and performance­based physical fitness. The reproducibility of self­assessed level
of fitness and muscle strength was good (ICC=0.80), whereas endurance, flexibility
and balance showed only moderate reproducibility (ICC=0.62-0.69).
Discussion: The finding of the preventive effect of high isometric back extension
endurance in relation to increased LBP intensity is supported by several other
studies showing that low back extension endurance, in particular, is significantly
associated with an increased risk of LBP. The study did not support a doses-response relation between back extension endurance and increased risk of increased LBP intensity, as the risk for moderate and low endurance was shown to
be approximately the same. The reduced risk of increased LBP intensity among
those with a moderate level of self­assessed fitness in relation to those with a high
level of self­assessed fitness was surprising and contradicted our hypothesis. One
possible explanation could be that those who estimate their level of fitness as low
are also more aware of their fitness level and, accordingly, have a behavioural
pattern where they avoid the physically straining work tasks that they believe
they lack the physical fitness to perform. They thereby reduce the risk of exces­
sive strain and, consequently, reduce the risk of increased back pain. In the long
term, this is not a sustainable solution for avoiding LBP, as the individual’s level
of physical fitness will fall in accordance with the reduced mechanical stimulation,
whereby the load required to strain the tissue is also reduced. The weak to moderate agreement between performance­based and self­assessed physical fitness and
the contradicting associations with LBP in the prospective study point towards
performance­based and self­assessed physical fitness both measuring something
different, despite their association.
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Physical Fitness and Low Back Pain
DANSK RESUMÉ (SUMMARY IN DANISH)
Baggrund: Forekomsten af lænderygbesvær (LBP) er særlig høj indenfor pleje- og
sundhedssektoren. Adskillige fysiske og psykiske arbejdsrelaterede faktorer som
hyppigt optræder i denne sektor er identificeret som risikofaktorer for LBP. På
trods af at pleje- og sundhedssektoren er karakteriseret ved høje fysiske arbejdskrav, er der endnu ikke skabt evidens for, at god fysisk kapacitet forebygger LBP,
da resultaterne er inkonsistente hvad angår sammenhængen mellem lav fysisk
kapacitet og øget risiko for LBP. Isometrisk rygekstensionsudholdenhed er et af
de fysiske kapacitetsparametre med flest påviste sammenhænge til LBP, men da
flere andre studier ikke finder, at dårlig udholdenhed øger risikoen for LBP, er der
endnu ikke konsensus om dens evne til at forudsige fremtidigt LBP.
Formål: Det overordnede formål med ph.d.-afhandlingen var at undersøge, om
målt og selvvurderet fysisk kapacitet er associeret med udviklingen og forekomsten af LBP blandt personer, som er beskæftiget eller under uddannelse indenfor pleje- og sundhedssektoren. Ydermere var formålet at teste konvergens- og
divergensvaliditeten af selvvurderede dimensioner af fysisk kapacitet mod testede
fysiske kapacitetsparametre og om LBP influerede på disse sammenhænge.
Metoder: Sammenhængen mellem fysiske kapacitetsparametre (testede og selvvurderede) og stigningen i LBP-intensiteten (>2) efter 30 måneder (followup)
blev undersøgt i en kohorte af personer, der arbejdede med fysisk og psykisk
handicappede mennesker (n=327). Konvergens- og divergensvaliditeten blev
testet blandt kommende social- og sundhedsassistenter og -hjælpere (SOSU-studerende), personer, der arbejdede med fysisk og psykisk handicappede samt
hospitalspersonale. Associationer mellem 12-måneders prævalensen af LBP og
fysiske kapacitetsparametre (testede og selvvurderede) og indflydelsen af LBP
på sammenhængen mellem korresponderede målte og selvvurderede kapacitetsmål blev undersøgt i et tværsnitsstudie blandt SOSU-studerende (i deres første
uge på uddannelsen) (n=612). I alle studierne (forløbsstudiet, validitetsstudiet og
tværsnitsstudiet) blev rygudholdenheden, rygbevægeligheden samt balanceevnen
testet. Udover disse tests blev konditionen og den isometriske rygstyrke målt i validitetsstudiet. Selvvurderet fysisk kapacitet blev i alle studiepopulationer målt på
samme måde ved hjælp af illustrerede, visuelle, analoge skala?er (VAS), (som var
et nyudviklet uprøvet selvvurderingsværktøj). Reproducerbarheden af selvvurderet fysisk kapacitet (uge til uge) blev testet i en separat gruppe SOSU-studerende
(n=159). Konvergens- og divergensvaliditetsanalysen blev udført ved hjælp af
lineær regression (aldersjusteret og stratificeret i forhold til køn) for at undersøge
konvergensen og divergensen mellem relaterede og ikke relaterede selvvurderede
og testede parametre.
Resultater: Isometrisk rygekstensionsudholdenhed målt med en modificeret ver­
sion af Biering-Sørensens procedure var den eneste fysiske kapacitetsparameter,
der udviste egenskaber som risikofaktor i forhold til stigende LBP-intensitet. Den
overordnede association mellem rygekstensionsudholdenhed og øget LBP-intensitet (likelihood ratio test) var kun tæt på statistisk signifikans (p=0,067). Personer
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Physical Fitness and Low Back Pain
med middelniveau af rygekstensionsudholdenhed var i signifikant større risiko
for at udvikle øget LBP­intensitet ved follow­up (OR=2,7; p=0,034) set i forhold
til personer med høj udholdenhed. De personer, som havde dårligst udholdenhed, var kun tæt på at være i signifikant højere risiko (OR=2,37; p=0,076) end for
personer med højt niveau. Selvvurderet kondition viste en modsat sammenhæng
med LBP. Personer, der vurderede deres kondition til middel, var i reduceret
risiko for øget LBP-intensitet i forhold til personer med høj selvvurderet fysisk
kapacitet (OR=0,37; p=0,02), mens dem med dårligst niveau ikke var i signifikant
mindre risiko (OR=0,58; p=0,23). Selvvurderet muskelstyrke, udholdenhed og
bevægelighed viste alle samme tendens i forhold til LBP som selvvurderet kondition, men associationerne var ikke statistisk signifikante. I tværsnitsstudiet hang
oplevet LBP indenfor det sidste år signifikant sammen med lavere scorer i selv­
vurderet kondition, bevægelighed og balance og testet rygbevægelighed. I validitetsanalysen fandt vi, at kondition, muskelstyrke og bevægelighed associerede
svagt til moderat med de korresponderede målte parametre. Sammenhængen
mellem selvvurderet fleksibilitet og testet rygbevægelighed blev også signifikant
bekræftet i tværsnitsstudiet. Ydermere var selvvurderet udholdenhed og balance
signifikant associerede med de korresponderede målte parametre, og LBP var ikke
en konfounder i sammenhængen mellem selvvurderede og testede fysiske kapacitetsparametre. Reproducerbarheden for selvvurderet kondition og muskelstyrke
var god (ICC=0,80), mens udholdenhed, bevægelighed og balance viste moderat
reproducerbarhed (ICC=0,62-0,69).
Diskussion: At høj isometrisk rygekstensionsudholdenhed er præventiv i forhold
til øget intensitet og forekomst af LBP understøttes af andre studier. Undersøgelsen understøttede ikke en dosis-responssammenhæng mellem rygekstensionsudholdenhed og øget risiko for øget LBP-intensitet, idet risikoen for middel og dårlig
udholdenhed var nogenlunde den samme. Den påviste reducerede risiko for øget
LBP-intensitet blandt personer med middelhøj selvvurderet kondition (i forhold
til personer med høj) var overraskende og i modsætning til vores hypotese. En
mulig forklaring kan være, at personer, som vurderer deres kapacitet til at være
lavere, er særligt opmærksomme på deres kapacitet og derfor udvikler et adfærdsmønster, hvor de undgår de fysisk belastende arbejdsopgaver, de ikke mener, de
besidder fysisk kapacitet til at udføre. Derved reducerer de risikoen for at blive
overbelastet, og risikoen for at få mere ondt i ryggen mindskes. På længere sigt er
det dog ikke en holdbar løsning til at undgå LBP, fordi den fysiske kapacitet vil
blive mindre i takt med, at den mekaniske stimulering reduceres, hvorved den
ydre kraft, der skal til for at overbelaste bevægeapparatet, også mindskes. Den
svage til moderate overensstemmelse mellem målt og selvvurderet fysisk kapacitet samt de modsat rettede sammenhænge med LBP i det prospektive studie tyder
på, at testet og selvvurderet fysisk kapacitet måler noget forskelligt på trods af, at
de er associerede.
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Physical Fitness and Low Back Pain
LIST OF PAPERS
This thesis is based on three papers. They will be referred to in the text by their
roman numerals.
I.
The role of physical fitness as risk indicator of increased Low Back Pain
intensity among people working with physically and mentally disabled
persons: A 30-month prospective study.
Jesper Strøyer1, Lone Donbæk Jensen2.
Submitted to SPINE November 2006, and accepted for publication September 2007.
1
National Research Centre for the Working Environment, Denmark, 2University Hospital of
Aarhus, Denmark.
II.
Construct Validity and Reliability of Self-assessed Physical Fitness.
Jesper Strøyer1, Morten Essendrop2, Lone Donbæk Jensen3, Susan Warming4, Kirsten Avlund5, Bente Schibye1.
Perceptual and Motor Skills, 2007, 104, 519-533.
1
2
National Research Centre for the Working Environment, Denmark, Group Clinical develop3
ment, ALK-Abello Hørsholm, Denmark, Department of Occupational Medicine, University
4
Hospital of Aarhus, Denmark, Clinical Unit of Health Promotion Bispebjerg University Hospi5
tal, Denmark, Institute of Public Health, University of Copenhagen
III
Is the cross­sectional association between self­assessed physical fitness
and performance­based physical fitness among health care students influ­
enced by low back pain?
Jesper Strøyer1, Annemarie Lyng Eskelund-Hansen2 , Kirsten Schultz Larsen3 and Niels Erik Ebbehoej2.
Manuscript.
1
2
National Research Centre for the Working Environment, Denmark. Clinic of Occupational
3
and Environmental Medicine, Bispebjerg University Hospital, Copenhagen, Institute of Public
Health, Department of Social Medicine, University of Copenhagen.
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Physical Fitness and Low Back Pain
1. INTRODUCTION
Low back pain (LBP) is both a Danish and a global public health problem and
results in many days of sick leave, reduced work capacity or, ultimately, expulsion
from the labour market, leading to extensive human and socioeconomic consequences (44,129).
Health care workers seem to be particularly exposed to LBP. International studies
reported their 12-month prevalence of LBP as above 60%, compared to employees
in general who showed a prevalence closer to 50% (22,63,70,92,99,113,117). Of the
social and healthcare helpers and assistants who were under education (healthcare
students) in Denmark in 2004, 51% (n=5700) had experienced LBP during the
previous year (95), whereas a much higher 12-month prevalence of 69% was found
among experienced healthcare who were in employment during the same period
in Denmark (n=8038) (personal communication). This difference between those
under education and those in employment indicates that the high prevalence
of LBP may be caused by factors experienced when performing the work at the
workplace. From comparisons with school teachers it has been shown that 43% of
the musculoskeletal disorders in the back and knee among employees in the home
care and 24-hour care centres can be ascribed to the occupational environment.
This implies that these musculoskeletal disorders can be prevented (9). In addition, on the bases of more than 300 studies, the Occupational Safety and Health
Administration (OSHA) concludes that with implemented ergonomic interventions in different occupations, 66% of the sick leave due to musculoskeletal disorders can be prevented in the health care sector (1). Thus, in the health care sector
the potential of prevention seems to be large.
Healthcare workers are highly exposed to both physical and psychosocial work
factors. Awkward, rotated and flexed work positions take place during patient
handling and patient care, and cleaning are often carried out at difficult accessible
spaces (18,62,63,123). In addition, the working environment are characterised by
high workload, high time pressure, low job control, low social support and conflicts with patients (11,18,19,57,62,63).
Many attempts have been made to establish predictors of LBP with the aim of
enhancing prevention. Physical workload is identified as a risk indicator of LBP,
both as a general factor compared to jobs with low physical demands, and as more
specific work factors such as frequent bending or twisting of the back, heavy lif­
ting and patient handling (15,20,49,53,61,65,67,82,106). Psychological work factors,
for instance low social support, are also suggested as risk indicators of LBP; howe­
ver, the interpretation of the evidence regarding the risk of LBP are contradictory
(26,51,54,78). The inconsistency in results regarding psychological work factors
might be due to the many different ways of measure psychological work factors
or lack of controlling for potential confounding from occupational biomechanical
demands as discussed by Davis and Heaney (26).
A conceptual model for the complex interaction between different work factors
present in the work environment, the way they interact with the individual and
the way it leads to pain, impairment and further to disability is illustrated in the
model in figure 1 (96). The model which is further presented and described in the
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Physical Fitness and Low Back Pain
comprehensive work by the National Research council (97) deals with two broad
categories: workplace factors (the workplace) and characteristics of the person
(the person). The person is identified as the central biological entity subject to
biomechanical loading with the various physical, psychological and social features
associated with the individual that may influence the biological and clinical re­
sponse, and disability response. The issue for this thesis is the association individual factors in the interaction with the biomechanical loading (which is a result of
the external loads) and the internal tolerances. The theory is that this interaction is
crucial to generate pain.
The biomechanical mechanism that explains how the external load leads to mechanical loadings which further result in mechanical strain, tissue damage and LBP
was described by McGill (88). McGill defines a failure tolerance which is the limit
an applied load shall exceed to cause injury or failure of the tissue. The distance
between the applied load and the failure tolerance is the margin of safety. McGill
defines injury as the full continuum from the most minor of tissue irritation to the
grossest of tissue failure and presumes that such damage generates pain. A bigger
margin of safety implies a reduced risk of pain.
According to this theory, LBP caused by physical work demands can be prevented
by two strategies: one is to decrease the applied physical load (external loads in
figure 1); the other is by increasing the failure tolerance. Both strategies increase
the margin of safety and thereby reduce the risk of tissue damage and LBP.
To reduce the physical load in the health care sector, as suggested in the first stra­
tegy is difficult because the job (as for example health care worker) involves work­
ing physically with people who may be unmanageable and unpredictable objects.
The introduction of mechanical lifts devises has reduced some of the major loads,
but it is difficult to standardize physical work tasks in the health care sector so that
technical aids can be used exclusively. Another way of reducing the applied load
is by better patient handling techniques. Better patient handling technique has
shown to reduce the applied load significantly in isolated methodological studies
(81,111); however, to date, no effect on LBP was found when instruction in patient
handling was implemented in the workplace (50,52,114).
The second strategy, which is the focus of this PhD, is the opportunity to improve
the failure tolerance by increasing the person’s level of physical fitness. In the pre­
sent thesis physical fitness is defined as the dimensions of aerobic fitness, muscle
strength, muscle endurance, flexibility and balance. This definition was inspired
by the early work of Fleishman (37) and the definition by Miller (91). Both con­
cepts reflect the belief in a multidimensional nature of physical fitness. Physical
fitness was in the study clearly distinguished from physical activity and physical
training.
Physical fitness belongs to the “Individual factors” box of the model (figure 1) and
as the arrows in the model indicate it can interact with the physiological pathway
on several levels. In the thesis the focus is on the connection with the biomechanical box. By increasing the individual’s physical fitness the relative load decreases
as do the internal loads which match the external loads.
The relationship between low level physical fitness and increased risk of future
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Physical Fitness and Low Back Pain
Figure 1. A conceptual model of the possible roles and influences that various factors may have in the
development of musculoskeletal disorders. (National Research Council and the Institute of medicine.
Musculoskeletal Disorders and the Workplace. Washington, D.C.: National Academy Press, p. 1-492,
2001. Originally adapted from National Research Council. Work-Related Musculoskeletal Disorders:
Report, Workshop Summary, and Workshop Papers. Steering Commitee for the workshop on WorkRelated Musculoskeletal Injuries: The Research Base. Washington, DC: National Acedemy Press. 1999)
LBP as the theory by McGill and the conceptual model support, has been attempted to be established in several studies introducing a wide spectrum of physical fitness tests: Aerobic fitness that is the physical fitness component with the
most unambiguous definition of maximal oxygen uptake (VO2) per kilo of body
weight (mlO2*min-1*kg-1) has not been found as a independent risk factor of LBP
(13,28,72,118). Despite the very exact definition, the testing of aerobic fitness is not
unproblematic under field test conditions because the direct measurement of ma­
ximal oxygen uptake is both time and equipment consuming and the maximally
testing can furthermore imply a health risk for the participants, which makes it
less applicable as a test among the working population and for people in all ages.
Consequently, many of the epidemiological studies used indirectly methods for
the assessment of aerobic fitness as the widely used Åstrand test (7,8) or the UKK
2-KM Walk test (100), although they are still time consuming in large scale studies.
The indirectly test introduces markedly standard errors for predicting the maximal oxygen uptake that makes the relationship with LBP harder to find. In general
more knowledge exists about the effect of physical activity, which traditionally
has been assessed by questionnaires, compared to aerobic fitness in relation to
16
Physical Fitness and Low Back Pain
the incidence and especially treatment of LBP (102). However, due to the genetic
factor of aerobic fitness (104) and the fact that physical exercising at a low level not
necessary increases the maximally oxygen uptake, the effect of physical exercise
and aerobic fitness in relation to health outcomes have to be distinguish. The as­
sociation between leisure time physical activity and LBP is not within the scope
for this thesis.
The remaining components of physical fitness are more wide defined than aerobic
fitness and a diversity of methods have been applied to test for associations with
future LBP.
Muscle strength in relation to LBP have primarily been measured by isometric
strength tests (3,12,16,21,77,79,83,98,107,124) but also isokinetic tests have been
used (31,41,45,72,75,98,122). Despite that five studies found poor trunk muscle
strength as predictive for low back pain (10,16,42,83,122) the evidence is inconclusive for a relation between trunk muscle strength and the risk of low back
pain (47) due to inconsistent results. The remaining studies found no relationship
between muscle strength and LBP or found the opposite result, that high muscle
strength was a risk factor for LBP (77,83).
Muscle endurance has both been measured in dynamic setups (77,79,105,109) and
static setups (3,17,41,45,64,68,79,115,118,122). The dynamic test procedures vary
much between studies whereas static endurance in relation to LBP is almost solely
evaluated by the back extension endurance test by Biering Sørensen (16) that is a
kind of golden standard although it has several modified but closely related versi­
ons (27). A static back flexion endurance test has also been evaluated as predictor
of LBP (68). Three of the four studies using dynamic setups (77,79,105) and six of
the ten studies using the static setup of Biering Sørensen (3,41,64,68,115,118) and
the study testing static back flexion endurance (68) found no relationship between
muscle endurance and the risk of LBP. That five studies found low level of isome­
tric back extension endurance to be a risk factor of LBP (17,45,64,79,122) indicate
that this particular test could have the quality as a risk factor of LBP, although
inconclusive evidence is stated (48).
Another physical fitness component that traditionally has been linked to the risk
of LBP is the flexibility of the spine that has been evaluated with a variety of met­
hods. Best known for epidemiological approaches are the modified Schöber test
(80), the finger­to­floor method (40) and the sit­and­reach test (4). In general, all
methods and LBP outcomes together, there is weak evidence for no relation between flexibility and the risk of LBP. Although most of the studies that examined
this relationship found no relation with LBP (3,14,17,42,45,79,83,105,115,118,122),
one study found high flexibility to increased risk of LBP (among men) (16) whe­
reas two other studies found the opposite result that low flexibility increased the
risk of LBP (3,122).
The last component of physical fitness that is examined in this thesis is balance
which is the most sparsely examined physical fitness parameters in relation to risk
of future LBP. Balance is a very complex motor skill and there is no solid consensus regarding the definition of balance control or globally “gold standards” for
measuring it (103). No studies have to my knowledge found balance to be associated with future LBP among healthy persons (118)
In conclusion no moderate or strong evidence is to date identified for any compo­
17
Physical Fitness and Low Back Pain
nents of physical fitness as predictors of LBP despite a variety of studies, although
some components have showed more promising ability as a predictor of LBP than
others.
Physical fitness is traditionally assessed using objective performance­based tests as
in the aforementioned studies. However, despite performance-based tests having
several advantages, including high reproducibility and sensitivity to change, they
also have the disadvantages of being time-consuming, and requiring adequate
space, special equipment and trained examiners. Hence, performance-based tests
are not always suitable for epidemiological studies involving a large number of
subjects. In such cases, self­assessed physical fitness can be a practical and cost­
effective alternative. The question is whether performance­based physical fitness
can be substituted by self­assessed physical fitness.
Several instruments for self­assessment of physical fitness are designed, and
compared with performance­based parameters of physical fitness (2,32,69,74,90,
101,127,130). However, despite well-designed studies and the multidimensional
structure of physical fitness (91), participants have experienced difficulties in diffe­
rentiating between different physical fitness components when they completed the
self-assessment and the validity of the different components showed great variety.
Visual analogue scales (VAS) is a feasible, valid, and reliable method when assessing subjective experience of pain (84) but VAS has also showed to be applicable
in other areas such as fatigue (76), appetite (38) and disability (5). In the present
thesis the use of VAS for self­assessment of physical fitness was tested.
18
Physical Fitness and Low Back Pain
AIM
The overall aim of this thesis was to expand the knowledge about physical fitness
as a risk indicator of LBP. Physical fitness parameters were assessed by both ob­
jective tests and by self-assessment using a newly developed questionnaire using
visual analogue scales.
In addition the agreement between the performance-based tests and the self-assessed questions was evaluated, and the influence of LBP and individual factors on
this relationship was further studied, to examine the applicability of the self-assessment instrument for use in future larger surveys.
Aims of the specific papers:
I.
The aim of paper I was to test if a low level of self-assessed or performancebased physical fitness was associated with an increase in LBP intensity at
30-month follow-up among people working with physically and mentally
disabled persons.
II.
The aim of paper II was to analyse the convergence and divergence between
self­assessed and performance­based physical fitness and to test the reliabi­
lity of the instrument for self­assessment of physical fitness using VAS.
III.
The aim of paper III was first to analyse if the association between LBP and
self­assessed physical fitness differed from the association between LBP and
performance­based physical fitness. Second, it was analysed if LBP confoun­
ded the adjusted associations between corresponding dimensions of self-assessed and performance­based physical fitness.
19
Physical Fitness and Low Back Pain
2. MATERIALS AND METHODS
2.1 STUDY POPULATIONS AND DESIGNS
The different study populations all comprised people who were either employed
in areas or training to work in areas that involved working physically with people.
A short description of the study populations and the performance-based dimensions and methods we applied are listed in table 1. Although all study populations
comprised both women and men, it should be noticed that the number of men
are low in all populations. The associations of performance-based and self-assessed physical fitness with LBP outcomes were analysed in a prospective design
with a 30­month follow­up (“the prospective study” or paper I). The validity and
reliability analyses were based on secondary analyses of cross-sectional data from
several surveys comprising performance-based and self-assessed parameters assessed within the same month (“the validity study” or paper II). The differences
and similarities between self­assessed and performance based physical fitness in
relation to each other and LBP were further examined among healthcare students
in their first week of training (“the cross­sectional study” or paper III).
2.2 PERFORMANCE-BASED PHYSICAL FITNESS
All the performance-based tests were selected according to the following criteria:
that, as far as possible, they were reported as valid and reliable (inter- and intra-tester), that they provided well distributed scores with limited “ceiling” and “floor”
effects, and that they were easy to administer in a field setting. There should be
minimal health risk involved with participation in the test battery; additional,
participants should be able to perform the tests in everyday clothing.
2.2.1 Maximal Oxygen Uptake
Aerobic fitness (ml O2*kg-1* min-1) was measured using the Åstrand ergometer
bicycle test (7,8) in conjunction with the Åstrand age-correction factor (24). The
subjects cycled on an ergometer bicycle for 6 minutes on a pre-selected power that
was estimated to give a steady-state heart rate of at a least 120 beats per minute.
The standard error for predicting the maximal oxygen uptake was reported as
between 10% and 15% (8,24), and this has been shown to have good reliability
(r=0.83-0.93) (24,66).
2.2.2 Back Muscle Strength
Maximal isometric voluntary contraction (MVC) of the back extensor and flexor
muscles was measured with a strain­gauge dynamometer fixed to the wall with
the subjects fixed in a standing position (33,34). Lever arm for torque calculations
at L4/L5 level was defined as the vertical distance from the middle of the strap to
the upper edge of the iliac crest. The reliability of the isometric strength tests was
reported as good (Intraclass Correlation Coefficient (ICC)=0.91­0.96) (33).
20
Physical Fitness and Low Back Pain
21
Physical Fitness and Low Back Pain
2.2.3 Back extension Endurance
Isometric back extension endurance was measured using a modified version of
the Sørensen test (16,27,93). The subjects were placed on their stomach with their
navel over the edge of a padded sloping board, which was 70 cm in length and 15
cm high in the raised end. The subject’s feet were pressed down to the floor by an
assistant and the subject was instructed to fold his/her arms across the chest and
hold the upper body in a horizontal position for as long as possible. The sloping
board implied that the hip flexion was approximately 12º during the test.
The determination time differed between the studies. In paper II the test was
continued for a maximum of 360 seconds, whereas the time limit in paper III was
reduced to 180 seconds due to a tight schedule. The time limit of 180 seconds was
decided on the basis of previous test data from which we expected that approximately 50% of the test persons would hold the position for 180 sec. Consequently,
the test result was treated as a continuous variable when using 360 seconds and
as a dichotomous variable when using 180 seconds (passed/not passed 180 sec).
Among healthy subjects and with a time limit of 240 seconds, the reliability was
reported to range from 0.54 to 0.99 (ICC) and among physically active LBP subjects the ICCs ranged from 0.82 to 0.96 (66,93) and, in general, can be regarded as
moderate to good.
2.2.4 Back flexion Endurance
The methods used to test isometric back flexion endurance differed between the
studies. In paper I and II , isometric back flexion endurance was tested with the
subjects in a supine position with 90Ÿ hip and knee flexion and with their feet
supported on a chair (56,60). The subjects were instructed to curl up until a band
fixed around the chest, at the height of the inferior angulus of scapulae, was free
of the floor. This position was held for as long as possible or to a maximum of 360
sec. The reliability of the test has been reported as good (r=0.93) (56,60).
In paper III, isometric back flexion endurance was measured using the test develo­
ped by McGill (85,89). The subject was positioned in a sit-up posture with the back
resting against a jig angled at 60º from the floor. Both knees and hips were flexed
90º, the arms were folded across the chest, and the toes were secured under toe
straps. To begin, the jig was pulled back 10 cm and the person held the isometric
posture for as long as possible or to a maximum of 180 sec. The test was evaluated
as passed or not passed the 180 sec maximum. An unspecified reliability coeffi­
cient of 0.97 on the basic of five people only was reported by McGill (89).
2.2.5 Flexibility
Sagittal flexibility was measured using the modified finger­to­floor method
(40,108) and calculated as the distance from the fingertips to the floor in a fully
flexed position when standing on a 30 cm measuring box without shoes. Positive
values indicate that the subject was unable to reach the top of the measuring box
(floor level) during a full forward bending; negative values indicate that the sub­
ject was able to reach further down the side of the box (below floor level) during a
full forward bending. The reliability was reported as good (ICC=0.93) (40).
22
Physical Fitness and Low Back Pain
2.2.6 Balance
In paper I and II the ability to balance and co-ordinate the trunk muscles was
evaluated by a balance test in a sitting position on a wobble board. This test was
developed by Essendrop and Hye-Knudsen at the National Institute of Occupational Health, Denmark. The wobble board was placed on a table, which allowed
the legs to hang down freely over the edge of the table. Two trials were given
to familiarize the subject with the wobble board and thereafter the subject was
asked to keep the wobble board in balance through movements of the hip and the
back. If any body segment or the edge of the wobble board touched the table the
stopwatch was stopped, but not zeroed, and one attempt was counted. Number
of attempts was counted until the subject had been balancing for one minute or a
maximum of 15 attempts was reached. McGill et al. have previously used a similar
set-up (87). In a test-retest setup at the National Institute of Occupational Health,
Copenhagen, we tested the reliability, which was found to be good (r=0.90, n=36
health care students, not published).
23
Physical Fitness and Low Back Pain
In paper III, the balance was tested as the ability to stand on one leg with the eyes
open for 60 seconds (one-leg-standing test) (116,120). The participants placed the
heel of the opposite foot against the inner side of the supporting leg at the level of
knee joint. The arms hung as relaxed as possible down the sides. The subjects were
first instructed to familiarize themselves with the balance position and to choose
the supporting leg where they felt most comfortable. The subjects were told to
stand in the position for as long as possible and as steadily as possible. The reliability was reported as acceptable for field testing of fitness (ICC=0.76)(120).
2.3 SELF-ASSESSED PHYSICAL FITNESS
Identical designs for self­assessment of physical fitness were applied in all studies.
The five components of physical fitness that were considered essential when co­
ping with physically demanding tasks were self­assessed: aerobic fitness, muscle
strength, endurance, flexibility, and balance. The answers were measured by five
VAS of 100 mm with illustrations and verbal anchoring of the extreme situations
(Figure 2). The subjects were asked: “How would you score the following compo­
nents of physical fitness in relation to people of your own age and sex?” and they
indicated their replies as vertical marks on the VAS. The VAS scores were computerized by a digitizer (Intuos A4 regular, Wacom Co Ltd.) which showed a high
degree of intraobserver reliability (ICC2.1=1.00, 115 double VAS readings) and
interobserver reliability (ICC2.1= 1.00, 26 VAS readings by six observers). The selfassessment instrument was designed in a group of PhD’s, associate professors and
candidates with an educational background of sports science or physiotherapy
from the department of work physiology at the national institute of occupational
health. The final graphic design was made by Christian Hye­Knudsen.
2.4 LOW BACK PAIN
Low back pain was defined as tiredness, discomfort or pain in the low back region
with or without radiating symptoms to the leg or legs. The low back region was
defined as the region of the back between L1 and the gluteal folds.
In the prospective study (paper I), cases were defined as subjects with an increase
of more than two at follow-up in self-reported rating of the LBP intensity during
the past twelve months (one indicating no pain and 10 indicating worst possible
pain). It has been shown that an increase in LBP intensity above two was clinically
relevant among chronic LBP patients (35). Due to the high prevalence of LBP in
the study population it would not had been possible to include only the “never ex­
perienced LBP” subgroup if the analyses should have sufficient statistical power.
Only subjects with LBP intensity below 6 at baseline were included in the analyses
to ensure that all subjects had the possibility of being a case.
In the cross-sectional study (paper III), the 12-month prevalence of LBP was included in the analyses with physical fitness parameters as independent variable.
Although the 12-month prevalence deals with the experience of LBP during the
previous year it was regarded as a measure of current status of non­specific LBP.
LBP questions were assessed using the Standardized Nordic Musculoskeletal
Questionnaire (30,73).
24
Physical Fitness and Low Back Pain
2.5 COVARIATES
The definition and classification of selected covariates are described. For a com­
plete description of the covariates see the original papers.
In paper I, seniority was defined as years working with physically and mentally
disabled people. 0-10 yr (low), >10 yr (high). Body mass index (BMI) was classified into three groups: <=24.9 (normal weight), 25­30 (overweight), >30 (severe
overweight). Leisure time physical activity was assessed by a modified version of
the item of Saltin and Grimby (110): 0-4 hours/week (low level activity), >4hours/
week (high level activity). Previous LBP history was defined as: no period ever
with 3­month persistent LBP (no); at least one 3­month period with persistent LBP
(yes).
The frequency during the workday of bent back was classified to: never­some­
times (seldom); often­very often (often). The frequency during the workday of
rotated back was classified to: never­sometimes (seldom); often­very often (often).
Physical workload during patient-related tasks was assessed by a continuous scale
(0­14) and dichotomised into: 0­5 (light); 6­14 (strenuous). The psychosocial scales:
influence at work, quantitative job demands, emotional job demands, and cogni­
tive job demands, were measured using the Copenhagen Psychosocial Questionnaire (COPSOQ) (71) and classified into tertiles.
In paper III, A history of physical demanding job was defined as more than 6
month in the past on the basis of an open question about their previous occupations and duration. Educational attainments was categorized as low (≤9 years)
or high (>9 years). Height was measured by an electronic height measuring unit
(SOEHNLE Professional GmbH & Co. KG Postfach 1308, D - 71536 Murrhardt).
BMI was categorized as in paper I.
2.6 STATISTICS
The prospective associations with increased LBP intensity as outcome (paper I)
was tested with multivariate logistic regression analyses. Only physical fitness
parameters with a sex and age adjusted association to the outcome at p<0.20 were
tested further. This was done in parallel multivariate logistic regression analyses
including those covariates in each analysis that showed an isolated association
(sex and age adjusted) to the outcome at p<0.20. Sex and age were fixed factors in
all the analyses. The choice of the relatively high significance level as the screening
criterion for variable selection was to ensure that all important variables were
identified (55). Due to the relatively few data and thereby limited statistical power
no interactions terms were included in the analyses.
The validity of self­assessed physical fitness examined in paper II was evalu­
ated by convergent and divergent validity analyses based on age-adjusted linear
regression analysis of self­assessed physical fitness parameters with performance­
based parameters (43,59). Convergent validity was evaluated by the size of the
age­adjusted correlation coefficient between corresponding parameters according
to the criteria of Innes, (59) who defines r<0.30 as poor, r≥0.30 as moderate, and
r≥0.60 as good convergent validity. If the convergent validity was satisfactory, the
divergent validity was further evaluated by correlations of self-assessed compo25
Physical Fitness and Low Back Pain
nents with non­corresponding parameters of performance­based physical fitness.
Because different components of physical fitness would not be totally unrelated,
the criterion for a satisfactory divergent validity was a markedly lower correlation
between non-corresponding parameters than corresponding parameters and was
not necessarily a non­significant relationship. The construct validity analyses were
stratified by sex because of the very skewed distribution between women and
men. The reliability was assessed by ICC values that take the number of subjects
and test sessions into account (29), and by confidence intervals of the difference
between test sessions to test for systematic changes. The ICC values were evaluated according to the reliability criteria of Innes (58), who defines ICC<0.75 as
poor to moderate, ICC>0.75 as good, and ICC≥0.90 as “required for clinical ap­
plication to ensure valid interpretations of findings”. A significance level of p<0.05
was chosen.
In paper III the association between LBP and the physical fitness parameters was
described stratified to sex but analysed in the total group of both sex. The corre­
lations between LBP and the continuous physical fitness variables (the self­asses­
sed components and performance­based flexibility) were tested by GLM analyses
with the physical fitness score as the dependent variable and LBP and covariates
as independent variables. The correlations between the dichotomy physical fitness
variables (the back endurance and balance tests) were tested by a logistic regression model with the test result (failed/passed) as dependent variables and LBP
and covariates as independent variables. A sex adjusted and a full model analysis,
that included all the covariates, were performed.
The ability of self­assessed endurance, flexibility and balance to discriminate the
persons who failed the corresponding performance-based test from the ones who
passed the test, were tested by general linear models (GLM) with the self-assessed score as the dependent variable and the performance-based (passed/failed)
and covariates as independent variables. To examine the effect of including the
covariates, an analysis only adjusted to sex was performed first, then an analysis
including the remaining covariates except from LBP, and then finally a full model
including LBP. Significance level of p<0.05 was chosen. The SAS statistical soft­
ware was used for all the analyses (PROC GLM was used for linear regression
analyses and PROC GENMOD for logistic regression analyses).
26
Physical Fitness and Low Back Pain
3. RESULTS
The main results from the different studies are presented under the following
headings: Basic characteristics of the study populations, Associations between
Performance­based physical fitness and LBP, Associations between Self­assessed
physical fitness and LBP, Covariates with associations to LBP, and Characteristics
of self­assessed physical fitness.
3.1 BASIC CHARACTERISTICS OF THE STUDY POPULATIONS
Some basic characteristics of the study participants are listed in table 2. In all the
study populations the women were in the majority, ranging from 81 to 95%. The
people working with physically and mentally handicapped persons of study IA
and IIA V were older than the other study populations, which were relatively
equal in age (31-35 years). No differences were found in BMI.
Table 2. Basic characteristics of the study participants. Mean (SD) or n (%)
Paper
I
II
II
II
II
III
Study
status of
population occupation
IA
IIA
IIB
IIC
IID
IIIA
in job
in job
students
in job
students
students
Sex
Women
271 (83%)
530 (81%)
91 (95%)
170 (91%)
149 (94%)
511 (83%)
Men
Age
years
BMI
kg*m-2
56 (17%)
123 (19%)
5 (5%)
17 (9%)
10 (6%)
101 (17%)
47 (9)
45 (9)
31 (12)
34 (9)
35 (11)
33 (10)
25 (5)
25 (5)
24 (5)
24 (4)
24 (4)
25 (5)
3.2 ASSOCIATIONS BETWEEN PERFORMANCE-BASED
PHYSICAL FITNESS AND LBP
Isometric back extension endurance measured using the Sørensen method was
the only performance-based parameter that showed any kind of association with
increased LBP intensity in the prospective study (paper I). In the multivariate
analyses of the prospective study comprising people working with physically
and mentally disabled persons, the general association with increased LBP intensity was only close to significance (p=0.067) (table 4). However, medium level
isometric back extension endurance resulted in a more than two-fold higher risk
of increased LBP intensity after the 30-month follow-up compared to those with
high level back extension endurance (OR=2.7, p=0.034, Wald test) (table 4). Low
level isometric back extension endurance showed an almost similar but insignifi­
cantly higher risk of increased LBP intensity (OR=2.4, p=0.076, Wald test). After
stratification by frequency of rotated back, which was the only covariate included
in the multivariate model except for age and sex, neither the subgroup exposed to
low physical demands nor the subgroup exposed to high physical demands was
significantly associated with increased LBP intensity.
27
Physical Fitness and Low Back Pain
Table 3. Associations of physical fitness parameters at baseline with increased LBP intensity
at 30-month follow-up, adjusted to age and sex (paper I).
N
Performance­based physical fitness
Back extension endurance
High
116
Medium
104
Low
96
Back flexion endurance
High
108
Medium
97
Low
104
Sagittal flexibility
High
104
Medium
103
Low
109
Balance
High
93
Medium
89
Low
97
Self­assessed physical fitness
Aerobic fitness
High
109
Medium
112
Low
106
Muscle strength
High
111
Medium
107
Low
109
Endurance
High
115
Medium
112
Low
100
Flexibility
High
121
Medium
106
Low
100
Balance
High
111
Medium
113
Low
103
% cases
OR
95% CI
p (Wald)
8
15
15
1
2.26
2.18
0.94-5.47
0.87-5.44
.
0.069
0.095
9
12
14
1
1.58
1.92
0.63-3.92
0.79-4.67
.
0.33
0.15
14
11
11
0.74
0.85
0.32-1.71
0.37-1.96
.
0.48
0.70
11
11
16
1
0.97
1.56
0.37-2.52
0.63-3.86
.
0.94
0.33
17
8
12
1
0.37
0.58
0.16-0.87
0.26-1.29
.
0.02
0.18
15
13
9
1
0.82
0.51
0.38-1.78
0.22-1.18
.
0.62
0.11
16
10
12
1
0.54
0.66
0.24-1.21
0.29-1.46
.
0.13
0.30
16
9
12
1
0.54
0.74
0.24-1.23
0.34-1.62
.
0.14
0.45
12
12
14
1
0.91
1.02
0.40-2.08
0.44-2.34
.
0.83
0.97
p (LR)
0.12
0.33
0.77
0.47
0.059
0.27
0.29
0.32
0.96
Wald: Wald test for level vs. reference level. LR: Likelihood ratio test for type3 signifi cance.
However, the ORs among those highest exposed were markedly higher than the
lowest exposed (3.9/3.2 vs. 2.0/1.8). Isometric back flexion endurance, sagittal fle­
xibility and balance showed no significant prospective associations with increased
LBP intensity.
In the cross-sectional study (paper III), LBP was included in the analyses as independent variable and tested for associations with self-assessed and performance28
Physical Fitness and Low Back Pain
based physical fitness (table 5). Both in the sex­adjusted and in the full model
analyses (adjusted to educational attainments, history of physical demanding job,
BMI, height, age and sex), LBP during the previous year was significantly associ­
ated with poorer flexibility. Although, the associations of LBP with the back endu­
rance tests and the balance test did not reach statistical significance (p=0.12­0.15),
a general picture was found that the percentages of persons who passed the tests
were highest in the group without experience of LBP during the previous year.
Table 4. Multivariate logistics regression analyses of physical fitness parameters adjusted to
sex, age, and rotated back with increased LBP intensity at 30-month follow-up (paper I).
OR
Full model
95%CI
p
Performance-based back extension endurance
High level
1
Medium level
2.71
1.08-6.79
Low level
2.37
0.91-6.14
0.067 A
Self­assessed aerobic fitness
High level
Medium level
Low level
0.066A
1
0.37
0.58
0.15-0.88
0.27-1.38
0.034
0.076
0.02
0.23
Likelihood ratio test.
A
3.3 ASSOCIATIONS BETWEEN SELF-ASSESSED
PHYSICAL FITNESS AND LBP
In the prospective study, persons with medium level self­assessed aerobic fit­
ness were at a significantly lower risk of increased LBP intensity than those
with high level self­assessed aerobic fitness (OR=0.37, p=0.02) (table 4). Persons
with low level self­assessed aerobic fitness did not reduce their risk significantly
(OR=0.58, p=0.23) and the general association of aerobic fitness was also insig­
nificant (p=0.066) (table 4). Self­assessed muscle strength, endurance, flexibility
and balance were not strongly enough associated with increased LBP intensity to
be included in multivariate analyses (p>0.20). However, it was notable that the
ORs of self­assessed strength, endurance and flexibility were all below 1 (table 3),
indicating an association between low level of LBP and decreased risk of increased
LBP intensity, which was the same as that found for aerobic fitness.
In the cross-sectional study of health care students, experience of LBP in the previous 12­month was highly significantly associated with lower score of self­asses­
sed aerobic fitness, both in the sex­adjusted and the full model analysis (table 5).
Additionally, LBP was significantly associated with lower self­assessed flexibility
and balance. The reduction in significance level when the association between LBP
and self-assessed balance was analysed in the full model compared with the sexadjusted was due to the inclusion of BMI in the analysis.
29
Physical Fitness and Low Back Pain
Table 5. Cross­sectional associations between LBP and physical fitness variables (paper III)
12-month prevalence of Association between LBP
LBP
and the physical fitness
All n=612
parameter†
yes
no
sex
full model‡
adjusted
Self­assessed physical fitness
Aerobic fitness (mm), mean (SD)
Muscle strength (mm), mean (SD)
Endurance (mm), mean (SD)
Flexibility (mm), mean (SD)
Balance (mm), mean (SD)
43 (19)
50 (18)
53 (20)
49 (21)
55 (19)
51 (20)
52 (18)
56 (19)
55 (19)
59 (20)
p<0.0001
p=0.39
p=0.14
p=0.0012
p=0.0061
p<0.0001
p=0.28
p=0.18
p=0.0082
p=0.026
Performance­based physical fitness
Back ext. endurance, n (%) passed 180s
114 (50%) 217 (58%) p=0.17
p=0.15
Back flex. endurance, n (%) passed 180s
93 (40%) 182 (49%) p=0.10
p=0.15
Flexibility (cm above floor level), mean (SD)
­1 (10)
­3 (9)
p=0.0011 p=0.0066
Balance, n (%) passed 60s
188 (82%) 299 (86%) p=0.13
p=0.12
†GLM performed for continuously variables and multivariate logistic regression analyses for dichoto­
mous variables, physical fi tness as dependent variable. ‡ Adjusted to educational attainments, history
of physical demanding job, BMI, height, sex and age.
3.4 COVARIATES WITH ASSOCIATIONS TO LBP
Frequency of rotated back was the only covariate of the prospective study that
was sufficiently associated with increased LBP intensity to be included in the mul­
tivariate model (table 3). It remained insignificant in the multivariate analyses but
changed the estimate of performance-based back extension endurance (table 4).
Among women, the risk of increased LBP intensity was almost three times higher
in the prospective study, but it did not reach significance (OR=2.9 p=0.083) (data
not shown). The association between LBP and covariates was not tested in the
prospective study
3.5 CHARACTERISTICS OF SELF-ASSESSED PHYSICAL
FITNESS
3.5.1 Distribution, sex differences and inter-item correlations
The full lengths of the VAS were satisfactory utilized among both women and
men. We observed that the participants showed a quick and intuitive feeling
of where to score their physical fitness on the VAS. Normal distributions of all
VAS scores were assumed due to skewness and kurtosis values between one and
minus one (study population IIA­C). Men’s mean score of self­assessed aerobic
fitness, muscle strength, endurance, and balance were significantly higher than the
women’s in the study populations of both paper II and III. The men scored their
flexibility significantly lower than did the women of paper II, whereas no diffe­
rence was found according to sex in paper III.
It appears from table 6 that strong inter-item correlations were observed between
the self­assessed physical fitness components (r=0.29­0.58, paper II).
30
Physical Fitness and Low Back Pain
3.5.2 Convergence and divergence of self-assessed and performance-based
fitness
In women, poor-to-moderate convergent validity was found for self-assessed
aerobic fitness, muscle strength and flexibility (r=0.30­0.36), whereas endurance
and balance showed poor convergent validity (r=0.05-0.16) (table 7). The divergent
validity of self­assessed aerobic fitness, muscle strength and flexibility was satis­
factory due to markedly weaker correlations with non-corresponding performance­based parameters (table 7). In men, aerobic fitness and muscle strength showed
moderate­to­good convergent validity (r=0.51­0.64) and flexibility showed low­
to­moderate convergent validity (r=0.31). Self­assessed aerobic fitness and muscle
strength showed satisfactory divergent validity, whereas self­assessed flexibility
was highly correlated, although not significantly, to oxygen uptake and isometric
back flexion strength.
The analyses of paper III supported a convergence between the performance-based flexibility result and the self­assessed flexibility score (table 8) due to the abi­
lity of the self-assessed score to discriminate between the persons who performed
in the lower half from those who performed in the higher half. The associations of
both back endurance tests and the balance test with their corresponding self-assessed scores were also highly significant, and no confounding of LBP was found in
any of the analyses.
The reliability of self­assessed aerobic fitness and muscle strength was good
(ICC=0.80), whereas the reliability of flexibility, endurance, and balance was only
moderate (ICC=0.62-0.69) (Paper II).
Table 6. Inter­item correlation coefficients* of self­assessed physical fitness
scores. In total 935 women and men (study population IIA-C).
N=935 women and men*
1. Aerobic fitness
2. Muscle strength
3. Endurance
4. Flexibility
5. Balance
1
2
0.41
3
0.58
0.49
*All correlations were significant at p<0.0001.
31
4
0.34
0.29
0.35
5
0.31
0.32
0.38
0.36
Physical Fitness and Low Back Pain
Table 7. Age­adjusted correlations coefficients of self­assessed components with performan­
ce­based parameters of physical fitness. (paper II).
Group/Measure
Self­assessed components of physical fitness
Aerobic Muscle Endurance Flexibility Balance
fitness
strength
Women
Study population IIC (n=170)
Maximal VO2
0.36‡
0.11
0.20†
­0.02
­0.02
Back extension strength
-0.01
0.30‡
0.11
0.08
0.05
Back flexion strength
­0.01
0.34‡
0.16
0.05
0.01
Study population IIA+B (n=620)
Back extension endurance
0.21‡
0.12†
0.16‡
0.22‡
0.08
Back flexion endurance
0.20‡
0.08*
0.18‡
0.12†
0.09*
Sagittal flexibility
0.17‡
0.17‡
0.14‡
0.36‡
0.06
Balance
0.07
-0.01
0.01
0.06
0.05
Men
Study population IIC (n=17)
Maximal VO2
0.64†
0.41
0.77‡
0.32
0.32
Back extension strength
0.04
0.51*
0.19
0.18
0.45
Back flexion strength
0.03
0.67†
0.31
0.50
0.61*
Study population IIA+B (n=128)
Back extension endurance
0.18*
0.10
0.15
0.27†
0.20*
Back flexion endurance
0.04
0.01
0.08
0.11
0.08
Sagittal flexibility
0.30‡
0.13
0.13
0.31‡
0.08
Balance
0.02
0.13
0.03
0.08
0.25
Note. Vertical, performance based measures of physical fitness. Correlation coefficients
between corresponding parameters in boldface. * p<0.05, † p<0.01, ‡ p<0.001.
Table 8. Stepwise adjusted associations between performance-based and self-assessed
physical fitness (paper III).
Back ext. endurance (passed 180s)
All n=612
Association with self­assessed fitness
VAS score
sex
full model‡
test result
n mean (SD) adjusted without LBP with LBP
failed
263
51 (19)
p<0.0001
p=0.002
p=0.001
passed
330
57 (20)
Back flex. Endurance (passed 180s)
failed
passed
314
279
50 (19)
59 (19)
p<0.0001
p<0.0001
p<0.0001
Flexibility (dichotomized)†
low
high
293
297
48 (20)
58 (18)
p<0.0001
P<0.0001
p<0.0001
Balance (passed 60s)
failed
passed
90
503
49 (23)
59 (18)
p<0.0001
p=0.0004
P=0.0008
Mean (SD). †The Sagittal flexibility scores were dichotomized according to the distribution
in the absence of a “passed value”. ‡ Adjusted to educational attainments, history of physi­
cal demanding job, BMI, height, sex and age in a GLM analysis.
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Physical Fitness and Low Back Pain
4. DISCUSSION
4.1 PERFORMANCE-BASED PHYSICAL FITNESS AS
RISK INDICATOR OF LBP
Among the four performance-based parameters we measured in the prospective
study, only isometric back extension endurance showed significant association
with increased LBP intensity at follow-up.
The result that lower back extension endurance was associated with the risk of
increased LBP intensity is in concordance with our hypothesis of an association
between poor physical fitness and higher risk of increased LBP intensity. Howe­
ver, there was no indication of a dose-response relationship and the almost similar
ORs for low and medium level compared to high level back extension endurance
suggest that those with low and medium level were in same risk of increased
LBP. The mechanism leading to LBP among those with lower physical fitness was
described by McGill (88). Poor physical fitness implies a higher relative physi­
cal workload during the workday (other factors being equal). Consequently, the
margin of safety, defined as the distance between the failure tolerance and the ap­
plied physical workload decreases, and the risk of exceeding the failure tolerance
increases. A load that exceeds the failure tolerance produces injury, which causes
pain.
McGill showed also that previous history of LBP is related to lingering deficits in
biomechanical, physiological, and motor control characteristics (87). Hence the
level of physical fitness may be due to a history of LBP. Persons with low physical
fitness may simply be less physically active during leisure time. It is unknown
which approach dominates, but it is hoped that the aggravation of LBP can be
prevented in both groups by physical fitness training.
The observation in this study that low back extension endurance measured using
the Sørensen method, associates with increased risk of LBP are in agreement with
some previous studies (16,17,45,79). However, other studies have found no association between back extension endurance and LBP (3,64,115,122). A significant
association between LBP and back extension endurance was found in both representative (16,45), and specific populations (79), among men (16,17), and in mixed
populations (45,79) and using different LBP outcome definitions. These results
strongly indicate that isometric back extension endurance could be an important
risk indicator of LBP, regarding different LBP outcomes. Although there is a lack
of representative studies among those studies who found no relationship between
physical fitness level and LBP there are too many studies with negative findings to
consider back extension endurance as a consistent risk indicator of LBP.
One reason for the strong indication of a causal relationship between back extension endurance and LBP is that the Sørensen method or a modified version
has become the preferred method for measurement of back extension endurance
(27,93), which has increased the comparability between studies. In contrast no
33
Physical Fitness and Low Back Pain
consensus exists regarding the definition and measurement of balance (103). Espe­
cially in field research where the use of heavy and advanced technical equipment
is limited, only very few tests are applicable. In the prospective study balance was
measured using a newly developed method that was designed to test a balance
parameter with particular relation to the low back region; however, no association
with increased LBP intensity was found and the lack of comparability with other
studies limits a deeper exploration. Although the balance test we chose for the
cross-sectional study has been evaluated to be a reliable and feasible health-related
balance test (119,120) and to be associated with back health (among women) (121)
we found no association with LBP in the cross-sectional study (paper III).
Back flexion endurance and sagittal flexibility was not associated with increased
LBP intensity in the prospective study. According to my knowledge, only one study has examined the association of isometric back flexion endurance with future
LBP (68) and another one the association of dynamic trunk flexion with future LBP
(later register based disability due to LBP) (109). Both studies found no association with future LBP. Contradictory results about the association between sagittal
flexibility and LBP­related outcomes have been reported in three studies. Sørensen
(16) found that high sagittal flexibility among men measured by using the Schober
test, associated with first­time occurrence of LBP, and that high flexibility among
women measured using the finger­to­floor method was associated with recurrent
LBP. Takala (122) and Adams (3) found that low flexibility was associated with
the occurrence of LBP, while high flexibility among women was associated with
medical consultation. In addition low sagittal flexibility has been found to be a
predictor of future back pain as reported in a study of 3,020 aircraft manufacturing
employees (14). Several other studies find no relationship between flexibility and
future LBP.
One of the hypotheses underlying this thesis was that the level of physical work
factors may influence the association between physical fitness parameters and
LBP. This hypothesis was not significantly supported by the results of the prospec­
tive study. However, when the prospective data were stratified by “rotated back”,
as an indicator of physical work load, the insignificant ORs among the highly
exposed became twice as high compared to the insignificant ORs among the low
exposed. This finding was considered to be an indication of poor back extension
endurance as a stronger risk indicator among those exposed to high physical
demands compared to those with low physical demands at work. More research
is needed to further explore these findings, especially because a recent study by
Hamberg­van Reenen et al. find no differences in risk estimates in relation to
LBP between two groups with poor back extension endurance that were exposed
to either low or high physical demands (46). It would be interesting to design a
future study with enough statistical power to include interaction terms between
physical fitness and physical work factors.
The cross-sectional study could not add knowledge to this area due to the lack of
work factors among students; however, the inclusion of the person’s previous ex­
perience with a physical demanding job in the analysis was to test if it influenced
34
Physical Fitness and Low Back Pain
the cross­sectional association between LBP and the physical fitness parameters. It
did not.
4.2 SELF-ASSESSED PHYSICAL FITNESS AS RISK
INDICATOR OF LBP
It was unexpected and contrary to our hypothesis that medium level of self-assessed aerobic fitness significantly decreased the risk of increased LBP intensity at fol­
low­up compared with those with high level self­assessed aerobic fitness. Self­as­
sessed muscle strength, endurance and flexibility indicated a similar relationship
but did not reach a statistically significant level.
The cross-sectional study (paper III) showed, in contrast to the prospective study,
an association between experience of LBP during the previous year and lower selfassessed aerobic fitness, flexibility and balance in both the sex­adjusted and full
model. An explanation of the association in the prospective study could be that
self­assessed physical fitness was different associated with the LBP variable in the
two baseline populations. However, this was not the case.
Baseline analyses of the study participants in the prospective study (only those
with follow­up data) showed that poor self­assessed physical fitness was asso­
ciated with higher LBP intensity (r=-0.07-(-0.11), p=0.045-0.21 raw correlations
between the five dimensions of self­assessed physical fitness and reported mean
LBP intensity during the previous year). Some of the explanation can be found in
the characteristics of those who drop out during the follow-up time. They were
characterized by having more pain but the same level of self-assessed physical
fitness as the remaining group (paper I). This suggests that the drop out persons fit
better with our hypothesis of low physical fitness as associated with increased LBP
than the remaining group in the study.
A hypothetical explanation of the finding of poor self­assessed fitness as pre­
ventive against increased LBP intensity could be that those who self-assess their
physical fitness to be low are particularly aware of their reduced capacity and act
accordingly. Consequently, they may change their behaviour at the workplace to
reduce their physical work demands, which further reduces the risk of their failure tolerance being exceeded, due to a increased margin of safety (88). As a result,
the risk of increased LBP intensity at follow-up decreased, not because of their
lower level of physical fitness, but because of a changed behaviour that reduces
their physical exposure. Not even detailed physical exposure assessment during
the study period, which is an extremely expensive solution, would register such
changes in behaviour with certainty. An alternative way to exploring the association would be to combine the quantitative survey with a qualitative approach, for
example by interviewing those with low self­assessed level of aerobic fitness who
reported a decreased intensity of LBP at follow-up.
To further elucidate the association of physical work demands and the association between LBP and level of physical fitness, the variable “history of physical
35
Physical Fitness and Low Back Pain
demanding job” was included in the cross­sectional analyses (paper III) of health
care students in training (free of physical work demands). However, the inclusion
of physical demanding job in the analyses between LBP (12-monmth prevalence)
and self­assessed physical fitness did not affect any of the associations. Another
variable introduced in the analyses of paper III was educational attainments as a
proxy for the social context for the individual. Educational attainments were only
significantly associated with self­assessed aerobic fitness but did not affect the as­
sociation between LBP and self­assessed aerobic fitness or any other of the self­as­
sessed dimensions.
Other methodological approaches could be introduced to understand the nature of
self­assessed physical fitness better and to explore why poor self­assessed fitness
decreases the risk of LBP aggravation. The introduction of a cognitive-behavioural
approach as that used when explaining the cognitive-behavioural model of fear of
movement/(re)injury (128) could inspire to a model that explain the link between
the awareness of the physical fitness level and a possible change in behaviour
which may lead to either reduced or increased risk of LBP.
More knowledge is needed about which psychological factors and how they influ­
ence the self­assessment of physical fitness using VAS to elucidate what we really
measure and how context dependent the measures are compared to objectively
measured performance­based physical fitness (39,94,131).
4.3. SELF-ASSESSED COMPARED WITH
PERFORMANCE-BASED PHYSICAL FITNESS
4.3.1 Associations between self­assessed and performance­based physical fitness
No self-assessed dimensions were strongly related to their corresponding performance-based measures. Weak to moderate convergence was found for the dimensions of aerobic fitness, muscle strength and flexibility which also had satisfactory
divergence, except for men’s self­assessed flexibility.
The three self-assessed dimensions which showed the best convergence with their
performance-based counterparts had correlations ranging 0.30-0.36 which corresponded to a shared variance of 9-13%. A higher convergence was found among
the 17 men in study population IIC (26-41% of the variance was shared) but due
to the group size these results can be regarded only as indicative. This degree of
convergence seems too low for an interchange of performance-based physical
fitness with self­assessed fitness at either individual level or when comparing
smaller groups, which neither was expected. However, in larger epidemiological
studies comprising a large number of persons, self-assessed aerobic capacity, muscle strength and flexibility may give a useful estimate of the actual physical fitness
level.
The analyses of association between corresponding physical fitness parameters in
paper III supported an association between the result of the flexibility test and the
VAS score which was highly significant. Additionally, the analyses also showed
significant associations between isometric back extension and flexion endurance,
and balance (one-leg standing), which were not found in the validity study.
36
Physical Fitness and Low Back Pain
However, the results are difficult to compare directly due to the dichotomy test
result for the test used in paper III and the consequently different statistical analyses. Interpreting the results of the agreement examined in paper III one might
argue that if the VAS scores are highly inter-correlated it would not makes any
difference which VAS score we use to discriminate between those who failed and
passed a test. In general, this was not the picture. For all performance-based tests,
except for back extension endurance, the corresponding VAS score discriminated
best (largest difference) between those who passed and failed the test compared
to the non-corresponding VAS scores (data not shown). In relation to the back
extension endurance test, self­assessed aerobic fitness and self­assessed endurance
showed both a significant difference of 6 mm between those who failed and those
who passed the test. Thus indicating a failed divergence validity. This result fits
well with the finding in paper II of a lacking validity for self­assessed endurance
that showed a high inter­item correlation with self­assessed aerobic fitness.
The face validity was in general good. Most of the participants only used few
seconds to consider where to put the marks on the VAS and they differentiated
clearly between the different dimensions, even though high inter-item correlations
were found. However, the absence of convergence for self-assessed endurance
in relation to the performance­based back extension and flexion endurance tests
may be due to a lack of content validity of the endurance questions in relation to
the isometric back endurance tests. The endurance question was illustrated with
two persons climbing a mountain, and did not correspond with local back muscle
endurance parameters. In future studies redesigning the instrument to measure
a more specific muscle endurance component will be considered. Self­assessed
balance did not correspond with its performance-based counterpart in the validity
study. However, as discussed in paper II it was a positive finding, at least among
women, that self-assessed balance did not correlate with non-corresponding performance-based measures, which indicated that self-assessed balance may have
the promising quality of possessing specificity. Paper III supported this finding by
the highly significant association between the balance test result and self­assessed
balance, only among women.
It seems that the one­leg balance test correspond more well with people’s per­
ception of balance, although a differential reporting of self-assessed balance, in
particular might be present.
This additional result supports that self-assessed balance share enough variance
with performance-based balance to be a rough estimate and that the degree of
concordance can be dependent on sex. If both performance-based and self-assessed balance is differential in relation to sex it should be considered to use different
balance test for evaluating women and men. This could increase the variation of
the test score among each sex and thereby increase the statistical applicability.
No other instruments for self­assessment of physical fitness among healthy people
used VAS to score the replies, whereas an earlier study was found that used VAS
for measuring of functional capacity (overall, chosen and pre-selected function)
among patients with rheumatoid arthritis receiving long-term treatment (112).
37
Physical Fitness and Low Back Pain
Two different strategies are used for self­assessment of physical fitness. The one
is to ask for a relative scoring of the respondent’s physical fitness using peers, as
we did when the respondents were asked to compare their fitness with people at
same age and sex. The other strategy is to use absolute standards, by asking the
respondents to estimate how well they can perform a specific physical fitness task
or how much or for how long time they can sustain a given physical fitness task
(e.g. walking, biking, carrying bags, walking steps, etcetera).
Due to the different types of instruments, the variability, and especially the different degree of awareness of physical fitness in the population being measured, it is
difficult to compare the few studies that examine the association between self­as­
sessed and performance­based physical fitness.
For instance one study comprises soldiers which is a group with particularly
good conditions for knowing their own level of fitness (69) while another study
comprises subjects with exactly same age and only the extreme groups selected
on the basis of a physical fitness test performed 15 years earlier. Thus the conside­
ration for age was eliminating in addition with an implementation of an artificial
big contrast in the data (as the authors mentioned them selves) (90). One of the
better validated (two studies) and promising instruments with better convergence
between corresponding parameters is the one described by Abadie (2) which
found valid assessment among younger persons (<50 years) of cardio-respiratory
endurance (aerobic fitness) (r=0.43­0.61), muscular strength (r=0.47) and muscular
flexibility (r=0.53). The components of physical fitness they found most valid was
the same as we found most valid in our validity study (paper II) but the validity
of their instrument was better than the one we found. However, the instrument by
Abadie proved unsatisfactory for use among elderly people (2,126) and the divergence among younger persons has not been reported. No studies were found that
examined the association between one of the above mentioned instruments for
self­assessment of physical fitness with LBP, neither cross­sectionally nor prospec­
tively.
4.3.2 Predictive validity
The comparison of self­assessed and performance­based fitness’ ability as risk
indicators of increased LBP intensity in the prospective study can be regarded as a
test of the predictive validity of the self-assessed instrument. None of the self-assessed physical fitness parameters were able to show the expected association
between low level and increased LBP intensity. Hence, poor predictive validity according to the hypothesis was found for aerobic fitness, muscle strength, enduran­
ce, and balance. As previously mentioned it is debatable if low or high flexibility
prevents against LBP or if an association exists at all, however, neither high nor
low level of self­assessed flexibility was associated with increased risk of increased
LBP intensity.
The low correlation of the five physical fitness VAS items with their corresponding
performance-based parameters and the high correlations between the self-assessed physical fitness dimensions suggest that the VAS items could be combined in
a scale measuring overall physical fitness. The fact that four out of five VAS items
show the same inverse association with LBP intensity also points in this direction.
38
Physical Fitness and Low Back Pain
The internal consistency was assessed using the Cronbach coefficient alpha (25).
On the basis of data from study population IIA-C (n=935) a value of 0.76 was
calculated and using data from study IIIA (n=612) yielded a value of 0.79. This
supports the idea of combining the five VAS items in one scale. The argument for
combining these items in a scale is that it results in a more precise and consistent
measurement of physical fitness, yielding better analyses of associations with
relevant outcomes. Several scale construction approaches exist (36). For these
continuous VAS items factor analysis would be logical to start with, and analysis
of differential item functioning could be done in this framework. Item response
theory models (125) for VAS items have also been proposed (23).
4.4 METHODOLOGICAL CONSIDERATIONS
4.4.1 Limitations and strengths of the thesis
One of the main limitations of this thesis was the low numbers of men present in
the study populations (5-19%). This implies that the results are much more comparable to other groups or studies of women than of men. However, the ratio of men
reflects the actual ratio of men employed and studying in the health care sector. In
the validity study the analyses were stratified to sex because of different ratios of
men in the subpopulations of paper III.
Due to a higher ratio of men in the prospective (paper I) and cross-sectional study
(paper III) compared to the validity study (paper II), the data from both sexes were
in these studies examined together in age-adjusted analyses, although many of the
data are presented in the tables stratified by sex.
The many study populations used in the thesis can both be seen as a limitation
and as a strength. The validation study would have been stronger if all performance based parameters were assessed in the same population. The validity of self-assessed aerobic fitness and muscle strength was tested in one population while the
validity of endurance, flexibility, and balance was tested in another. Consequently,
the differences in validity between these two groups of self-assessed parameters
could be due to differences between the study populations. However, the different
study populations used in this thesis made it possible to examine the consistency
of the distribution pattern, the sex dependency and the cross-sectional association
with LBP. This ensured that the basic characteristics for the instrument for selfassessment of physical fitness were not linked to one particular subpopulation
that might possess specific qualities. Although the thesis contained many study
populations, the study populations, as described in the method section, all comprised people who were either employed in areas or training to work in areas
that involved working physically with people. Thus, compared to the variety of
existing occupations the study populations were relatively homogenous.
Selection bias occurred in study population IIIA with a follow-up population only
comprising 30% of the basis population. The drop out groups all experienced
more LBP and were younger compared to those who remained in the survey. The
consequences of this selection on the results are difficult to determine, but it might
39
Physical Fitness and Low Back Pain
be more difficult to find an association with LBP among the remaining healthier
part of the employees, resulting in an underestimation of the association strength.
In the cross­sectional study comprising health care students in their first week of
training (paper III) the response rate was high (89% with completed the questionnaire, 80% were tested and 69% with complete test data and questionnaire) which
imply that the results obtained were representative of the population of health
care students examined.
4.4.2 Differences in test procedures and conditions
When comparing the results between the prospective study comprising people
working with disabled persons and the cross-sectional study comprising health
care students we must be aware that beyond differences in study designs and
study populations also methodological differences existed. Those differences are
important to elucidate if performance-based functional tests are viewed as behavioural assessments in which a subject’s performance during testing is considered
within the unique assessment context (43). The procedures for measuring performance­based back flexion endurance and balance differed between studies. The
isometric back flexion endurance test described by Ito and Hyytianen (56,60) ap­
plied in the prospective study was replaced by the test described by McGill 2003
(87) in the cross-sectional study (i.e. baseline in a RCT) because the test was found
to be more standardizable due to the sit-up posture and furthermore the posture
reduced the load of the neck muscles (i.e. some complained about soreness in the
neck after test termination). The way of measuring balance was changed from the
wobble-board procedure in the prospective study to the simpler one-leg-standing
test in the cross-sectional study to ensure that all participants were able to perform
the test regardless of LBP status, motor skills and courage. The one-leg-standing
test was also believed to be less influenced by the participants’ ability to con­
centrate which was an important factor in the cross-sectional study because the
students were tested together in a gym which unavoidably causes more noise and
it was unfortunately not possible to avoid some interactions between the students
who were tested and those who waited for their turn at one of the five test stati­
ons. However, the test sessions were carried out in a very positive and competitive atmosphere. The limited time allocated to the test sessions was the reason to
the shorter test termination time for back extension endurance and back flexion
endurance (maximum of 180 seconds) in the cross-sectional study compared to
the prospective (maximum of 360 seconds). More people than expected reached
the time limit (of 180 seconds) which may be result of the competitive atmosphere
during the test session. This choise between a well-established but less suitable
method, and an untested but more specific method is a problem that often appears
when selecting performance tests for a survey.
4.4.3 Test­retest reliability of self­assessed physical fitness
Although the reliability of the self-assessment instrument was found to be acceptable it was somewhat lower than we desired. Together with the re-test questionnaire the respondents also received two additional questions concerning
LBP: “Have you experienced LBP during the previous 7 days?” and “score your
average LBP intensity during the previous 3 month” which allowed us to examine
40
Physical Fitness and Low Back Pain
if respondents with low agreement (>20mm) between the repeated occasions had
specific characteristics regarding age, sex or LBP. However, no systematic trends
were found between low agreement of self­assessed physical fitness and LBP
status, changes in LBP status (comparing 7-day and 3-month prevalence at the
two occasions), age or sex. The only systematic trend seen was that those with low
agreement in one dimension more frequently showed low agreements in other
dimensions too.
41
Physical Fitness and Low Back Pain
5. CONCLUSIONS
Physical fitness as risk indicators of LBP
The performance-based and self-assessed physical measurements used in the
study did not show any strong associations with LBP outcomes. Only low back
extension endurance was found to be moderately associated with increased LBP
intensity at follow­up. This result is in agreement with previous findings indica­
ting that back extension endurance measured by the Sørensen method could be
considered a risk indicator of LBP. Opposite to our hypothesis, low level of selfassessed aerobic fitness was shown to reduce the risk of increased LBP intensity.
No other performance­based or self­assessed physical fitness parameters showed
statistical significant associations with increased LBP intensity.
The applicability of the instrument to self­assessment of physical fitness
The weak to moderate agreement between performance-based and self-assessed
physical fitness and the contradicting associations with LBP in the prospective
study point toward self­assessed physical fitness to be a concept different from
performance­based physical fitness. The strong interrelationships found between
the five different dimensions of self­assessed physical fitness further support such
an understanding.
More knowledge about how work factors and other individuals factors influence
the self­assessed dimensions of physical fitness are needed before the instrument
for self­assessment of physical fitness can be implemented in surveys as an alter­
native to performance­based physical fitness. The instrument for self­assessment
might possess characteristics that differ so much from performance-based physical
fitness’ that it shall be considered as a related but independent risk indicator of
LBP.
42
Physical Fitness and Low Back Pain
6. PERSPECTIVES
More prospective studies are still needed to establish more knowledge about
which physical fitness components that can prevent first time occurrence, recur­
rent or aggravation of LBP. Future study populations should optimally comprise
both men and women who are exposed to varied level of physical work demands
during their workday to take the physical exposure level into account. Also
intervention studies are needed to examine how much we can influence the LBP
aggravation by inducing better physical fitness and if a changed level of physical
fitness is as important as the level it self. As a step in that direction the analyses of
the follow-up data of study population IIIA will show if it is possible to improve
the individual capacity already during the training and thereby hopefully reduce
the risk of new LBP episodes upon entry the labour market.
A way of further elucidate the changes in the self­assessed fitness over time and
especially in relation to LBP would be to combine our quantitative designs with
qualitative analyses. These quantitative analyses could comprise interviews with
the marginal groups which self-assess their capacity as low and opposite to our
hypothesis had an unchanged or even decreased LBP intensity. This would give
valuable information about possible changes in behaviour during the follow-up
as we hypothesized could be one of the causes of the unexpected relationship between low self­assessed fitness and decreased risk of increased LBP intensity.
Another possible approach for further research is to use the self-assessment instrument among chronic LBP patient or patients in a rehabilitation process in which
cases the use of performance-based test are limited.
Finally I will support the statement by Bill Marras that the treatment of LBP and
the finding of the causality is a multidisciplinary discipline. Hence we all have to
combine our knowledge irrespective of our scientifically approach to make an ef­
fective prevention strategy.
43
Physical Fitness and Low Back Pain
44
Physical Fitness and Low Back Pain
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52
Physical Fitness and Low Back Pain
The role of physical fitness as risk indicator of increased Low Back
Pain intensity among people working with physical and mentally
disabled persons: a 30-month
prospective study.
Authors:
Jesper Strøyer1 and Lone Donbæk Jensen2
1
National Research Centre for the Working Environment, Denmark,
3
Department of Occupational Medicine, University Hospital of
Aarhus, Denmark
Running title: Physical fitness and LBP
Journal:
SPINE Accepted for publication September 2007.
Corresponding author:
Jesper Strøyer, M.Sc., Ph.D. stud.
National Research Centre for the Working Environment, Denmark.
Lersø Parkalle 105
DK-2100 Copenhagen, Denmark
E-mail: jsa@nrcwe.dk
Tel: (+45) 3916 5477
Fax: (+45) 3916 5201
53
Physical Fitness and Low Back Pain
Abstract
Study Design. A prospective cohort study. Objective. To study if low level of
physical fitness was associated with increased low back pain (LBP) intensity at
30-month follow-up. Summary of Background Data. The evidence of low physical fitness as a risk factor for LBP is inconclusive due to contradictory results.
Methods. The study participants were 327 employees (women=271, men=56) at
institutions for physically and mentally disabled persons. Physical fitness was
measured by tests of back extension and flexion endurance, flexibility and ba­
lance and by self­assessed aerobic fitness, muscle strength, endurance, flexibility
and balance, using visual analogue scales. Low back pain, lifestyle parameters,
and physical and psychosocial work factors were assessed by questionnaires at
baseline and at follow­up. Outcome was defined as an increase above 2 steps
in average LBP intensity during the previous year (0-10). Results. Persons with
low level back endurance showed an insignificantly higher risk of increased LBP
intensity (OR=2.4, p=0.076), whereas persons with medium level back endurance
were at significantly higher risk (OR=2.7, p=0.034) compared with high level back
endurance. The general association between isometric back extension endurance
and increased LBP intensity was insignificant (p=0.067). Persons with medium
level self­assessed aerobic fitness were at lower risk of increased LBP intensity
compared to those with high level (OR=0.37, p=0.02), although the general association of aerobic fitness was insignificant (0.066). Performance­based back flexion
endurance, flexibility, and balance and self­assessed muscle strength, endurance,
flexibility, and balance were not associated with increased LBP intensity. Conclusions. The significant association between medium level back extension endurance
and increased LBP intensity supports the finding of other studies that particularly
back extension endurance is an important physical fitness component in preven­
ting LBP and that the subcomponents of physical fitness are related in different
ways to LBP.
Keywords: Self­assessed physical fitness, Visual Analogue Scale, back extension
endurance, physical job demands, prospective study.
Key Points
•
The evidence of physical fitness as a risk factor for LBP remains unclarified
due to contradictory results in the literature.
•
Associations of low levels of performance­based and self­assessed physical
fitness with increased LBP intensity were studied in a prospective cohort
study with a 30-month follow-up.
•
Persons with medium level back endurance had at significantly higher risk
of increased LBP intensity at follow-up compared with persons with high
level back endurance.
•
The study supports the findings of other studies that back extension endu­
rance, in particular, is an important physical fitness component in the pre­
vention of LBP and that the subcomponents of physical fitness are related in
different ways to LBP.
54
Physical Fitness and Low Back Pain
Mini Abstract/Précis
Performance­based and self­assessed physical fitness associations with increased
LBP intensity were examined prospectively. Persons with medium level back
endurance were at significantly higher risk of increased LBP intensity at follow­up
compared with high level endurance. No other self-assessed or performance-based parameter prevented against an increase in LBP intensity at follow-up.
Introduction
Low back pain (LBP) is a common musculoskeletal disorder leading to extensive
human and socioeconomic consequences 1;2. Many attempts have been made to
establish predictors in order to enhance prevention. Physical work load, such as
working with the back in bent and rotated positions and patient handlings have
been identified as risk factors for LBP 3-7.
Low physical fitness has also been suggested as a risk factor for LBP; however,
evidence is weak due to contradictory results in the literature 8-13. There are many
reasons for these contradictions e.g. the ratio between women and men differs
between studies, some stratify to sex and others do not, the type of population,
the way the physical fitness parameters are measured, the definition of LBP, the
follow-up length, and the level and assessment of physical workload which may
influence the relationship between physical fitness and LBP 14.
Furthermore, the strength of the association between physical fitness and LBP
may depend on the chosen subcomponent of physical fitness, which implies that
a general consensus about physical fitness as a risk factor of LBP is irrelevant. One
of the subcomponents showing promising results as a risk factor of LBP is performance-based back extension endurance12;15­17 , although several studies find no
association between the test performance and LBP10;11;13;18.
This study comprised people working in healthcare which are exposed to relatively high physical demands. Additionally, the healthcare sector is characterized
by a high prevalence of LBP 19-21.
Our aim was to test if a low level of different subcomponents of physical fitness
was associated with an increase in LBP intensity at 30-month follow-up among
people working with physically and mentally disabled persons.
55
Physical Fitness and Low Back Pain
Materials and methods
Design and population. The study was a prospective cohort study comprising a
self­administered questionnaire and performance­based physical fitness tests at
baseline and a questionnaire at 30-month follow-up. The cohort participation rate
at baseline and follow­up are outlined in figure 1. We invited 1106 employees
from all institutions for physically and mentally disabled persons in the county
of Aarhus, Denmark to participate. Of these 800 (72%) accepted, 620 (78%) underwent a physical fitness test. At the 30­month follow­up, 440 of the tested persons
completed the questionnaire. The study group was reduced to 327 persons (women=271, men=56) before entering the analyses, because 48 persons had incomplete LBP data and 65 persons reported LBP intensity above 5 at baseline which
excluded them from the analyses according to the case/non-case criteria. All
participants gave a written, informed consent and the local ethics committee at the
University of Copenhagen, Denmark approved the study.
Outcome Variable
Increased LBP intensity. LBP was defined as tiredness, discomfort or pain in the
low back region. The low back region was defined as the region between L1 and
the gluteal folds. Cases were defined as subjects with an increase of more than two
steps in self-reported rating of the LBP intensity during the past twelve months
follow­up (“State your average level of LBP during the previous 12 months on
the scale below”, one indicating no pain and 10 indicating worst possible pain). It
has been shown that an increase in LBP intensity above 2 was clinically relevant
in chronic LBP patients 22. Table 1 shows the distribution of the LBP intensity at
baseline in the source population and the number of cases at follow-up at each
level of LBP intensity. Subjects with low back pain intensity above 5 at baseline
were excluded from the analyses to give all the participants a change to become a
case. The cut-off limit of 5 also ensured that all cases had had a markedly relative
increase in pain intensity. Of the 327 persons included in the analyses at followup, 27 % improved their LBP intensity, 42 % were unchanged and 31 % got worse.
As table 1 shows, 41 persons (12 %) got worse by more than 2 and were classified
as cases (women=37 and men=4).
Determinants
Performance­based physical fitness. Before the test session, the participant was
interviewed, using a standardized questionnaire guide, to elicit musculoskeletal
pain, diseases and other circumstances that could involve a health risk or might
affect the results of the tests. The most frequent occurrences for exclusion were:
musculoskeletal pain at the test day in the regions tested, history of severe low
back pain, under treatment for high blood pressure, fever, headache and pregnancy. Isometric back extension endurance (Modified Sorensen test): the subjects were
lying prone on a sloping board (70x40x15 cm). The feet were pressed down to the
floor by an assistant and the subjects held their upper body in a horizontal posi­
tion with the arms folded across the chest and with a hip flexion of approximately
12º. Isometric back flexion endurance: a band was fixed around the subject’s chest
at the height of the inferior angulus of scapulae. In supine position with the feet
supported on a chair (90º hip and knee flexion) and the arms folded across the
56
Physical Fitness and Low Back Pain
chest, the subject was instructed to curl up until the band was free of the floor.
The two tests were held for as long as possible, but to a maximum of 360 sec. 23;24.
Flexibility: The modified finger­to­floor method was used25;26. The subjects stood on
a 30 cm high box and bent forward while pressing a horizontal measurement slide
downwards. Sagittal flexibility was defined as the distance from the fingertips to
the box level in the fully flexed position. Thus negative values indicated that the
subject was able to reach further down than the box level. Balance: was evaluated
by a balance test with the subject sitting on a wobble board that was placed on a
table allowing the legs to hang freely over the edge of the table. Two trials were
given to familiarize the subjects with the test. Subjects were asked to keep the
wobble board in balance through movements of the hip and back. If any body
segment touched the table, the stopwatch was stopped but not zeroed, and one
attempt was counted. Number of attempts was counted until the subject had been
balancing for one minute or a maximum of 15 attempts was reached. A similar
setup has recently been developed27.
Self­assessed physical fitness. Aerobic fitness, muscle strength, endurance, flexibility,
and balance were self-assessed using Visual Analogue Scales (VAS) of 100 mm
with illustrations and verbal anchoring of the extreme situations (figure 2). The re­
spondents were asked: “How would you score the following components of phy­
sical fitness compared to people of your own age and sex?”. Replies appeared as
vertical marks on the VASs. The questions were developed by the Department of
Work Physiology at the National Research Centre for the Working Environment,
Denmark 28. The VASs were completed the day before the tests and computerized
using a digitizer.
The classification of the performance­based and self­assessed physical fitness para­
meters were based on the distribution in the population (tertiles) and appear in
table 2. The three classification levels enabled us to test for signs of dose­response
relationships.
Covariates
Seniority (yr): defined as years working with physically and mentally disabled
people. 0-10 yr (low), >10 yr (high). Body mass index (BMI): the subjects were classified into three groups according to BMI (kg/m2): <=24.9 (normal weight), 25­30
(overweight), >30 (severe overweight). Leisure time physical activity: assessed by a
slightly modified version of the question of Saltin and Grimby 29 with illustrations:
0-4 hours/week (low), >4hours/week (high). Previous LBP history: defined as: no
period ever with 3-month persistent LBP (no) or at least one 3-month period with
persistent LBP (yes).
Previous LBP history is a known risk factor of low back pain 30.
Physical factors at Work. Bent back: the frequency during the workday of bent
back: never-sometimes (seldom) or often-very often (often). Rotated back: The
frequency during the workday of rotated back was assessed; never­sometimes
(seldom) or often-very often (often). Physical workload during patient related tasks:
was assessed by a continuous scale (0-14) and dichotomised into 0-5 (light) or 6-14
(strenuous).
57
Physical Fitness and Low Back Pain
Psychosocial factors at work. Psychosocial work factors were measured by the
Copenhagen Psychosocial Questionnaire (COPSOQ) 31. Influence at work consisted
of four questions dealing with decision latitude. Quantitative job demands consisted
of five questions on quantitative job demands concerning the relationship between
the amount of work and the time allotted to do the work. The quantitative job
demands scale was reduced from seven to five items on the authors’ recommenda­
tion 32. Emotional job demands consisted of three questions regarding how the work
affected the respondent emotionally. Cognitive job demands consisted of four items
concerning memory and reflection demands. The psychosocial work factors were
classified in three levels according to the distribution in the population (tertiles).
Participation and drop-out. Drop-out occurred four times during the study period
(figure 1). The first was the group of non­respondents at baseline (N=306), whom
we have no information about, the second was the persons who completed the
questionnaire but did not undergo the performance-based tests (N=180). Compared with the persons with test data at baseline (620) the persons without test data
had more sick leave due to LBP during the previous year (p=0.005), they had to
be spared more at work because of LBP during the previous year (0.007), they had
a higher LBP intensity during the previous 3 months (p=0.005) and additionally,
they were younger (4.8 yrs, p<0.001) with lower seniority (3.9 yrs, p<0.001). The
third drop-out was the persons with complete performance-based test and questionnaire data at baseline but without questionnaire data at follow-up (N=180).
Compared with the tested persons at baseline who completed the questionnaire
at follow-up (n=440), the persons without follow-up data had more sick leave due
to LBP during the previous year at baseline (p=0.027), they were younger (3.4 yrs,
p=0.001) and had lower seniority (3.5 yrs, p<0.001). However, no differences were
seen in any of the self­assessed or performance­based physical fitness parameters.
The fourth drop-out was the combined group of persons with incomplete LBP
data to be included in the analyses and persons with LBP intensity above 5, in
total 113 persons. The drop-out group was older (2.1 yrs, p=0.028) and performed
worse in the isometric back extension endurance test (p=0.001).
Data analyses. The data were analyzed for associations between each physical
fitness parameter (determinant) at baseline and an increased intensity of pain in
the low back region (dependent variable) at follow-up. Separate logistic regression
analyses were performed using the GENMOD procedure of the SAS 8.02 software.
The criterion for including a determinant or a covariate in the multivariate model
was an association with the LBP outcome at p<0.20 level in an age and sex adjusted logistic regression analysis. This relatively high significance level was chosen
as screening criterion for variable selection to ensure that all important variables
were identified 33. The final model included all significant covariates, sex and age.
Due to the relatively small study population, we had to be aware of the number of
variables entered in the final model as the recommended ratio between cases and
independent variables is 1:10 34. No interaction terms were included in the model.
The effects of the physical fitness parameters were tested separately; hence, we
must consider chance findings. All variable levels were coded so that the reference
level (OR=1) represented the hypothetical advantageous level concerning incre58
Physical Fitness and Low Back Pain
ased LBP. All determinants and covariates were defined as class variables in the
logistic regressions analyses. The Wald test was performed to test the significance
of each classification level compared with the reference level, and the Likelihood
Ratio test was performed to test for a general effect (type 3) if the variable had
more than two levels. A significance level of p<0.05 was chosen.
Results
The drop out analyses showed that we dealt with a restricted study population of
relatively healthy persons.
Results from the age and sex adjusted logistic regression analyses appear in
table 3. Performance­based back endurance (p=0.12), self­assessed aerobic fitness
(p=0.059) and frequency of rotated back (p=0.17) fulfilled the criterion for inclu­
sion in the multivariate analyses. Sex was significantly related to increased LBP
intensity (p=0.048, OR=2.9 of being a woman), whereas age did not associate with
increased LBP intensity.
Performance­based back flexion endurance, flexibility and balance were not as­
sociated with increased LBP intensity, neither were self-assessed muscle strength,
endurance, flexibility and balance.
Results from the multivariate analyses appear in table 4. When adjusted to “ro­
tated back”, age and sex, persons with low level back endurance showed an
insignificantly higher risk of increased LBP intensity (OR=2.4, p=0.076, Wald test),
whereas persons with medium level back endurance were at significantly higher
risk (OR=2.7, p=0.034, Wald test). The general association between isometric back
extension endurance and increased LPB intensity was insignificant (p=0.067, Like­
lihood ratio test).
To elucidate the effect of adjusting by frequency of rotated back, the analysis was
stratified by frequency of rotated back. After stratifying, neither the subgroup
exposed to low physical demands nor the subgroup exposed to high physical
demands were significantly associated with increased LBP intensity. However,
the OR in the highly exposed group was markedly higher compared to the low
exposed group (3.9/3.2 vs. 2.0/1.8) (table 5).
Persons with medium level self­assessed aerobic fitness were at lower risk of in­
creased LBP intensity compared to those with a high level (OR=0.37, p=0.02, Wald
test), although the general association of aerobic fitness was insignificant (0.066,
Likelihood ratio test) (table 4).
Discussion
Employees with medium level back extension endurance were at significantly
higher risk of increased LBP intensity after 30 months compared to those reporting a high level. No significant associations were found between any level of the
remaining performance-based tests and increased LBP intensity.
Employees with medium level self­assessed aerobic fitness were at significantly
lower risk of increased LBP intensity at follow-up compared to those with high
level. No other self­assessed physical fitness components were associated with
increased LBP intensity.
When evaluating the findings of multiple analyses we cannot exclude that the
results can be due to chance finding. However, it was not nine identical analyses
59
Physical Fitness and Low Back Pain
but tests of five different dimensions of physical fitness, where some of the dimen­
sions were assessed by two different methods.
The relationship between high level of self­assessed aerobic fitness and higher risk
of increased LBP intensity was not expected. The association was not due to an opposite association between self­assessed and performance­based aerobic fitness. In
a previous study that examined the validity of the instrument for self-assessment
of physical fitness using VAS28, positive associations were found of self-assessed
aerobic fitness, muscle strength and flexibility with their corresponding perfor­
mance-based measures, although the associations were only weak to moderate.
That low aerobic fitness prevented against increased LBP intensity and that self­
assessed muscle strength, endurance and flexibility showed similar tendencies
indicate that the self­assessed physical fitness parameters in general are differently
related to increased LBP intensity than the performance-based parameters. The
respondents may take several psychosocial factors into account when they answer
the questions of self­assessed fitness 35-37, factors that we did not control for in the
analyses and that influence the relationship to increased LBP intensity. Another
explanation we can not exclude is despite the weak to moderate convergent
validity between some performance-based and self-assessed parameters the two
methods may measure different latent constructs with different relationships to
changes in LBP intensity. A third explanation could be that those who self-assess
their physical fitness to be low are particularly aware of their reduced capacity
and consequently change their behavior at the workplace in a way that reduce
their excessive loading. Thereby they may reduce the excessive physical load and
the subsequent occurrence of LBP. Consequently, this group may reduce their risk
of increased LBP intensity due to a changed behavior that reduces their physical
exposure. A way of elucidating this hypothesis would be to include a quantitative
approach. By interviewing the marginal group who reported low self-assessed
fitness in combination with decreased or unaltered LBP intensity changes in beha­
vior may be explained.
We can neither rule out the possibility that some participants, and especially those
with low physical fitness, were motivated by the baseline measurements and
increased their leisure time physical activities subsequently. A weakness of the
study is the relatively few cases, which reduces the power of the analyses. A better response rate would have enhanced the possibility of more cases and thereby
more power. We believe that the low power and consequently higher risk of a statistic type 2 error, also was the explanation for the non­significant relationship we
found between low level isometric back extension endurance and increased LBP
intensity, and not because of a physiological difference between the persons in the
low and medium level groups. The odds ratios were almost the same for medium
and low level (2.71 vs. 2.37 compared with high level) which might reflect the exi­
stence of a threshold value opposite to a dose response relationship between back
extension endurance and increased low back pain intensity.
The study population at follow-up included only 30% of the respondents at
baseline, which underlines that the study population was a selected group. The
drop-out of a younger group with more LBP during the follow-up period suggests
that those without the ability to sustain the work demands were selected out of
the occupation, and those who stayed had particular qualifications. It might be
60
Physical Fitness and Low Back Pain
reflected by the result that only medium level back endurance was significantly as­
sociated with increased LBP intensity, whereas low level only tended to be so. The
consequences of this selection on the results are difficult to interpret, but it may be
more difficult to find an association with LBP among the remaining healthier part
of the employees, resulting in an underestimation of the association strength. To
test how sensitive the results of the logistic regression analyses were to the classifi­
cation of the physical fitness variables, the analyses were succeedingly performed
with the physical fitness variables as continuously variables and then dichotomi­
zed (in contrast to the tertile classification). The sensitivity analyses showed that
the significance of the results was sensitive to the cut­off point’s definitions, espe­
cially regarding self­assessed aerobic fitness. However, back extension endurance
was the performance-based components with the strongest association to higher
risk of aggravated LBP and low level self­assessed physical fitness was in general
associated with a lower risk of aggravated LBP, irrespective of the cut-off point
definition. The study population was originally supposed to be relatively highly
exposed to physical demands to ensure the relevance of high physical fitness to
correspond to the demands. However, only 27% were classified as having high
physical demands according to the variable “rotated back”, and despite contrast
in the material normally being a quality, we would have preferred a higher ratio
of employees with high physical job demands to ensure the need for high physical
fitness.
Our outcome definition was an increase in LBP intensity and not the incidence
or recurrence of LBP. Hence, when comparing our results with other studies, we
must consider that differences in results can be due to differences in case definiti­
ons.
The preventive effect of high level isometric back extension endurance and
development of LBP supports the findings of other studies 12;16;17;38, and although
several studies fail to prove the relationship between back endurance and LBP
10;11;13;18
, the number of positive studies suggests that this is a promising physical
fitness factor in preventing LBP.
Two earlier studies indicated that the relationship between physical fitness and
LBP may be stronger among persons exposed to high physical demands 39;40. This
was not supported in a recent study by Hamberg-van Reenen et al 41 which found
equal and statistically significant risks for two subgroups with a poor back exten­
sion endurance, but exposed to different levels of physical demands.
When the analysis of back extension endurance in the present study was stratified
by exposure level, both strata were non­significantly associated with increased
LBP intensity. However, interestingly, the ORs among the highly exposed group
were almost twice as high as among the low exposed group, which indicates that
an effect of the physical exposure level might exist.
This study supports the findings of other and that back extension endurance is an
important physical fitness component in the prevention of LBP. Although non­sig­
nificant findings between performance­based tests and increased LBP dominated,
we suggest enhancing the focus of physical fitness among healthcare personal,
especially the muscles involved in back functioning in order to withstand the
physical exposure during the workday. It might be a step in the right direction to
reduce the high prevalence of LBP among healthcare personnel.
61
Physical Fitness and Low Back Pain
Tables and figures
Table 1. The distribution of LBP intensity during the previous year reported at
baseline. Only subjects with pain intensity below 6 were included in the analyses.
Level
1
2
3
4
5
6
7
8
9
10
Total
LBP intensity
baseline
N
%
130
33
54
14
57
15
48
12
38
10
23
6
20
5
12
3
9
2
1
1
392
100%
Cases
increase of pain>2 units
N
%
14
33
9
21
11
26
3
7
4
10
1
2
0
0
0
0
0
0
0
0
42
100%
62
Physical Fitness and Low Back Pain
Table 2. The classification of the performance­based and self­assessed physical
fitness parameters.
Predictor variable levels
Performance­based physical fitness
Back extension endurance (sec)
Low
Medium
High
Range
N
12-105
107-157
158-360
96
104
116
Back flexion endurance (sec)
Low
Medium
High
0-45
46-87
88-360
104
97
108
Sagittal flexibility (cm above floor level)
Low
3-32
Medium
(-6) -2
High
(-22)- (-7)
109
103
104
Balance test (number of attempts 15=maximum)
Low
15
Medium
6-14
High
1-5
97
89
93
Self­assessed physical fitness using VAS
Aerobic fitness (mm)
Low
3-41
Medium
42-54
High
55-97
106
112
109
Muscle strength (mm)
Low
Medium
High
10-47
48-63
64-99
109
107
111
Endurance (mm)
Low
Medium
High
5-49
50-65
66-100
100
112
115
Flexibility (mm)
Low
Medium
High
4-42
43-60
61-99
100
106
121
Balance (mm)
Low
Medium
High
4-45
46-60
61-100
103
113
111
63
Physical Fitness and Low Back Pain
Table 3. Age and sex adjusted analyses of physical fitness parameters and cova­
riates with increased LBP intensity at 30-month follow-up. $Percentage of cases
at each classification level, † Wald test performed for each level,‡ Likelihood Ratio
test performed for general effect (type 3),*adjusted only to age, #adjusted only to
sex.
Variables
N=327 % cases$
Determinants
Performance­based physical fitness
Back extension endurance
High
116
8%
Medium
104
15 %
Low
96
15 %
Back flexion endurance
High
108
9%
Medium
97
12 %
Low
104
14 %
Flexibility
High
104
14 %
Medium
103
11 %
Low
109
11 %
Balance
High
93
11 %
Medium
89
11 %
Low
97
16 %
Self­assessed physical fitness
Aerobic fitness
High
109
17 %
Medium
112
8%
Low
106
12 %
Muscle strength
High
111
15 %
Medium
107
13 %
Low
109
9%
Endurance
High
115
16 %
Medium
112
10 %
Low
100
12 %
Flexibility
High
121
16 %
Medium
106
9%
Low
100
12 %
Balance
High
111
12 %
Medium
113
12 %
Low
103
14 %
Covariates
BMI
<=25
184
13 %
25-30
88
10 %
>30
50
16 %
64
OR
95% CI
p Wald † p LR‡
1
2.26
2.18
0.94-5.47
0.87-5.44
.
0.069
0.095
1
1.58
1.92
0.63-3.92
0.79-4.67
.
0.33
0.15
1
0.74
0.85
0.32-1.71
0.37-1.96
.
0.48
0.70
1
0.97
1.56
0.37-2.52
0.63-3.86
.
0.94
0.33
1
0.37
0.58
0.16-0.87
0.26-1.29
.
0.02
0.18
1
0.82
0.51
0.38-1.78
0.22-1.18
.
0.62
0.11
1
0.54
0.66
0.24-1.21
0.29-1.46
.
0.13
0.30
1
0.54
0.74
0.24-1.23
0.34-1.62
.
0.14
0.45
1
0.91
1.02
0.40-2.08
0.44-2.34
.
0.83
0.97
1
0.81
1.3
0.35-1.86
0.53-3.16
.
0.61
0.56
0.12
0.33
0.77
0.47
0.059
0.27
0.29
0.32
0.96
0.67
Physical Fitness and Low Back Pain
Leisure time physical activity
Low
297
12 %
High
30
13 %
Low back pain History (3-month period)
No
287
12 %
Yes
40
15 %
Frequency of work time with bent back position
Never-sometimes
222
11 %
Often-very often
96
16 %
Frequency of work time with rotated back position
Never-sometimes
233
11 %
Often-very often
87
17 %
Physical workload during patient-related tasks
Light (0-5)
102
9%
Strenuous (6-14)
222
14 %
Influence at work
High
96
10 %
Medium
135
14 %
Low
96
13 %
Quantitative demands
High
114
11 %
Medium
106
13 %
Low
107
13 %
Emotional demands
High
99
10 %
Medium
145
12 %
Low
83
16 %
Cognitive demands
High
98
10 %
Medium
88
17 %
Low
140
11 %
Seniority
High
155
11 %
Low
172
14 %
Age*
<36 yr
36
6%
36-45 yr
92
15 %
46-55 yr
135
13 %
>55 yr
64
11 %
Sex#
Male
56
5%
Female
271
14 %
65
0.89
1
1.08
0.35-3.32
.
0.89
1
1.34
0.51-3.48
.
0.55
1
1.38
0.68-2.80
0.37
1
1.65
0.81-3.35
.
0.16
1
1.57
0.71-3.47
.
0.26
1
1.35
1.19
0.59-3.09
0.48-2.93
.
0.47
0.71
1
1.08
1.11
0.48-2.45
0.49-2.51
.
0.85
0.80
1
1.13
1.55
0.49-2.62
0.63-3.78
.
0.77
0.34
1
1.64
1.07
0.69-3.92
0.46-2.49
.
0.27
0.88
1
1.54
0.74-3.18
.
0.25
1
3.11
2.75
2.15
0.67-14.5
0.60-12.5
0.42-11.0
.
0.15
0.19
0.36
1
2.93
0.87-9.90
.
0.083
0.56
0.37
.
0.17
0.25
0.77
0.97
0.60
0.45
0.24
0.41
0.048
Physical Fitness and Low Back Pain
Table 4. Multivariate associations of physical fitness parameters at baseline adju­
sted to sex, age, and rotated back during the workday with increased LBP intensity at 30-month follow-up.
Full modelA
OR
CI
Performance-based back extension endurance
High level
1
Medium level
2.71
1.08-6.79
Low level
2.37
0.91-6.14
p
0.067 B
0.034
0.076
Self­assessed aerobic fitness
0.066B
High level
1
Medium level
0.37
0.15-0.88
0.02
Low level
0.58
0.27-1.38
0.23
A
Adjusted to rotated back during the workday, sex, and age. BLikelihood ratio test.
Table 5. Multivariate association of performance-based back extension endurance
with increased LBP intensity stratified by level of frequency of workday with
rotated back. Adjusted by sex and age.
Low physical exposure
n=233
OR
CI
p
Performance-based
back extension endurance
High level
Medium level
Low level
High physical exposure
n=87
OR
CI
p
0.43
1
1.97
1.81
0.66-5.85
0.54-6.06
66
0.22
0.33
0.25
1
3.88
3.26
0.61-24.6
0.59-18.1
0.15
0.18
Physical Fitness and Low Back Pain
Figure 1
Figure 1. Flow chart of the study population.
67
Physical Fitness and Low Back Pain
Figure 2
Figure 2. The design of the self-assessment instrument using VAS. The subjects
were asked to score the physical fitness components with reference to people of
their own age and sex by setting a vertical mark on each VAS.
68
Physical Fitness and Low Back Pain
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Physical Fitness and Low Back Pain
Is the cross-sectional association between self-assessed physical
fitness and performance­based physical fitness among healthcare
students influenced by low back pain?
Jesper Strøyer1, Annemarie Lyng Eskelund-Hansen2 , Kirsten Schultz Larsen3 and
Niels Erik Ebbehoej2.
1
National Research Centre for the Working Environment, Denmark. 2Clinic of Occupational and Environmental Medicine, Bispebjerg University Hospital, Copenhagen, 3 Institute of Public Health, Department of Social Medicine , University of
Copenhagen.
Manuscript
Prepared for submission to: Occupational Medicine
Corresponding author:
Jesper Strøyer Andersen, M.Sc., PhD. Stud.
National Research Centre for the Working Environment, Denmark
Lersø parkallé 105,
DK-2100, Copenhagen Ø
E-mail: jsa@nrcwe.dk
89
Physical Fitness and Low Back Pain
Abstract
Background Conflicting results has been found for the predictive value of self­as­
sessed and performance­based physical fitness regarding low back pain (LBP).
Further exploration to better understand the nature of self-assessment of physical
fitness and its applicability in occupational medicine is needed. Aims First, differences in associations of LBP with self-assessed and performance-based physical
fitness were examined. Second, it was examined if the association between cor­
responding dimensions of self­assessed and performance­based physical fitness
were confounded by LBP. Methods Of the 885 healthcare students invited, 612
completed a questionnaire and a physical fitness test program in their first week of
training. Aerobic fitness, muscle strength, endurance, flexibility and balance were
self­assessed using visual analogue scales. Back endurance, flexibility and balance
were in addition tested. Results LBP was significantly associated with self­asses­
sed aerobic fitness, flexibility and balance, and performance­based flexibility in
both the sex­adjusted and full model analyses. The significance levels of the as­
sociations between self­assessed endurance, flexibility and balance with their cor­
responding test results were not affected to any appreciable degree when LBP or
any other covariate (history of physical demanding job, educational attainments,
BMI, height and age) were included in the analyses. The significant association
between self-assessed and performance-based balance was only due to the women’s scoring. Conclusion LBP showed almost similar associations with self­as­
sessed and performance­based physical fitness. LBP did not confound the highly
significant associations between corresponding self­assessed and performance­
based physical fitness parameters among healthcare students. More knowledge is
needed about which factors that influence the self­assessment of physical fitness.
Keywords: Visual analogue scale, Biering Sørensen test, finger­to­floor method,
one-leg standing balance.
90
Physical Fitness and Low Back Pain
Introduction
The prevalence of Low Back Pain (LBP) is particularly high in the healthcare sector
(1­5). Various physical work­related aspects typical of the sector (2;6­8) have been
identified as risk factors of LBP (9­15). About 43% of the musculoskeletal disorders
in the low back and knee among home care helpers has been ascribed to the occupational environment (16;17).
Despite high physical work demands being characteristic of the healthcare sector,
to date, there is no conclusive evidence that a high level of physical fitness can
prevent LBP (18-23). It has been discussed if the inconsistent results concerning
the relation between physical fitness and LBP are due to the great variability in
physical fitness tests applied, the different types of populations investigated or
insufficient control of physical exposure variables. Another reason could be that
physical fitness is indirectly associated with LBP through the individual’s strategy
for handling the physical workload experienced in the occupational environment.
Such a hypothesis emphasis the need for a more complex understanding of the
relationship between physical fitness and the cause of LBP, including the inter­
play between individual factors, background variables and the perception of own
physical fitness.
Instruments for self­assessment of physical fitness have primarily been used in
large epidemiological studies as a substitute for physical fitness measurements.
Self­assessed physical fitness measures have shown moderate correlations in
relation to corresponding performance-based measures. At the same time strong
inter-item correlations between the different components of self-assessed physical fitness has been found (24­30). These findings, together with the unexpected
results of a previous follow-up study pointing to low level self-assessed physical
fitness as protective against aggravated LBP (31), indicate that performance­based
and self­assessed physical fitness might represent related but different concepts of
physical fitness.
If self­assessed physical fitness is to be considered as a risk indicator of LBP in the
healthcare sector, substituting performance­based physical fitness, more know­
ledge are needed about which factors that influence the associations with LBP and
performance­based physical fitness before the self­assessed tool can be imple­
mented in prevention programme.
In the present study it was first analysed if the association between LBP and
self­assessed physical fitness differed from the association between LBP and
performance­based physical fitness. Second, it was analysed if LBP confounded
the adjusted associations between corresponding dimensions of self-assessed and
performance­based physical fitness.
Methods
The study was based on secondary analyses on baseline data of a randomized
controlled intervention study among healthcare students in the community of Copenhagen in 2004-2005. Students from 37 classes (n=885) were invited to complete
91
Physical Fitness and Low Back Pain
a comprehensive questionnaire, and to participate in physical fitness test in their
first week of training. A total of 612 (69%) students with complete physical fitness
test data were analysed. All participants gave written informed consent and the
local ethics committee at the University of Copenhagen, Denmark approved the
study (journal: 2003-41-3508).
Prevalence of LBP was defined according to a questionnaire (32;33) as tiredness,
discomfort or pain in the low back region with or without radiating symptoms to
the leg or legs (34) during the previous 12 month. A history of physical demanding job was defined as more than 6 months in the past, on the basis of an open
question about their previous occupations and duration. Educational attainments
was categorized as low (≤9 years) or high (>9 years).
Five components of physical fitness (Aerobic fitness, muscle strength, endu­
rance, flexibility, and balance) were self­assessed according to peers using Visual
Analogue Scales (VAS) of 100 mm with illustrations and verbal anchoring of the
extreme situations (figure 2)(30). The respondents were asked: “How would you
score the following components of physical fitness compared to people of your
own age and sex?”. The VASs were scored the day before the tests and computeri­
zed using a digitizer.
Four different physical fitness tests were conducted in classes during a 90 minutes
test session. Isometric back extension endurance was tested using a modified
Sorensen test (35-38). The subjects lie prone on a sloping board (70x40x15cm) with
the feet pressed down to the floor by an assistant. Their upper body was in a ho­
rizontal position, the arms folded across the chest, and the hip flexed 12º. This po­
sition was hold for a maximum time of 180 sec. Isometric back flexion endurance
was assessed with the subjects positioned in a sit-up posture with the back resting
against a jig angled at 60º from the floor, the arms folded across the chest, and
with knees and hips 90º flexed (39). To begin the jig was pulled back 10cm and the
person holds the isometric posture in line with the jig as long as possible, but maximally 180 sec. The modified finger­to­floor method was used to measure sagittal flexibility (40;41), defined as the distance from the fingertips to the floor level
when the subject flexed the spine and hip maximally without bending the knees.
The person stood on a 30 cm high box without shoes. Balance was evaluated by
the one-leg-standing test testing the ability to stand on one leg with the eyes open
in 60 seconds (42;43). The two back endurance tests and the balance test were
coded as passed/not passed. The flexibility test was analysed as a continuously
variable or dichotomized, dependent of the analyses. Weight was measured with
hidden display and height was measured by an electronic height measuring unit
(SOEHNLE Professional GmbH & Co. KG Postfach 1308, D - 71536 Murrhardt).
Body mass index (BMI) was calculated as weight (kg)*height (cm)­2 and classified
into three groups. A short one-to-one screening interview was performed before
the testing to identify reasons for wholly or partly exclusion. E.g.: musculoskeletal
pain at the test day in the regions of testing; history of severe low back pain; under
treatment for high blood pressure; fever; headache; or pregnancy.
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Physical Fitness and Low Back Pain
Descriptive analyses were performed for all the variables. Differences according
to sex were tested with students t-test and chi-square test for continuously and
dichotomous variables respectively.
The association between LBP and the physical fitness parameters was descri­
bed stratified to sex but analysed in the total group of both sex. The correlations
between LBP and the continuous physical fitness variables (the self­assessed
components and performance­based flexibility) were tested by GLM analyses with
the physical fitness score as the dependent variable and LBP and covariates as
independent variables. The correlations between the dichotomy physical fitness
variables (the back endurance and balance tests) were tested by a logistic regression model with the test result (failed/passed) as dependent variables and LBP
and covariates as independent variables. A sex adjusted and a full model analysis,
that included all the covariates, were performed.
The ability of self­assessed endurance, flexibility and balance to discriminate the
persons who failed the corresponding performance-based test from the ones who
passed the test, were tested by general linear models (GLM) with the self-assessed
score as the dependent variable and the performance-based (passed/failed) and
covariates as independent variables. Flexibility was dichotomized according to the
distribution due to the lack of a “passed” value. To examine the effect of including
the covariates, an analysis only adjusted to sex was performed first, then an ana­
lysis including the remaining covariates except from LBP, and then finally a full
model including LBP. Significance level of p<0.05 was chosen. The SAS statistical
software (v. 9.1) was used for all the analyses (PROC GLM was used for linear
regression analyses and PROC GENMOD for logistic regression analyses).
Results
Of the 885 healthcare students invited to participate in their first week at school,
790 (89%) agreed to complete the questionnaire. Among the 705 (80%) students
who agreed to be tested, 612 (69%) students performed all physical fitness test and
were included in the analyses, whereas 89 students where excluded from the analysis due to an incomplete physical fitness test program. Four students were ex­
cluded due to pregnancy or disease. The students who refused to be tested (n=85)
were not different from those who accepted (n=705). The students with incomplete
test data (n=89) scored their self­assessed aerobic fitness, muscle strength and
balance significantly lower than those who completed the test program, whereas
their 12­month prevalence was not significantly higher (47% vs. 38%).
The LBP prevalence was the same in women (39%) and men (34%) (table 1). Selfassessed aerobic fitness, muscle strength, endurance and balance were significan­
tly higher in men than in women. The women were significantly more flexible,
whereas the men had significantly better isometric back flexion endurance. The
men were significantly taller compared to women, whereas BMI was identical.
Table 2 displays the self­assessed and performance­based physical fitness scores
among those with and without LBP during the previous year, both stratified by
sex and in the total group. The associations between the physical fitness parame­
ters and LBP and the effect of including the covariates in the model were analysed
93
Physical Fitness and Low Back Pain
in the total group. Higher prevalence of LBP during the previous year was significantly associated with lower self­assessed aerobic fitness, lower self­assessed
flexibility, lower self­assessed balance and lower performance­based flexibility in
both the sex­adjusted and full model analyses (table 2). Only the significance of
the association between LBP and self-assessed balance differed between the sexadjusted and the full model analysis which was due to the inclusion of height in
the model (p=0.05). BMI was highly significantly associated with most of the phy­
sical fitness variables but did not affect the strength of the associations between
physical fitness and LBP in any of the analyses. Educational attainments were
associated with self­assessed aerobic fitness (p=0.02) but did not influence aerobic
fitness’ association with LBP.
The ability of self­assessed endurance, flexibility and balance to discriminate
between persons who failed or passed the corresponding performance-based test
was tested by adjusted correlations between self-assessed scores and performancebased test results (passed/failed). Educational attainments, history of physical
demanding job, BMI, height, sex and age as covariates were included (table 3).
Highly significant associations were found between all physical test results and
their corresponding self­assessed mean scores. The highly significant association
between the self-assessed balance score and the balance test result was only due to
the women’s ability to discriminate, as no difference was found between the men
who failed and those who passed the balance test.
The inclusion of LBP in the model did not change the significance level in any of
the associations between the self-assessed scores and corresponding test results
(table 3).
The only appreciable change in significance level was seen when the covariates
(except from LBP) was included in the analysis of self-assessed endurance with
back extension endurance (p<0.0001 vs p=0.002) which was elucidated to be due
to the inclusion of BMI in the model (p=0.06).
To examine how specific the corresponding self­assessed physical fitness score
was compared with the non-corresponding scores for each performance-based
test, the non-corresponding mean scores were computed for all VAS scores were
computed in relation to each compared in the groups who passed and failed
To examine if the non-corresponding VAS scores discriminated as well between
those who failed or passed the tests all VAS score means were calculated in relation to success of each test result. For all performance-based tests, except for back
extension endurance, the corresponding VAS score discriminated best (largest difference) between those who passed and failed the test compared to the non-corresponding VAS scores (data not shown). Self­assessed aerobic fitness discriminated
as well (6 mm) as self-assessed endurance between those who failed and passed
the back extension test.
Discussion
LBP showed almost similar associations with self-assessed and performance-based
physical fitness parameters. More self­assessed than performance­based para­
meters were significantly associated with LBP, however all the physical fitness
parameters showed the same picture of better physical fitness among those wit­
94
Physical Fitness and Low Back Pain
hout experience of LBP during the previous year compared to those with LBP. The
similarities were further pronounced by the significant associations of both self­as­
sessed and performance­based flexibility with LBP and the lack of significance for
self­assessed and performance­based endurance. However, LBP was significantly
associated with self­assessed balance in contrast to a non­significant association
with performance-based balance.
LBP did not confound the associations between corresponding parameters of selfassessed and performance­based physical fitness. The history of physical deman­
ding job was included in the analyses to test if the ability to self-assess physical
fitness was linked to a previous experience of a physical demanding job. However,
it was insignificantly associated with all physical fitness variables and did not
confound any of the associations. Educational attainments did neither affect the
discriminate ability of self­assessed physical fitness. The rationale for including
educational attainments was that it could be a proxy for the individual’s social
context. The social context might influence the way to consider own physical
fitness in interplay with the experiences and strategy for handling musculoskeletal
pain. BMI was included because it is known factor by many people and thereby
could be a reason for the ability to self­assess physical fitness if BMI in addition
associates with performance­based physical fitness. BMI only slightly confound
the association between self-assessed endurance and back extension endurance,
whereas the remaining associations were unaffected.
The highly significant associations between corresponded parameters, irrespective
of variables included in the analyses, indicate an independent and robust association between self­assessed and performance­based physical fitness, although
the major part of the variation in self­assessed physical fitness still remain to be
explained. However, the ability of self­assessed aerobic fitness to discriminate
according to the result of the back extension strength test indicate that self-assessed endurance was not specific enough to be used as a measure of back extension
strength. This result is in concordance with a previous result (30) that did not find
a satisfactory convergence between self-assessed endurance and back extension
endurance.
The general higher means of the self­assessed physical fitness parameters among
men compared to women was identically with the result of a previous study using
the same instrument among people working in the healthcare (30). Due to the lack
of reference values for the applied fitness test, a survey in a reference population
is needed to examine if this difference is due to differential perceptions of physical
fitness between men and women or is due to the fact that men are relatively more
fit. Differential reporting by women and men was not found regarding self­re­
ported disability (44). More women than men reported disability and functional
limitation which corresponded with their poorer performance scores. When using
peers as reference, as in the present study, women and men in a reference population are supposed to score identical values in average, if the instrument is free of
differential and systematic bias.
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Physical Fitness and Low Back Pain
One of the limitations of this thesis was the low numbers of men who appear in
the study population (17%). However, the ratio of men reflects the actual ratio of
men studying in the health care sector. Another weakness is the application of two
different statistical analyses when comparing the self­assessed physical fitness
scores with the performance-based test result. It can not be excluded that differences in significance level between performance­based tests with dichotomy and
continuously test results can be due to different statistical power when performing
logistic regression (non-parametric statistics) compared with GLM (parametric statistics). An advantage was the high participation rate of 80 %. In the analyses we
included the students with complete test data only (69%) to avoid that differences
in statistical results between physical fitness parameters were due to different
sub-samples analysed. The strength of the present study was that the subjects
were chosen independently of their physical fitness status. Some studies examine
the correspondence between self­assessed and performance­based physical fitness
among soldiers (25) or marginal groups selected on the basis of earlier fitness test
results (29) which induce an artificial big contrast in the analyses and limit the
comparability. Healthcare students should not possess particular qualities of selfassessing physical fitness, advocating for the instruments applicability in other
occupations than among healthcare students. Although, studies only involving
subjects with the same education or occupation in general limit the generalization
of the results to some degree.
To conclude, almost similar associations of LBP with self-assessed and performance­based physical fitness were found. LBP did not confound the highly significant
associations between corresponding self-assessed and performance-based physical
fitness parameters among healthcare students. The results support that a real and
independent association between self­assessed and physical fitness exists and that
self­assessed physical fitness can be measured without taking, non­specific LBP,
educational attainments and morphological factors into account. As concluded
by other authors as well (45;46), it seems that although self­assessed and perfor­
mance measures are associated they are also distinct and should in higher degree
be combined than substituted. A performance-based measure is not necessary
superior (46) or more objective (47) compared with self-assessed measures in relation to an health outcome which should be elucidated in future surveys. Hence
more knowledge is needed about how work factors and other individual factors
influence the self­assessed dimensions of physical fitness before the instrument for
self­assessment of physical fitness can be implemented in surveys as an alternative
to performance­based physical fitness.
The population studied is a unique group of students tested in the beginning of
their training before they are educated and before entering the labour marked. It
would be highly interesting to follow changes in physical fitness and LBP status
during their training and intervention period and subsequently when entering the
labour marked.
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Physical Fitness and Low Back Pain
Table 1. Characteristics of the study participants.
Women
n=511
Age
32.3 (10)
12-month prevalence of LBP
196 (39%)
Self­assessed physical fitness
Aerobic fitness (mm)
46 (20)
Muscle strength (mm)
50 (18)
Endurance (mm)
52 (19)
Flexibility (mm)
52 (20)
Balance (mm)
56 (19)
Performance­based physical fitness
back ext. endurance (% passed 180s)
286 (56%)
back flex. endurance (% passed 180s)
225 (44%)
Flexibility (cm above floor level)
­3.3 (9)
Balance (% passed 60s)
432 (85%)
Height (cm)
164 (6)
BMI (kg*m-2)
Normal (<25)
289 (57%)
Overweight (25-30)
138 (27%)
Severe overweight (>30)
80 (16%)
Educational attainments (>9 years school)
127 (25%)
History of physical demanding work (>6month)171 (37%)
Men
n=101
37.6 (34)***
34 (34%)
56 (21)***
58 (17) ***
65 (19) ***
56 (21)
64 (19) ***
51 (50%)
57 (56%)*
1.8 (11)***
85 (84%)
178 (7) ***
61 (60%)
26 (26%)
14 (14%)
31 (31%)
34 (35%)
Mean (SD) or n (%). *P<0.05, **P<0.01, ***P<0.001 between men and women.
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Physical Fitness and Low Back Pain
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Physical Fitness and Low Back Pain
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Physical Fitness and Low Back Pain
Figure 1
Figure 1. The design of the self-assessment instrument using VAS. The subjects
were asked to score the physical fitness components with reference to people of
their own age and sex by setting a vertical mark on each VAS.
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Physical Fitness and Low Back Pain
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