PHYSICAL ACTI\TITY AND HEALTH
IN CHILDREN AND ADOLESCENTS
Oded Bar-Or
McMaster University
Prepared for the CSEP and Health Canada Guidelines
Hamilton, August. 1999
Revised Dec 1999
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system
or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or
otherwise without prior written permission of the Canadian Society for Exercise Physiology.
TABLE OF
CONTE~TS
INTRODUCTION AND SCOPE
DEFINITIONS
METHODOLOGICAL CONSIDER.A.TIONS
ACTIVITY
PATTER~S
OF CHILDREN AND ADOLESCENTS
TRACKiNG OF PH'{SICAL ACTIVITY BEHAVIOUR
FACTORS THAT 0il0DIFY ACTIVITY BEHAVIOUR AND ENERGY
EXPENDITURE
.-\CTIVlTY AND HEA.LTH DURING CHILDHOOD AND ADOLESCENCE
IMPLICATIONS OF CHILDHOOD ACTIVITY AND SEDENTARISM TO
ADULT HEALH
PUBLISHED GUIDELI\:ES FOR ENHANCED PHYSICAL
,-\CTIVITY AMONG YOUNG PEOPLE
CONCLUSIOJ"S AND RECOtvlME"\:OATIONS
REfERE:\CES
TABLES 1-10
lNTRODUCTION AND SCOPE
Research conducted mostly in the last :2 decades has shown a strong association
between sedentarism in adult years and a high risk for several chronic diseases (Barlow et
al., 1995: Kannel et al.. 1985: :YlcGinnis. f oege, 1993: Paffenbarger et al., 1986:
Paffenbarger et al., 1991: Powell et al., 1987). Even though atherosclerotic
cardiovascular disease is seldom manifested clinically before middle age, pathological
vascular changes appear as early the first decade of life (Barker. 1999; Pate, Blair, 1978;
Voller. Strong. 1981). A question remains as to whether an increase in the physical
activity level of children will help to reduce the risk of future atherosclerotic and other
chronic adulthood diseases. This report is intended to:
•
summarise merhcdological aspects in the study of physical activity and the healthrelated effects of enhanced physical activity.
•
document current physical activity and energy expenditure patterns among children
and adolescents. with emphasis on European. US and Canadian data.
•
analyze physical and environmental factors that affect activity behaviour in young
peop le. (Psychosocial factors are outside the scope of this document. )
•
review the evidence on the relevance of children's physical activity to their current
health. as well as to the risk of their acquiring chronic diseases in adulthood
•
summarise published guidelines for the enhancement of physical activity among
children and adolescents.
Search strategies in the preparation of this document included:
•
MEDLI:\'"E searches of last 5 years for: Activity (physical Activity) AND
Children (Adolescents): Activity (Physical Activity) AiVD Children AND
Risk Factors: Energy Expenditure AJVD Age (Childhood, Adolescence):
Energy Expenditure A1VD Childhood Obesity; Peak Bone Mass AND
Physical Activity.
•
Scan of Research Quarterlv for Exercise and Sport (last 5 years) for articles
on the above topics.
•
Scan of Pediatric Exercise Science (all issues) and Proceedings ofPediatric
fVork Physiology (last 10 years) for articles on the above.
•
Author's reprint library and Reference Manager, which includes over 7,500
entries. mostly regarding children and exercise.
DEFINITIONS
Current and past literature on the subject of this review has been rather
inconsistent in the choice ofterms. For clarity's sake. the following are definitions of
terms as used in this dOCU111ent.
•
Energv Expenditure (EE). The biochemical energy used by the body in order to
perform its functions. EE can be expressed as work (e.g. Joules) or as power (e.g.,
kcal per min).
•
Exercise. The subset of physical activity used within the context of a training
program or a laboratory test.
•
Exercise Energy Expenditure (EEE). The energy expended for the performance of
physical activities.
5
•
.\!ET. The ratio between exercise energy expenditure during an activity, and resting
energy expenditure (EEE:REE). It is expressed is a non-dimensional number.
•
sloderate to I'igurolls .ictivitv. :-\ctivity that requires at least as much effort as brisk
or tast walking.
Oxygen Uptake (VO:). The volume of oxygen consumed by the body in a given
•
period. Also used as a surrogate measure of energy expenditure.
•
Physical Activity (FA), Any bodily movement produced by skeletal muscles resulting
in energy expenditure. PA has mechanical. physiologic and behavioural components.
•
Phvsical Activitv Level rP-iLJ. The ratio between 24-hour total energy expenditure
and resting energy expenditure (TEE/REE). It is used as an index of energy
expenditure as related to physical activity, in an attempt to cancel out the effect of
body size.
•
Pin-sica I Fitness. A set of attributes that relates to the ability to perform physical
activity.
•
PUlver. \Vork divided bv time. (Also - force times velocity). Expressed usually in
Watts. but also in liters per minute of oxygen uptake.
•
Prepubertal. A child who has not yet shown changes in secondary sex
characteristics.
•
Reactivitv. Changes in the spontaneous activity behaviour of a person. which result
from the knowledge that he or she is being observed.
•
Resting Energy Expenditure (REE). Energy expenditure while the subject is fully
rested. Usually taken at the supine position, first thing in the morning prior to
breakfast.
o
•
Total Energy Expenditure (TEE). Energy expenditure over a 24-hour period.
•
fVork. Energy used by the body during a physical task. Usually expressed in Joules.
Can be mechanica! energy or metabolic energy.
•
Trucking. Longitudinal stability of a certain variable. or the maintenance over time
of a relative position within a distribution of values in the observed population.
•
Young People. This term will be used here to denote children and adolescents
combined.
\lETHODOLOGICAL CONSIDERA.TIONS
There are several methodological constraints in the study of children' s physical
activity and its immediate and long-term effects. One needs to recognize these
constraints. in order to appreciate the inherent difficulties and limitations of such studies.
This section will briet1y review methodo logical issues in the aSSeSS111ent of PA and EE
among children and adolescents. highlighting their respective strengths and weaknesses.
Over the years. numerous methods have been suggested for the aSSeSS111ent of PA
and/or EE in pediatric populations. Table 1 is a summary of such methods, and their
strengths and drawbacks. The sequence of methods presented in the table is from the
simplest to the most sophisticated. Unfortunately, the simpler. less expensive methods
are also less reproducible and valid than the more sophisticated, expensive ones.
Questionnaires have often been used in large-scale studies. Their main strength
is a low cost and relative simplicity, but they depend on the recall ability of the child, or
of an adult who is supposed to be familiar with the child's activity pattern. One approach
La
improving the quality of such recall is through interviews. By using appropriate
questions. the interviewer can refresh the child's (or the adult's) memory, thus improving
the quality of the recall. A major drawback of the interview method is its high manpower
cost. Diaries kept by the subject reduce the recall time. Information can be logged at
various. predetermined intervals. One disadvantage of this method is the child's reactivity
(see Definitions section). Another is the reliance on the child's interpretation of her or
his activities. Direct observations, by an observer or through time-lapse/video
photography. overcome the need to rely on the quality of recall or on the reporting skills
uf the child. Direct observations by an observer are extremely expensive, which makes
this method unsuitable for large-scale studies. Photographic approaches require that the
subject be confined to a limited space. Direct observations are the Gold Standard of PA
behaviour. against which other methods should be evaluated.
\Vhile the above four methods are useful for the assessment of activity patterns,
they are not suitable for the determination of energy expenditure. Notwithstanding,
attempts have been made over the years to assign energy equivalents to various activities.
Such conversion tables. almost invariably, ignore differences in energy expenditure that
are due to differences in body mass and skill level of the subjects.
Objective methods have been developed to assess the mechanical aspects of
activities . Vlovement counters can be attached to the child to record the number of
movements. One example of these devices is the pedometer, which is worn on the waist,
to count the number of strides. The main disadvantage of this inexpensive method is that
it ignores the intensity of the steps, which does not allow to tell whether the child was
walking or running, and at what speeds. Nor is it possible to tell whether
walking/running was done on the level. uphill or downhill. Accelerometers, in addition
to counting movements. yield information on the intensity of each movement. The
derived data can then be translated to mechanical energy expenditure.
\J one of the above methods is aimed at measuring the metabolic EE. This can be
done indirectly by heart-rate monitoring. This method is based on the linear
relationship between a person's metabolic level and heart rate. With the advent of
miniaturiscd, inexpensive telemetric devices, this method has been used in many studies.
A major disadvantage, often overlooked, is that heart rate depends not only on EE, but
also on factors such as excitement, fear. hydration status, time after meal and. most
i mportantly. climate. Vleasuring oxygen uptake is a more direct method of determining
EE. A major disadvantage of this approach is that the equipment (metabolic cart,
ventilated hood, respiration chamber) can only be used in the laboratory, and it does not
allow for observations of free-living conditions. Recently developed, small portable
analysers are intended to OVerC01TIe this constraint.
The Gold Standard for the determination of EE under free-living conditions is the
doubly-labelled water method. All the subject needs to do is drink water enriched by
stable isotopes of oxygen and hydrogen. and then provide 1-2 urine or saliva samples
over a period of 10-14 days. The main disadvantage is an extremely high cost of the O 2
isotope and the cost of using a mass spectrometer. In addition. this method provides only
a single value of EE over the entire period. with no information of changes in EE from
une day to the next.
In SUlTI1TIary, none of the currently used methods can, by itself, provide.
information on the behavioural, mechanical and metabolic components of physical
activitv. Two or 1110re methods should be combined for this purpose. However. very few
researchers in the field have adopted a multi-method approach.
.- \CTIVlTY PATTERNS OF CHILDREN AND ADOLESCENTS
Age- and gender-related changes occur in the pattern and amount of physical
activity and in daily energy expenditure. This section will briefly review activities most
practised by children and adolescents and then outline age- and gender-related differences
in the amount of PA and EE.
Data on these topics are based on several longitudinal cohort studies or large-scale
cross-sectional surveys. For some reason. 1110st longitudinal studies were done in
Wesrern Europe. while cross-sectional surveys were carried out in North America.
Notable among the longitudinal projects is the Amsterdam Growth Study. It yielded data
on activity and energy expenditure. as well as on relationships between health indicators.
PA and EE over a period of 15 years. starting at age 12 (Kemper. 1995: Kemper et al.,
1990: Van Mechelen, Kemper. 1995). Another Dutch longitudinal study. the Nijmegen
Study. was carried out over 6 years (ages 6- i 2) (Saris et al.. 1986).
In the US. there have been several nation-wide cross-sectional surveys of PA in
young people. The first of these is the National Children and Youth Fitness Study
(NCYFS 1) (Ross. Gilbert. 1985: Ross et al., 1980). performed in the early 1980s. That
survey focused on the second decade of life. In a sequel. the same investigators
conducted in the mid 1980s
~
nation-wide survey of activity and fitness among children
lO years and younger (Ross. Pate. 1987). A more recent, ongoing project is the Youth
\U
Risk Behavior S urvaillence (CDC. 1996), launched by the Centers for Disease Control
.md Prevention (CDC) in Atlanta. This survey, which is distributed periodically.
addresses several behaviours. including physical activity.
In Canada, a major longitudinal study focused on a school-based intervention
program centered in Trois Rivieres and Pont Rouge(Jequier et al., 1977). Two nationwide cross-sectional surveys were the 1981 Canada Fitness Survey (Canada Fitness
Survey. 1983) and the 1988 Campbell's Survey on Well Being (Stephens. Craig, 1990).
:\nother. more recent study is the 1990 Canada Health Promotion Survey (Stephens,
1993)
Favourite Activities among Children and Adolescents
First Decade.
In the first decade of life PAis often spontaneous and non-
organized, and is comprised of intermittent brief bouts.
In contrast, older children and
adolescents resort more often to organised activities of a more prolonged nature. For
example. 6- to 1O-year-old girls and boys were found from direct observations
to
perform
predominantly intermittent activities. The median duration for light-to-moderate activity
bouts was 6 seconds (Bailey et 211.. 1995). Most of the high-intensity bouts did not exceed
3 seconds and 95% of these lasted less than 15 seconds.
As reported by Russell et al. (Russell et 211.. 1992), the Canadian Institute of Child
Health published in 1989 a survey of 433 licensed day-care centres. The most frequently
reponed spontaneous outdoor activities included: climbing, running races. jumping,
cycling. swinging and sliding. Structured activities included walks, games, field trips, ball
games and cycling. Even though the intensity of activities was not reported in this survey,
It
it seems that the free, non-structured activities were higher in intensity than the structured
ones.
Based on NCYFS II (Ross. Pate, 1987), which assessed 1st to 4th graders in a
nation-wide US sample, approximately 84%
of the children took pan in activities
available in C0111111t.mity organizations such as parks & recreation. span leagues & tea111S,
churches. YMCAs, YWCAs, scouts and farm clubs. Parents reported the following
activities were practised 1110St commonly in these organizations by their children:
swimming. racing/sprinting, baseball/softball, cycling and soccer (Ross et al., 1987).
Second Decade. An example of the choice of recreational activities of young
people IU years or older has been provided by the 1988 Campbell Survey of Canadians
(Stephens, Craig, 1990). Using a questionnaire, participants were asked to check
activities "that are not related to work". As seen in Table 2, the most popular activity for
boys ages 10-14 years was cycling, compared with swimming for the girls. In the 15-19
years age range, cycling was most popular among the boys and walking among the girls.
in the US. Based on NCYFS I (Ross et al.. 1985a). cycling is the activity
that takes the largest portion of time outside physical education for boys grades 5 through
9, and for girls grades 5 and 6. Basketball is the 1110St popular activity for boys in grades
10, 11 and 12. For girls in grades 7-12, swimming is the most popular sport.
Other
popular activities among these US youth include tackle football and baseball/softball for
the boys. and disco/popular dance, baseball/softball and jogging/fast walking among the
girls.
It thus seems that. in the second decade of life, cycling is the most popular
recreational PA in Canada and the US, alike.
12
lee hockey and outdoor winter sports are conspicuous by their absence from the
above US and Canadian lists. One may well wonder whether this is a ret1ection of the
way the questionnaires were structured, or of the seasons during which most of the data
collection took place.
Effect of Age and Gender on Activity Behaviour
Al1110st without exception. studies on activity behavior and EE have shown a
decline with age. particularly in the second decade of life. Some suggest that the decline
starts as early as age 7-8 years (Taylor et al., 1948). In general, the reduction in PA
among girls starts earlier. and is faster. than in males. For example. in The Amsterdam
Growth Study activity level of Dutch adolescents. as assessed through an interview (Van
Mechelcn. Kemper, 1995), was considerably lower in girls than in boys at age 13 years.
This difference was apparent mostly in the high-intensity activities. In an earlier
observation from that project (Verschuur, Kemper, 1985), the decline in EE (assessed by
HR monitoring) was apparent at age II in the females and only at age 14 among the
males. A survey of nine studies performed in various countries on 6- to 18-year-old girls
and boys has concluded that the males were 140,/0 1110re active. based on questionnaires
(Sallis et al., 1993). The difference increased to 23% in favor of the boys when objective
measures such as HR monitoring were used. In that survey, the average decline in
activity each year wets 2.6-7.-+% in the females, compared with
1.8-2.7~:o
in the males
The NCYFS -1 l Ross et al., 1985a: Pate et ai., 1994) sampled 8,800 grade 5-12
US students and assessed their activity through questionnaires. Unlike other studies.
there was no age-related trend in total daily PA:
s" and 6th graders reported spending 1.9
hours/day on physical activities. compared with 2.0 hours/day in the
111
\ .0 hours/day among the 10 to 1:2
111
r" to 9 th graders and
graders. The girls. however. were consistently less
.ictive than the boys. with the difference ranging from 5 to
15~!o.
In contrast. based on the 1990 YRES (CDC. 1992) and shown in Table 3, the
th
percentage of 9 th to 12 graders who participated in vigorous PA three or more days per
week declined consistently with age and was markedly lower in the girls than in the boys.
.-vnothcr finding shown in this table is that Afro-Americans (and. to a lesser extent,
Hispanics) are considerably less active than the Caucasians. Similarly, based on 1995
Jam. the YRBS (CDC. 1996) indicated that activity in the US declined markedly during
adolescence. For example, at age 12-13 years, 31 % did not fulfill the criterion of 3-perweek periods of moderate to vigorous activity. This rate increased to 62S/o at age 18-21
years. It is not clear whether the above discrepancy between the NCYFS and the YRBS
emanates from differences in the criterion for "activity", or is a reflection of real changes
that OCCUlTed over 10-15 years.
1..+
Canadian data show a similar pattern of age-related decline in physical activity
und of a lower activity level among females compared with males. Based on the 1981
Canada Fitness Survey (Canada Fitness Survey, 1983), which used questionnaires to
report 1'ro111 age 10 years on. both genders show a decline in activity levels between ages
10-11 and 18-19 years. The decline is particularly apparent in the "active" category (i.e.,
3 hours or 1110re of activity per week. for 9 or more months of the year). While 760/0 of
the males at age 10-11 reported being active. this rate dec lined to 600/0 at age 18-19 years.
The respective rates for the females \vere
73~/0
and 65S/o. As reponed in other countries.
the Canadian females started the decline earlier (age 14-15) than did the males. The latter
maintained a rate of 76-7TYo until age 16-17 and then had a precipitous decline. The
results of the Canada Fitness Survey were further analyzed by Shephard (Shephard.
I986). who calculated the energy equivalents of the above trends. Accordingly, energy
expenditure at age 13-15 was considerably higher ( 13.4 and 10.9 kJ per kg per day for
males and females, respectively) than at age 16-19 years (8.8 and 7.5 kJ per kg per day)
and age 20-29 years (6.3 and 4.2. kJ per kg per day, respectively). These figures represent
~l
50-6UI)/;) reduction i11 EE per kg body weight from ages 13-15 to 20-29 years.
The 1988 Campbell Survey (Stephens. Craig, 1990) used another criterion for an
"active status: the expenditure of 3 or more kilocalories of energy per kg body weight
per day. The results are reported for two age categories during adolescence ( 10-14 and
15-10 years). Seventy-two percent of the males at age 10-14 belonged
groups. compared with
69~/o
IS
and
39~<J
69~/o
to
the active
among the females. At age 15-19 the respective rates were
and at age 20-24 they further declined to 47 % and 260/0, respectively.
Even though the activity criteria differ in the 1981 and 1988 surveys. The decline among
the females in 1988 is more dramatic than in 1981. Unfortunately, one cannot compare
the two surveys because of differences in the criteria. In the Quebec Family Study,
moderate to vigorous activity was estimated in 1978-1981 among 424 boys and 366 girls,
ages 8
to
9 years. using a 3-day diary in which the subjects divided the day into 15-Inin
blocks (Bouchard et al.. 1983). The activity level (using categorical units) for the boys in
the l)-l3 vr age group was 2-1-3. it declined to 193 at age 13-15 and to 176 at age 16-18.
The respective values for the girls were 167, 135. and 105 (Eisenmann et al., 1999).
Interestingly. when energy expenditure (k.loule per kg per day) was calculated from the
same data base. there was no age-related decline. The girls' mean energy expenditure
was some 6-8% lower than in the boys, across all ages.
The age-related decline in PA occurs not only with spontaneous activities. but
also with participation in physical education classes. For example, the NCYFS I in the
US (Ross et al., 1985: Ross. Pate. 1987) has shown that. while 97% of 1st through 6
th
graders takes part in physical education classes. only 49% enroll in grades 11 and 12.
According to the 1993 \rRBS. in grade 9. 71 ~!o of students (girls and boys combined)
enro lied in physical education classes. This rate declined to 590/0 in grade 1O.
42~/o
in
grade 11 and 3 8~/0 in grade 12. In the above samples. girls' enrolment was sorne 100/0
lower than among the boys throughout these grades. Based on the 1997 Guidelines of the
US CDC (Centers for Disease Control and Prevention. 1997), daily participation in
physical education among US high school students dropped from 420/0 in 1991 to 25~/0 in
1995. In several Canadian school systems. participation in physical education classes is
compulsory for one term only during the entire last three years of high school. As a
16
result. the great majority of pupils opt out of physical education during these years .
.-\ccording to a 1997 Gallop Poll. only 7°<} of parents in the US consider physical
education as basic
to
the school curriculum (presentation by V. Seefeldt. East Lansing
conference. 1999)
Based on the above reports, children worldwide seem to be more physically active
than adults. A question remains though as to whether the above activity levels are
sufficient. The answer will depend on the criteria used to denote "sufficient activity". If
one uses 3 days per week of moderate to vigorous activity as a benchmark (see section on
published guidelines. below). then the activity levels for many young people are
insufficient. A 1995 survey in the US (CDC, 1996), for example, has shown that 480/0 of
girls and 26% of boys were not active 3 out of the preceding 7 days, at a moderate to
v igorous
intensity. Using the same criterion, a 1993 national survey in England has
shown that. in the 16 to 24 years age group, 520/0 of the females and
32~/o
of the males
were insufficiently active. In Canada. based on the 1990 Canada Health Promotion
Survey (Stephens. 1993).
26~/O
of adolescent males and 330/0 of adolescent females were
insufficiently active. These rates of inactivity seem lower than in the US and UK studies,
which used a similar criterion for "sufficient activity". Because of methodological
differences. however. one cannot state that the above differences are valid for the
population at large.
\7
TRA.CKING OF PHYSICAL ACTIVITY BEHAVIOUR
Does physical activity behaviour track from childhood through adolescence into
theadult years'? This is a most relevant question if one assumes that physically active
children. compared with sedentary children. have a greater potential to beC0111e active
adults. There are several original studies and reviews in which tracking of PA from
childhood or from adolescence was determined (Andersen. Haraldsdonir,
.-\ndersen. 1996: Atkins et al., 1997: Kelder et al., 1994: Kemper et al.,
1996: Pate et al., 1996: Raitakari et al.,
1994~
1993~
1990~
Malina,
Sallis et al., 1995; Saris. 1986; Taylor et al.,
1999: Van Mechcieu, Kemper. 1995). As a general rule. short-term tracking (i.e.. over 23 years) is stronger than long-term tracking. Beginning at about 4 years of age, inter-age
correlations range from 0.16 to 0.57 (Sallis et al., 1995; Pate et al., 1996). Physical
activity tracks moderately during childhood and during the transition into adolescence, as
it does over a span of 3 years during adolescence. However, the correlation coefficients
are lower over spans of 5 and 6 vears, and they further diminish when one compares
activity between early adolescence and young adulthood. In the Amsterdam Growth
Study. for example (Table 4 L correlation coefficient for boys were 0.44 from age 13 to
! 6.0.20 from age 13 to 21 and 0.05 from age 13 to 27. The same general pattern was
apparent for the females (Van Mechclen. Kemper, 1995). A similar trend has been found
in Finland. based on the Cardiovascular Risk in Young Finns Study (Raitakari et al.,
(994) and in a retrospective US study (Taylor et al., 1999). Moderate to low correlation
coefficients have been found also among Danish females and males, whose activity
behaviour was assessed (using a questionnaire) at ages 15 to 19 years and again 8 years
later (Andersen. Haraldsdottir. 1993).
\~
Factors that may favorably affect adulthood activity include the childhood skill
ieve l and participation in organized sports. In contrast. adulth exercise behaviour is
negatively related to being forced and/or being encouraged to exercise during childhood
and adolescence (Malina, 1996). Studies on the probability of remaining active or
inactive in adolescence and in adulthood. also suggest moderate tracking. It thus seems
that tracking may be higher in groups that are at the extremes of the activity-inactivity
contmuum.
fACTORS THAT MAY MODIFY ACTIVITY BEHAVIOR AND ENERGY
EXPE\:OlTURE
.-\.S summarised in Table 5. there are various biological, familial. psychologicaL
societal and cultural factors that 111ay influence activity behavior and energy expenditure
of children and adolescents. PA and EE may also be modified by variations in the
physical environment. such as cl imate. weather and seasonal changes.
Studies that have addressed these relationship are usually based on correlational
.maiysis. which ret1ects associations, but not cause and effect. The following is a brief
discussion of S0111e biological factors and those related to the physical environment, that
may modify PA and EE during growth and maturation. Discussion of the psychological,
social and cultural factors is beyond the scope of this review.
Biological Factors. Heredity. Based on their observations from the 1981 Canada
Fitness Survey. Perusse et al. (Perusse et al., 1988)concluded that environmental
!lJ
influences on PA are stronger than hereditary ones. In a subsequent project, the same
group (Perusse et al . 1989) studied 375 families of French descent in the greater Quebec
urea. Using a self-recorded activity log (Bouchard et al., 1983). they assessed the "level
o t habitual physical activity" (all daily activities, ranging from those that require a very
low EE to those that require a high metabolic level) and "exercise participation"
(activities that require at least 5 MET). Associations were computed for pairs that
included "biological relatives" (parent vs natural child and biologic siblings, including
dizygotic and monozygotic twins) and "non-biological relatives" (e.g. spouses. uncle vs
nephew or niece. and siblings by adoption). The authors concluded that the level of
habitual PA is significantly influenced by heredity (290/0 heritability). In contrast, there
was no genetic int1uence on exercise participation.
Two earlier twin children studies in the USA (Scarr, 1966~ Willerman, 1973) have
also shown heritability of the overall activity pattern. and less so for the type of activity
chosen by the children. In one of these studies (Scarr, 1966), it was found that. even when
parents erroneously labelled their monozygotic twins as dizygotic and vice versa. the
concordance in PA was stronger in the real monozygotes than in the real dizygotes.
One can therefore conclude that there is a significant genetic effect -- the strength
of which varies among studies -- on a child's general activity pattern. However. the
intensity of children's activities, and the specific type of sports they are engaged in. are
strongly int1uenced by environmental factors. It is the latter relationship that is of public
health importance . More research is needed to determine whether the above phenotypical
expressions can be generalized to all age groups.
20
Both undernutrition and obesity are associated with physical hypoactivity of
children and adolescents. Protein-calorie undernutrition often results in reduced resting
metabolism and 2-t-hour EE (Torun. 1990: Torun et al., 1996), which may reflect the
body's attempt to conserve energy. Some, but not all, studies have shown a reduction in
2-t-hour EE per kg body mass among obese infants, children and adolescents (Bar-Or et
al.. 1998). This issue is discussed in further detail in the section on activity and health
during childhood and adolescence. Health status is a major determinant of PA. As a
group. children with a chronic disease or a physical/mental disability are less active than
their healthy peers (Bar-Or. 1983: Longrnuir, Bar-Or. 1994: Van Den Berg-Emons et al.,
[995). The pathological condition in itself may limit the child's activity. but
psychological and societal forces are often even more important. Interestingly, the extent
of hypoactivity is often unrelated to the severity of the disease or disability (Bergman,
Sta111111, 1967).
Pubertal changes. mostly in females, are often accompanied by a reduction in PA.
This has partially be explained by psychosocial factors (e.g .. maturing girls become
interested in pursuits other than sports: or the notion that "success in sports requires
masculinity" (Butcher. 1983)). However, reduction in PA may result also from biological
changes. such as an increase in adiposity, widening of the pelvis. discomfort before and
during the menstrual period. and a reduction in blood hemoglobin.
Motor skills mav determine PA patterns (Malina. Bouchard. 1991). Although
athletic success reflects training, it also depends on innate skill. Children and adolescents
would pursue activities in which they are skilled and successful.
Does phvsicaltitness affect PA? intuitively one would assume that this is so (and
that PA affects fitness). However. research so far has yielded equivocal findings. Some
studies show no relationship, while others show only fair correlations between activity
.ind fitness ( Morrow. Freedson, 1994). For details. see the section Activity and Health
during Childhood and Adolescence.
The Phvsical Environment
Availability ofactivitvjacilities. This author could not find references which
specifically reported on the extent by which children's activity depends on the relevance
of the proximity to playground, parks and recreation centres. It is conceivable though
that. because of safety and logistical consideration, this is an important factor.
Seasonal and climatic variation 111ay affect activity behavior (Shephard et al.,
1980) (Blanchard, 1987). In temperate and cold climatic regions, the level of PA often
increases in the summer months. Indeed, according to the NCYSF L 5th- to 12th-grade
boys spent twice as many hours per week on after-school activities during the summer,
compared with the winter.
Among girls, the ratio was 2.3: 1 in favour of summer time
activities. This seasonal trend was not age-dependent. In the same survey (Ross et al..
19~5b).
90%
of the students reported an appropriate PA pattern during the summer,
compared with only 6~o;~) reporting so for the winter. Rates for fall and spring were 74%
and 79~/O. respectively. The same pattern was obtained for activities with a carry-over
value for the future and activities that induce "sweating and hard breathing". This pattern
lS similar for girls and boys.
in Canada. climatic and seasonal variations may be a significant factor due both to
the range of climatic regions and the sharp inter-seasonal climatic variations. The effect
of fewer daylight hours during the winter deserves particular attention as this is a further
limitation on children's ability to play outdoors. The relevance of the seasons to a child's
activity was addressed in the Trois Rivieres regional experiment, in which 546 girls and
boys rrornwere divided into a physical education enrichment group (additional 5 hours
per week) and a control group (40 min of required PE) (Jequier et al., 1977). Based on a
=:4-72 hours recall questionnaire. given in spring and fall, urban children devoted more
time to "very light" activities. but less time to "light" activities. in the spring than in the
fall (Shephard et al., 1980).
The authors do not provide an explanation for this,
somewhat inconsistent. pattern. It is unfortunate that they did not monitor activity
patterns in winter and summer, when differences may be most apparent because of
climatic variation and the much shorter winter days. Indeed, in the 1981 Canada Fitness
Survey. there \VaS an almost 2: 1 ratio in the number of hours devoted to participation in
the 10 most popular activities during the summer compared with winter (Shephard. 1986)
(PP. I 17-1 1S). These data however are presented for the population as a whole. rather
than for age groups. A similar trend was reported for children from Finland. a country
similar to Canada in its winter-to-summer climatic differences. In that study (Telama et
al.. 1985). parents reported twice as much outdoor activity in the summer than in the
winter for 3- and 6-year-old girls (6.7
t'S
3.3 hours per day) and boys (7.0 vs 3.7 hours per
Seasonal variations may occur also in relation to summer vacation and prolonged
holidavs. \Vhile most children are more active during these periods, others (whose main
.icrivitv is school-based: show a reduction in activity. The greater activity level during the
summer probably reflects the finding that children are more active when outdoors, as
shown in US (Klesges et al., 1990)and in J apaneses (Mimura et al., 1991) studies.
Spontaneous activity during weekends seems to be greater than during . . veekdays.
for example. in the above mentioned Trois Rivieres study, vigorous activities during the
spring were performed less during weekdays than during weekends (Shephard et al.,
19tSO). Evaluation of children' s activity patterns and energy expenditure should therefore
cover weekends and weekdays, alike.
Satety concerns. particularly in urban areas. have become dominant in the
decision of parents as to whether a child would be allowed to spend time outdoors, or to
walk to and from school (Bar-Or et al.,
1998~
Centers for Disease Control and Prevention.
1997).
Schoof \'s work site
is another environmental factor to be considered. Ilmarinen &
Rutenfranz from Germany conducted a longitudinal follow up of 25 girls and 26 boys
between ages 1-1- and 17 (llmarinen. Rutenfranz, 1980). They analyzed the possibility
that PA declines once a person leaves school and starts to work. At age 16. 73% of the
boys and
52~';)
of the girls quit school and joined the labour force. When observed one
year later (recall questionnaires), the boys who started working did not differ in their PA
fr0111
those staying at school. However, among the girls. the occupational group was half
as active as those remaining at school. The main difference was the extent of
participation in sports. The authors speculate that. once a person joins the work force.
there is less time available for leisure activities and a greater fatigue at the end of the
working day. particularly among females.
ACTIVITY AND HEALTH DURING CHILDHOOD AND ADOLESCENCE
A major issue to be addressed in this review is whether PA during childhood is
accompanied by health-related benefits. The next section will focus on the possible longterm benefits that beC0111e apparent only in adult life. The current section addresses the
evidence for a relationship between PA levels and health indicators, as measured in
children and adolescents.
Several reviews and monographs (e.g.,(Armstrong, WelS111an, 1997) (Bar-Or,
1983: Bar-Or, 1985: Despres et al., 1990; Malina, 1990) have addressed the relationships
between health and PA at a young age. However, the most systematic analysis of this
topic has been provided by a consensus statement published in 1994 (Sallis, 1994). This
is the first comprehensive attempt to apply objective criteria to the strength of such
evidence, based on firm epidemiological considerations. The document includes reviews
on the relationship between adolescent PA on the one hand. and aerobic fitness. skeletal
health. adiposity, blood pressure. blood lipids and psychological variables. on the other.
Tables 6-10 summarise the main findings in each review.
Aerobic Fitness and Physical Activity. Intuitively, one would assume that,
because training can increase aerobic fitness, children and adolescents who are fit are also
the active people among their peers. This apparently is not the case. While in adults
aerobic fitness is often taken as a surrogate indicator of habitual activity (e.g., (Blair et
ul.. 1989)), it seems that there is a weak association among children and adolescents
25
between activity and aerobic fitness .. As seen in Table 6. which summarises data from
~
I studies ( Morrow. Freedson. \99..+ L neither cross-sectional, nor longitudinal
(observational or intcrventional: results show a strong association between fitness and
activity. ~loITOW and Freedson concluded that, a "typical relation" is r = 0.16-0.17.
These data refer mostly to males. because very few studies were conducted with females.
The authors further suggest that. even though one cannot determine an intensity threshold
beyond which aerobic fitness would improve. a walking activity per se does not seem to
induce a high enough training effect. Nor is it clear what volume of activity is needed to
enhance aerobic fitness. The above review focused on adolescents. \Vhile there are no
similar reviews on children. there are indications that aerobic trainability in
prepubescents is similar to (Pate. \Vard. 1990), or even lower than (Bar-Or, 1983) that
observed in adolescents or adults.
Even though the authors of the above review (Morrow, Freedson, 1994) proposed
a list of recommendations regarding volume and intensity of training that is required in to
enhance aerobic fitness. these are not based on evidence bOD1e out by the reviewed
studies. Following are the three recommendations:
l . Youth and adolescents should be involved in regular (preferably daily)
physical activity. Additional aerobic fitness benefits may be achieved with
moderate to vigorous physical activity. For example, a minimum of 3 days
per week for a minimum of 30 min at an intensity of 75~/o of heart rate reserve
- that is. resting heart rate
T
O. 75(maxilllul heart rate <resting heart rule)
The total "volume-, of physical activity in which youth and adolescents engage
may be important in terms of aerobic fitness .... Children and youth may be
20
active but not in the same ways that adults are active. High-intensity activity
of a sustained nature is unlikely to be part of their typical daily routines,
unless they are training for a specific purpose, , ,.
s . The physical activity should use large muscles and provide an aerobic
stimulus (e.g., running, cycling, rope jumping, swimming).
Bone Health and Physical Activity. Most reviews about the health
consequences of activity or inactivity at a young age have focused on coronary risk and
obesity It is important, though, to realise that the level of one's physical activity may
have a far reaching effect on one's skeletal health. Much literature is available on this
relationship in adults and the elderly, and some evidence has emerged in recent years
about the possible role played by activity in childhood and adolescence on bone
accretion, particularly bone mineral density (BMD) and peak bone mass. Peak bone mass
is achieved usually in the third decade of life, but sometimes earlier. Thus bone accretion
during adolescence is a major determinant of peak bone mass (Ott, 1991), thereby
.utecung bone health in later years. The possible effect of physical activity on peak bone
mass is therefore of major importance.
:-\ review of the evidence for a relationship between skeletal health and physical
activity of young people (Bailey, Martin, 1994) is summarised in Table 7. The
conclusions, as listed in the table. are based on 21 studies published until 1993. More
recent reviews (Barr. McKav. 1998; Bass et al., 1998; Blimkie et al.. 1996) and several
original studies (Chad et al.. 1999) (Cheng et al., 1998; Courteix et al., 1998: Jone,
Dwyer. 1998; Khan et al., 1998; Matkin et al., 1998; Nickols-Richarson et al., 1999;
:'\ordstr0111 er al.. 1998: Rubin et al.. 1999) (Teegarden et al.. 1996; Valdimarsson etal.,
1999) have provided similar information.
The general cone lusion 1'1'0111 the above studies is that PA during childhood and
adolescence is associated with, and seems to contribute positively to, bone mineral
content and peak bone mass. As seen in Table 7, this relationship is particularly strong in
prospective observational studies and in unilateral studies (where one limb serves as
control to the contralateral limb). More specific conclusions are:
•
Weight-bearing and high-impact activities. such as gymnastics. figure skating, soccer,
basketball. volleyball. racquet sports and ballet are more beneficial than non weightbearing activities such as swimming (Bailey et al., 1997; Cassell et al., 1996; Courteix
et al., 1998: Grimston et al.. 1993: Khan et al., 1998; McCulloch et al., 1992~ NickolsRicharson et al., 1999: Nordstrom et al., 1998). In point of fact, there is some
evidence that swimmers have a lower bone mineral density than.do non-athletes
(Grimston et al.. 1993: McCulloch et al., 1992~ Risser et al., 1990~ Slemenda et al.,
1991). A possible explanation is that competitive swimmers spend many hours in
relative weightlessness.
•
Activities that increase muscle strength should be promoted, because there is a
relationship between bone mineral content and strength (Cheng et al., 1998:
Nordstrom et al., 1998).
•
Immobility and immobilization should be prevented as much as possible, because
they 111ay disrupt bone accretion and/or enhance its resorption (Bailey et al., 1997:
Biewener. Bertram. 1994: Frost. 1988).
28
•
The benefits of physical activity may be local. depending on muscle forces on
specific bones. For example. bones in the dominant limb 111ay have a higher mineral
density than in the non-dominant limb (Faulkner er al.. 1993)
•
In girts. the beneficial effects of activity may be counteracted by low estrogenic
activity, which may be secondary to excessive low-energy diets (Baer et al., 1992).
•
The beneficial effects of enhanced activity mayor may not apply to children of all
ages. In a recent randomized contro lled intervention, o-month-old infants were
assigned to either a gross motor program or a tine motor program for one year
lSpecker et al., 1999). Results suggested that the gross motor program was
accompanied by a lower bone mineral content, particularly among infants who also
had a low calcium intake. The authors suggested that the gross motor program may
have led to reduced bone accretion during fast growth, when calcium intake was
moderately low.
Adiposity and Physical Activity. Even though this document focuses on the
general population. special attention should be paid to a large segment of our young
people. who are obese. Juvenile obesity in North America has reached epidemic
dimensions and its prevalence is still on the increase (Bar-Or et al.. 1998). Almost one
quarter of children in the US are now considered obese. This represents S0111e 200/0
increase in prevalence over one decade (Troiano et al., 1995). Exact prevalence values
for Canada are not available. but it is reasonable to assume that the above trend is also
true for Canada and other technologically advanced countries. According to the 1988
Campbell Survey. .+3°0 of females ages 15-19 years had excess fat (taken as sum of 5
29
skinfolds). The respective rate for males was
27~/0
(Stephens, Craig, 1990). The above
survcv did not provide data for people younger that 15 years. Forty percent of obese
children and
70~tQ
of obese adolescents are at risk of being obese as adults (Guo et al.,
1994: DiPietro et al.. 1994). Therefore. juvenile obesitv is a major public health
challenge.
There is a plethora of research on the relationship between adiposity and physical
activity in children and adolescents. Much of ithas been reviewed in recent years (BarOr. Baranowski, 1994: Bar-Or et al.. 1998: Gutin, Humphries, 1998). Table 8
:,umrnarises the evidence derived from 21 pertinent studies performed until 1993 on
children and adolescents ages 11 to 21 years. As seen in the table, the relationship
between adiposity and physical activity (or energy expenditure) differs in those who are
ulreadv obese and the general non-obese population.
In cross-sectional comparisons between young athletes and non-obese sedentary
children or adolescents. the athletes usually have a lower adiposity level. This, however,
may reflect pre-selection rather than a direct effect of physical activity. Indeed. in a
landmark review of 55 intervention studies that lasted 6 to 104 weeks. Wilmore
(\Vilmore. 1983) concluded that the overall effect of enhanced physical activity on
percent body fat in non-obese young people was minimal (approximately 10/0).
The effect is greater for young people who (Ire alread.. . obese. Cross-sectional
J
studies have clearly shown that obese children and adolescents are less active than their
non-obese peers (even though the evidence is less clear for energy expenditure).
l.ikewsie. longitudinal interventions. with and without low-calorie diets. have been
30
efficacious in causing a reduction in adiposity. Following are some of the more specific
findings:
•
Obese children and adolescents are often less physically active than their non-obese
peers. particularly in spontaneous, non-structured activities (Bullen et al., 1964;
Corbin. Pletcher. 1968; Markuske, 1969).
•
Other studies found little or no relationship between the degree of adiposity and the
activity level (Beunen et al.. 1992; Marti. Vartiainen. 1989: Sallis et al.. 1988: Saris.
1986: Stunkard. Pestka, 1962). Such a discrepancy 111ay ret1ect the variety of
methods used to determine activity and the criteria used to define obesity, as well as
the ethnic and social background of the samples.
•
\Vhen calculated in absolute units. total daily energy expenditure of obese children is
the same. or even higher. compared with non-obese controls (Bandini et al.. 1990;
Bar-Or et al., 1998). This reflects the larger body mass of the obese. rather than a
higher activity level. One way of cancelling out body 111aSS is by dividing total
energy expenditure by resting energy expenditure. This ratio is called "physical
activitv level" or PAL. In a study of 1.5- 4.5-year-old girls and boys, PAL was
inversely related to adiposity (Davies et al.. 1995), which suggests that the activityrelated energy EE is lower in young obese children. This is an important finding,
because total energy expenditure was determined by doubly-labelled water, which is
considered the Gold Standard for the assessment of EE under free-living conditions.
Likewise. in another study which used this technique, total energy expenditure minus
basal metabolic rate was inversely related to body adiposity in 12- to 18-year-old girls
and boys. once body mass was partialled out (Bandini et al., 1990).
31
•
There is a strong association between the risk of becoming obese and the extent of TV
viewing, as shown
111
large-scale observations in the US (Dietz, Gortrnaker.
1985~
Gortmaker et al.. 1~)96: Pate, Ross. 1987). ln one study. based on a nation-wide
survey. the authors (Dietz. Gortmaker. 1985) suggested that each additional hour of
TV viewing per week increases by 2 ~/o the risk of being obese. A 1986-1990 survey
has shown that the odds of being obese are 4.6 times greater for those adolescents
who watch TV 5 hours or 1110re per day, compared with those who watch 0-2 hours
(Gortmakcr et al.. 1996). It was further shown in that study that the likelihood of
recovery from juvenile obesity is inversely related to the extent of TV watching
•
Although TV viewing represents a sedentary activity (and in one study, energy
expenditure decreased below "resting" levels when children watched TV (Klesges et
al., 1993 )). the increased intake of food while watching TV and the numerous foodrelated ads that the viewers watch are important factors as well (Bar-Or et al., 1998).
In that regard. one should note that a sedentary lifestyle covaries with other
behaviours. including fat and alcohol consumption (Kelder et al., 1994).
•
The optimal structure of an intervention regimen has yet to be determined. It seems,
though. that activities that include "lifestyle" changes are more efficacious than
regimented exercise prescriptions (Epstein et al., 1990; Epstein et al., 1982).
•
The importance of TV watching is further shown by the outcomes of intervention
programmes in which a decrease of TV viewing was a major C0111pOnent. Epstein and
colleagues (Epstein et al., 1997: Epstein et al., 1998) have shown that behaviour
modi fication. where obese children who decrease the extent of TV watching are
rewarded. yields better weight control and longer adherence than does an actual
32
exercise prescription. Although such an approach is most promising. the above
results have yet to be confirmed by other groups.. In a recent randomly controlled
th
trial (Robinson. 1999)yJ and 4 graders in California. who reduced their TV.
videotape and video game time over 7 months, had a reduction in body adiposity.
•
\Vhile there is no information on the dose-response relationship between the success
of enhanced physical activity (Bar-Or, Baranowski. 1994), programmes that last over
one year seems to be more efficacious than shorter interventions (Sasaki et al., 1987).
•
There are no data to suggest
a11
optimal increase in energy expenditure. It seems
though that obese children and adolescents. with a reasonable motivation. can
increase their overall daily expenditure by about 100/0 in a 45- 60-min session (Blaak
et al., 1992).
•
It is reasonable to recommend activities that will use large muscle groups and last
long enough to induce sufficient energy expenditure. Indeed. most intervention
studies have used endurance-type activities. There seems to be one potential
exception: when obese children are prescribed a very low-calorie diet they are likely
to lose fat-free mass and not only fat mass (Blaak et al., 1990). It is possible that. by
including resistance training. one may be able to reverse this undesirable effect
( Loftin et al.. 1996). This concept is based on the anabolic effect of resistance
training. which may be more efficacious in adolescents than in children (Blimkie,
1993). There are no definitive studies. however. which have demonstrated the
efficacy of resistance training in preservation of fat-free mass in obese young people.
•
Based on the clinical experience of this author, the selection of activities for obese
children 111USt take into account their own preferences. An activity that. from a
33
physiologic point of view. may not have a high energy equivalent (e.g., bowling or
baseball). but is liked by the child is better than a metabolically more demanding
activitv that the child dislikes.
Blood Pressure and Physical Activity. Even though hypertension is mostly seen
among adults and the elderly. it also occurs in children and adolescents. For example,
nearly 3 million US young people have excessive blood pressure levels (National High
Blood Pressure Education Program \Vorking Group on Hypertension Control in Children
and Adolescents, 1996). In Canada. based on the 1988 Campbell Survey (Stephens,
Craig. 1990). "less than l5~/0" of children 10 to 14 have a diastolic pressure of 2:90
111n1Hg and a systolic pressure of 2: 1-1-0 mml-lg. The rates for the 15 to 19 years group are
"less than 10" for both systolic and diastolic pressures. These reported rates do not really
provide information about the actual prevalence of hypertension in Canadian children and
youth. Obese individuals. in particular. are prone to high blood pressure (Holl, 1992;
\V i lliams et al., 1992),
Tracking of blood pressure from adolescence to adulthood has been studied in
various countries (Akerblorn et al., 1999: Andersen, Haraldsdottir, 1993: Malina, 1990;
Palti et al., 1988: Raitakari et al.. 1994: Twisk et al., 1997). For example. when Danish
adolescents were tested S years apart (starting at age 15-19 years) correlation were 0.49
and 0.44 for systolic and diastolic pressures. respectively, among the males and 0.54 &
0.38 among the females. Fifty-three percent of the males and 380/0 of the females stayed
in the upper quintile for systolic pressure. Respective values for diastolic pressure were
-1-3 u (J and 360/0 (Andersen. Haraldsdottir, 1993). Similar tracking coefficients were shown
3..+
for Dutch adolescents (Twisk et al.. 1997). In this context, high blood pressure in early
years is of a definite public health concern.
Table 9 is a summary of the evidence regarding a possible relationship between
activity and blood pressure. It is based on 29 studies published until 1993. The general
message is that in children and adolescents with a normal blood pressure. enhanced
physical activity does not induce a further decrease in pressure. In contrast, children and
adolescents with a high blood pressure do benefit from enhanced physical activity,
particularly if it includes aerobic exercise. The benefit is a reduction in both systolic and
diastolic pressures. albeit not necessarily to normal levels, The major findings are as
to llowing:
•
Based on cross-sectional comparison, endurance-trained children and adolescents
sometimes have a lower blood pressure than untrained controls (Fraser et al., 1983;
Fripp et al., 1985; Panico et al., 1987). However, when variables such as body mass
and adiposity are partialled out. the relationship between blood pressure and fitness
diminishes. or disappears altogether (Armstrong et al.. 1991 ).
•
Training studies in young people with a normal blood pressure show a minimal effect,
if any. on blood pressure (Eriksson. Koch. 1973; Hansen et al., 1991 a).
•
Endurance training interventions in adolescents with hypertension or a borderline
hypertension induced a reduction in systolic and/or diastolic pressures. Of special
importance is a randomly controlled study of Danish 9- to l l-year-old girls and boys
who were given 3 extra 50-tnin physical education sessions per week, over 8 tnonths.
(Hansen et al., 1991 b). While after 3 months there was no effect on blood pressure,
there was a significant change after 8" months: systolic pressure declined by 6 mml-lg
35
and diastolic pressure by 3 1111nHg. The controls had no change in blood pressure.
The above study suggests a dose-response effect of the duration of an intervention.
Another randomly controlled intervention, within the Bugalosa Heart Study in the US
(Frank et al.. 1982). yielded a dcline of 8 mml-lg in both systolic and diastolic
pressures over 8 months. This intervention, however, included dietary measures and
not only enhanced physical activity.
•
\Vhile a bout of resistance-type exercise (e.g., strength training, weight lifting) caused
a marked increase in arterial blood pressure (Macdougall et al., 1985), resistance
training following an aerobic training program induced a further reduction in blood
pressure of adolescents with hypertension (Hagberg et al., 1984). Another 2-month
weight lifting programme did not induce changes in blood pressure (Laird et al.,
1979).
•
There are no data as to what constitutes an optimal activity program for the
prevention or for the treatment of hypertension in young people.
Blood Lipids and Physical Activity. One of the major risk factors for
atherosclerosis and coronary heart disease is an abnormal blood lipid profile.
Specifically, high levels of plasma cholesterol and of low-density lipoprotein (LDL) are
highly correlated with atherosclerosis and coronary heart disease. while high-density
lipoprotein (HDL) levels are inveresly related to these conditions (Expert Panel on Blood
Cholesterol Levels in Children and Adolescents. 1992). Based on the Framingham study
in the US. the ratio LDL/HDL is a major predictor of coronary heart disease morbidity
and mortality (Castelli. 1984). While coronary heart disease and other diseases
associated with atherosclerosis are usually not manifested before middle age,
atherosclerosis can be detected already in adolescence and even in childhood (Strong et
ul.. 1995). Improvement of the lipoprotein profile at a young age is therefore an obvious
public health challenge. This section will briet1y review possible relationships between
lipoprotein profile and physical activity. Studies in this field encounter several
methodological hurdles. For example, some programmes for young people with risk such
as obesity or dyslipidernia include a combined intervention of activity and dietary
changes. With such a design, it is hard to tell what is the relative role of each.
Furthermore, some of the intervention programmes induce weight (or body fat) loss
and/or improvement in physical fitness. It is then hard to distinguish between the direct
effects of the intervention and secondary effects due to weight loss or an increase in
fitness, even if one can statistically partial out these factors (Craig et al., 1996).
Table lOis a summary of the evidence regarding a possible association between
blood lipids and physical activity in children and adolescents and the changes that may
occur in their blood lipids as a result of activity programs. The table is based on 32
studies. pub lished until 1993 t Armstrong. S imons- Morton. 1994). Cross-sectional
analyses suggest that trained individuals have a more favourable lipoprotein profile than
do their untrained peers. However. activity-based interventions have little or no effect on
this profile. The latter is true for the general. healthy population as well as for young
people with high risk (e.g., obesity. diabetes and familial aberrations in the lipoprotein
profile). Studies since 1993 have shown the same general pattern, although some suggest
that certain activity-based interventions may be efficacious (Gutin, Humphries, 1998).
Following are S011le specific conclusions:
37
•
The relationship between lipoprotein profile and physical activity seems to be
stronger in adults than it is in children and adolescents.
•
.-\ relationship exists between blood lipids and aerobic fitness. irrespective of body
fatness (Gutin et al., 1997).
•
There is
110
information about any dose-response relationship between blood lipids
and the volume or intensity of training.
\laturation and Physical Activity. Skeletal maturation is neither accelerated
nor delayed by enhanced PA, nor does sport participation affect the age at peak height
velocitv (Beunen et al.. 1992). It is clear. though, that menarche occurs later among some
urhleric groups than in the general female population (Malina et al., 1973). Noteworthy
are gymnasts and figure skaters who. on the average, reach their menarche some 2 years
or more later than the general population (Beunen et al., 1992). The mechanism for such
a delay is not clear. One school of thought (e.g., (Malina, 1983) promotes a two-part
hypothesis. of pre-selection and of social forces. There is evidence. for example. that
girls who are selected into competitive gymnastics programs have body characteristics
(c.g .. shan stature) that are associated with late maturation (Bajin, 1987). The social
component of this hypothesis states that. unless female gymnasts are late maturers, they
will prefer to retire from competition, rather than perform poorly once maturational
changes have taken place.
The other school of thought suggests that the training regimen itself may cause a
delay in menarche by interfering with the normal hormonal sequence of puberty. A
negative energy balance that often occurs among young athletes has been postulated as a
possible trigger for such an aberration (Frisch et al.. 1981). More recent studies have
suggested a possible direct link between physical exertion and the hypothalamicpituitary-gonadal axis, but the variability in outcomes does not allow any definitive
conclusions about a causal relationship between primary amenorrhea and athletic training
(rogol, 1996).
IMPliCATIONS OF CHILDHOOD ACTIVITY AND SEDENTARISM TO
ADULT HEALH
The intent of this section is to review possible relationships between future health
outcomes (i.e., those manifested during adult life) and physical activity during childhood
and adolescence. The ideal design for studying such relationships is a long-term,
randomly controlled intervention. For obvious ethical and logistical reasons, such an
approach is impossible to implement in humans. Another approach is to conduct a
longitudinal observation of cohorts of children and adolescents who vary in their PA
level. Such data have been emerging in recent years and provide the bulk of information
on this topic (Andersen. 1996) (Beunen et al., 1983) (Post et al., 1997) (Twisk et al.,
I <,)97: Van Lenthe et al.. 1997). Some studies have provided retrospective data where
adults reported their childhood and adolescence activity behaviours (Kriska et al., 1988~
Talmage. Anderson, 1984). Such observations, although important, depend on the recall
ability of the subjects, with its inherent limitations. At best they can provide information
about activity behaviours. but not on energy expenditure.
One of the factors that limit our knowledge about long-term health-related effects
of activity behaviour is the poor-to-fair tracking of risk factors from childhood to
39
adulthood (Akerblorn et al.. 1999; Guo et al., 1993; Twisk et al., 1997; Van Lenthe et al.,
1996). \Vith such low tracking it is hard to draw conclusions about the long-term effect
01' physical activity. unless one conducts randomly controlled interventions.
Another methodological constraint is the lack of consistency in activity behaviour
over years (Malina, 1996). As shown, for example, in the Amsterdam Growth Study
tracking of daily activity from age 13 to 27 was quite low (r=0.34) (Twisk et al., 1997),
which suggests that individuals modify their activity pattern over time in an unpredictable
manner. Such inconsistency in activity behaviour makes it hard to characterise
individuals as "active" or "inactive" over long periods of time. This, in tum, detracts from
the validity of observational -- cohort or retrospective -- studies. Furthermore, because
1110st physiologic effects of a training program are short-lived once the program ceases,
an inconsistent activity pattern is not very likely to induce long-standing effects.
In conclusion. evidence to date does not suggest that childhood and adolescence
PA per se determines health indicators in later years. One exception may be the
beneficial effect on peak bone mass.
Adult Stature and Physical Activity. Even though some studies have suggested
that participation in sports lTIay affect growth, there seems to be no relationship between
adult stature and PA or EE at a young age (Malina, 1980; Malina, 1990). Indeed,
differences in adult stature between athletes and non-athletes seem to reflect preselection. rather than a training effect.
-w
Adult Physical Fitness and Childhood Physical Activity. As discussed earlier.
.ictive children and adolescents are not necessarily rnore fit than their sedentary peers. A
question remains as to whether physical activity in childhood and adolescence affects
physical fitness in later years. The scant information available suggests that this may not
be so. unless physical activity is sustained over the years. A case in point is a study on
competitive, elite girl swimmers form Sweden (Astrand et al., 1963). While training and
competing, their aerobic fitness was well above average. However. when tested several
years after retirement (Eriksson et al.. 1971), their fitness was not markedly different
fr0111 that of age-matched women who had not competed during childhood and
adolescence. In the Amsterdam Growth Study (Kemper, 1995),98 females and 84 males
were observed periodically between ages 11 and 27 years. They were assigned an
activity score. based on their overall activity pattern over the years (not only during
childhood). as determined from a questionnaire. When comparing the groups with the
highest and lowest activity tertiles (Kemper, Van Mechelen, 1995), the former were
generally more fit. lt is noteworthy, however, that there also was an interaction between
group and time. such that the difference in fitness became apparent mostly during young
adulthood and less so during adolescence. Maximal oxygen uptake was the fitness
component that showed the greatest activity-related difference. although the more active
rertile also had higher muscle strength. flexibility and speed when they reached young
adulthood. The authors did not report the fitness levels of those who were active during
ado lescence but became sedentary at ages 11 and :2 7. The jury therefore is still out
regarding any carry-over effect of activity during childhood and/or adolescence on the
fitness level in adult years.
-tl
PUBLISHED GUIDELINES FOR E:\JHANCED PHYSICAL ACTIVITY AMONG
YOU:\G PEOPLE
This section wil] briet1y review' guidelines that have been published recently in
the US and in Canada. regarding the enhancement of physical activity among children
.ind adolescents,
US-Based Guidelines.
The first authoritative document is the Healthy People 2000 report. published in 1991
by the US Department of Health and Human Services (US Department of Health and
Human Services. 1991), \'/hile addressing physical activity for all age groups. the
document includes several guidelines with relevance to children and adolescents, as
follows:
!.
Increase to greater than or equal to
equal
to
30~,\)
the proportion of people aged greater than or
6 years who engage regularly'. preferably daily. in light to moderate physical
activi ty for greater than or equal to 30 minutes per day.
Increase to greater than or equal to 20% the proportion of people aged greater than or
equal to i 8 years and to greater than or equal to 750/0 the proportion of children and
adolescents aged 6-1 7 years who engage in vigorous physical activity that promotes
the development and maintenance of cardiorespiratory fitness. greater than or equal to
3 days per week. or greater than or equal to 20 minutes per occasion.
3.
Reduce to less than or equal to 15~/0 the proportion of people aged greater than or
equal to 6 years who engage in no leisure-time physical activit:'.
-1-.
Increase to greater than or equal to
40~/0
the proportion of people aged more than or
equal to 6 years who regularly perform physical activities that enhance and maintain
l11USC1.tlar strength, muscular endurance, and flexibility.
5.
Increase to greater than or equal to 50% the proportion of overweight people aged
more than or equal to 12 years who have adopted sound dietary practices combined
with regular physical activity to attain an appropriate body weight.
6.
Increase to greater than or equal to 500/0 the proportion of children and adolescents in
1st through 12th grade who participate in daily school physical education.
7.
Increase to greater than or equal to 50% the proportion of school physical education
class time that students spend being physically active. preferably engaged in lifetime
physical activities.
8.
Increase community availabili ty and accessibility of physical activity and fitness
t~1C ilities.
l).
Increase to greater than or equal to 50~/0 the proportion of primary care providers who
routinely assess and counsel their patients regarding the frequency, duration. type.
and intensity of each patient's physical activity practices.
Although the Healthy People 2000 document pronounces. as a national priority, the
importance of enhanced physical activity to the health of the nation. the above nine items
represent more of a "wishful thinking" list than specific evidence-based guidelines. Frain
a public health point of view. however. this is a landmark document.
In 1993. a group of US. Canadian and British Scientists. with support from the US
CDC. convened for a consensus meeting, which was summarised as the 1994 Physical
Activity Guidelines for Adolescents (Sallis. 1994). This document includes nine chapters
that examine the evidence regarding benefits of physical activity for people 11 to 21
years old. both in the general. healthy population and those with a chronic illness. Details
of this evidence are discussed in the previous section on Activity and Health during
Childhood and Adolescence. Although the participants were encouraged to arrive at
specific evidence-based recommendations, such evidence did not yield any detailed,
quantitative guidelines. The document did include, however, two broad guidelines for the
general population:
1. All adolescents should be active daily, or nearly every day, as part of play, games,
sports. work. transportation. recreation, physical education. or planned exercise, in
the context of family. school. and C0111111unity activities.
Adolescents should engage in three or 1110re sessions per week of activities that
last 20 min or more at a time and that require moderate to vigorous levels of
exertion. (vlodcrate to vigorous activities are those that require at least as much
effort as brisk or fast walking. )
The gist of the above recommendations is that adolescents should combine daily
activities, in which intensity and duration are de-emphasized, with 3 or more weekly
sessions in which the intensity and duration are sufficient to induce a training effect. The
rationale for the first recommendation is that: "dail» weight-bearing activities. ofeven
brietduration. during adolescence are critical for enhancing bone development that
atjects skeletal health throughout lite. Substantial daily energy expenditure is expected to
reduce risk otobesitv and tnav have other positive health effects that have not been
.locumented", The rationale for the second recommendation is that: "there is evidence
that regular participation ill continuous moderate to vigorous physical activity during
adolescence enhances psychological health, increases HDL cholesterol, and increases
cardiovascularfitness. Physical activity probably improves other health variables that
have not yet been investigated in adolescents. It is not known whether morefrequent,
<horter sessions ofphysical activitv would pro ".. ide some ofthe same benefits".
In 1997 the CDC published the Guidelines for School and Community Programs to
Promote Lifelong Physical Activity Among Young People (Centers for Disease Control
and Prevention, 1997). This is a detailed document, which focuses on the relevance of
children's physical activity to health and the means that one should follow to enhance the
physical activity level of chi ldren and adolescents. It includes 10 "broad
recommendations". These are not limited to the child or the family members, but also to
policy makers, the community. the school and the health establishment. In that sense,
this is the most far-reaching authoritative document regarding physical activity for young
people. The recommendations are as follows:
1. Policy: Establish policies that promote enjoyable, lifelong physical activity
~H110ng
young people.
Environment: Provide physical and social environments that encourage and
enable safe and enjoyable physical activity .
..+)
3. Physical Education: Implement physical education curricula and instruction that
emphasize enjoyable participation in physical activity and that help students
develop the knowledge. attitudes. motor skills. behavioral skills. and confidence
needed to adopt and maintain physically active lifestyles.
-t.
Health Education: Implement health education curricula and instruction that help
students develop the knowledge. attitudes, behavioral skills, and confidence
needed to adopt and maintain physically active lifestyles.
5.
Extracurricular Activities: Provide extracurricular physical activity programs
that meet the needs and interests of all students.
6. Parental Involvement: Include parents and guardians in physical activity
instruction and in extracurricular and community physical activity programs, and
encourage them to support their children's participation in enjoyable physical
activities.
7. Personnel Training: Provide training for education. coaching, recreation, healthcare. and other school and community personnel that imparts the knowledge and
skills needed to effectively promote enjoyable. lifelong physical activity among
young people.
~.
Health Services: Assess physical activity patterns among young people, counsel
them about physical activity, refer them to appropriate programs, and advocate for
physical activity instruction and programs for young people.
9. Community Programs: Provide a range of developmentally appropriate
community spans and recreation programs that are attractive to all young people.
10. Evaluation: Regulariy evaluate school and COn11TIUnity physical activity
instruction, programs, and facilities.
Canadian Guidelines
This author could not find Canadian-based guidelines that reflect scientific
evidence. There are, however. two published documents which suggest a line of action .
.:\ 1989 "blueprint" document. published by Fitness Canada's National Children and
Youth Fitness Office (Fitness Canada. 1989), contains six goals. as listed below. that will
help "to realize the vision of active living among Young Canadians". Note: words in
bold type appear in the original dOCU111ent.
1. To raise the awareness of all Canadians concerning the benefits of a healthv,
~
J
physically active lifestyle for children and youth.
To facilitate the transition trorn awareness to the adoption of healthy
attitudes and behaviours concerning participation in physical activity among
children and youth.
3. To expand the range of phys ical activity participation opportunities
available
to
all children and youth.
-J.. To facilitate delivery and coordination of quality physical activity
opportunities for children and youth .
.+7
"
To ensure. support and maintain the quality and quantity of leadershp
necessary to develop and deliver physical activity opportunities to children
and youth.
6. To identify and undertake ongoing research that addresses active living
among children and youth.
The above document includes 30 references.
1110Stly
of reports. lectures and
pronouncements. The list also includes few books and chapters. but it is unclear to what
extent this blueprint was based on these references.
In 1996, Health Canada issued an Active Living Guide for Parents (Health
Canada, 1996). This pamphlet, which emphasizes the connection between health and an
active lifestyle. lists several roles that a parent can play to help the child be active:
1.
Be a role model
Help your child learn skills
3.
Encourage your child
-t.
Provide your child with opportunities to be active
5.
Participate together in activities
CONCLUSIONS
This review is not comprehensive. it addresses certain issues that. in my opinion,
L11ay provide background for the preparation of the CSEP - Health Canada Guidelines.
Even though I firmly believe in the importance of physical activity to health. an effort has
been made in this document to adhere to the evidence. Following are the main
conclusions that one can derive 1'r01TI this review:
•
There is no single tool for the assessment of physical activity or energy
expenditure. particularly if one wants to include metabolic. mechanical and
behavioural aspects. The plethora of assessment tools, and the inherent
imprecision and low validity of SOITIe of them. make it hard to pool data from
different studies or to compare their findings.
•
There is a major decline in physical activity with age and maturation.
particularly in the second decade of life. This decline seems to be pervasive,
irrespective of the country in which a study was performed.
•
Even though there are no universal criteria for "sufficient" physical activity
and energy expenditure levels. it seems that a large percentage of young
people are insufficiently active. This percentage increases markedly in the
second decade of life. Some information suggests that 1110re young people in
Canada are sufficiently active. compared with those in the US.
Methodological differences. however. do not allow us to conclude that this
indeed is the case.
•
Activity patterns may change markedly
•
Adolescent females are at particular risk of being sedentary. Girls are less
WIth
the change of season.
active than boys and their decline in physical activity starts earlier than in
boys. Girls ~ rate of participation in physical education classes is lower than
that of boys.
•
Other groups with a high rate of sedentarism among children and adolescents
are those with obesity, motor disabilities and chronic diseases. In the US,
minorities (Afro-Americans in particular) have a low rate of physical activity.
•
Of particular public health importance is the recent increase in juvenile
obesity that seems to be associated with a decline in physical activity. The
extent of TV watching is strongly related to the prevalence of juvenile obesity
and to the likelihood that obesity will be sustained over years.
•
Indices for risk of atherosclerosis can be observed as early as the first decade
of life.
•
\Vhile enhanced physical activity in adults has been shown to reduce
morbidity and mortality from several chronic diseases. the association in
children and adolescents has not yet been established.
•
Cross-sectional studies often show that active young people have a better
profile of health indices (e.g., bone mineral density, blood pressure, adiposity,
blood lipid profile) than their sedentary counterparts. However. training
studies in heulthv young people do not seem to improve this profile.
•
In contrast, S0l11e beneficial effect of enhanced physical activity can be
discerned in young people who. to start with, are at a high risk (for example,
hypertension. obesity, blood lipid aberrations).
•
There is hardly any information on the possible carry-over of activity-related
benefits trorn childhood to adulthood.
•
One possible exception is a higher peak bone mass that. based on retrospective
studies, is higher among adults who were active in earlier years. \Veightbearing activities may be of particular benefit.
•
There are no data about dose-response effects of enhanced physical activity on
health indicators in children and youth. This is true both for the intensity and
the volume of activities. Nor is it known whether there is a threshold of
training intensity and volume beyond which activity becomes beneficial to
health.
•
Recent guidelines for recommended activities a1110ng adolescents address the
need to combine daily activities, in which the volume and intensity are not
important. with three-per-week exercise bouts that last at least 20 minutes and
are of "moderate-to-high" intensity.
•
To achieve a greater involvement of young people in physical activities and
sports, one needs to address societal and environmental issues such as:
•
Greater recognition by the educational authorities regarding the
importance of physical activities at school as a 111eanS of health
promotion
•
Enhanced use of sports facilities after school hours, for all young
people and not only for athletes
•
•
Increase of child safety in playgrounds, parks and roads
Reduction in the exposure of children to sedentary activities such as
TV. video, and computer games
5l
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Table 1 Strengths and Drawbacks of Commonly Used Methods for the Assessment of
Physical Activity tPA) and Energy Expenditure (EE).
. METHOD
i STRENGTHS
:\SSESSING
DRA.WBACKS
I COMMENTS
Simple. low cost;
Relies on memory;
I
suitable for large-
hard to quantify:
recall period the
scale studies
low validity
higher the validity
More valid than a
Relies on memory
Interviewer can
!
Questionnaire
I
PA
PA
Interview
I
I
i
PA
Diary
.
.
questionnaire
Short recall time
The shorter the
corroborate info
Interactive
depends on child's
interpretation
Direct
I PA, EE(?)
Observation
PA, EE(?)
Time- Lapse/
(or Video)
No need for recall;
Expensive; depends
"Gold Standard" for
context documented
on observer's skill
behavioural aspects
Objective, hard
Child is limited to
Less expensive than
record available
predetermined area
direct observation
Objective. little
Does not detect
interaction: low cost
specific movements
Photography
PA. (EE?)
Movement
Counters
Acce Iero metry
PA. EE(?)
I
Does not detect
Some validity vs
plus acceleration
specific activities
measurements of El
Little interaction;
HR affected not
Needs individual
inexpensive
only by metabolism
"calibration" vs VC
Measures
Limited activities:
Useful for
metabolism
need for mouth-
ergometry & VO 2-
I Same as counters.
I
HR Monitoring
EE
.
: V0 2 -
EE
.
I
!
Metabolic Cart
i
i
81
i
piece or face-mask
II
I
I
I V0 2 -
portable
I
EE
I
: Equipment
EE
Chamber
DoublyLabeled Water
EE
I
Measures
Highly interactive:
metabolism away
expensive
I Limited pediatric
use in prolonged
observations
Precise
Very limited
Validating other
measurement of EE
quarters; expensive
tests; ideal for BMR
Best measure of EE;
Very high cost;
'"Gold Standard" for
not interactive
requires at least one
average EE, but not
week
for profile of EE
R1v1R = resting metabolic rate. BMR = basal metabolic rate.
82
I
!
I
from the lab
Respiration
I HR "calibration"
Table 2. Recreational Physical Activities among 10- to 19- Year-Old Canadaians.
Values are in
S/o
of each age- and gender-group sample. Based on the 1988 Campbell
Survey (Stephens, Craig, 1990).
Gender and age
I
(yrs)
I
Walking
57
Boys 10-14
I
Girls 10-14
: Boys 15-19
,
I
I
II
Swimming
Bicycling
34
77
90
25
26
87
86
46
58
74
38
1_
70
67
I
I
83
Dancing
I
62
I
51
I
.+0
!
Girls 15-19
!
Gardening
75
I
I
30
II
I
I
59
Table 3. Percentage of US High School Students who Participated in Vigorous Physical
Activity 3 or more Day per \Veek. by Sex. Racc/Ethnicitv, and Grade. Based on 11.631
students sampled for the United States Youth Risk Behaviour Survey, 1990 (CDC, 1992)
Category
Females
Males
Total
9 th
30.6
51.1
40.1
io"
27.1
54.6
40.1
11th
23.4
50.2
40.7
12th
17.3
43.8
36.0
\
Grade
31,7
RacelEthnicity
Whilte
27.5
51.4
39.3
Afro- American
17.4
42.7
29.2
Hispanic
20.9
49.9
34.5
24.8
49.6
37.0
Total
84
Table 4. Tracking Correlation Coefficients in Total Weekly Habitual Physical Activity
of Males and Females. Activity was Assessed by Interiews. Adapted from Van
Mechelen & Kemper (van mechelen. Kemper, 1995).
Age Span, years
Males (n=84)
13-16
0.44
13-21
0.20
13-27
0.05
I
Females (n=98)
0.58
I
0.18
I
85
0.17
Table 5. Factors that have been Shown. or Suggested. to Affect Activity Behavior and
Energy Expenditure of Children and Adolescents.
Biological
Psychological
Heredity
Self-efficacy
Adiposity & Nutrition
Self-schema for activity
Health status
Perception of barriers to activity
Sexual maturation
Attitudes about activity
Motor skills
Beliefs about activity
Physical Fitness
Social & Cultural
The Physical Environment
Parent attitudes & Behaviours
Availability of activity facilities
Peer attitudes and behaviors
Seasonal variation
Socioeconomic status
Climatic changes
Cultural & ethnic values
Day of the week & holidays
Time spent on TV viewing
Safety considerations
Time spent on computer games
Work vs Studies
Sport as a societal value
86
Table 6. Relationship between Aerobic Fitness and Physical Activity
(Morrow & Freedson, 1994)
No.
Level
Studied
Trends/impact
PA related
to outcome
Dose-response
data
Basis for
recommendations *
Recommendations
20% (10%)
40% (20%
40% (20%
Volume: total volume of
physical activity may be
important
Frequency: ~ 3 d/wk
Intensity: ~ 75% max HR
Time: minimum 20 min
Type: aerobic
Boys
<5
>10
<5
I
II
III
t
t
t
Some
Some
Some
100%(50%)
Total
Girls
<5
5-10
<5
Total
I
II
III
t
t
t
Some
Some
None
20% (10%)
40% (20%)
40% (20%)
100%(100%)
Volume: total volume of
physical activity may be
important
Frequency: ~ 3 d/wk
Intensity: ~ 75% max HR
Time: minimum 20 min
Type: aerobic
Note: I =controlled trial: II = observational study; III = descriptive study of report based on expert opinion or clinical
j udgemenr. i = some apparent evidence of beneficial trend of PA related to outcome.
*Percentage represents the relative weightings used to reach conclusions. First entry is by gender. Percents in
parentheses represent relative weighting across gender.
Table 7. Relationship between Skeletal Health and Physical Activity (Bailey & Martin, 1994)
Group
General adolescent
population
.._.-...
Evidence
Level of
No.
Quality
Studies
<5
<5
5-10
5-10
5-10
I
IlA
IlA 1
lIB
liB,
_-_._-----.-_._._--------
Strength of
Association
t
II
II
I
I
Amount of
dose-response
data
Some
None
Some
Some
None
Basis for
recommendations
Recommendations
100% adolescent data
0% adult data
0% expert opinion
Avoid immobilization; wt.
bearing activity; involve all large
muscle groups; regular short
intense activity better than
irregular prolonged activity
._----.-.-.-----------------
--------_.• _----------------
Note: I = controlled trial; IIA = prospective observational study; lIA) = unilateral control study: 1113 = cross-sectional observational study; IlB, = retrospective
questionnaire study. t = some apparent evidence of beneficial trend of physical activity related to outcome: t t - good evidence.
-~,--,
Table 8. Relationship between Adiposity and Physical Activity (PA) or Energy Expenditure (EE)
(Bar-Or & Baranowski, 1994)
. _.. •.
......_----------,
----,..--- -,-------------------_, _,,"._-"" .. "
Evidence
No.
Level of
Studies
quality
Group
General adolescent
population
Obese ado lescents
>10
<5
>10
5-10
>10
1
IIA
IIR
I
lIB
Strength of
Association
Amount of
dose-response data
Basis (or
recommendations
t
Scant
None
None
Duration may be important
None
100%
100%
100%
100%
100%
~
t
tt
tt
~
----_
. . ..,.
._.
-~~
.....
-
for PA
for EE
""'-'.
I == controlled trial; IIA = prospective observational; liB = cross-sectional observational;
evidence.
Note:
-4 ==
adolescents
adolescents
adolescents
ado lescents
adolescents
Recommendations
None at present
Increase daiIy EE by ~ 10%,
combined with nutrition and
behaviour modification; deemphasize
exercise intensity
.
no evidence for association; t == some evidence; tt = good
Table 9. Relationship between Arterial Blood Pressure and Physical Activity
(Alpert & Wilmore, 1994)
Level of
No.
Studies
quality
Strength of
association
Dose/response
data
Basis for recommendations
- -.
<5
5-10
I
II
-t
t
None
None
<5
<5
I
II
titt
Some
None
C-.-
no apparent evidence of beneficial trend of physical activity;
-------~----
General adolescent
population
100% adolescent data
0% adult data
0% expert opinion
None at this time.
tUW-risk population
100% adolescent data
0% adult data
0% expert opinion
..__._.,__
Note: -t
evidence.
-----¥~._.-
Aerobic exercise of the following
FITT: (a) z 60% VO z
__n
i
Recommendation
max, (b) z 30 min/d. ( c) z 3 d/wk.
Probably no need to limit isometric
or resistance exercise if there is no
evidence of end-organ damage
(e.g. renal failure, stroke, increased
left ventricular mass). Note: This
may not be the minimum dose.
= some evidence of beneficial trend of physical activity to outcome;
it = good
Table 10. Cont.
STRENGTH OF ASSOCIATION
EVIDENCE
No.
Studies
Level of
Quality
TC
HDL
LDL
TG
HDL
TC
.-.- ....•
TC
HDL
_-_._
..
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Basis for
Recommendations
Recommendations
FITT
100% expert
opinion
Increase habitual physical activity
High-Risk Population
Obese
M
<5
<5
I
~
I
~
t
t
Diabetes
F+M
<5
I
~
-~
Familial Links
F I M
M (PA)
F(PA)
M (fit)
F (fit)
<5
<5
<5
<5
<5
I
liB
liB
liB
liB
t
t
~
~
~
~
~
~
~
~
~
t
~
t
~
~
~
~
F
~
t
t
~
t
Fr =-:: 4 times per wk; 1"-:: 75-80% of max
HR; Ty = dynamic whole body aerobic
exercise; Ti = 30 min per session
Note: M = male; F = female. IA = randomized control trial; IB = non-randomized trial; HA = prospective observational study; liB = cross-sectional observational study.
For strength of association: ~ = no apparent evidence of beneficial trend of physical activity; t = some apparent evidence of beneficial trend of physical
activity to outcome. Fr = frequency; I = intensity; Ty = type; Ti = time.