A REVIEW OF THE RELATIONSHIP BETWEEN LEG POWER AND
CHRONIC DESEASE IN OLDER ADULTS
by
Sara Elizabeth Strollo
BS, Virginia Polytechnic Institute and State University, 2011
Submitted to the Graduate Faculty of
the Department of Epidemiology
Graduate School of Public Health in partial fulfillment
of the requirements for the degree of
Master of Public Health
University of Pittsburgh
2013
i
UNIVERSITY OF PITTSBURGH
GRADUATE SCHOOL OF PUBLIC HEALTH
This essay is submitted
by
Sara Strollo
on
April 18, 2013
and approved by
Essay Advisor:
Elsa S. Strotmeyer, PhD, MPH
Assistant Professor
Department of Epidemiology
Graduate School of Public Health
University of Pittsburgh
_________________________________
Committee Member:
Nancy W. Glynn, PhD
________________________________
Research Assistant Professor of Epidemiology
Director, Master’s Degree Programs
Department of Epidemiology
Graduate School of Public Health
University of Pittsburgh
Committee Member:
Bret H. Goodpaster, PhD
_________________________________
Associate Professor of Medicine
Assistant Professor of Health and Physical Education
Department of Endocrinology
Department of Health and Physical Education
School of Medicine
School of Education
University of Pittsburgh
ii
Copyright © by Sara Strollo
2013
iii
Elsa S. Strotmeyer, PhD, MPH
A REVIEW OF THE RELATIONSHIP BETWEEN LEG POWER AND CHRONIC
DISEASE IN OLDER ADULTS
Sara Strollo, MPH
University of Pittsburgh, 2013
ABSTRACT
Increased life expectancy in developed countries has heightened the importance of
preventing disability and preserving physical function in older adults. Lower-extremity muscle
power is a relatively novel, but reliable predictor of physical function in older populations. Leg
power has been shown to decline earlier than muscle strength with aging. Therefore, leg power
measurements may provide earlier predictors of decreased functionality. Most leg power studies
exclude participants with acute or severe chronic conditions. However, chronic diseases are
prevalent among adults aged 65 years or older and should be considered when assessing muscle
function of aging individuals.
This master’s essay reviews the existing literature on the relationship between leg power
and chronic disease in older adults. Various leg power measures and the age-associated chronic
conditions of osteoarthritis, diabetes mellitus, and cardiovascular disease are assessed to
characterize associations and determine gaps in the literature. Identifying future priorities of
research, aimed to improve late life independence among older adults, is of high public health
importance. This review emphasizes the important role of prevalent age-associated chronic
conditions when assessing lower-extremity muscle power and physical function in older adults.
iv
TABLE OF CONTENTS
PREFACE .................................................................................................................................................. vi
1.0
INTRODUCTION ............................................................................................................................1
2.0
LEG POWER ....................................................................................................................................3
3.0
4.0
2.1
KEISER PNEUMATIC LEG PRESS ........................................................................................... 4
2.2
NOTTINGHAM POWER RIG .................................................................................................... 5
2.3
SIT-TO-STAND MOVEMENT ................................................................................................... 7
2.4
COUNTERMOVEMENT JUMP .................................................................................................. 8
2.5
STANDARD MEASURE OF LEG POWER.............................................................................10
CHRONIC DISEASE..................................................................................................................... 11
3.1
OSTEOARTHRTIS.....................................................................................................................13
3.2
DIABETES MELLITUS..............................................................................................................16
3.3
CARDIOVASCULAR DISEASE ................................................................................................20
CONCLUSION ............................................................................................................................... 25
BIBLIOGRAPHY .................................................................................................................................... 27
v
PREFACE
I would like to express my appreciation to my faculty advisor, Dr. Elsa Strotmeyer, for her
assistance with this literature review and the opportunity to gain field experience in
administering various lower-extremity power measurements. I would also like to thank Dr.
Nancy Glynn and Dr. Bret Goodpaster for serving on my master’s essay committee. Their
willingness to assist me with this endeavor largely contributed to its success and timely
completion.
vi
1.0
INTRODUCTION
In the 21st century, the United States (US) and other developed countries are experiencing
changing demographics with a rapid increase in adults age 65 years or older1, 2. Increased life
expectancy has impacted the health care system3 and intensified the demand for medical
services4. As a result, an emphasis has been placed on age-associated disability prevention and
the preservation of physical function among older adults5-7. Healthy aging enables older adults to
retain functional independence and a higher quality of life. Research aimed to improve late life
independence has considered muscle power and chronic disease among older adults as important
determinants of disability and physical function decline8. However, the relationship between
chronic disease and leg power in older adults has yet to be comprehensively characterized.
Muscle power, an indication of the ability to exert force quickly, is a relatively novel
research measure for predicting late-life disability8-10. Muscle power is used for most habitual
daily activities11. It has been shown that aging adults experience muscle power decline earlier
than muscle strength12-15. Therefore, leg power measurements may provide earlier predictors of
decreased functionality. Various measures of leg power have been used throughout research.
Traditional measures include leg extension machines such as the Keiser leg press and
Nottingham power rig. Task-based leg power measures have emerged in recent years. They are
commonly performed on a force plate and include sit-to-stand and jumping movements.
1
Chronic diseases have been found to be essential underlying causes of physical disability
in older adults16. They have a strong influence on middle-aged adults making a healthy transition
into old age17. In addition to physical activity reduction contributing to significant impairments
in function, chronic illnesses diminish quality of life and predispose individuals to disability18.
Although leg power has been established as a reliable predictor of physical function among older
adults, few leg power studies have assessed the relationship between muscle power and prevalent
chronic diseases. Muscle power studies often exclude acute or chronic illnesses and focus on
healthy participants or mobility-limited community-dwelling older adults. Therefore, conclusions
often have limited generalizability to the large proportion of older adults with one or more acute
or chronic illnesses.
Studies that include participants with prevalent age-associated chronic diseases such as
osteoarthritis (OA), diabetes mellitus (DM), and cardiovascular disease (CVD), often lack
analysis of the diseases’ associations with leg power. Most merely provide the percentages of
individuals with comorbidities. OA, DM, and CVD are known to contribute to physical function
limitations in older adults18. A comprehensive review of the literature can identify future
research directions for investigating leg power as a predictor of age-associated disability and
physical function. In addition, further research in this area will provide opportunity to identify
public health priorities in aging and the influences of age-associated prevalent chronic diseases.
2
2.0
LEG POWER
Impairments of muscle power and lower-extremity muscle power’s relationship with physical
function have gained attention in recent years8, 19-21. While muscle strength indicates a person’s
ability to exert maximal muscular force, muscle power indicates the ability to exert force quickly
(Power= Force x Velocity)22. Muscle power and muscle strength decrease as adults age and
predict physical limitations23, 24. However, decreases in muscle power occur to a greater extent
than muscle strength with aging12,
15, 25, 26
. In addition, a stronger association exists between
muscle power and certain physical performances measures, such as habitual gait speed and stair
climb time22, 27.
Age-associated alterations in muscle power are related to a reduction in muscle mass,
which is mediated by the loss of type II fast twitch fibers28, 29. Type II fibers are capable of
producing four times the amount of peak power output of type I fibers30. Therefore, the loss of
type II fibers are believed to be associated with advanced aging and result in muscle power
impairments27, 31. Substantial loss of nerve function is quite common in older adults (25%)32 and
related to leg press power33 and physical performance34
35, 36
. As a result of slower nerve
conduction velocity, peak force development may be delayed and result in lower power
production37.
Jozsi et al. (1999) have attributed many age-associated changes in skeletal muscle to
sedentary lifestyles, injury, or disease19. Each standard deviation increase in knee extensor power
3
has been associated with a reduction of disability likelihood by 27-42% (P< 0.004)38.
In
addition, research has shown leg power to be predictive of self-reported disability among
community dwelling older women23. In general, studies have revealed elderly men to have
higher muscle power levels than women12, 26, 39.
Commonly used traditional leg power measures include leg presses (Keiser pneumatic),
power rigs (Nottingham, Bassey), and dynamometers. These measures are often performed in
seated positions and involve high velocity extensions of one or two legs at low resistances to
determine power. Additional novel task-based leg power tests were designed to mirror daily
mobility tasks40. Sit-to-stand movements41 and standing jumps14 are the most frequently used
task-based measurements and can be performed at home or other alternative non-clinic locations
without expensive equipment21, 42. Although the Keiser pneumatic leg press, Nottingham power
rig, sit-to-stand movement, and countermovement jump all measure lower extremity leg power,
differences exist between protocols and measurement movements. Each testing method’s
strengths and weaknesses differ based on participant characteristics and study objectives43.
2.1
KEISER PNEUMATIC LEG PRESS
Keiser pneumatic resistance machines are frequently used to assess lower-extremity power
measurements of older adults. The Keiser pneumatic leg press is a computerized machine that
measures the amount of force a participant can exert21. Force is applied through air resistance
and can be easily altered between leg extensions. Prior to conducting measurements, a
participant fully extends one or both legs with no load to determine a full range of motion
(ROM)21, 44. The ROM begins with leg(s) at a 90-degree angle and the participant’s arms crossed
4
over their chest. Participants proceed to straighten their leg(s) against the foot plate until full
extension is reached then gently return to the starting position over a period of 2, 1, and 2
seconds, respectively21. A one-repetition maximum measurement (1-RM) must be obtained prior
to assessing leg power42. The 1-RM is achieved by progressively increasing the resistance
between each repetition until the participant can no longer extend their leg through the full
ROM21. Participants are given a 30- to 45-second rest between maximal effort repetitions21.
After identifying a participant’s 1-RM, lesser intensities of the maximal force (from 40% up to
90%) are performed as fast as possible through the full ROM to determine power42. Peak power
is recorded based on the highest mean power produced throughout the trials45. Generally, peak
lower extremity muscle power is acquired at 70-75% of the 1-RM21, 22.
The Keiser pneumatic leg press has been described and validated with excellent
reliability in older adults23, 46-48. The simple control panel and computer program allows for quick
and efficient measurements. In comparison to weighted machines, air-pressure resistance allows
for a smooth and consistent movement that may prevent injuries49. It is the most frequently used
leg power measurement in populations that assess chronic disease, especially OA37,
45, 50-52
.
However, obtaining 1-RM measurements can be challenging in older adults with disability and
comorbidities associated with chronic pain. Older adults may have difficulty getting in and out of
the seat. Although it is easy to administer, this measurement technique is limited because the
equipment is not portable, has a high cost42, and a long length of test time.
2.2
NOTTINGHAM POWER RIG
The Nottingham power rig is a reproducible and validated measure of leg extension power53, 54. It
has been become a popular leg power measure since it was commissioned for use in a National
5
Fitness Survey in 199055. The power rig involves a seated leg extension movement while the
participant’s arms are folded over his or her chest53, 56. A flywheel that is accelerated by the
participant pushing a single foot against a footplate is used to measure average power from the
final velocity53. A maximum of nine trials are performed; however, testing is often stopped once
five successful trials are completed53. The two highest measures should be within 5% of each
other53.
Bassey et al. (1992) found that leg power, measured with the Nottingham power rig, was
correlated (p<0.001) with the functional measures of gait speed (r=0.80), chair-rise time (r=0.
65), and stair-climb time (r=0.81)26. The Nottingham power rig is a preferable measure for the
elderly because it requires less balance than other measurements, such as jumping, stair
climbing, and walking26, 53, 54, 57, 58. The equipment has been found safe and acceptable for all age
groups and levels of physical capability41,
53
. In addition, the machine’s seat is suitable for
individuals with back problems26, 27, 54, 57, 58. The Nottingham power rig is the most frequently
used measure when assessing leg power in older adults with CVD59-62 and has been considered
the “gold standard” for measuring power41, 53. Caserotti et al. (2008) concluded that explosivetype heavy-resistance training with the Nottingham power rig can even be safe and acceptable in
healthy 80 to 89-year-old women14. However, the Nottingham power rig has not been used to
measure leg power among older adults with OA. Disability and comorbidities associated with
chronic diseases, such as OA, may interfere with obtaining leg power measurements. In addition,
the equipment is expensive, lacks portability, and can be difficult to administer.
6
2.3
SIT-TO-STAND MOVEMENT
Also known as a chair rise movement, the sit-to-stand movement involves the use of complex
motor acts63 through the collaboration of muscle actions at both the hips and knees64. The sit-tostand movement is an essential daily movement and relevant predictor of an elderly person’s
ability to live independently65. Individuals must be capable of producing enough strength and
power to raise their body’s center of mass to perform this measure65. To assess leg power, the
movement can be performed on a platform that measures the ground reaction of the feet41, 63, 65,
66
. Participants cross their arms over their chest while sitting on an armless, backless chair67. The
chair is attached to a force platform, and the chair height is determined by the height of the
participant’s lateral knee joint67. Legs are positioned at approximately a 90° angle with feet
centered on the force plate and eyes fixed straight ahead66. Participants are instructed to rise as
fast as possible into an upright position41, 65, 66. At least 3 trials, with resting breaks in between,
are conducted to achieve a maximal power measurement67. Power is calculated from the vertical
ground reaction force of body weight, the time it takes to stand upright, and body height changes
during the sit-to-stand transfer41, 65.
The sit-to-stand transfer is a functional task-based measure41 that older adults face on a
daily basis68. Lindemann et al. (2003) suggest that using this leg power measure, instead of the
Nottingham power rig, is sensible, portable and low cost41. However, power measuring force
plates are large and heavy to transport. Sit-to-stand leg power measures can conveniently be
performed on the same platform as the countermovement jump measurements. Thus far, this
method has not been commonly used to evaluate leg power among older adults with chronic
disease. However, Kuh et al. (2005) measured functional leg power by assessing chair-rise time
with a stopwatch in a cohort of 53-year-old British men and women69. Participants rose from a
7
seated position into a standing position with their back and legs straightened69, 70. The movement
was completed in the seated position and was performed for 10 complete trials to obtain a
minimum time69,
70
. Participants with physical limitations such as balance impairments and
bodily pain may have difficulty performing this measure71. The test is quick and easily
administered in a clinical setting among able participants.
2.4
COUNTERMOVEMENT JUMP
A countermovement jump is a vertical jump performed on a force platform to estimate maximal
muscle power output14, 39, 72. Vertical velocity is calculated in conjunction with the vertical force
expended to push both feet from the platform72. It is a multi-joint movement and closely related
to functional performance73. Participants begin in an upright standing position and perform a fast
and fluid downward preparatory knee movement (to approximately 90°) prior to pushing
vertically off of the platform25, 72-74. Participants are instructed to jump as high as they can and
land gently on the platform73. At least 3 maximal jumps are performed, rests occur in between
jumps, and the highest jump is selected for analysis 25, 72-74.
In addition to an estimation of muscle power output, the jumping movement can
represent functional capability72. The majority of daily human tasks are weight-bearing and
incorporate similar accelerations and decelerations of body mass72. Assessing jumping power has
potential advantages in comparison to other methods of leg power measurement75. The
movement only involves the same intrinsic resistance that the participant encounters on a daily
basis rather than exerting muscle power against an eternal resistance75. Although the
8
countermovement jump may result in higher mechanical outcome variability than single joint
tests, reproducibility has been relatively high in studies (r≥0.95)73,
76
. In addition, maximal
vertical jump tests have been assessed in numerous studies among older adults without
methodological issues39, 73, 76-79.
A study by Runge et al. (2004) compared age effects on muscle power output among men
(N=89, 18-79 years) and women (N=169, 20-88 years) of different age groups. Muscle power
was assessed by countermovement jumps and chair rises. Jumping peak power specific to body
mass was estimated to be lower by more than 50% in women (r=0.81 p<0.001) and men (r=0.86,
p<0.001) between the ages of 20 and 80 years75. In addition to a continuous and linear muscle
power decline across the age range, frail, older persons who were incapable of performing the
chair-rising movements were able to complete the jumping mechanography75. Therefore, these
observations may suggest that countermovement jumps are preferable for muscle power
assessments in comparison to chair rises75.
The countermovement jump has yet to evaluate leg power differences among older adults
with common chronic conditions such as OA, DM, or CVD. However, the countermovement
jump has a shorter test time than most leg power measurements and can be performed on the
same platform as the sit-to-stand movement. Safety precautions, such as using a stabilization
harness or additional staff members as spotters, must be considered when conducting jumping
measurements among elderly participants41.
9
2.5
STANDARD MEASURE OF LEG POWER
Future research could benefit from the identification of a simple, standard measurement of
lower-extremity muscle power66. Authors of current literature infrequently address the strengths
and weaknesses of specific leg power measurement techniques. Cost and ease of administration
are essential components of studies in older adults. Although fewer studies utilize task-based leg
power tests, the measures may be less expensive and burdensome than traditional leg power
measures. Force platforms avoid uncomfortable seated positions and utilize daily functional
movements. Future research should determine the feasibility of using task-based tests to assess
leg power among older adults with chronic disease.
10
3.0
CHRONIC DISEASE
As life expectancy increases throughout the US, so does the prevalence of age-related chronic
health conditions37, 80, 81. A higher proportion of older adults18, 82 is accompanied with increased
disability, reduced quality of life (QOL), and heightened costs of health care and long-term
care18. Approximately 80% of older Americans are living with at least one chronic condition, and
50% are living with at least two2. Women tend to live longer than men in a disabled state23, 83.
According to the American Heart Association, 70.2% of men and 70.9% of women between 6079 years old have CVD84. OA affects 33.6% of individuals age 65 years and older, and women
tend to have higher rates of OA than men in late life85. Symptomatic knee OA affects 13.6% of
women and 10.0% of men age 60 years or older86. Among individuals age 20 years or older,
11.8% of men and 10.8% of women have DM87. DM has a dramatically higher prevalence
among men and individuals age 65 years or older (26.9%)87. Consequences of living with
chronic illness in late life may include limitations of daily activities, loss of function, and pain88.
In addition to health concerns, the prevalence of chronic conditions and the number of
Americans age 65 years or older is negatively impacting the United State’s health care system37.
As a result, chronic diseases consist of 95% of older Americans’ health care expenditures89.
Frailty, has been considered a synonymous term for disability, comorbidity, and other
physical limitations90. Fried et al. (2001) used data from the Cardiovascular Health Study (n=
5417) and found frail (n=368) men and women ≥ 65 years have significantly higher rates of
11
CVD, DM, and OA than those who were not frail or intermediately frail90. The significant
associations with comorbid chronic disease included arthritis (70.6% prevalence, p<0.001), DM
(25.0% prevalence, p<0.001), myocardial infarction (13.3% prevalence, p<0.001), angina
(28.8% prevalence, p<0.001), congestive heart failure (13.6% prevalence, p<0.001), peripheral
vascular disease (3.8% prevalence, p<0.001), and hypertension (50.8% prevalence, p<0.001)90.
Arthritis, CVD, and DM are among the five most common chronic illnesses in older
people88 and are known causes of disability and hospitalization among seniors91,
92
. Strength
measurements can have limitations when performed on older adults with chronic conditions and
their associated comorbidities. Symptomatic OA can result in chronic pain, muscle weakness,
limited range of motion of the joints, and inhibited daily activities93. According to the Centers for
Disease Control and Prevention (2005), approximately 60-70% of diabetic men and women have
mild to severe forms of nervous system diseases such as peripheral neuropathy (PN)94. PN can
further limit strength testing abilities95. Additionally, muscle weakness and physical function
impairments are associated with DM95. Reduced aerobic endurance, changes in body
composition, bone loss, skeletal muscle atrophy, and weakness commonly occur with declining
cardiovascular status as well18.
In contrast to the high resistance required to evaluate muscle strength, leg power tests are
capable of assessing muscle capabilities at low resistances with high velocity movements.
Although the relationships between OA, CM, CVD and leg power have yet to be
comprehensively delineated, the literature occasionally addresses the influence of chronic illness.
The associations between the burden of OA, DM, and CVD on leg power are particularly
important to describe since these are relatively preventable conditions96-98.
12
3.1
OSTEOARTHRTIS
Arthritis and similar arthritic illnesses are the leading causes of disability in the US99. As the
most prevalent chronic condition among older adults, OA affects over half of all individuals over
the age of 6518, 93. The incidence is known to increase with age100, and symptomatic knee OA
occurs at the rate of 240 per 100,000 person years101. Osteoarthritis is an incurable chronic
condition102 with various medical and disablement comorbidities. Muscle strength is commonly
studied as an indicator of late-life disability among older adults. However, pain and joint
stiffness as a result of disease may inhibit testing, limit compliance, cause adverse events, and
result in deviations in protocols. Muscle power may be a more feasible form of measurement
because it involves moving less resistance. In addition, leg power measurements have been used
safely in mobility-limited older adults and persons 80 years and older103. Although hand
osteoarthritis is frequently considered in muscle power studies, lower-extremity osteoarthritis has
yet to be well studied23, 56.
Clark et al. (2010) collected self-reports of knee osteoarthritis while comparing leg
extension torque and power with a Cybex-II dynamometer104. Participants (N=89) were recruited
to fulfill three specific groups: healthy middle-aged adults (MH, N=29, 40-55 years, mean age ±
SD= 47.2 ± 4.7 years), healthy older adults (OH, N=28, 70-85 years, mean age ± SD= 74.0 ± 3.6
years), and mobility-limited older adults (OML, N=32, 70-85 years, mean age ± SD= 78.1 ± 4.5
years)104. The Short Physical Performance Battery (SPPB) was used to classify the older adults
into their respective healthy or mobility-limited categories. Older adults ranking ≤ 9 (out of 12
points) on the SPPB were classified as mobility-limited. In this population, prevalence of selfreported knee OA was 0% among middle-aged adults, 7% among older adults, and 16% among
mobility-limited older adults104. OA was not adjusted for in the analysis. Impaired power in the
13
older mobility-limited adult age group was shown once leg extensions were performed above 60
degrees (p<0.0001)104. The greatest leg power differences occurred at 240 degrees-second, at
approximately MH=275 W, OH=200 W, and OML=100 W104.
Clark et al. (2010) suggest future investigation of the potential neural mechanisms and
how the neuromuscular system contributes to declines in mobility function104. In addition, further
consideration of OA in the analysis may provide insight on the relationship with physical
function and muscle power measurements across the various groups. Measuring lower-extremity
muscle power of participants with OA may assist in determining the most feasible measuring
technique, how the disease affects muscle power measurements, and whether poor results and
conclusions are related to the burden of disease.
De Vos et al. (2005) considered osteoarthritis prevalence as an adverse event when
conducting a randomized controlled trial of resistance training programs for 8-12 weeks45. Of the
112 healthy older adults (mean age ± SD= 69 ± 6 years) evaluated with a Keiser leg press, 20
adverse events were reported among 17 participants. The rate of adverse events related to power
training (0.25%) were less than strength training (0.34%). Exacerbation of OA accounted for
15% of all adverse events45. All events were resolved with alterations in regimens, antiinflammatory medication, or analgesic medication45. However, adverse events related to OA may
decrease with lower resistance muscle power training in comparison to strength training.
In a study by Orr et al. (2006), 38 type 2 diabetic patients were randomized into a 4month tai chi (mean age ± SD= 65.9 ± 7.4 years) and sham exercise (mean age ± SD= 64.9 ±
8.1 years) controlled trial to determine associations with muscle power50. Peak muscle power
was measured using a Keiser pneumatic leg press. Throughout the entire population, OA (84%)
and hypertension (76%) were prevalent comorbidities50. Information on how the researchers
14
defined diagnosed type 2 DM and comorbidities was not included in the study report. Mobility
impairments were determined by measures of balance and gait speed. In this older, obese cohort
with type 2 DM, mobility impairments were found to be associated with low muscle power
(P=0.043 to <0.0001)50. The high prevalence of obesity and OA among participants may have
compromised an optimal measurement technique within the intervention groups. Although the
number of comorbidities was considered, the report did not specify how these conditions were
defined. An additional shortcoming was the lack of analysis or adjustment for comorbidities.
Robertson et al. (1998) assessed muscle function in a cross-sectional study of patients
waiting for a primary unilateral knee replacement for OA (n=26, mean age ± SD= 72 ± 8
years)43. Using a Kin-Com dynamometer, leg extensor power was found to be a repeatable
measure of muscle function among men (n=13) and women (n=13) (mean difference 0.03 ±
0.24, p=. 60)43. Patients’ affected legs (Test 1=57.7 ± 31.8 W, Test 2=59.3 ± 29.9 W) had lower
power than the unaffected legs (Test 1=85.0 ± 39.7W, Test 2=82.1 ± 40.7W)43. Although
statistically significant (p<0.05), the lack of a comparison group consisting of participants
without OA of the knee limited the study’s generalizability.
Sayers and colleagues (2010, 2012) compared two studies and found that older adults
with knee OA52 and community dwelling older adults without an OA diagnosis51 improved
muscle performance measures in a similar manner. The primary study consisted of 38 healthy,
community-dwelling older men and women without an OA diagnosis51. In a follow-up study, 33
older adults (mean age ± SD= 67.6 ± 6.8 years) with knee OA were randomized into high-speed
power training (HSPT, n=12), slow-speed power training (SSPT, n=10), and control (CON,
n=11) groups52. Greater improvements in peak muscle power among those with knee OA,
measured by a Keiser pneumatic leg press, were reported within the HSPT group compared to
15
those in the CON group (p= 0.04)52. The intervention groups had large increases of peak power
measures, though not mobility-based functional tasks52.
In a review of high-speed power training, Sayers et al. (2007) concluded that chronic
diseases like knee OA remain overlooked in the literature when investigating the importance of
movement speed and rapid force-producing capability in older adults37. The limited amount of
current literature on the relationship between OA and leg power in older adults is restricted by
small sample sizes, interactions with other chronic diseases and the lack of a standard measure of
leg power. Traditional leg power extension measures propose difficulties with pain and range of
motion of joints. Therefore, strength measurements may not be feasible among individuals with
OA. OA has been evaluated in several randomized controlled trials and is frequently included in
participant characteristics. Future studies may benefit from using task-based power
measurements to evaluate muscle power between older adults with and without OA and the
potential correlations with mobility-based functional tasks.
3.2
DIABETES MELLITUS
According to the Centers for Disease Control and Prevention, 10.9 million (26.9%) individuals in
the US, age 65 years or older, have DM87. A large increase in prevalence among those 75 years
or older is expected in the future2. Incidence of DM has been estimated to be 4.2/1000 according
to data standardized to the 2000 US population97. Older adults and minorities are estimated to
have the fastest increases in DM diagnoses, and it is estimated that diagnoses will increase by
16
165% between 2000 and 2050 in the US105. Among adults age 65 years and older, DM is the
sixth leading cause of death18. Development of disability is approximately two to three times
greater in diabetic older adults than those without this condition106. Physical disability often leads
to additional use of health services, institutionalization, reductions in QOL, and a lowered overall
health status107. Studies have established a decrease in strength and function with DM108, 109. In
addition, type 2 diabetic older adults have displayed lower “muscle quality”110 and often have
multiple comorbidities in old age50.
Hilton and colleagues (2008) studied associations of performance and function in
individuals with obesity, DM, and PN111. Differences in muscle power were reported for 6
participants (mean age ± SD= 58 ± 10 years) and their 6 age- and sex-matched controls (mean
age ± SD= 58 ± 9.2 years). Increased risk of DM, CVD, disability, and mortality is associated
with obesity111-114. A Biodex Multi-joint System 3 Pro isokinetic dynamometer assessed
concentric isokinetic and isometric ankle plantar-flexor and ankle dorsiflexor strength and
power111. The ratio of maximal plantar-flexor muscle power output to lower-extremity muscle
volume was lower in the participants with obesity, DM, and PN in comparison to controls at
angular velocities of 60°/s (0.133 vs. 0.0.031, P<0.05) and 120°/s (0.184 vs. 0.0.036, P<0.05)111.
Calf intermuscular adipose tissue (IMAT) volumes were 2.2-fold higher in participants with
obesity, DM, and PN (mean volume ± SD= 120 ± 47 cm3) in comparison to the control group
(mean volume ± SD= 54 ± 41 cm3)111. Participants who were obese with chronic DM and PN
had significantly weaker ankle dorsiflexors and plantar-flexor muscle power than the control
group111. Ankle dorsiflexor muscle power was 83-86% lower (1.3 vs. 9.6, P<0.05 to 2.1 vs. 12.4,
P<0.05), and ankle plantar-flexor muscle power was 73-77% lower (13.4 vs. 50.0, P<0.05 to 15.9
vs. 69.3, P<0.05)111. Limitations included the small sample size and cross-sectional design;
17
however, the findings revealed the extent of muscle power impairments among those with
obesity, DM, and PN111. Expanding this preliminary data with prospective studies and optimal
leg power measurement techniques may better determine if muscle power predicts the onset of
disability in people who are obese with DM and PN110, 111.
Men with type 2 DM participating in a twice weekly, whole-body progressive resistance
training (PRT) program showed peak power output improvements74, 115. Ibanez et al. (2008) used
the same intervention plan while comparing type 2 diabetic, older sedentary men (n=9, mean age
± SD= 66.6 ± 3.1 years) to a control group of their age-matched healthy counterparts (n=11,
mean age ± SD= 64.8 ± 2.6 years)106. Post-training gains were found in leg muscle power output
among control participants (323.5 ± 122.1 W vs. 394.4 ± 147.2 W, P<0.001) and diabetic
participants (239.5 ± 53.9 W vs. 312.3 ± 52.8 W, P<0.01)106. However, no significant posttraining difference between the diabetic group and the non-diabetic control group was observed
in the power output of leg extensor muscles at 30% of their 1-RM half-squat106. Post-training 1RM half-squat strength gains occurred among control participants (103.1 ± 25.9 kg vs. 135.0 ±
32.5 kg, p<0.001) and diabetic participants (106.3 ± 8.3 kg vs. 124.1 ± 8.0 kg, p<0.001)106. The
results suggest that older type 2 diabetic men might develop fewer strength gains, but achieve
similar muscle power gains with resistance training106. However, the authors theorized that
increases in intensity and maximal strength targets might have been too stressful and physically
exhausting for the diabetic older men106. Therefore, diabetic individuals may benefit from power
training instead of maximal strength training. However, data are difficult to interpret due to the
study’s small sample size.
Orr et al. (2006) randomized 38 type 2 diabetic patients into a 4-month tai chi (n=17,
mean age ± SD= 65.9 ± 7.4 years) and sham exercise (n=18, mean age ± SD= 64.9 ± 8.1 years)
18
controlled trial to determine associations with muscle power50. Although not statistically
significant, peak muscle power was obtained with a Keiser pneumatic leg press and changed by
4.8 ± 18.1% in the tai chi group and -0.4 ± 16.7% in the sham exercise group from baseline to
follow-up50. Among all participants, the most common participant comorbidities were
hypertension (76%) and OA (84%)50. At baseline, a higher number of comorbidities and poorer
muscle power were related to impaired balance in addition to old age, high percent body fat, poor
cognition, low QOL, low exercise capacity, slow gait speed, and decreased muscle contraction
velocity (p=0.043 to <0.0001)50. Existing comorbidities between the tai chi group (6.9 ± 6.7) and
control group (6.1 ± 8.8) were not analyzed independently in relation to the leg power
measurements50. The study concluded mobility impairments in older, obese individuals with type
2 DM are associated with low muscle power50. The high prevalence of obesity and OA among
participants may be a study limitation. Comorbidities may have affected the results, but were not
adjusted for in analyses.
Volpato et al. (2012) performed a cross-sectional analysis of 835 community-dwelling
adults age 65 years or older enrolled in the InCHIANTI population-based study62. Indicators of
muscle performance, including muscle power, were assessed among 11.4% of the cohort with
prevalent DM (mean age ± SD =73.9 ± 6.2), in addition to the non-diabetic (mean age ± SD
=73.8 ± 6.5) participants62. Lower-extremity muscle power was measured by the Nottingham
power rig and tended to be lower among participants with DM (-6.9 ± 4.2 W) in comparison to
those without (65.8 ± 1.8) after adjusting for age and sex (p=0.102)62. Lower muscle power (65.8
± 1.8 vs. -12.7 ±5.2), quality, density, and strength were found in participants with DM when the
analysis was restricted to those on pharmacological therapy (p<0.05)62. A temporal or causal
relationship could not be determined because of the cross-sectional and observational study
19
design62. However, the researchers suggested that clinical and biological evidence revealed
potential negative effects of DM on muscle physiology and walking performance62.
Small sample sizes, obesity, OA, and cross-sectional designs were obstacles to effectively
assessing the relationship between DM and leg power in older adults. No studies evaluated
minorities, investigated differences between men and women separately or used task-based
measures to test lower-extremity leg power. However, individuals with DM appeared to have
few complications with measuring leg power. Additionally, statistically significant findings of
low muscle power among participants with DM were consistent throughout. Longitudinal data is
needed to strengthen conclusions and determine causal relationships.
3.3
CARDIOVASCULAR DISEASE
CVD is a prominent cause of disability and the leading cause of death worldwide116. CVD is
responsible for one in three reported deaths in the US each year83,
116
. Classifications of the
disease include myocardial infarction (MI), coronary artery disease (CAD), congestive heart
failure (CHF), peripheral vascular disease (PVD), and hypertension18, 117. The Framingham Heart
study reported a doubling of prevalence of chronic heart failure with each additional decade of
life and a 7-10% prevalence level among persons 80 years or older18, 118, 119. Heart failure occurs
among 7.8% of men and 4.5% of women age 60-79 years84. In the same age bracket, 21.1% of
men and 10.6% of women have CAD84. Hypertension is a prominent risk factor of heart disease
and stroke120,
121
. In the US, 72.6% of men and 71.9% of women age 60-79 years have
hypertension83, and prevalence rates increase with age83. According to data from the National
20
Health and Nutrition Examination Survey (NHANES) 2005-2008, men and women have nearly
equal hypertension prevalence rates83. PVD rates are similar among men and women as well, and
increase drastically with age122. Approximately 13.7% of men and 15.0% of women age 70 years
and older develop PVD122.
Adults 65 years of age and older, in the US, account for >50% of all acute MIs and
coronary bypass operations123, 124. The American Heart Association has concluded that men are
more frequently hospitalized for heart disease and acute MI while women have higher
hospitalization rates for CHF and stroke83. As medical advancements increase the survival rate
following acute cardiovascular events, rates of individuals living with chronic CVD will
increase116.
Foldvari et al. (2000) used baseline data from a 1-year randomized controlled clinical
trial of community-dwelling, sedentary and independent elderly women (mean age ± SD = 74.8 ±
5.0 years) with a variety of chronic medical conditions and disabilities (3.2 ± 1.9)23. They aimed
to test whether peak muscle power, assessed by a Keiser pneumatic leg press, was closely
associated with self-reported functional status23. Out of all physiological factors tested, including
leg strength (r=0.43, p=0.0001), leg power was found to have the strongest univariate correlation
to functional status (r= -.47, p<0.0001)23. Hypertension (55%), PVD (19%), and CAD (15%)
were among the most common participant medical diagnoses23. The study’s relatively small
sample size and homogeneous sample limited generalizability. Although it is important to
acknowledge the high prevalence of CVD among women age 70 years and older, relationships
with men should be assessed as well. Additional longitudinal analyses of chronic disease
prevalence, CVD, and leg power measurements should be a future research direction.
21
Kuh et al. (2005) assessed data from a British cohort study (N=2956) that consisted of
participants aged 53 years69. Functional leg power was measured by chair rise time. Men
(11.7%) and women (12.2%) were categorized into a “disabling/life threatening conditions”
health group based on a diagnosis of at least one health condition of DM, cancer, epilepsy,
and/or cardiovascular problems69. Chair rise time was collected from 1332 men and 1363
women. After adjusting for height, weight, physical activity, musculoskeletal symptoms,
respiratory symptoms, and social class, differences between participants in the “disabling/lifethreatening conditions” health group and chair rise times were found among men (r=-.36,
p=0.020) and women (r=-.39, p=0.003)69. Overall, chair rise times were reported as slower in
adults with poorer health. The effects were significant for the “disabling/life threatening
conditions” health group. The investigators limited eligibility to healthy participants to prevent
incomplete physical performance tasks by individuals with more functional limitations, health
problems, and sedentary lifestyles.
Approximately two thirds (67.0%) of participants (N=1753, ≥ 60 years age) involved in a
study by Kuo et al (2006) were hypertensive as determined by a self-report of a physician’s
diagnosis38. Medical histories of CVD (MI, CHF, and angina) and DM were reported. All
chronic conditions were adjusted for prior to the analysis of muscle power and disability. Knee
extensor force production was assessed by a Kin-Com isokinetic dynamometer. The study
findings provided additional evidence of the importance of leg power impairments as a clinical
indication of individuals at risk of disability38. A weakness of this study was failing to examine
how these cardiovascular comorbidities affected leg power measurements.
McDermott et al. (2004) sampled community-dwelling men and women age 60 years and
older from two Italian cities59. Of these participants, 109 had lower extremity PVD as defined by
22
<0.90 on the ankle-brachial index (ABI)59. The researchers measured lower extremity leg power
with a Nottingham power rig and examined PVD associations. Participants with PVD (an
average of 83.69 watts, mean age= 78.27 years) were found to have reduced muscle power in
the lower extremities in comparison to participants without PVD (an average of 103.51 watts,
mean age = 73.78 years, p<0.001)59.
Additionally. McDermott et al. (2008) measured knee extension power, strength, and
handgrip of 424 participants with PVD (mean age ± SD =71.39 ± 7.56) and 271 participants
without PVD (mean age ± SD = 74.97 ± 8.24)60. They aimed to determine associations of
strength and lower extremity performance among individuals with lower ABI levels. The study
was conducted with a cross-sectional design and Nottingham leg power measurements. Lower
knee extension power was found to be associated with lower ABI values; however, these results
were not statistically significant60. Participants with PVD had lower knee extension power than
those without PVD (93.49 ± 52.73 W vs. 117.17 ± 68.6 W, p<0.001) after adjusting for age, sex,
race, comorbidities, body mass index, smoking history, leg symptoms, and recruitment cohort60.
Although comorbidities were adjusted for statistical analyses, the prevalence of each disease was
not reported. No significant associations were found between knee extension isometric strength
and ABI60. Longitudinal studies could provide further opportunity for determining associations
between PVD and leg power among older adults.
Saunders et al. (2008) conducted a cross-sectional, observational study using baseline
data from a randomized controlled trial61. Participants were community dwelling and
independently ambulatory (n=66, mean age ± SD= 72.10 ± 9.91 years) after a stroke61. The
researchers aimed to determine associations of activity limitations and lower-limb explosive
extensor power with a Nottingham power rig61. Cardiovascular comorbidities of ischemic heart
23
disease (n=22), left ventricular failure (n=2) and hypertension (n=31) were considered as
potential confounders; however, comorbid disease(s) were not found to be predictive of lowerlimb extensor power61. Restrictive eligibility criteria and the homogeneity of the sample may
have limited the ability to find an association between comorbid disease and lower limb extensor
power.
Limited data exists on CVD and muscle power among older adults. A majority of the
studies had cross-sectional designs and limited generalizability. Restrictive eligibility criteria
limited analysis between leg power and cardiovascular comorbidities. However, reliable data
exists to show that PVD is associated with lower-extremity muscle power.
24
4.0
CONCLUSION
Recently, muscle power (Power=Force x Velocity)22 has become a more common predictor of
late-life disability and a frequent measure of physical function among older adults6, 17-19. Chronic
diseases have been found to be essential underlying causes of physical disability in late life as
well8-10. However, few studies have assessed the relationship between muscle power and the
prevalent chronic diseases of OA, DM, and CVD. In addition to the most feasible leg power
measure for older adults remaining undetermined, many questions have yet to be answered about
the relationship between leg power and chronic disease in older adults.
The study designs assessed in this review varied when considering the relationship
between leg power and chronic disease. A large proportion of the studies used cross-sectional
analyses. Although these were convenient to conduct, the investigators could not determine
causal relationships. Most of the controlled trials involved randomizing participants to exercise
or training programs; however, comparing participants with chronic disease to case-matched
controls may give the most accurate and reliable results. Alternatively, large cohort studies of
ethnically and geographically diverse older adults with thorough measurements of chronic
disease may offer a higher quality design. The primary weakness among all study designs was
that the main hypotheses did not directly relate to chronic disease and its relationship with lowerextremity muscle power.
25
In general, the studied populations were not representative of the general population of
aging adults. Age ranges and the mean age varied greatly between studies. Most eligibility
criteria required participants to be considered healthy and community dwelling. Health status
was frequently determined by self-report, rather than a clinic evaluation. In addition, the majority
of leg power studies exclude acute and chronic diseases. Therefore, a limited amount of studies
directly compared chronic disease and leg power. Homogeneous samples were common and
races were rarely specified, creating limited generalizability.
Comprehensive measures of disease were weak throughout the studies. Studies involving
participants with PVD did not determine inclusion based on a standard ABI level and did not
specify how the disease was identified. Few DM studies ascertained disease by the standard
American Diabetes Association criteria, and self-report remained the standard for CVD
diagnoses. The definition of CVD differed between studies, and few directly addressed its
relationship with leg power. The measures of leg power were not comprehensive, and studies
rarely justified why they chose one technique over the other. With a variety of leg power
measures performed throughout the various studies, it became difficult to determine which was
the superior method of measurement among older adults with chronic conditions.
Future research should focus on conducting larger prospective studies and determining
the optimal leg power measure for adults with one or more chronic diseases. Analysis of the
relationships between chronic diseases and leg power should include acute and severe
conditions.
26
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