Journal of Gerontology: MEDICAL SCIENCES
2007, Vol. 62A, No. 8, 866–871
Copyright 2007 by The Gerontological Society of America
Knee Strength Maintained Despite Loss of Lean
Body Mass During Weight Loss in Older
Obese Adults With Knee Osteoarthritis
Xuewen Wang,1 Gary D. Miller,2 Stephen P. Messier,2 and Barbara J. Nicklas1
1
Section on Gerontology and Geriatric Medicine and 2Department of Health and Exercise Science,
Wake Forest University School of Medicine, Winston-Salem, North Carolina.
Background. The effects of weight loss on muscle function in older adults have not been well studied. This study
determined the effects of a 6-month weight-loss intervention on muscle strength and quality in older obese adults with
knee osteoarthritis.
Methods. Participants were randomized to a weight loss (WL) (n¼44, 70 6 6 years) or weight stable (WS) (n¼43, 69 6
6 years) group. The WL intervention consisted of weekly educational meetings, a meal replacement diet, and a three-sessionper-week structured exercise program to achieve 10%–12% weight loss. The WS intervention included bimonthly group
meetings and newsletters. Body composition and knee extensor strength were measured at baseline and after intervention.
Results. The WL group decreased body weight, lean body mass, fat mass, and percent body fat ( p , .001 for all).
Concentric extension strength increased 25% in WL ( p . .05), whereas eccentric extension decreased 6% in WS ( p ¼
.028). Concentric muscle quality (strength per kg body weight or lean body mass) increased in WL ( p , .05), whereas
eccentric muscle quality decreased in WS ( p , .05). Changes in lean body mass and fat mass were inversely associated
with changes in most muscle strength and quality measures ( p , .05). Men and women did not differ in response to the
intervention in knee strength outcomes.
Conclusions. Hypocaloric dieting in combination with exercise training had beneficial effects on muscle strength/
quality, despite loss of lean body mass in this sample of older men and women. Greater fat loss was associated with greater
gains in muscle strength and quality. More studies are needed regarding the mechanisms by which loss of fat mass
increases muscle strength and quality.
T
HE human aging process is accompanied by a significant decline in neuromuscular function and performance characterized by the inevitable reduction in skeletal
muscle mass and strength (1). Studies have shown that
isometric, concentric, and eccentric muscle strength declines
with advancing age (2,3). Muscle quality (strength per unit
of cross-sectional area or muscle mass) also declines with
age (4,5). Moreover, the prevalence of joint dysfunction and
arthritis, osteoarthritis in particular, increases progressively
with age (6,7). All these factors contribute to the loss of
functional mobility and independence present in many older
adults.
The global epidemic of obesity presents an increasing
health concern. Among persons older than 60 years, 70.8%
are overweight or obese, with 32.9% being obese (8).
Excess body fat is a recognized risk factor for many chronic
age-related diseases such as cardiovascular diseases and
diabetes, and growing evidence from cross-sectional and
longitudinal studies shows that obesity is associated with
declines in physical function and greater onset of disability
(9–13). Obesity is also associated with an increased risk of
knee osteoarthritis (14,15), the leading cause of physical
disability in older adults (16). Thus, being overweight or
obese may exacerbate age-related loss of functional mobility
and independence.
Treatment of obesity via hypocaloric dieting has not been
widely advocated in older adults because of concerns
866
regarding potential adverse health consequences associated
with weight loss. Unintentional weight loss is a common
complication of some age-related diseases (17,18) and it not
only decreases fat mass but also decreases lean body mass
(muscle mass and bone mass), which could further impair
physical function via potential loss of muscle strength and/
or quality (19,20). The effects of intentional weight loss on
muscle strength or quality have not been well studied,
especially in older persons. Some data show that weight loss
from hypocaloric dieting combined with moderate physical
activity increased muscle power, strength, quality, and
physical performance in adults (21–25), but other data show
weight loss by hypocaloric diet alone did not change muscle
strength in young adults (26).
Given the concerns related to weight loss in older persons, the purpose of this study was to determine the effects
of a 6-month weight-loss intervention composed of caloric
restriction and exercise training on muscle strength and
quality in older obese adults with knee osteoarthritis.
METHODS
Study Design
This study was a randomized clinical trial, the Physical
Activity, Inflammation, and Body Composition Trial,
designed to compare the effects of assignment to a
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867
Figure 1. Progress of participants through the study.
weight-stable (WS) control group or to a weight loss (WL)
group on measures of body composition and physical
function in older adults with knee osteoarthritis. The
primary results of the study, showing greater improvements
in physical performance and self-reported physical function
and pain (Western Ontario and McMaster University
Osteoarthritis Index, WOMAC) in the WL than the WS
group, are published elsewhere (27). It was approved by the
Wake Forest University Institutional Review Board. We
report here on the effects of the intervention on muscle
strength and quality.
Participants and Recruitment
Participants were recruited over an 18-month period as
previously described (27). The eligibility criteria included:
60 years of age; body mass index 30.0 and 45.0 kg/m2;
osteoarthritis in at least one knee; stable weight over the
previous 3 months; sedentary without actively participating
in 20 minutes of formal exercise 1 session per week or
expending 200 calories per week in moderate- to highintensity activities within the past 6 months; and self-reported
difficulty in at least one of the following activities attributed to
knee pain: lifting or carrying groceries, walking one-quarter
mile, getting in and out of a chair, or going up and down stairs.
Individuals were excluded if they: (i) had an unstable
medical condition or a condition where rapid weight loss or
exercise was contraindicated, (ii) were unwilling to modify
diet or physical activity patterns, or (iii) would not be able to
comply with the intervention.
Interventions
Participants were randomly assigned to one of two groups
(WS or WL) and were instructed to continue to take their
medications. For participants in the WS group, weights were
measured at bimonthly group meetings where pertinent
health topics were discussed. They were encouraged to
maintain their weight throughout the 6-month duration.
Newsletters were sent in months 1, 3, and 5 describing facts
on pertinent general health topics.
Participants in the WL group underwent a 6-month
weight-loss intervention to achieve a 10%–12% loss in
initial body weight. Body weight was monitored at weekly
behavioral and educational sessions. Diet was set based on
providing a daily energy deficit of 1000 kcal, as determined
by predicted energy expenditure, with a calorie distribution
goal of ;20% from protein, ;25% from fat, and ;55%
from carbohydrates. A maximum of two meal replacements
(SlimFast shakes and bars) containing about 220 kcal with
7–10 g of protein, 33–46 g of carbohydrates, 1.5–5 g of fat,
and 2–5 g of fiber were provided. For the third meal,
a weekly menu plan with recipes was given for participants
to follow. They also participated in a facility-based structured exercise program 3 days/week for 60 minutes consisting of aerobic and strength training. The primary mode
of aerobic training was walking with 50%–85% of the agepredicted heart rate reserve. Strength training included leg
extension, leg curl, heel raise, and step-ups using ankle cuff
weights, weighted vest, and resistance training equipment.
In addition, behavioral interventions including techniques to
incorporate physical activity into daily life was emphasized
on other days.
Measurements
Age and race were acquired by self-report. The outcome
measures of this article, knee strength and body composition, were collected at baseline and 6-month visits.
Knee concentric/eccentric extension muscle strength was
assessed using a Kin-Com 125E isokinetic dynamometer
(Chattanooga Group, Hixon, TN) at a velocity of 30 s1
through a joint arc from 908 to 308 (08 ¼ full extension). The
first and last 108 were subsequently deleted to account for
acceleration and deceleration of the dynamometer at the
ends of the range of motion and also to account for possible
inconsistent effort. Two maximal reproducible trials were
averaged, and the maximum number of each test was six
with 30- to 60-second intervals. The tested leg was the one
most affected by arthritis according to the participant.
Weight and height were obtained (with shoes and outer
garments removed) using a scale calibrated weekly. Percent
body fat, lean body mass, fat mass, and total body mass
were measured by dual-energy absorptiometry (DXA)
(Delphi QDR; Hologic, Bedford, MA).
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WANG ET AL.
Statistical Analyses
Statistical analyses were performed using SAS 9.1 (SAS
Institute, Cary, NC). Descriptive statistics were calculated,
and values are reported as means 6 standard deviations
(SD). We used factorial analysis of variance with a repeated
measure on one factor, to compare mean body composition
measures and knee strength outcomes across groups and
before and after intervention. A Group 3 Time interaction
was included in the model. Mean percent change was
calculated as the average of individual differences before
and after intervention. In the supplementary analysis, we
also examined the main effect of gender and the interactions
of Gender 3 Group, Gender 3 Time, and Gender 3 Group 3
Time. Pearson’s correlations between changes in body
composition and strength measurements were examined. An
a level of 0.05 was selected.
RESULTS
Retention and Adherence
Forty participants (90.9%) in WL and 33 (76.7%) in WS
completed the study (Figure 1). The primary reasons for
dropping out of the study were unhappiness with group
assignment, inability to attend intervention due to personal
reasons, poor health (unrelated to study outcomes), and
moving from the area. Body composition and knee strength
measurements were available from 73 and 64 participants,
respectively, at both baseline and after intervention. Participants in the WL group attended an average of 77.5% of
the exercise sessions and 75% of the nutrition sessions over
the 6-month study duration.
Table 1. Demographic Information, Body Composition, and Muscle
Strength at Baseline and After a 6-Month Intervention for the
Weight Loss (WL) and Weight Stable (WS) Groups
Variable
Age, y
Female, %
Non-White, %
WL (N ¼ 40)
WS (N ¼ 33)
69.9 6 5.7
62.5
12.5
68.8 6 5.7
63.6
6
97.8 6 16.3
89.7 6 16.1*
8.1 6 7.2
98.1 6 15.3
98.3 6 15.9
0.2 6 3.9
35.0 6 5.8
31.8 6 5.1*
8.8 6 8.7
34.7 6 4.3
34.8 6 4.6
0.2 6 3.9
59.1 6 12.1
57.4 6 11.9*
2.9 6 3.7
58.5 6 12.6
58.9 6 12.8
0.8 6 4.1
39.8 6 9.7
34.6 6 11.2*
13.8 6 12.7
41.0 6 7.5
41.0 6 8.0
0.01 6 6.8
40.3 6 7.4
37.4 6 7.7*
2.9 6 2.8
41.5 6 6.3
41.3 6 6.3
0.2 6 1.7
Weight, kg
Baseline
Postintervention
Change, %
Body mass index, kg/m2
Baseline
Postintervention
Change, %
Lean body mass, kg
Baseline
Postintervention
Change, %
Fat mass, kg
Baseline
Postintervention
Change, %
Percent body fat, %
Baseline
Postintervention
Change, %
Concentric extension, Newton
Baseline
Postintervention
Change, %
255 6 99
282 6 96
25 6 64
271 6 145
241 6 125
0 6 53
378 6 126
387 6 112
8 6 33
404 6 164
358 6 143y
6 6 34
Eccentric extension, Newton
Demographics and Body Composition
The WL and WS groups had similar mean age, percentage of women, and percentage of non-White participants
( p . .05 for all) (Table 1). Their baseline body weight,
body mass index, lean body mass, fat mass, and percent
body fat were also similar ( p . .05 for all), and interactions
of Group 3 Time were significant ( p , .001 for all). The
intervention significantly decreased the above measurements in WL only ( p , .001 for all). The total amount of
weight loss in WL was composed of 61.9 6 41.5% fat and
19.9 6 25.0% lean mass.
Muscle Strength and Quality
For absolute concentric extension, the Group 3 Time
interaction was significant ( p ¼ .02), with a 25% increase in
WL only ( p . .05) (Table 1). For absolute eccentric
extension strength, the Group 3 Time interaction tended to
be significant ( p ¼ .062). It decreased 5% in WS ( p ¼ .028),
but increased 8% in WL ( p . .05).
We also examined muscle quality expressed as knee
strength relative to body weight and lean body mass. The
Group 3 Time interactions for both concentric and eccentric
extension strength per kilogram of body weight were
significant ( p ¼ .004 and .011, respectively) (Figure 2A
and B). Concentric extension strength per kilogram of body
weight increased from baseline to after intervention in WL
( p ¼ .006), but remained unchanged in WS ( p . .05).
Baseline
Postintervention
Change, %
Concentric extension/body weight, Newton kg1
Baseline
Postintervention
Change, %
2.64 6 1.04
3.20 6 1.10y
37.2 6 72.8
2.74 6 1.34
2.45 6 1.21
1.0 6 56.0
Eccentric extension/body weight, Newton kg1
Baseline
Postintervention
Change, %
3.91 6 1.28
4.40 6 1.28z
18.2 6 38.3
4.12 6 1.47
3.70 6 1.36y
5.3 6 35.8
Concentric extension/lean mass, Newton kg1
Baseline
Postintervention
Change, %
4.36 6 1.53
5.05 6 1.65y
29.6 6 67.8
4.53 6 1.98
4.02 6 1.77
0.1 6 53.9
Eccentric extension/lean mass, Newton kg1
Baseline
Postintervention
Change, %
6.51 6 1.80
6.97 6 1.90
11.8 6 35.6
6.92 6 2.30
6.14 6 1.92y
6.2 6 34.3
Notes: All values are mean 6 standard deviation. Percent changes were
calculated as the average of individual percent changes before and after
intervention. Lean body mass, fat mass, and percent body fat, n ¼ 37 for WL and
n ¼ 32 for WS; concentric extension, n ¼ 36 for WL and n ¼ 27 for WS;
eccentric extension, n ¼ 34 for WL and n ¼ 28 for WS.
*p , .001, yp , .05, zp ¼.072, compared to baseline.
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869
Table 2. Relationships Between Changes in Body Composition and
Knee Extensor Strength in the Entire Sample
Knee Extensor Strength
Lean
Mass
Fat
Mass
Lean
Mass*
Fat
Mass*
0.318y
0.179
0.366y
0.256z
0.371y
0.281z
0.307z
0.191
0.362y
0.289z
0.330y
0.248z
0.358y
0.192
0.401y
0.267z
0.414y
0.297z
0.336y
0.163
0.394y
0.267z
0.361y
0.221§
Concentric extension
Eccentric extension
Concentric extension/body weight
Eccentric extension/body weight
Concentric extension/lean mass
Eccentric extension/lean mass
Notes: All are Pearson’s correlation coefficients.
*Partial correlation coefficients after adjusting age, gender, and race.
y
p .01, zp .05, §p ¼ .09.
Figure 2. A, Concentric extension knee strength per kilogram of body
weight (Newton kg1) at baseline and after a 6-month intervention by group.
The Group 3 Time interaction was significant, p ¼ .004. B, Eccentric extension
knee strength per kilogram of body weight (Newton kg1) at baseline and after
a 6-month intervention by group. The Group 3 Time interaction was significant,
p ¼ .011.
Eccentric extension strength per kilogram of body weight
tended to increase in the WL group ( p ¼ .072), but
decreased in the WS group ( p ¼ .047). Similar results were
obtained for knee strength per kilogram of lean body mass
( p ¼ .013 and p ¼ .023 for Group 3 Time interactions for
concentric and eccentric extension strength, respectively).
Concentric extension strength relative to lean body mass
increased in WL only ( p ¼ .030), whereas eccentric extension strength per kilogram of lean body mass decreased in
the WS group only ( p ¼ .044).
In addition, we examined whether there was a difference
in response to the intervention between men and women
by including the Gender 3 Time, Gender 3 Group, and
Gender 3 Group 3 Time interactions. There were no statistically significant gender interactions ( p . .05) for
absolute and relative knee strength, indicating similar
gender responses for all knee-strength outcomes.
Body Composition and Muscle Strength and Quality
Changes in lean mass and fat mass were related to
changes in absolute concentric and all relative knee-strength
outcomes ( p , .05). Adjusting for age, gender, and race did
not affect the correlations (Table 2).
DISCUSSION
This randomized, controlled trial demonstrated that
weight loss via hypocaloric dieting in combination with
moderate exercise training maintained absolute knee extensor muscle strength and improved muscle quality in older
persons with osteoarthritis. These favorable effects occurred
despite a significant loss of lean body mass during the 6month intervention. We also found that greater fat loss was
associated with greater gains in muscle strength and quality.
Very few prior studies have reported the effects of
a weight-loss intervention on muscle strength or quality in
either younger or older persons. Our results are in line with
those reported by Sartorio and colleagues (24), showing that
a 3-week hypocaloric diet (1200–1500 kcal/day) with lowintensity aerobic exercise in 18- to 77-year-old obese
persons improved motor control and lower limb muscle
power. Previous intervention studies have shown that
resistance exercise training can significantly improve muscle
strength even if increases in muscle mass are small (28,29).
These results suggest that there are factors other than muscle
mass contributing to gains in muscle strength, although
muscle mass is important in determining muscle strength in
elderly persons (30).
Some observational data show an inverse association
between adiposity and muscle strength and quality. In the
Health, Aging and Body Composition Study (Health ABC),
a cohort of well-functioning community-dwelling older
persons aged 70–79 years at baseline, body fat is an
independent contributor to age-related declines in muscle
strength and quality (leg and arm) (5). Sartorio and
colleagues (23) have found improved isotonic strength after
loss of fat mass in both men and women (mean age 29.3 6
7.0 years). Moreover, a quadratic relationship was found
between percent body fat and leg muscle quality, with
individuals at the high and low extremes of percent body fat
having lower levels of leg muscle quality than individuals in
the midrange (5).
Although the resistance training component of the exercise intervention in our study was modest, it may have had
beneficial effects on muscle strength and quality, independent of weight loss. In addition, the loss of lean body mass
may have been less than if a resistance training component
was not incorporated into the intervention. However, as our
data show that the magnitude of the changes in strength and
quality were related to the loss of body fat, there are likely
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WANG ET AL.
mechanisms by which loss of adipose tissue may also improve muscle strength and quality. Biologically, fat may
contribute to the regulation of protein metabolism and
ultimately to accumulation of lean mass (31,32). In addition,
there were no relationships between changes in pain
(measured by the Western Ontario and McMaster University
Osteoarthritis Index) and changes in knee strength (data not
shown), suggesting that improvement in knee pain did not
contribute to the maintenance/improvement in knee strength
in our study.
Another mechanism may be through loss of intramyocellular triglyceride with weight loss. Observational data
show that a lower muscle density, that is, higher muscle
lipid content, assessed by an x-ray attenuation on computed
tomography scan, is associated with lower voluntary isokinetic knee extensor strength independent of subcutaneous
and midthigh adiposity (33). Prior studies have also shown
that weight loss reduces muscle lipid content (34,35). In
addition, as weight loss reduces biomarkers of inflammation
(36), and there is a link between inflammation and muscle
mass and strength (37–39), a loss of adipose tissue may
benefit muscle strength through a reduction in inflammation.
Therefore, the reduction in intramyocellular triglyceride and
inflammation may be potential mechanisms by which
muscle strength and quality improved with weight loss.
Gender differences in muscle strength and quality with
aging have been found (3); however, very few studies
examined whether there are gender difference in the effects
of diet and exercise-training intervention on muscle strength
and quality. One study showed that a 3-week hypocaloric
diet and aerobic and strength training in younger adults
improved absolute maximum leg power output in women
but not in men (23). However, improvements in leg power
relative to body weight or fat-free mass, and total isotonic
strength, were similar between men and women. Our study
found that responses to the intervention were not different
between genders for absolute or relative knee extensor
strength.
As is the case with most randomized trials, more
participants in the treatment group completed the intervention compared to the control group. However, we chose not
to perform an intent-to-treat analysis because our primary
purpose was to assess whether actual amount of weight loss
affected muscle strength. Although we found that greater fat
loss was associated with greater gains in muscle strength
and quality, we were not able to determine if this was
a causal relationship. In addition, the intervention for the
WL group included caloric restriction and a structured
exercise program; therefore, we were not able to examine
their individual effect on the outcome variables.
Conclusion
Results from this randomized, controlled trial in older
persons show that intentional weight loss via hypocaloric
dieting and exercise training improved knee extensor
strength and quality despite loss of lean body mass. Thus,
it may be appropriate to prescribe weight loss in older obese
persons, particularly those with osteoarthritis. However,
additional studies examining the effects of intentional
weight loss on other measures of physical function, and
especially on onset of physical disability, are needed.
Moreover, more research is needed regarding the mechanisms by which loss of fat mass increases muscle strength
and quality.
ACKNOWLEDGMENTS
This work was supported by the SlimFast Nutrition Institute, Wake
Forest University, the Claude D. Pepper Older American Independence
Center (National Institutes of Health [NIH] Grant P30 AG21332), and the
Wake Forest University General Clinical Research Center (NIH Grant
M01-RR07122).
We thank Pamela Moser for her work with the intervention and Gretchen
Austin, Tina Ellis, Shannon Brown, Kristen Klingler, and Meghan
Provonost for the data collection.
CORRESPONDENCE
Address correspondence to Xuewen Wang, PhD, Section on Gerontology
and Geriatric Medicine, Wake Forest University School of Medicine,
Medical Center Blvd., Winston-Salem, NC 27157. E-mail: xwang@
wfubmc.edu
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Received May 28, 2006
Accepted November 10, 2006
Decision Editor: Luigi Ferrucci, MD, PhD