Arthritis & Rheumatism (Arthritis Care & Research)
Vol. 53, No. 5, October 15, 2005, pp 659 – 665
DOI 10.1002/art.21466
© 2005, American College of Rheumatology
ORIGINAL ARTICLE
Exercise, Self-Efficacy, and Mobility Performance
in Overweight and Obese Older Adults With
Knee Osteoarthritis
BRIAN C. FOCHT,1 W. JACK REJESKI,2 WALTER T. AMBROSIUS,2 JEFFREY A. KATULA,2
STEPHEN P. MESSIER2
AND
Objective. To examine changes in mobility-related self efficacy following exercise and dietary weight loss interventions
in overweight and obese older adults with knee osteoarthritis (OA), and to determine if self efficacy and pain mediate the
effects of the interventions on mobility task performance.
Methods. The Arthritis, Diet, and Activity Promotion Trial was an 18-month, single-blind, randomized, controlled trial
comparing the effects of exercise alone, dietary weight loss alone, a combination of exercise plus dietary weight loss, and
a healthy lifestyle control intervention in the treatment of 316 overweight or obese older adults with symptomatic knee
OA. Participants completed measures of stair-climb time and 6-minute walk distance, self efficacy for completing each
mobility task, and self-reported pain at baseline, 6 months, and 18 months during the trial.
Results. Mixed model analyses of covariance of baseline adjusted change in the outcomes demonstrated that the
exercise ⴙ dietary weight loss intervention produced greater improvements in mobility-related self efficacy (P ⴝ 0.0035),
stair climb (P ⴝ 0.0249) and 6-minute walk performance (P ⴝ 0.00031), and pain (P ⴝ 0.09) when compared with the
healthy lifestyle control intervention. Mediation analyses revealed that self efficacy and pain served as partial mediators
of the beneficial effect of exercise ⴙ dietary weight loss on stair-climb time.
Conclusion. Exercise ⴙ dietary weight loss results in improved mobility-related self efficacy; changes in these taskspecific control beliefs and self-reported pain serve as independent partial mediators of the beneficial effect of exercise ⴙ
dietary weight loss on stair-climb performance.
KEY WORDS. Arthritis; Efficacy beliefs; Physical activity; Physical function.
INTRODUCTION
Knee osteoarthritis (OA) is a chronic degenerative disease
that affects approximately one-third of all older adults in
the United States. The joint damage and pain accompanying knee OA are primary causes of activity restriction and
physical disability (1) and have a profound impact on
quality of life in the elderly (2). Obesity is a modifiable risk
Supported by the National Institute of Aging (grants
AG14131 and 5P60 AG10484) and the General Clinical Research Center (grant M01-RR07122).
1
Brian C. Focht, PhD: Ohio State University, Columbus;
2
W. Jack Rejeski, PhD, Walter T. Ambrosius, PhD, Jeffrey A.
Katula, PhD, Stephen P. Messier, PhD: Wake Forest University, Winston-Salem, North Carolina.
Address correspondence to Brian C. Focht, PhD, Division
of Health Behavior and Health Promotion, School of Public
Health, Ohio State University, 320 West 10th Avenue, Columbus, OH 43210. E-mail: bfocht@sph.osu.edu.
Submitted for publication May 19, 2005; accepted in revised form June 2, 2005.
factor for the development and progression of knee OA (3).
Epidemiologic evidence suggests that weight loss may prevent the incidence of knee OA and alleviate adverse symptoms accompanying the onset of the disease (4). Consequently, weight loss interventions are now advocated in
the treatment of overweight or obese patients with OA of
the knee (5–7).
With the growing recognition of the importance of
weight loss interventions for arthritis patients, the recently
completed Arthritis, Diet, and Activity Promotion Trial
(ADAPT) examined the effects of exercise and dietary
weight loss interventions, both separately and in combination, in the treatment of overweight or obese older
adults with knee OA. Findings from ADAPT demonstrated
that combining exercise and dietary weight loss resulted in
significant improvements in self-reported measures of
physical function and pain symptoms and performance
measures of mobility (8). The current investigation addresses the influence of the interventions in ADAPT on
changes in mobility-related self efficacy and examines
whether changes in self efficacy and self-reported pain
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660
mediate and/or are independent predictors of improvement in performance-based measures of mobility.
Perceptions of personal capabilities are important cognitive mechanisms that influence the health status and
physical functioning of patients with arthritis. For example, self-efficacy beliefs and knee pain are independent
predictors of activity restriction among older adults with
knee OA (9). Performance-related self-efficacy beliefs have
also been shown to be prospectively related to functional
decline among older adults with knee pain (10). Recent
research with physical activity also suggests that changes
in performance-related control beliefs possess particularly
important implications for the effectiveness of exercise
interventions in the treatment of knee OA. Notably,
Rejeski and colleagues (11) demonstrated that exercise
therapy resulted in significant improvements in self efficacy for the performance of functional tasks. Moreover,
changes in these mobility-related control beliefs and knee
pain were found to mediate the improvements in performance measures of mobility achieved with exercise therapy.
Taken collectively, these findings reinforce the position
that changes in mobility-related self-efficacy beliefs and
perceptions of relevant physical symptoms, such as pain,
are determinants of the functional benefits accompanying
exercise participation. Therefore, in the present investigation, we examined the effects of exercise and dietary
weight loss interventions, both separately and in combination, on mobility-related self efficacy in the ADAPT
trial. A secondary objective of this study was to examine
whether self efficacy and pain mediated the beneficial
effects of the exercise and dietary weight loss interventions on performance measures of mobility.
MATERIALS AND METHODS
Design. Complete details of the ADAPT design and
methodology have been reported elsewhere (8,12,13). In
brief, ADAPT was a single-blind, 18-month, randomized,
controlled trial examining the effects of 4 interventions on
various measures of physical function: exercise alone, dietary weight loss alone, exercise in combination with dietary weight loss, or healthy lifestyle control. ADAPT was
conducted at the Claude D. Pepper Older Americans Independence Center of Wake Forest University, WinstonSalem, NC. All participants provided written informed
consent that had received approval of the university institutional review board prior to participation.
Participants. The eligibility criteria for participation in
the study were age ⬎60 years; calculated body mass index
ⱖ28 kg/m2; self-reported knee pain on most days of the
month; sedentary activity pattern with ⬍20 minutes of
formal exercise per week during the past 6 months; selfreported difficulty with at least 1 of the following activities
due to knee pain: walking 0.25 miles (3– 4 city blocks),
climbing stairs, bending, stooping, kneeling, shopping,
house cleaning, getting in or out of bed, standing up from
a chair, lifting and carrying groceries, or getting in or out of
a bathtub; radiographic evidence of tibiofemoral OA as
Focht et al
determined by a single observer on the basis of weightbearing anteroposterior radiographs; and willingness to
undergo testing and intervention procedures.
Exclusion criteria included a serious medical condition
that precluded safe participation in an exercise program
such as coronary artery disease, severe hypertension, peripheral vascular disease, stroke, congestive heart failure,
chronic obstructive pulmonary disease, insulin-dependent
diabetes, psychiatric disease, renal disease, liver disease,
active cancer other than skin cancer, and anemia; a MiniMental score ⬍24; inability to complete the 18-month
study or unlikely to be compliant; inability to walk without a cane or other assistive device; participation in another research study; excessive alcohol consumption of
ⱖ14 drinks per week; or inability to complete the trial
protocol, in the opinion of the clinical staff, because of
frailty, illness, or other reasons.
A total of 2,209 older adults were prescreened via telephone interviews. Of this population, 1,596 individuals
did not meet 1 or more of the eligibility criteria and another 297 refused to be contacted any further. A total of
316 participants were randomized into the study with the
following group assignments: 82 in dietary weight loss
alone, 80 in exercise alone, 76 in the combination exercise
and dietary weight loss, and 78 in the healthy lifestyle
control.
Measures. Measures of mobility. Participants completed 2 performance-based mobility tasks: a 6-minute
walk task and timed stair-climb task. The 6-minute walk
task was conducted in a gymnasium measuring 70 feet by
88 feet in area. Each individual was instructed to walk as
far as possible in 6 minutes. Participants began walking at
the command “go” and continued walking until they received the command “stop.” Participants were not allowed
to carry a watch and were not provided any feedback
during the test. Performance was measured as the total
distance covered in feet. The stair-climb task involved
ascending a set of 5 stairs, turning around on the top of the
platform, and then descending. Performance was measured as the total time (in seconds) necessary to complete
the task. These performance-based tasks have been shown
to be valid and reliable tests of physical function in older
adults with knee OA in previous research (9).
Walking self efficacy. Prior to performing the 6-minute
walk task, participants were asked to rate their confidence
in their ability to walk around the gymnasium 2 times
without stopping. This measurement was subsequently
repeated for 5 additional levels of difficulty for the anticipated distances of completing 4 laps, 6 laps, 8 laps, 10
laps, and 12 laps without stopping. For each level of difficulty, participants were presented with a confidence ladder with 10 steps ranging from 0 (completely uncertain) to
10 (completely certain). Walking self-efficacy scores were
calculated by summing the participant’s confidence ratings across the 6 levels of difficulty and multiplying this
result by 2 to produce a score ranging from 0 to 100. This
hierarchical measurement protocol is consistent with the
protocol developed by Bandura (14), and the walking self-
Exercise and Self-Efficacy in Knee OA
efficacy scale has demonstrated adequate psychometric
properties in several previous investigations (9,11,15).
Stair-climbing self efficacy. Stair-climbing self efficacy
was assessed using the same hierarchical measurement
approach used for walking self efficacy. Specifically, prior
to completing the stair-climb task, participants were asked
to rate their level of certainty on the 0 –10 confidence
ladder that they could complete the task 2 times, 4 times,
6 times, 8 times, and 10 times without stopping. Stairclimb self-efficacy scores were also calculated by summing
across the 5 levels of difficulty and multiplying this result
by 2, yielding a score ranging from 0 to 100.
Pain. Pain was assessed with the pain subscale of the
Western Ontario and McMaster Universities Osteoarthritis
Index (WOMAC) (16). Participants were asked to indicate
the pain severity they had experienced during the past 48
hours due to knee OA on a scale ranging from 0 (none) to
4 (extreme). The WOMAC pain subscale consists of 5 items
and total scores range from 0 to 20, with higher scores
indicating greater pain.
Procedures. Following recruitment, participants completed a series of clinic visits to verify their eligibility for
the study. The baseline assessments were obtained during
the screening visits and eligible participants were assigned, using a variable-block randomization procedure,
into 1 of the 4 treatment arms: exercise alone, dietary
weight loss alone, exercise and dietary weight loss, or
healthy lifestyle control. Assessments of mobility-related
self efficacy and physical function were obtained at data
collection visits conducted at baseline and at 6 and 18
months after randomization.
Interventions. Exercise. Each participant assigned to
the exercise alone or exercise and dietary weight loss
treatment arms participated in three 60-minute exercise
sessions per week. Specifically, each session comprised an
aerobic phase (15 minutes), a resistance training phase (15
minutes), a second aerobic phase (15 minutes), and a cool
down (15 minutes). The first 4 months of the 18-month
intervention were facility based. At any time after the
initial 4 months, participants who wished to exercise at
home underwent a 2-month transition phase in which the
participants alternated between exercising at the facility
and exercising at their home. Hence, some participants
remained in the facility-based program, others chose the
home-based program, and some participants opted to engage in a combined facility/home-based program. At the
beginning of a home-based phase, exercise leaders visited
homes to work with participants in tailoring their individualized exercise regimens that were consistent with the
study protocol.
The aerobic exercise involved walking within a heart
rate range of 50 –75% of heart rate reserve. The resistance
training consisted of 2 sets of 12 repetitions of the following exercises: leg extension, leg curl, heel raise, and step
up. Cuff weights and weighted vests were used to provide
the resistance and a 1–1.5-minute rest interval was maintained between each exercise. Additional home visits or
facility-based booster sessions were scheduled to assist
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participants who were having difficulty complying with
the home-based exercise intervention. Attendance logs for
center-based exercise and self-reported exercise logs for
home-based sessions were used to gather adherence data
and monitor progress. Exercise adherence was defined as
the number of exercise sessions completed divided by the
total number of prescribed sessions.
Dietary weight loss. The dietary intervention was designed to produce and maintain an average weight loss of
5% across the 18-month intervention. The intervention
was based on principles from the group dynamics literature (17) and social-cognitive theory (14). The major emphasis of the intensive phase was to heighten awareness of
the importance of and need to change eating habits to
lower caloric intake. Behavior change was facilitated
through the use of self-regulatory skills including self
monitoring, goal setting, cognitive restructuring, problem
solving, and environmental management. One introductory individual session was followed by 16 weekly sessions consisting of 3 group sessions and 1 individual session each month. The transition phase included a session
every other week for a total of 8 weeks. The goals for this
phase included assisting participants who had not reached
their weight loss goals in establishing new goals, and
maintaining and preventing relapse in those participants
who had reached their weight loss goals. The maintenance
phase included monthly meetings and phone contacts,
alternated every 2 weeks.
Exercise ⫹ dietary weight loss. The exercise ⫹ dietary
weight loss intervention involved completing the procedures previously described for both the exercise alone and
the dietary weight loss alone programs. Delivery of the
combined intervention was provided to participants consecutively on the same day and at the same location.
Healthy lifestyle control. The healthy lifestyle control
intervention served as a usual care comparison group with
the 3 treatment arms and was designed to provide attention, social interaction, and health education. The group
met monthly for 1 hour during the first 3 months. A health
educator, who scheduled videotaped presentations and
physician discussions on topics concerning OA, obesity,
and exercise, organized the healthy lifestyle program.
Monthly phone contacts were maintained during months
4 – 6 and bimonthly phone contacts during months 7–18.
Compliance was defined as the number of sessions attended divided by the total number of sessions offered.
Statistical analysis. The effects of the diet and/or exercise interventions on changes in self efficacy for walking
and stair climbing at the 6-month and 18-month assessments were analyzed using mixed-model analyses of covariance (ANCOVA). Time (2 levels) and group (4 levels)
were included as factors. We used the logarithm of stairclimb time and the Arcsin of the square root of self efficacy/100 as variance stabilizing transformations (18). All
analyses were conducted using PROC Mixed in the SAS
software version 8 (SAS Institute, Cary, NC), a procedure
that analyzes all available followup information by providing maximum likelihood estimates of missing data. In
each of these models, change from baseline in self efficacy
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Focht et al
Table 1. Mean ⴞ SD self-efficacy scores by task and treatment condition
Treatment group
Self-efficacy scores for stair climb
(range 0–100)
Health education
Diet only
Exercise only
Diet plus exercise
Self-efficacy scores for 6-minute walk
(range 0–100)
Health education
Diet only
Exercise only
Diet plus exercise
Baseline
Followup
Change
Adjusted score
at followup
70.24 ⫾ 28.75
68.29 ⫾ 27.94
70.19 ⫾ 26.22
63.84 ⫾ 29.25
72.28 ⫾ 27.11
72.01 ⫾ 27.12
72.90 ⫾ 26.02
77.25 ⫾ 24.28
2.04
3.72
2.71
13.41
83.20
81.64
83.07
89.70
67.88 ⫾ 31.95
68.91 ⫾ 32.53
67.49 ⫾ 34.21
66.44 ⫾ 33.26
72.90 ⫾ 31.89
74.03 ⫾ 30.16
82.03 ⫾ 24.71
84.95 ⫾ 24.59
5.01
5.11
14.54
18.51
83.85
83.79
92.02
95.24
was used as the outcome and age, sex, and the baseline
self-efficacy value were included as covariates. The TukeyKramer adjustment for multiple comparisons was used
when examining pair-wise differences. We examined intervention by time interactions and found that none were
significant (all P ⬎ 0.35); therefore, they were not included
in the analyses. We calculated the adjusted means for
treatment averaging over time. Participants who completed baseline and at least one followup assessment were
included in the analyses. Additionally, all analyses were
conducted using the intent-to-treat principle. Following
the primary analyses that tested the effects of the interventions on change in mobility-related self efficacy, secondary
analyses were conducted to determine if self efficacy and
pain mediated the effects of interventions on 6-minute
walk and stair-climb performance. Specifically, a series of
separate mixed-model ANCOVA models were tested to
examine the effects of the interventions on change in selfreported pain and 6-minute walk and stair-climb performance; the effects of the potential mediators (self efficacy
for walking and stair climbing and self-reported pain) on
6-minute walk and stair-climb performance; and a composite model testing the effects of the treatments on
6-minute walk and stair-climb performance while controlling for self efficacy for mobility and self-reported pain.
RESULTS
Participant attrition and adherence. Of the 316 participants randomized into the trial, 252 (80%) completed the
trial. As reported previously (8), retention of participants
did not differ among the 4 intervention groups. Adherence
was also not significantly different among the groups, with
rates of 75% in the healthy lifestyle control, 72% in the
dietary weight loss, 60% in the exercise only, and 64% in
the combined intervention.
Effects of the interventions on self efficacy for mobility.
The main analyses in this study were conducted to examine the effect of the exercise and dietary weight loss interventions on change in the measures of self efficacy for
mobility. Analysis of change in stair-climbing self efficacy
yielded a significant main effect for treatment group
(F[3,206] ⫽ 2.65, P ⫽ 0.05). Inspection of the group means
provided in Table 1 demonstrates that participants in the
exercise ⫹ dietary weight loss intervention significantly
improved their stair-climb self efficacy compared with the
healthy lifestyle control group. Analysis of change in
walking self efficacy also revealed a significant main effect
for treatment group (F[3,210] ⫽ 6.04, P ⫽ 0.0006). As
illustrated in Table 1, participants in the exercise ⫹ dietary weight loss and exercise alone interventions significantly improved their walking self efficacy compared with
the healthy lifestyle control group.
Although changes in mobility-related control beliefs are
relevant outcomes of lifestyle interventions among older
patients with knee OA, the minimal clinically significant
difference (MCSD) associated with changes in mobilityrelated self efficacy has yet to be established. Accordingly,
we estimated the MCSD for the mobility-related self-efficacy measures. To estimate the MCSD, we calculated the
mean and SD of the transformed data for each self-efficacy
outcome at baseline. We subsequently calculated small,
medium, and large effect sizes. The following MCSD values represent the back transformed differences obtained by
using half of the small, medium, and large effect sizes
above and below the observed baseline mean: for walking
self efficacy the effect sizes were 8.08, 20.04, and 31.60,
whereas for stair-climb self efficacy the effect sizes were
6.70, 16.65, and 26.38, respectively.
Effects of the interventions on performance-related mobility and knee pain. As reported in previous findings
from the ADAPT trial (8), significant group main effects
were observed for pain (F[2,232] ⫽ 3.37, P ⫽ 0.0193),
6-minute walk distance (F[3,196] ⫽ 12.75, P ⬍ 0.0001),
and stair-climb time (F[3,212] ⫽ 3.18, P ⫽ 0.0249). Specifically, the exercise ⫹ dietary weight loss intervention resulted in significantly greater improvements in pain
(t[232] ⫽ 2.35, adjusted P ⫽ 0.09) relative to the healthy
lifestyle control group. In regard to the performance measures of mobility, the exercise ⫹ dietary weight loss intervention yielded a more favorable improvement in stairclimb time (t[212] ⫽ 2.85, adjusted P ⫽ 0.0249) relative to
the healthy lifestyle control group, and both the exercise ⫹
dietary weight loss intervention (t[196] ⫽ ⫺4.97, adjusted
P ⬍ 0.0001) and the exercise alone intervention (t[196] ⫽
Exercise and Self-Efficacy in Knee OA
663
Table 2. Mean ⴞ SD for mobility performance by task and treatment condition
Treatment group
Stair-climb time
Health education
Diet only
Exercise only
Diet plus exercise
6-minute walk distance
Health education
Diet only
Exercise only
Diet plus exercise
Baseline
Followup
Change
9.44 ⫾ 4.91
9.77 ⫾ 5.70
9.85 ⫾ 4.56
10.38 ⫾ 7.26
9.86 ⫾ 5.56
9.86 ⫾ 8.78
9.15 ⫾ 4.70
8.85 ⫾ 5.35
0.41
0.09
⫺0.70
⫺1.53
1,422 ⫾ 269
1,406 ⫾ 254
1,417 ⫾ 251
1,360 ⫾ 280
1,411 ⫾ 261
1,433 ⫾ 260
1,551 ⫾ 297
1,524 ⫾ 316
⫺4.55, adjusted P ⬍ 0.0001) resulted in significantly better
changes in walking distance when compared with the
healthy lifestyle control group (Table 2).
The independent and mediational role of self efficacy
and pain on performance. To evaluate the independent
effects of self efficacy and pain on change in stair-climb
time, composite mixed-model ANCOVAs were constructed using change in stair-climb time and 6-minute
walk distance as outcome variables. In these models, covariates included baseline scores for each outcome (visit,
age, sex, and treatment effect) whereas baseline status of
pain or self efficacy, as well as change in pain or self
efficacy, were considered for any independent effect that
they had on change in each outcome. Results of these
analyses revealed that baseline pain (F[1,191] ⫽ 7.60, P ⫽
0.0064) and baseline self efficacy (F[1,191] ⫽ 7.03, P ⫽
0.0087), together with change in pain (F[1,191] ⫽ 10.44, P
⫽ 0.0015) and change in self efficacy (F[1,191] ⫽ 35.93, P
⬍ 0.0001), were all independent predictors of stair-climb
time beyond the treatment effect and other covariates mentioned above. In regard to the baseline predictors, these
findings suggest that participants reporting less pain and
higher self efficacy at baseline demonstrated superior stairclimb performance at followup. Additionally, the observation that change in each outcome was an independent
predictor of performance suggests that participants demonstrating the greatest increase in self efficacy and decrease in pain exhibited the most favorable change in
stair-climb performance. Similar results were found for
6-minute walk distance except that baseline pain was not
statistically significant (F[1,178] ⫽ 22.27 for baseline self
efficacy, P ⬍ 0.0001; F[1,178] ⫽ 9.05 for change in pain, P
⫽ 0.0030; F[1,178] ⫽ 28.47 for change in self efficacy, P ⬍
0.0001).
Collectively, the primary results of this study and the
findings reported previously by Messier et al (8) demonstrate that the ADAPT interventions had a significant effect
on both the proposed mediators (self efficacy for mobility
and knee pain) and the outcomes of interest (performance
measures of mobility). Thus, we conducted secondary analyses in the present investigation to examine whether
change in self efficacy and pain mediated the effects of the
exercise and dietary weight loss interventions on change
in the performance measures of mobility or whether there
⫺11
27
134
163
Adjusted score
at followup
8.60
8.24
7.80
7.54
1,417
1,447
1,559
1,575
was evidence that change in self efficacy and pain had
independent effects on the outcomes of interest. First,
mixed-model ANCOVA analyses were conducted to test
the effect of the proposed mediators on the measures of
mobility task performance. Results of the model examining change in 6-minute walk performance demonstrated
that baseline walking self efficacy (F[1,178] ⫽ 25.75, P ⬍
0.0001), change in walking self efficacy (F[1,178] ⫽ 35.81,
P ⬍ 0.0001), and change in pain (F[1,178] ⫽ 6.43, P ⫽
0.0121) were significant predictors of change in baselineadjusted 6-minute walk distance. Additionally, results of
the model examining change in stair-climb performance
demonstrated that baseline stair-climb self efficacy
(F[1,192] ⫽ 7.96, P ⫽ 0.0053), change in stair-climb self
efficacy (F[1,192] ⫽ 38.63, P ⬍ 0.0001), baseline pain
(F[1,192] ⫽ 7.45, P ⫽ 0.0069), and change in pain (F[1,192]
⫽ 9.90, P ⫽ 0.0019) were significant predictors of change
in baseline-adjusted stair-climb time. Thus, the findings
from these models demonstrate that self efficacy and pain
are significant independent predictors of change in walking and stair-climb performance.
Having established the required univariate relationships
between the exercise and dietary interventions, self efficacy and pain, and the performance measures of mobility,
we then tested a composite model examining the effect of
the interventions on change in the measures of mobility
performance after controlling for self efficacy and pain in
the model. Results of the mixed-model ANCOVA analyses
of stair-climb performance revealed that controlling for
stair-climb self efficacy and pain in the original model
attenuated, but did not eliminate, the significance of the
treatment group main effect (F[3,192] ⫽ 2.78, P ⫽ 0.0425).
Results of the ANCOVA analyses of walking performance
revealed that controlling for walking self efficacy and pain
in the original model did not reduce the significance of the
treatment group main effect (F[3,178] ⫽ 11.46, P ⬍
0.0001). However, in each analysis, the changes in self
efficacy and pain remained independent predictors of mobility-related performance after including terms for the
group effects.
DISCUSSION
In the ADAPT trial, the combination of exercise and dietary weight loss resulted in significant improvements in
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self-reported measures of physical functioning, pain, and
performance measures of mobility. The present study examined the influence of the ADAPT intervention on
changes in mobility-related self-efficacy beliefs. Moreover,
we were interested in whether baseline levels or change in
self-efficacy beliefs and pain were independent predictors
of change in performance-based measures of function
and/or whether change in self efficacy and/or pain mediated the effect that the interventions had on these indices
of mobility disability.
Analyses revealed that both the exercise alone and the
diet ⫹ exercise groups experienced significant increases in
walking self efficacy. However, only the diet ⫹ exercise
group reported significant improvements in self efficacy
for stair climbing. Based on the estimated MCSD for the
self-efficacy measures, the improvements in mobility-related self efficacy following the exercise alone and diet ⫹
exercise interventions represented small to moderate effect size differences. The differential effects of the exercise
alone and combination interventions on change in self
efficacy for 6-minute walk and stair-climb performance
underscore the importance of specificity in the measurement of mobility-related self efficacy (14). Thus, the
present findings suggest that although exercise alone increases self efficacy for walking, exercise must be combined with weight loss to influence efficacy beliefs involving the performance of more challenging mobility tasks
such as stair climbing. Because the exercise only treatment
did not differ from the healthy education control group on
changes in stair-climb time in the ADAPT study (8), it
appears that physical activity and weight loss should indeed be the intervention of choice in older, overweight,
and obese adults with knee OA.
The results of this study underscore the importance of
self-efficacy beliefs and pain in the treatment of knee OA,
even within the context of lifestyle interventions. That is,
both baseline levels and changes in self efficacy and knee
pain were significant independent predictors of improvements in mobility performance. The only exception was
that baseline levels of pain did not predict change in
6-minute walk times. These data are consistent with growing empirical evidence indicating that control beliefs and
pain play key roles in the disablement process (10,11,19).
Therefore, lifestyle interventions aimed at improving
physical functioning in patients with knee OA should
include components that target self-efficacy beliefs and
pain management (19). In addition to maximizing effects
on physical functioning, these types of intervention strategies have the added potential of enhancing adherence
and the long-term maintenance of physical activity
(15,20).
In the present study, self efficacy and knee pain partially
mediated the influence of the interventions on change in
stair-climb performance. A previous study involving older
adults with knee OA, the Fitness and Arthritis in Seniors
Trial, found that self efficacy and pain completely mediated the effect of a physical activity intervention on stairclimb time (10). In the current study, the exercise only
group did not differ from the health education control
group on stair-climb time; thus, the effect in this study was
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driven by the combined exercise and dietary weight loss
group.
In conclusion, the present study demonstrated that a
combined dietary weight loss and physical activity intervention had unique effects on changes in self efficacy for a
weight-dependent stair-climb task as compared with exercise alone. Additionally, both baseline values and changes
in self efficacy and pain were significant predictors of
improvement in mobility disability above and beyond the
effects of the interventions. These findings add to the
growing body of evidence demonstrating that control beliefs and physical symptoms are crucial to understanding
and intervening the process of physical disablement (18).
Furthermore, when treating obese, older adults who have
compromised function due to knee OA, it is highly preferable to use a combined dietary weight loss and physical
activity intervention as compared with a physical activity
intervention alone.
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