Hindawi Publishing Corporation
Journal of Obesity
Volume 2012, Article ID 261974, 6 pages
TheAssociation of ObesitywithWalkingIndependentof Knee
Pain: TheMulticenterOsteoarthritis Study
Jingbo Niu,2MichaelNevitt,4Cora E.Lewis,5James Torner,6andK. DouglasGross7
1Department of Physical Therapy and Athletic Training, College of Health and Rehabilitation Sciences: Sargent College,
Boston University, Boston, MA 02115, USA
2Section of Clinical Epidemiology Research and Training Unit, Boston University School of Medicine, Boston, MA 02118, USA
3Department of Biostatistics, Boston University School of Public Health, Boston, MA 02118, USA
4Department of Epidemiology & Biostatistics, University of California-San Francisco, San Francisco, CA 94107, USA
5Division of Preventive Medicine, University of Alabama, Birmingham, AL 35294, USA
6Department of Epidemiology, University of Iowa, Iowa City, IA 52242, USA
7Department of Physical Therapy, MGH Institute of Health Professions, Boston, MA 02129, USA
Correspondence should be addressed to Daniel K. White, firstname.lastname@example.org
Received 17 September 2011; Accepted 19 January 2012
Academic Editor: Panagiota Nota Klentrou
Copyright © 2012 Daniel K. White et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
Practice guidelines recommend addressing obesity for people with knee OA, however, the association of obesity with walking
independent of pain is not known. We investigated this association within the Multicenter Osteoarthritis Study, a cohort of older
adults who have or are at high risk of knee OA. Subjects wore a StepWatch to record steps taken over 7 days. We measured knee
pain from a visual analogue scale and obesity by BMI. We examined the association of obesity with walking using linear regression
adjusting for pain and covariates. Of 1788 subjects, the mean steps/day taken was 8872.9 ± 3543.4. Subjects with a BMI ≥35 took
3355 fewer steps per day independent of knee pain compared with those with a BMI ≤25 (95% CI −3899, −2811). BMI accounted
for 9.7% of the variability of walking while knee pain accounted for 2.9%. BMI was associated with walking independent of knee
and the American College of Rheumatology for people with
knee osteoarthritis (OA) in order to promote healthy living
in a reduction in knee pain and improvement in functional
ability [3, 4]. Moreover, meeting physical activity guidelines
through activities like walking can reduce the risk of death
. This is particularly noteworthy given that knee OA is
associated with an increased risk of all-cause death  and
given the rising prevalenceof knee OA , this mortality risk
poses significant public health implications.
For older adults with knee OA, knee pain is associated
with difficulty walking , and often considered the primary
culprit for low levels of physical activity and walking .
Obesity is also associated with difficulty walking and low
levels of physical activity, and it is a primary risk factor for
knee OA [10, 11]. About 1/3 of the United States population
is obese with a BMI of at least 30 . Furthermore,
one in three obese adults have arthritis . However, the
association ofobesity independent ofknee pain withwalking
is not known. This is an important association to understand
since weight loss is rarely prescribed to patients in practice
 and instead clinicians typically focus on pharmacologic
therapies for knee pain. While such practice is in contrast to
treatment guidelines for OA, which recommend both weight
loss intervention and pain management , evidence
supporting these guidelines for an outcome of walking is
2 Journal of Obesity
Hence, the purpose of our study is to examine the
association of obesity with walking independent of knee
pain. We hypothesized that obesity would be associated with
walking independent of knee pain.
2.1. Sample. The cross-sectional study sample consisted of
a large multicenter longitudinal cohort study of community-
dwelling subjects who have or are at high risk of knee
OA . The MOST sample included adults aged 50 to
79 years were recruited from the communities in and
surrounding Birmingham, Alabama and Iowa City, Iowa.
Inclusion criteria, based on risk for knee OA, included the
presence of known risk factors, including being age over 50
years, female gender, previous knee injury or operation, and
body weight in excess of the median weight for each age-
and sex-specific group based on data from the Framingham
OA Study . The MOST study protocol was approved by
the institutional review boards at the University of Iowa in
Iowa City, University of California San Francisco, University
of Alabama in Birmingham, and Boston University Medical
2.2. Walking Subsample. Information on walking, pain, and
obesity were collected at the 60-month MOST follow-up
exam between June of 2009 and January of 2011. We
restricted the analysis sample to those participants who had
a minimum of 3 days of walking data since previous studies
have found this to be the minimum number of days needed
for a reliable estimate of physical activity .
Of the 2330 MOST participants attending the 60-month
follow-up visit, 16% (377) did not agree to wear the
StepWatch, and 2% (58) had monitor malfunctions. Of the
remaining 1895 participants who wore the StepWatch, 94%
(1788/1895) wore it for at least 3 valid days and represent
the study sample. The StepWatch was worn for 3, 4, 5, 6,
and 7 days by 3%, 4%, 7%, 12%, and 74% of participants,
respectively. In general, participants included in this analysis
were more likely to have better health status (e.g., lower BMI,
depressive symptoms, less muscular weakness, and fewer
comorbidities) compared with those not included in the
analysis (data not shown).
and body mass index (BMI) were collected, trained research
assistants followed a written protocol to fit the StepWatch to
verbal instructions for putting on the device each morning
and taking off the device at bedtime for the next 7 days.
TheStepWatchisasmall(70 ×50 ×20mm;38g),water-
proof, self-contained device that is worn on the ankle and
ing no feedback to the user. The StepWatch has high concur-
rent validity in comparison with several reference standard
measures of step frequency in older adults, high convergent
validity in comparison with SF-36 scores among subjects
with OA, and high test-retest reliability in adults [19, 20].
To determine if subjects wore the monitor long enough
to be counted as a full day, we adopted a published method
of monitoring as the minimum amount of time needed
to define a full day. The ten hour threshold represents
more than 66% of waking hours and has been utilized as a
threshold in studies of physical activity in the general adult
population  and people with knee OA . Time in use
was counted from the first step recorded in the morning
to the last step recorded in the evening. To exclude times
subjects may have taken off the StepWatch during the day,
we omitted times where the monitor registered no steps for
180 consecutive minutes during the day (see the appendix).
We quantified walking as the total number of steps taken
per day on average as a continuous outcome. We calculated
of monitoring divided by the number of valid days.
2.4. Independent Variables
2.4.1. Knee Pain Severity. We measured knee pain severity as
the average pain in the past 30 days on Visual Analogue Scale
were categorized according to the VAS pain score of the more
from standardized weight and height assessments. For anal-
yses, we treated BMI as a continuous factor and a categorical
factor by classified according to World Health Organization
(WHO) categories .
2.5. Covariates. The following factors were treated as covari-
ates based on existing literature linking them to function
or physical activity [16, 24–27]: age, sex, living situation
(alone or with someone), education (<college, ≥college),
race (white, nonwhite), radiographic knee osteoarthritis
(ROA) defined as a Kellgren and Lawrence score of ≥2 in
the tibiofemoral joint, pain in the hip, ankle, or foot (present
versus absent), comorbidities (0 versus ≥1) estimated with
the modified Charlson comorbidity index , depressive
symptoms (<16, ≥16) measured with the Center for Epi-
demiologic Studies Depression Scale (CES-D) , as well
as knee strength in tertiles from the mean of four isokinetic
knee extensor torque repetitions at 60deg/sec measured in
Newton-Meters (Cybex Inc. Medway, MA). All covariates
were collected at the 60 month clinic visit except for living
situation, which was collected at the baseline clinic visit.
2.6. Statistical Analysis. We examined levels of walking by
employing descriptive statistics and plotted a histogram of
the distribution of walking across our sample. Then, we
examined the association of pain and BMI with walking. We
first examined the independent effects of pain and BMI by
calculating effects estimates and 95% confidence intervals
(CI) using multiple linear regression adjusting for both pain
and BMI (categorically) as well as for covariates. We also
Journal of Obesity3
10002000300040005000 6000 700080009000
1100012000 13000140001500016000 17000 180001900020000 21000 2200023000
Figure 1: Histogram of steps taken per day. N = 1788.
adjusted for study site (Alabama or Iowa), to account for
differences in data collection of StepWatch data and other
study variables. We also examined the relative effect of pain
and BMI with walking from partial correlation coefficients
and standardized beta coefficients. We confirmed walking
was normally distributed from visual inspection of Figure 1.
The mean age the 1788 subjects included was 67.2 (sd =
7.7) years. Most participants’ (36%) were overweight with a
BMI ≥25 and <30 followed by 29% being obese with a BMI
≥30 and <35. The majority of subjects were women (60%)
and white (90%). Table 1 lists characteristics of the subjects
included in this study.
The average number of steps taken per day was 8872.9
(sd = 3543.4). Figure 1 shows the average step counts per day
taken by participants.
More pain and higher BMI were associated with fewer
steps taken per day (see Table 2). Each increase of 10 points
on the VAS for pain was associated with taking 167 fewer
steps per day. Subjects who were overweight (BMI ≥25 and
<30), in obese class I (BMI ≥30 and <35), and in obese class
II (BMI ≥35) took 989, 2069, and 3355 fewer steps per day
compared with those with a healthy weight (BMI <25).
Mutually adjusting for pain and BMI along with covari-
ates explained 28% of the variability of walking. Pain
accounted for 2.9% of the variability while BMI accounted
for 9.7% of the variability of walking. Similarly, a one
standard deviation increase in pain accounted for a decrease
of 0.07 of a standard deviation of walking, while a one
standard deviation increase in BMI accounted for a decrease
of 0.28 of a standard deviation of walking (see Table 3).
We found BMI to be strongly associated with walking
independent of knee pain. In particular, we found BMI to
account for 9.7% of the variability of walking in comparison
to only 2.9% for pain. These findings suggest that obesity has
animportant associationwithlow levelsofwalkinginpeople
with or at high risk of knee OA independent of knee pain.
when considered along with the effect of obesity. To put the
relative effect of pain into perspective, knee pain accounted
for only 10% of the total variability accounted by our
model (pain, BMI, and all covariates). In contrast, obesity
accounted for 35% of the total variability of the same
model. We found a similar trend from the standardized
beta coefficients with a one standard deviation increase in
BMI accounting for more change in walking than the same
increase in pain. This difference is notable given that knee
pain is a major cause of functional limitation in people with
knee OA [30–33]. However, from a conceptual perspective,
the performance of physical function, such as walking speed,
is distinctly different than how much physical activity one
performs on a daily basis. Furthermore, previous studies
have reported a weak association between knee pain and
physical activity [34–36]. One possible reason for this is
that people may have avoided walking for different reasons.
Those with low levels of knee pain did not walk for fear of
increasing their knee pain, while those with high knee pain
were unable to walk due to current pain levels. Disentangling
these associations is needed for future longitudinal studies.
We found obesity to have a strong association with
walking, which has been reported previously in adults who
are normal weight and obese and general population studies
[37, 38]. Subjects in the highest BMI category walked over
3000 steps less per day than those in the lowest BMI category.
The magnitude of this difference is clinically meaningful as it
approaches a one standard deviation difference for walking
in our sample. Given that our study is cross-sectional,
we cannot infer causal direction, and association between
obesity and walking is likely bidirectional. For instance, low
levels of walking or physical activity could result in obesity.
Similarly, people who are obese could have difficulty walking
and hence have low levels of walking. Irrespective of the
directionality, we found obesity to be strongly associated
with walking independent of pain, which underscores the
obesity epidemic in the United States and the importance of
addressing obesity to avoid future poor health outcomes.
Step counts collected in our study cannot be compared
with previous studies utilizing pedometers. Pedometers are
known underestimate the number of steps taken by older
adults up to 33% compared with a StepWatch , hence
step counts in our study are higher than pedometer based
studies. However, the average step counts in our study
are comparable with smaller studies that employed the
StepWatch in people with knee or hip OA and older adults
[40, 41]. For instance, Winter et al. reported 30 people
with radiographic knee OA walked 9350 steps/day, which is
similar to our finding of 9194 and 8598 steps/day for men
and women, respectively.
Our study has several strengths. First, we report daily
walking from a large cohort of people with or at high risk
of knee OA who wore a validated walking monitor. Second,
this is the first study to report the association of obesity with
walking independent of knee pain in people with or at high
risk of knee OA using a well-validated objective monitor.
There are some limitations to our study. First, subjects may
have changed walking habits with the knowledge that their
4 Journal of Obesity
Table 1: Summary of baseline characteristics across all subjects and within BMI categories.
(n = 1788)
(n = 269)
(n = 641)
(n = 529)
(n = 352)
24.2 (23.8) Knee pain (0–100) [Mean (sd)]
BMI [kg/m] [Mean (sd)]
BMI <25 [%]
BMI ≥25 and <30 [%]
BMI ≥30 and <35 [%]
BMI ≥35 [%]
Age [Mean (sd)]
Sex [% women]
Living situation [% Lives alone]
Education [% ≥ College]
Race [% White]
Site [% Alabama]
Radiographic Knee Osteoarthritis [%]
Pain in the hip, ankle, or foot [%]
No Comorbidity [%]
Depressive Symptoms [Mean CES-D (sd)]
Knee extensor strength [N-M/kg] [Mean (sd)]
Table 2: Change in the number of daily steps attributed to pain and BMI after adjustment for covariates.
Adjusted∗beta [95% CI]
−166.8 [−245.5, −88.1] Knee Pain (10 unit increments on 0–100 VAS scale)
<25 “healthy weight”
≥25 and <30 “overweight”
≥30 and <35 “obese class I”
≥35 “obese class II-III”
∗Mutually adjusted for pain and BMI as well as age, sex, living situation, education, race, study site, Radiographic Knee Osteoarthritis, pain in the hip, ankle,
or foot, comorbidity, depressive symptoms, and isokinetic knee extensor strength.
−989.4 [−1437.0, −541.7]
−2069.6 [−2540.1, −1599.2]
−3355.1 [−3899.4, −2810.8]
monitor, that is, when subjects are aware of how many
steps are being recorded [42–44]. We believe any increases
in walking due to a testing effect were minimized since
study participants did not know the number of steps that
were recorded. Second, we acknowledge we employed few
limited to measures already collected in the MOST study
and were not able to add measures of self efficacy or fear
avoidance, which are likely associated with walking. Lastly,
our sample consisted of people both with and at high risk of
directly generalizable to those with knee OA. We performed
a sensitivity analysis stratifying our sample by those with and
without ROA and found similar effects for pain and obesity
within each strata compared with our main findings.
We found BMI to be associated with walking independent of
pain in the studied sample of people with or at high risk of
knee OA. These findings support clinical practice guidelines
that obesity is an important modifiable factor to intervene
upon and particularly relevant for walking among people
with or at high risk of knee OA. Future research should
with walking in order to better understand the temporal
relationships between these factors.
Previous literature to distinguish periods of inactivity from
nonwear have employed monitors designed to measure all
types of physical activity (Actigraph). These studies have
employed thresholds of 60 minutes to 90 minutes of no
activity to represent nonwear. However, it is not known if
these same thresholds can be generalized to a monitor only
measuring step counts, as periods of physical inactivity likely
differ from periods of not walking worn by people with or at
high risk of knee OA. One previous study of adult bariatric
surgical candidates found a threshold of 120 minutes of no
Journal of Obesity5
Table 3: Percentage of variability of walking attributed to pain,
BMI, and covariates and expected change in walking for each
one standard deviation increase in pain, BMI, and continuous
Knee pain (0–100)
Radiographic Knee Ostearthritis
Pain in the hip, ankle, or foot
Isokinetic knee extensor strength
∗Calculated for continuous variables only.
3060 90120150 180210 240270300
Nonwear threshold values: minimum consecutive minutes of
no steps to classify nonwear time
steps was suggested as nonwear time . We examined
how increases in threshold values of nonwear time changed
reporting of the daily average wear time, steps, and time
walking at a moderate intensity.
As threshold values of nonwear time increased, the
monitor was counted as being worn for a greater duration
of time. Subsequently, the average number of steps/day
decreased. However, changes in steps/day did not change
appreciably using threshold values of nonwear greater than
180 minutes. As a result, we employed a 180 minute
not wearing the monitor (for more details see Figure 2).
This work is supported by NIH AG18820, AG 18832,
AG 18947, AG 19069, AR007598, NIH AR47885, NIAMS
Arthritis Foundation Arthritis Investigator Award, Boston
Claude D. Pepper Older Americans Independence Center
(P30-AG031679), and the Foundation for Physical Therapy:
Geriatric Research Grant.
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