Physical activity levels in patients with early knee osteoarthritis measured by accelerometry

Authors


  • ClinicalTrials.gov identifier: NCT00586300.

Abstract

Objective

Physical activity (PA) is recommended for osteoarthritis (OA) management to reduce pain and improve function. The purpose of this study was to objectively assess the level and pattern of PA in male and female knee OA patients to determine adherence to Centers for Disease Control and Prevention/American College of Sports Medicine and Exercise and Physical Activity Conference recommendations for PA.

Methods

Early OA patients (n = 255, 76% women, mean ± SD age 54.6 ± 7.1 years, mean ± SD body mass index 27.8 ± 4.3 kg/m2) with Kellgren/Lawrence-defined grade II (no higher) radiographic OA in at least 1 knee wore an accelerometer for 6–7 contiguous days. Light (LPA), moderate (MPA), and vigorous (VPA) PA intensities were defined as accelerometer recordings of 100–2,224, 2,225–5,950, and >5,950 counts per minute, respectively.

Results

Patients wore accelerometers for a mean ± SD of 6.8 ± 0.3 days and 13.8 ± 2.2 hours/day, and spent much more time (P < 0.001) in MPA (23.6 ± 17.2 minutes/day) than VPA (0.95 ± 3.5 minutes/day). Men spent significantly (P < 0.05) more time in all PA intensities than women. Only 30% of patients achieved recommended PA levels. The proportion of men (47%) achieving the recommendation was significantly (P = 0.04) higher than women (24%).

Conclusion

Knee OA patients accumulate little VPA and most (70%) do not achieve recommended levels for MPA or greater. New strategies to increase levels of PA in this population are needed.

INTRODUCTION

Osteoarthritis (OA) is the most prevalent form of arthritis in the US (1). Moderate to severe OA affects at least 15.8 million American adults between ages 25 and 74 years, accounting for more than 12% of the population in this age range (2). OA is more prevalent in women than men (3). Indeed, OA is currently the most prevalent chronic condition in women and the rate of self-reported cases is projected to increase in future decades (4). Along with age and female sex, obesity is a major risk factor for OA incidence and severity (5). Joint pain and physical impairment from OA are responsible for extensive use of medical and surgical resources in the US. The burden of OA is projected to affect 40 million Americans by the year 2020 (3). Clearly, OA has emerged as a major public health concern negatively impacting individuals' lives and socioeconomic situations.

The primary goals of OA management are to reduce pain and improve functional ability and quality of life. Patient education and exercise are recommended as 2 mainstays of treatment. The health benefits of physical activity (PA) are undisputed (6). Regular PA is associated with lower prevalence of obesity and comorbidities such as diabetes, cardiovascular disease, and hypertension (6–8). Accordingly, the US Centers for Disease Control and Prevention (CDC) (9) and the American College of Sports Medicine (ACSM) (10) have implemented recommendations for PA to promote health benefits for the general public. These recommendations are based on intensity of PA reported in standard units of metabolic equivalents (METS) (9, 10). Activities are categorized by intensity as light (LPA; <3 METS), moderate (MPA; 3–6 METS), or vigorous (VPA; >6 METS) (9). According to CDC/ACSM recommendations, adults should engage in a minimum of 30 minutes/day of at least moderate-intensity PA (≥3 METS) on at least 5 days of the week (9, 10). Work group recommendations from the 2002 Exercise and Physical Activity Conference (EPAC) advise patients with knee OA to accumulate 30 minutes of at least moderate-intensity PA (≥3 METS) on at least 3 days of the week (11). VPA is associated with increased health benefits (6). The pain and discomfort felt during certain types of weight-bearing VPA (i.e., running) may inhibit knee OA patients from choosing these modes of activity. However, non–weight-bearing modes of VPA (e.g., resistance exercise) may be safe and feasible for knee OA patients, depending on the local mechanical environment (12), and thus, the accumulation of 30 minutes/day of a combination of moderate- and vigorous-intensity PA (MVPA; ≥3 METS) is likely a beneficial and realistic PA goal for patients with knee OA.

Published intervention studies have demonstrated beneficial effects of PA on relevant outcomes in patients with knee OA (13–16). While it is generally assumed that the pain and impairment associated with knee OA limits PA, typical levels and patterns of PA in knee OA patients are poorly described. Thus far, no studies have objectively measured PA levels in the knee OA population, nor have they determined whether this population achieves recommended PA levels (9, 10). Existing studies are limited in their ability to identify specific knee OA populations that could benefit from more PA due to problems inherent in self-report measures of PA (17) and small samples with heterogeneity in disease severity and duration. Furthermore, despite the higher prevalence of OA in women (3), potential sex differences in the level and pattern of PA that may be implicated have yet to be examined. Decisions regarding the allocation of resources for promoting PA in knee OA patients should be informed by accurate estimates of knee OA population PA levels. Thus, the primary aim of the present study was to objectively assess the level and pattern of health-enhancing MVPA in patients with early knee OA and to determine the proportion of patients who achieve the PA recommendations implemented by the CDC/ACSM (9, 10) and EPAC (11). The secondary aim was to compare PA in male and female knee OA patients.

PATIENTS AND METHODS

Design.

Data used for this cross-sectional analysis were obtained from the Multidimensional Intervention for Early Osteoarthritis of the Knee Study (the Knee Study), a randomized clinical trial comparing the effectiveness of progressive resistance exercise against self-management and combined exercise/self-management on pain and functional ability in adults with early knee OA. The study was approved by the University of Arizona Institutional Review Board and was conducted in accordance with the Declaration of Helsinki. This report presents an analysis of PA levels in Knee Study participants prior to the intervention. Participants provided written informed consent and self-reported demographic information.

Study sample.

A total of 294 patients were recruited from the Tucson, Arizona area using mass mailings, media advertisements, periodic media coverage, and requests to local physicians for referral of their patients. Eligibility criteria were as follows: age between 35 and 65 years to ensure an early knee OA sample, pain on most days (i.e., ≥4 days in a week) in 1 or both knees for at least 4 months during the previous year, duration of symptoms <5 years (18, 19), grade II OA (and no higher) radiographic status in at least 1 knee as defined by Kellgren/Lawrence (K/L) criteria (20), and disability due to knee OA according to the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) (21). All participants met the American College of Rheumatology classification criteria for knee OA (22). Descriptions of knee OA severity, proportions of patients with grade II unilateral (left or right) and grade II bilateral knee OA, and knee pain are presented in Table 1. After prebaseline attrition (n = 20) and exclusion of cases from the analysis due to failure to meet PA assessment inclusion criteria (i.e., an accelerometer recording of at least 8 hours/day for at least 6 continuous days; n = 19), baseline PA data were successfully obtained from 255 patients (87% of the original sample), including 59 men and 196 women.

Table 1. Description of participants*
 Women (n = 196)Men (n = 59)All (n = 255)
  • *

    Values are the percentage unless otherwise indicated. OA = osteoarthritis; WOMAC = Western Ontario and McMaster Universities Osteoarthritis Index.

  • WOMAC pain subset comprises 5 items: 0 (lowest score) to 500 (maximum score).

  • Significant sex difference (P < 0.001, t-test for a difference between 2 independent means).

Age, mean ± SD years54.5 ± 6.955.3 ± 8.054.6 ± 7.1
Knee OA pain, mean ± SD WOMAC score89.8 ± 72.284.0 ± 73.288.5 ± 72.3
Height, mean ± SD cm164.8 ± 7.1179.9 ± 7.4168.3 ± 9.6
Weight, mean ± SD kg75.1 ± 13.792.4 ± 14.479.1 ± 15.6
Body mass index, mean ± SD kg/m227.6 ± 4.428.5 ± 3.727.8 ± 4.3
 Normal (<25)311728
 Overweight (25–30)364638
 Obese (>30)333734
Knee OA severity   
 Grade II unilateral left knee OA292027
 Grade II unilateral right knee OA263428
 Grade II bilateral knee OA454645
Race/ethnicity   
 White, non-Hispanic677669
 African American, non-Hispanic232
 Hispanic211219
 Other999
Education   
 ≤11 years202
 High school or equivalent373336
 College or equivalent262626
 Graduate school or equivalent354036
Employment status   
 Employed858184
 Unemployed824
 Retired71810
Income   
 <$20,00012310
 $20,000–$40,000211520
 $40,000–$60,000296043
 >$60,000386043
Marital status637867

Anthropometry.

Following standard protocols outlined in the Anthropometric Reference Standardization Manual (23), weight was measured to the nearest 0.1 kg using a calibrated scale (Seca model 770; Seca, Hamburg, Germany) and height was measured to the nearest 0.1 cm using a portable stadiometer (Shorr Height Measuring Board, Olney, MD) after full inspiration.

Pain.

Knee pain was assessed using the 5-item pain subscale of the WOMAC, which has been validated in knee OA patients (21). The WOMAC pain subscale uses visual analog scales (0–100) to elicit patients' reports of their pain severity during walking, stair use, lying in bed at night, sitting, and standing. Scored as the sum of items, possible scores range from 0 to 500; lower scores reflect less pain.

Physical activity.

Accelerometer-based estimates of PA were obtained using the MTI Actigraph (model 7164; Manufacturing Technologies, Inc., Shalimar, FL). This uniaxial electromechanical accelerometer is designed to record vertical plane acceleration and deceleration at a frequency of 10 × seconds−1 and a threshold of 0.033g. Additional specifications for this device are published elsewhere (24). A number of studies have demonstrated its durability, reliability, and validity in adults according to criterion methods such as heart rate monitoring and indirect calorimetry (25–29). The accelerometers were attached to a belt worn around the waist with accelerometer placement on the right hip. The manufacturer recommended accelerometer placement on the hip because this site permits measurement of whole body movement, does not interfere with daily activities, and is the most frequently used site in epidemiologic studies.

For each assessment, the MTI Actigraph was initialized and set to record data in 60-second epochs prior to placement on the patient. Patients were instructed to wear the accelerometer for 7 contiguous days during all waking hours, except during water activities. The following measures were obtained: number of days measured, registered time (hours/day), and average duration (minutes/day) spent in all health-enhancing PA intensities, including MPA (3–6 METS), VPA (>6 METS), and combined MVPA (≥3 METS). Validation studies demonstrating the accuracy and precision of the MTI Actigraph model 7164 against portable indirect calorimetry systems in adults (25–29) have reported different cutoff points due to the wide range of activity types and intensities used in the calibration studies. Guided by a recent review of calibration studies of accelerometer output for adults (30), we minimized sampling error by averaging the cutoff points reported by laboratory- and field-based calibration studies using the MTI Actigraph model 7164 (25–29) and applied the resulting cutoff points to discriminate between PA intensities. MPA (3–6 METS) and VPA (>6 METS) intensity cutoffs from these studies ranged from 1,809–2,430 and 5,725–6,403 counts per minute, respectively. The applied cutoff points for LPA, MPA, and VPA intensities were accelerometer recordings of 100–2,224, 2,225–5,950, and >5,950 counts per minute, respectively.

Leisure-time PA and exercise habits within the previous 3 months were assessed using the Aerobics Center Longitudinal Study Physical Activity Questionnaire (ACLS), which has been described elsewhere (31). The ACLS elicits self-reports of the frequency (sessions/week) and duration (minutes/session) of activities such as walking, running, using a treadmill, cycling, swimming, aerobics, yoga, weight lifting, and other sports (e.g., golf, tennis, soccer). Descriptions of activities are provided in Table 2.

Table 2. Physical activities reported on the Aerobics Center Longitudinal Study Physical Activity Questionnaire (31)*
ActivityResponse rate, n (%)Frequency, n (%)Sessions/week, mean ± SDMinutes/session, Mean ± SD
  • *

    Data were self-reported. Activities refer to exercise and physical activity habits performed at least once a week in the past 3 months.

  • Values were only calculated for respondents reporting a particular activity.

  • Golf (not riding), leisure volleyball, social dancing, softball.

  • §

    Basketball, boxing, football, lacrosse, rugby, soccer.

Walking238 (93)175 (74)4.2 ± 4.530.9 ± 20.3
Running237 (93)11 (5)2.6 ± 1.226.4 ± 15.3
Treadmill236 (93)26 (11)2.1 ± 0.929.4 ± 21.3
Cycling235 (92)28 (12)3.1 ± 3.030.1 ± 25.5
Swimming236 (93)18 (8)2.3 ± 1.331.6 ± 15.0
Aerobics238 (93)25 (11)2.4 ± 1.432.5 ± 16.4
Yoga235 (92)4 (2)3.0 ± 1.243.0 ± 7.3
Weight lifting234 (92)36 (15)2.5 ± 1.324.8 ± 13.2
Racquet sports236 (93)2 (1)1.5 ± 0.760.0 ± 0.0
Moderate sports235 (92)22 (9)1.7 ± 1.3114.0 ± 81.3
Vigorous sports§236 (93)3 (1)1.3 ± 0.643.2 ± 38.7
Other activities218 (85)44 (20)2.4 ± 1.873.6 ± 56.8

Statistical analysis.

All statistical analyses were performed using the Statistical Package for the Social Sciences for Windows, version 15.0 (SPSS, Chicago, IL). Data are presented as the mean ± SD and percentages. Skewed variables, including registered time (hours/day), MPA (minutes/day), VPA (minutes/day), and MVPA (minutes/day), were log transformed to normalize distributions prior to analyses. In the descriptive tables and text, only nontransformed data are presented. Sex comparisons on variables of interest were accomplished with the t-test for differences between 2 independent means. Comparisons between average durations (minutes/day) of time spent in MVPA intensities for weekday versus weekend days were accomplished using the paired t-test. Chi-square analysis was conducted to compare the proportion of patients with grade II unilateral versus grade II bilateral knee OA who met CDC/ACSM (9, 10) and EPAC (11) PA recommendations and to compare the proportion of men and women who achieved PA recommendations (9–11).

RESULTS

Participants are described in Table 1. The final sample comprised 255 patients (76% women, mean age 54.6 ± 7.1 years, mean body mass index [BMI] 27.8 ± 4.3 kg/m2) with early knee OA. Men were significantly (P < 0.001) taller and heavier than women; however, age and BMI were similar in men and women. No significant (P = 0.94) differences were observed when comparing average right and left knee K/L scores in all patients. Furthermore, analysis of covariance showed no significant (P > 0.10) differences in time spent in any PA intensities for patients with varying degrees of combined knee OA severity. Chi-square analysis indicated that the proportions of patients with varying degrees of combined knee OA severity who met the PA recommendations (9, 10) were not significantly different (P > 0.10). Within the entire group of knee OA participants, 45% had bilateral grade II OA, whereas 27% had grade II OA in their left knee only and 28% had grade II OA in their right knee only. No significant (P > 0.05) differences in time spent in any PA intensity were observed between patients with unilateral versus bilateral grade II knee OA. Furthermore, chi-square analysis showed no significant (P > 0.05) difference in the proportion of unilateral versus bilateral grade II knee OA patients who achieved the PA recommendations (9–11). Sex differences were not observed between patients with unilateral versus bilateral grade II OA, nor were they observed for right, left, or combined K/L scores. The mean WOMAC knee pain score was 88.5 ± 72.3. No significant (P = 0.51) sex differences were observed in self-reported knee pain between men (mean WOMAC score 84.0 ± 73.2) and women (mean WOMAC score 89.8 ± 72.2). MVPA was not significantly (P = 0.97) different among patients with moderate-intensity occupations (n = 33) versus patients with light-intensity occupations (n = 183), where occupation intensity was scored as METS using the compendium of physical activities (32). Marital status did not predict significant (P = 0.62) differences in MVPA.

The number of days measured and daily habitual accelerometer wearing times were compared between men and women with an independent group t-test. Patients wore the accelerometer an average of 6.8 ± 0.3 days for 13.8 ± 2.2 hours/day. The average durations of wearing time (hours/day) for all patients were almost identical for weekday and weekend days, and there were no significant (P = 0.39) sex differences in weekday and weekend wearing times. Likewise, patients with 6 versus 7 days of valid accelerometer data did not differ (all P values > 0.05) in any of the measured variables.

The numbers of knee OA women (n = 196) and men (n = 59) who achieved various amounts (minutes/day) of MVPA are depicted in Figure 1. The percentages of participants who met the recommendations for MVPA along with the average durations (minutes/day) spent in all health-enhancing PA intensities (MPA, VPA, MVPA) are shown in Table 2. Thirty percent of patients accumulated a minimum of 30 minutes/day of MVPA. The proportion of men (47%) who achieved the CDC/ACSM recommendations for PA (9, 10) was significantly (P = 0.04) higher than the proportion of women (24%). Likewise, the proportion of men (54%) who achieved the EPAC recommendations for PA (11) was significantly (P = 0.012) higher than the proportion of women (33%). As a group, early knee OA patients achieved a mean of 24.5 ± 19.1 minutes/day of MVPA. A small but significant (P < 0.05) inverse correlation (r = −0.13) was found between BMI and MVPA in women. Patients spent much more time in MPA compared with VPA (mean ± SD 23.6 ± 17.2 minutes/day versus 0.95 ± 3.5 minutes/day; P < 0.001). Men spent significantly more time in MPA, VPA, and MVPA intensities than women (P < 0.05) (Table 3).

Figure 1.

Number of female (n = 196) and male (n = 59) knee osteoarthritis patients who achieved different amounts of combined moderate- and vigorous-intensity physical activity (MVPA; >3 metabolic equivalents, ≥2,225 counts per minute) recorded in average minutes/day.

Table 3. Proportion of subjects meeting MVPA recommendation and times (minutes/day) spent in health-enhancing PA intensities*
 Women (n = 196)Men (n = 59)All (n = 255)P
  • *

    Values are the percentage or mean ± SD for duration (minutes/day) spent at health-enhancing intensities of PA unless otherwise indicated. MVPA = combined moderate- and vigorous-intensity physical activity (≥2,225 counts per minute); PA = physical activity; CDC = Centers for Disease Control and Prevention; ACSM = American College of Sports Medicine; EPAC = Exercise and Physical Activity Conference; METS = metabolic equivalents.

  • Chi-square statistics, sex differences.

  • Independent t-test, sex differences.

Achieved CDC/ACSM recommendations, % (9, 10)2447300.037
Achieved EPAC recommendations, % (11)3354380.012
Moderate PA (3.0–6.0 METS), mean ± SD21.5 ± 15.830.5 ± 20.023.6 ± 17.2< 0.001
Vigorous PA (>6.0 METS), mean ± SD0.68 ± 3.41.86 ± 4.00.95 ± 3.50.024
MVPA (≥3.0 METS), mean ± SD22.2 ± 17.432.4 ± 22.124.54 ± 19.1< 0.001

Average daily weekday and weekend levels of PA are shown for all patients in Table 4. The average duration (minutes/day) spent in MVPA intensities for all patients was significantly (P < 0.001) higher for weekdays versus weekend days. The weekly pattern of average MVPA (minutes/day) is illustrated in Figure 2, depicting PA levels for each day of the week in men and women. Men spent significantly (P < 0.05) more average time (minutes/day) in all PA intensities on weekdays and weekend days as compared with women, except for time spent in VPA on the weekends, which did not differ between sexes.

Table 4. Weekday versus weekend average PA (minutes/day) at different intensities for all participants (n = 255)*
IntensityWeekdayWeekend
  • *

    Values are the mean ± SD. See Table 3 for definitions.

  • P < 0.001 significant difference, paired t-test.

Moderate PA (3.0–6.0 METS)24.7 ± 17.920.8 ± 22.0
Vigorous PA (>6.0 METS)1.0 ± 4.00.80 ± 3.2
MVPA (≥3.0 METS)25.8 ± 19.921.6 ± 23.6
Figure 2.

Average duration (minutes/day) spent in combined moderate- and vigorous-intensity physical activity (MVPA; ≥3 metabolic equivalents, ≥2,225 counts per minute) for female (n = 196) and male (n = 59) patients with early knee osteoarthritis. Diamonds = female patients; circles = male patients.

DISCUSSION

This study is unique in its use of accelerometers to objectively measure PA levels in knee OA patients. A critical finding was that most patients (70%) did not achieve the minimum level of health-enhancing PA for adults (30 minutes/day of MVPA) recommended by the CDC (9) and ACSM (10). A similarly high percentage of patients (62%) failed to meet EPAC recommendations (11) formulated specifically for knee OA patients. Significantly more men achieved all recommendations for PA (9–11) than women.

Despite the importance of PA for health and function of weight-bearing joints, historically PA was believed to increase the risk of knee OA because the disease was labeled as “wear and tear arthritis” (33). Regular PA is now recognized as a safe, multifaceted therapeutic treatment to improve many of the factors that lead to disability in patients with knee OA (34). PA also has the potential to improve muscle strength, reflex inhibition, proprioception, and range of motion in the knee and decrease the risk of excess weight gain (6–8). As an established component in weight loss and maintenance, the importance of PA is underscored by the findings of Coggon et al (5) suggesting that a large proportion of severe knee OA is attributable to obesity. Weight reduction efforts are believed to prevent anywhere from 25.1% to 48.3% of knee OA in women over the age of 50 who account for 91.3% of all female knee OA cases (3). Our finding that only 24% of women achieved recommended levels of PA suggests that the higher prevalence of OA incidence (3) and obesity-linked OA severity (5) in women may involve inadequate levels of MVPA. Indeed, we demonstrated a small but significant (P < 0.05) inverse correlation (r = −0.13) between BMI and MVPA in women. This finding is congruent with findings from a weight loss intervention study of knee OA subjects in which the combination of diet and aerobic exercise resulted in significant improvements in self-reported physical function, 6-minute walk distance, stair-climb time, and knee pain as compared with the control group (35).

Three prior clinical trials have used subjective methods to describe PA in persons with knee OA (14, 16, 36). However, these studies failed to assess the duration and intensity of the PA that knee OA participants achieved. Instead, participants recalled their activities in weekly telephone interviews, introducing imprecision and inaccuracy in PA measures due to reactivity, recall bias, and social desirability bias (17). To overcome these limitations, objective measures of PA are necessary to determine the duration and intensity of PA in this population. The use of accelerometers such as the MTI Actigraph provides an accurate and nonreactive method for measuring PA. These devices have been shown to be precise under a number of laboratory- and field-based conditions (25–29). However, accelerometers worn at the hip may underestimate energy expenditure during certain types of activities, such as biking, climbing stairs, and weight lifting, and provide no estimate of PA during water activities such as swimming because they cannot be worn (30).

Differences exist between validation studies in accelerometer-based cutoff points for various intensities of PA. However, the combination of cutoff points from both laboratory- and field-based studies ensures the most accurate cutoff points for application in the population of interest (30). We averaged the cutoff points reported by 5 validation studies (25–29) to reduce sampling error and used the resulting cutoffs to accurately discriminate between PA intensities.

The low proportion (30%) of knee OA patients meeting the CDC (9) and ACSM (10) PA recommendations is not in line with the 45.4% of US adults whose self-reports of PA reach the recommended levels (37). However, there are limitations to making comparisons between PA data assessed subjectively and by accelerometer because subjective methods ask for activity effort and duration, whereas accelerometers detect body movement over user-specific epochs (i.e., 1 minute).

This is the first study to objectively measure week-long PA in knee OA patients using accelerometers. Surprisingly, only a few studies (38, 39) have performed week-long PA assessments using the MTI Actigraph model 7164 accelerometer to determine levels of PA in large samples of any adult population. One study measured PA in 72 nonobese (BMI <30 kg/m2) adults (38) with a mean ± SD of 39.8 ± 19.5 minutes/day of MPA and 5.4 ± 5.1 minutes/day of VPA. Our results suggest that knee OA patients achieve lower levels of PA than nonobese adults. Nonobese men were significantly (P < 0.05) more active than nonobese women for all health-enhancing PA intensities (38), which is consistent with the sex differences observed in our knee OA patients.

A more recent study by Hagströmer et al (39) used the MTI Actigraph model 7164 accelerometer to measure PA in 1,114 Swedish adults (56% women). This study found that 52% of subjects achieved the CDC (9) and ACSM (10) PA recommendations encouraging accumulation of a minimum of 30 minutes/day of MVPA. This proportion was much higher than the proportion of knee OA patients in our study (30%) who achieved recommended PA levels. On average, Swedish adults exceeded our knee OA patients in MPA by ∼5 minutes/day. However, the Swedish adult and knee OA samples were similar in that they both achieved very low amounts of VPA. Swedish men were significantly (P < 0.05) more active than Swedish women for all health-enhancing PA intensities, which is also consistent with our knee OA patients. Overall, comparisons with previous studies (37–39) indicate that individuals with knee OA achieve lower levels of PA than nonclinical adult populations. Furthermore, women achieve significantly lower levels of PA than men when accelerometers are used to measure PA.

There are several limitations in the present study. One limitation is that accelerometers may not capture activities that do not involve ambulation as accurately as activities that do, and the selection of different cutoff points would obviously change the estimates of PA in various intensities. Thus far, there have been no accelerometer calibration studies performed specifically in knee OA patients; therefore, by necessity, we based cutoff points on the literature. As noted, we averaged cutoff points from laboratory- and field-based studies with the aim of deriving generalizable cutoff points. Future validation studies in obese populations are needed because excess adipose tissue, especially around the waist, might affect the validity of accelerometer outputs. Another limitation is that 6 or 7 contiguous days of accelerometer recording may not be representative of habitual levels of PA. However, seasonal differences in PA may not be as important a source of variation in the southwest US as in other regions. In addition, it is possible that accelerometers may have become activated while riding or driving motor vehicles, and we did not measure or control this potential source of variance in accelerometer-measured PA. Finally, accelerometers cannot be worn during water activities (i.e., swimming), and therefore PA is underestimated in individuals who regularly participate in these activities.

In summary, knee OA participants were compliant wearers of the MTI Actigraph accelerometer, making it a good tool for objective assessment of PA in this population. Only 30% of patients with early knee OA achieved the PA recommendations implemented by the CDC/ACSM (9, 10) and only 38% achieved the EPAC recommendations (11) for persons with arthritis. Significantly more men than women met the CDC/ACSM (9, 10) and EPAC (11) PA recommendations. Greater average PA may be a contributing factor to the decreased prevalence of knee OA observed in men compared to women (3). Both men and women with early knee OA achieved very low levels of VPA, which may have resulted from pain during high-intensity weight-bearing activities. In the future, it will be important to develop strategies to encourage knee OA patients to participate in greater levels of at least moderate-intensity PA. In particular, these strategies should target female knee OA patients who clearly fall short of the recommended levels of PA. Furthermore, both female and male knee OA patients could benefit from increased VPA, which may be more feasible if non–weight-bearing modes of activity are encouraged.

AUTHOR CONTRIBUTIONS

Mr. Farr had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study design. Going, Cornett.

Acquisition of data. Farr, Kasle, Cornett.

Analysis and interpretation of data. Farr, Going, Lohman, Rankin, Kasle, Cussler.

Manuscript preparation. Farr, Going, Lohman, Rankin, Kasle, Cussler.

Statistical analysis. Farr, Going, Lohman, Cussler.

Acknowledgements

We would like to thank the knee OA participants, the project coordinators for participant recruitment, and the members of the Knee Study team. The results of the present study do not constitute endorsement by any professional organizations.

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