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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Arthritis Rheum. Author manuscript; available in PMC Dec 4, 2008.
Published in final edited form as:
PMCID: PMC2595140

Physical Activity Levels in Early Knee Osteoarthritis Patients Measured by Accelerometry



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 CDC/ACSM and EPAC recommendations for PA.


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


Subjects wore accelerometers for 6.8 ± 0.3 days and 13.8 ± 2.2 hours per day (hr/day) and spent much more (P < 0.001) time in MPA (23.6 ± 17.2 min/day) compared to VPA (0.95 ± 3.5 min/day). Males spent significantly (P < 0.05) more time in all PA intensities than females. Only 30% of subjects achieved recommended PA levels. The proportion of males (47%) achieving the recommendation was significantly (P = 0.04) higher than females (24%).


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

Osteoarthritis (OA) is the most prevalent form of arthritis in the United States (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 being female, obesity is a major risk 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 U.S. The burden of OA is projected to afflict 40 million Americans by the year 2020 (3). Clearly OA has emerged as a major public health concern negatively impacting individual's 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 two mainstays of treatment. The health benefits of physical activity (PA) are undisputed (6). Regular PA is associated with lower prevalence of obesity and co-morbidities such as diabetes, cardiovascular disease, and hypertension (6-8). Accordingly, the U.S. 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 min/day of at least moderate-intensity (≥ 3 METs ) PA on at least 5 days of the week (9;10). Work group recommendations from the 2002 Exercise and Physical Activity Conference (EPAC), advise knee OA patients to accumulate 30 minutes of at least moderate-intensity (≥ 3 METs) PA 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 min/day of a combination of moderate- and vigorous-intensity PA, hereafter called MVPA (≥ 3 METs), is likely a beneficial and realistic PA goal for knee OA patients.

Published intervention studies have shown 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 this study was to objectively assess the level and pattern of health-enhancing MVPA in early knee OA patients and to determine the proportion of subjects 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


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 (SM) and combined exercise-SM on pain and functional ability of adults with early knee OA. The study was approved by the University of Arizona Institutional Review Board and conducted in accordance with the Helsinki Declaration. This report presents an analysis of PA levels in Knee Study participants prior to intervention. Participants gave written informed consent and self-reported demographic information.

Study Sample

Two hundred and ninety-four subjects were recruited from the Tucson, AZ area using mass mailings, media advertisements, periodic media coverage, and requests to local physicians for referral of their patients. Eligibility criteria were: age between 35 and 65 years to ensure an early knee OA sample, pain on most days (i.e, 4 or more days in a week) in one or both knees for at least four months during the previous year, duration of symptoms of less than 5 years (18;19), radiographic status of grade II OA (and no higher) in at least one knee, as defined by Kellgren and Lawrence (K/L) criteria (20) and disability due to knee OA by the WOMAC Index (21). All participants met American College of Rheumatology classification criteria for knee OA (22). Descriptions of knee OA severity, proportions of subjects with grade II unilateral (left or right) and grade II bilateral knee OA, and knee pain are presented in Table 1. After pre-baseline attrition (n = 20) and cases excluded from the analysis for failure to meet PA assessment inclusion criteria (i.e., an accelerometer recording of at least 8 hr/day for at least six continuous days; n = 19), baseline PA data were successfully obtained from 255 subjects (87% of the original sample), including 59 males and 196 females.

Table 1
Description of participants.


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, 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.


Knee pain was assessed using the 5-item pain subscale of the Western Ontario and McMaster Universities Osteoarthritis (WOMAC) Index, which has been validated in knee OA subjects (21). The WOMAC pain subscale uses visual analog scales (0-100) to elicit subjects' 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 Actigraph MTI (Manufacturing Technologies, Inc., Model 7164, Shalimar, FL). This uniaxial electromechanical accelerometer is designed to record vertical plane acceleration and deceleration at a frequency of 10 × s−1, and at a threshold of 0.033 × g. Additional specifications for this device are published elsewhere (24). A number of studies have demonstrated its durability, reliability, and validity in adults against 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 epidemiological studies.

For each assessment, the MTI Actigraph was initialized and set to record data in 60-second epochs prior to placement on the subject. Subjects were instructed to wear the accelerometer for seven contiguous days during all waking hours, except during water activities. The following measures were obtained: number of days measured, registered time (hr/day), and average duration (min/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. Moderate (3-6 METs) and vigorous (>6 METs) PA 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 light, moderate, and vigorous PA intensities were accelerometer recordings of 100 to 2,224, 2,225 to 5,950, and > 5,950 counts per minute, respectively.

Leisure time PA and exercise habits within the previous three 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, 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).

Statistical Analysis

All statistical analyses were performed using The Statistical Package for the Social Sciences for Windows, Version 15.0 (SPSS, Inc., Chicago, IL). Data are presented as means, standard deviations (SD), and percentages. Skewed variables, including registered time (hr/day), MPA (min/day), VPA (min/day), and MVPA (min/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 two independent means. Comparisons between average durations (min/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 subjects with grade II unilateral versus grade II bilateral knee OA that achieved CDC/ACSM (9;10) and EPAC (11) PA recommendations and also to compare the proportion of males and females that achieved PA recommendations (9-11).


Participants are described in Table 1. The final sample comprised 255 subjects (76% women, 54.6 ± 7.1 years, BMI 27.8 ± 4.3 kg/m2) with early knee OA. Males were significantly (P < 0.001) taller and heavier than females, however, age and BMI were similar in males and females. No significant (P = 0.94) differences were observed when comparing average right and left knee K/L scores in all subjects. Furthermore, ANCOVA showed no significant (P > 0.10) differences in time spent in any PA intensities for subjects with varying degrees of combined knee OA severity. Chi-square analysis indicated that the proportions of subjects with varying degree of combined knee OA severity that met the PA recommendation (9;10) were not significantly different (P > 0.10). Within the entire group of knee OA participants, 45% of subjects had bilateral grade II OA, while 27% of subjects had grade II OA in their left knee only, and 28% of subjects had grade II OA in their right knee only. No significant (P > 0.05) differences for time spent in any PA intensities were observed between subjects 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 subjects that achieved the PA recommendations (9-11). Sex differences were not observed between subjects with unilateral versus bilateral grade II OA, nor were they observed for right, left, or combined K/L scores. Knee pain according to the WOMAC index was 88.5 ± 72.3. No significant (P = 0.51) sex differences were observed in self-reported knee pain between males (84.0 ± 73.2) and females (89.8 ± 72.2). MVPA was not significantly (P = 0.97) different among subjects with moderate-intensity occupations (n = 33) versus subjects 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 numbers of days measured and daily habitual accelerometer wearing times were compared between men and women with an independent groups t-test. Subjects wore the accelerometer an average of 6.8 ± 0.3 days and 13.8 ± 2.2 hr/day. The average durations of wearing time (hr/day) for all subjects were almost identical for weekday and weekend days and there were no significant (P = 0.39) sex differences in weekday and weekend wearing time. Likewise, subjects with 6 versus 7 days of valid accelerometer data did not differ (all Ps > 0.05) in any of the measured variables.

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

Figure 1
Numbers of female (n = 196) and male (n = 59) knee OA subjects who achieve different amounts of combined moderate- and vigorous-intensity physical activity (MVPA) (> 3 METs) recorded in average min/day.

Average daily weekday and weekend levels of PA are shown for all subjects in Table 3. Average duration (min/day) spent in MVPA intensities for all subjects were significantly (P < 0.001) higher for weekday versus weekend days. The weekly pattern of average MVPA (min/day) is illustrated in Figure 2, depicting male and female PA levels for each day of the week. Males spent significantly (P < 0.05) more average time (min/day) in all PA intensities on weekday and weekend days as compared to females, except for time spent in VPA on the weekends, which did not differ between sexes.

Figure 2
Average duration (min/day) spent in combined moderate- and vigorous-intensity physical activity (MVPA) (≥ 3 METs) for female (n = 196) and male (n = 59) subjects with early knee OA.
Table 3
Proportion of subjects meeting MVPA recommendation and times (min/day) spent in health-enhancing PA intensities.


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

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, PA's importance is underscored by findings of Coggon et al. (5) suggesting that a large proportion of severe knee OA is attributable to obesity. Weight reduction efforts in women 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 showed a small but significant (P < 0.05) inverse correlation (r = − 0.13) for females between BMI and MVPA. This finding is congruent with findings from a weight loss intervention study in 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 to the control group (35).

Three prior clinical trials have used subjective methods to describe PA in knee OA subjects (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 non-reactive 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 five 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 subjects meeting the CDC (9) and ACSM (10) PA recommendation is not in line with the 45.4% of United States adults whose self reports of PA reach the recommended levels (37). However, there are limitations to making comparisons between accelerometer and subjectively assessed PA data because subjective methods ask for activity effort and duration, while accelerometers detect body movement over user specific epochs (i.e. one minute).

This is the first study to objectively measure weeklong PA in knee OA patients with accelerometers. Surprisingly, only a few studies (38;39) have performed weeklong 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 non-obese (BMI < 30) adults (38), who averaged 39.8 ± 19.5 min/day of MPA and 5.4 ± 5.1 min/day of VPA. Our results suggest that knee OA patients achieve lower levels of PA than non-obese adults. Non-obese males were significantly (P < 0.05) more active than non-obese females 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) published in 2007 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 recommendation encouraging accumulation of a minimum 30 min/day of MVPA. This proportion of subjects was much higher that the proportion (30%) of our knee OA subjects that achieved recommended PA levels. On average, Swedish adults exceeded our knee OA subjects in MPA by about 5 min/day. However, the Swedish adult and the knee OA samples were similar in that they both achieved very little 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 subjects. Overall, comparisons with previous studies (37-39) indicate that knee OA subjects achieve lower levels of PA than non-clinical adult populations. Furthermore, females achieve significantly lower levels of PA than males, when accelerometers are used to measure PA.

There are several limitations in the present study, such as 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 specifically in knee OA patients and 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 are needed in obese populations because excess adipose tissue, especially around the waist, might affect the validity of accelerometer outputs. Another limitation is that six or seven 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 United States as in other regions. Also, 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. Lastly, 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 early knee OA subjects achieved the PA recommendation implemented by the CDC/ACSM (9;10) and only 38% achieved the EPAC recommendation (11) for people with arthritis.. Significantly more men than women met 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 males compared to females (3). Both males and females 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 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.

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


We would like to thank the knee osteoarthritis 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.

Grant Support by NIH/NIAMS: R01-AR-047595.

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