Osteoarthritis (OA) is a major debilitating disease that affects >27 million persons in the US (1). This number is expected to increase due to the growing obesity epidemic and because the prevalence of arthritis is highest in older adults and greater numbers of adults are reaching older ages (2–4). Approximately 92% of persons with OA will see a physician at least once during the year (5). There were >11.1 million outpatient visits for OA and 632,000 joint replacements due to OA, with accompanying hospital costs of $22.6 billion, in 2004 (6). Knee OA is a leading cause of arthritis-related limitations, which in turn, are a major driver of excess medical costs due to arthritis (7, 8).
Encouraging physical activity is an important public health intervention for improving the health of persons with arthritis. Physical activity is so crucial to optimal health outcomes, there are now federal recommendations for the general public, including those with arthritis (www.health.gov/paguidelines) (9). Evidence from randomized controlled trials (RCTs) has demonstrated general benefits and disease-specific benefits of physical activity. RCTs in the general adult population show that physical activity reduces mortality, improves cardiovascular fitness, and improves sleep (10–12). Physical activity can reduce the risk of developing many chronic conditions and can improve quality of life (13, 14–16). Among persons with knee OA, RCTs have shown that physical activity programs are effective for reducing pain, improving physical performance, and preventing or delaying disability. In addition, aerobic exercise was shown to prevent the development of depression and reduce depressive symptoms (17–19). While those studies demonstrated that physical activity is beneficial for improving health outcomes in persons with knee OA, those RCTs only tested single doses of physical activity.
Maintaining physical function is critical to independent community living for persons with arthritis. Given that goal, there is a critical need to determine whether physical activity has a graded relationship to functional outcomes in persons with arthritis. There is a gap in the OA literature with regard to the relationship between different doses of physical activity and health outcomes. The research priorities listed by the Department of Health and Human Services in its physical activity guideline report include the need to identify physical activity doses that can result in beneficial disease-specific outcomes (20). The recent public health agenda for OA identified a lack of data regarding the amount and type of physical activity needed to improve and maintain function. The present cross-sectional and short-term prospective study can fill this gap (21). Understanding the nature of the relationship between physical activity and function has important public health implications related to developing physical activity guidelines and clinical implications for counseling knee OA patients, particularly those for whom a recommended physical activity threshold appears unreachable. The aim of this study was to test whether there is a graded relationship between physical activity and functional performance in persons with knee OA.
- Top of page
- PATIENTS AND METHODS
- AUTHOR CONTRIBUTIONS
Of 2,678 OAI participants with baseline radiographic knee OA, 89 were excluded from the analysis (14 due to incomplete baseline performance data and 75 due to incomplete baseline interview data), leaving 2,589 available for baseline analyses. As shown in Figure 1, 2,301 completed year 1 assessments, and 2,013 completed year 2 assessments.
A total of 2,589 patients (ages 45–79 years) with radiographic knee OA at the baseline OAI visit participated in functional performance tests. This analytical sample was predominantly white (79%) and female (58%) with an average age of 62.4 years. The 89 patients with radiographic knee OA who did not participate in performance tests and/or who had incomplete interview data were primarily female (63%), with an average baseline age of 62.1 years, and tended to be nonwhite (62%).
At baseline, the total PASE score ranged from 0 to 465, with a mean ± SD of 155 ± 80, which indicates that this group with radiographic knee OA had substantial variability in physical activity behavior. Baseline characteristics of this cohort stratified by physical activity level groups are presented in Table 1. Compared to patients in the more active groups, adults with radiographic knee OA in the lowest physical activity group (level 1) tended to be older, nonwhite, female, and less educated, and they more frequently reported comorbidities but reported fewer prior knee symptoms and injuries and less alcohol consumption. There were no notable differences related to disease severity across the physical activity groups.
Table 1. Baseline characteristics of the 2,589 patients with radiographic knee OA at OAI baseline, by physical activity quartile*
| ||Quartile 1 (PASE score ≤93) (n = 644)||Quartile 2 (PASE score 93–146) (n = 659)||Quartile 3 (PASE score 146–206) (n = 641)||Quartile 4 (PASE score >206) (n = 645)||P for trend†|
|Age, years|| || || || ||<0.001|
| 45−54||14.60||16.69||20.28||44.34|| |
| 55−64||27.48||28.22||34.17||40.78|| |
| 65 or older||57.92||55.08||45.55||14.88|| |
|Race|| || || || ||<0.001|
| White||72.67||83.16||81.44||80.00|| |
| African American||24.07||15.02||15.91||16.90|| |
| Other||3.26||1.82||2.65||3.10|| |
|Education level|| || || || ||<0.001|
| Less than high school||5.59||4.10||3.59||2.79|| |
| High school graduate||42.39||41.73||37.75||37.36|| |
| College||52.02||54.17||58.66||58.66|| |
|Knee OA severity|| || || || ||0.54|
| JSN grade 0||30.59||28.98||32.76||29.30|| |
| JSN grade 1||46.89||50.83||46.33||51.01|| |
| JSN grade 2||22.52||20.18||20.90||16.69|| |
|Mean ± SD knee pain severity score‡||4.26 ± 4.02||3.86 ± 3.78||3.87 ± 3.87||4.05 ± 3.88||0.22|
|Presence of knee symptoms§||50.16||50.68||50.23||56.28||0.04|
|Prior knee injury||41.30||43.10||46.80||58.60||<0.001|
|Any hip pain||57.14||57.06||56.01||54.73||0.34|
|Any ankle pain||12.89||11.99||10.76||9.77||0.06|
|Any foot pain||13.51||11.08||12.64||11.47||0.43|
|Body mass index, kg/m2|| || || || ||0.21|
| <25||16.46||17.60||20.28||15.81|| |
| 25–29.9||35.25||40.52||41.03||40.00|| |
| ≥30||48.29||41.88||38.69||44.19|| |
|Presence of comorbidity¶||30.43||29.59||26.21||20.47||<0.001|
|High depressive symptoms#||12.73||10.17||7.49||11.63||0.27|
|Alcohol use|| || || || ||<0.001|
| 0 drinks/day||24.69||20.03||17.00||16.74|| |
| <1 drink/day||37.89||35.66||38.07||36.12|| |
| ≥1 drinks/day||37.42||44.31||44.93||47.13|| |
|Mean ± SD PASE score||62.29 ± 21.83||120.47 ± 15.55||173.15 ± 17.08||265.47 ± 47.65||<0.001|
|Gait speed, mean ± SD feet/second||4.02 ± 0.73||4.20 ± 0.69||4.32 ± 0.62||4.51 ± 0.64||<0.001|
Baseline functional performance measured by gait speed ranged from 0.80 to 6.83 feet/second with a mean ± SD of 4.26 ± 0.69 feet/second. Gait speed had a moderate correlation with raw PASE scores (r = 0.27). A positive relationship between graded baseline physical activity level groups and baseline functional performance is shown graphically by cumulative gait speed frequency curves in Figure 2. For example, more than half (51%) of the patients in the lowest physical activity level group (quartile 1) did not meet the threshold of a 4 feet/second walking speed corresponding to the minimum walking speed required to safely cross a street for which many pedestrian traffic lights are timed (42). In comparison, only 37%, 29%, and 19% of the patients in the more active groups 2, 3, and 4, respectively, did not meet this threshold. Notably, the 4 lines in Figure 2 are ordered by physical activity levels and are distinctly separated in the middle section of the distribution, which is consistent with a positive graded relationship between physical activity levels and gait speed performance.
Figure 2. Cross-sectional cumulative percentage of patients in each physical activity quartile with the indicated gait speed (feet/second) at baseline. Patients in physical activity group 1 were the least active, and those in group 4 were the most active. A total of 2,589 patients were assessed.
Download figure to PowerPoint
Statistical analyses evaluating a cross-sectional graded relationship are summarized in Table 2. The average improvement in baseline functional performance when compared to the lowest group level increased with membership in higher baseline physical activity levels (mean baseline gait speed 4.0, 4.18, 4.29, and 4.49 feet/second, respectively; P for trend < 0.001). These trends remained significant in multivariate analyses that simultaneously controlled for baseline demographic factors (age, sex, race/ethnicity, marital status, and education level) and health factors (BMI, knee OA severity, knee pain, presence of knee symptoms, prior knee injury, hip pain, ankle pain, foot pain, current smoking, current alcohol consumption, comorbidity, and high depressive symptoms). To provide perspective on the magnitude of these group differences, if each patient in this cohort increased his or her gait speed by 0.2 feet/second, the proportion of this cohort that walked fast enough to safely cross a street would increase from 66% to 75%. Subgroup analyses demonstrated statistically significant trends for a graded relationship between baseline physical activity group levels and performance for both men and women as well as by age (45–64 years and 65–79 years).
Table 2. Average differences in baseline gait speed (feet/second), by physical activity quartile*
| ||Quartile 1||Quartile 2||Quartile 3||Quartile 4||P†|
|Entire cohort (n = 2,589)|| || || || || |
| Unadjusted difference||Reference||0.18 ± 0.04||0.29 ± 0.04||0.49 ± 0.04||<0.001|
| Adjusted difference‡||Reference||0.09 ± 0.03||0.16 ± 0.03||0.25 ± 0.04||<0.001|
|Women (n = 1,508)|| || || || || |
| Unadjusted difference||Reference||0.24 ± 0.05||0.33 ± 0.05||0.51 ± 0.05||<0.001|
| Adjusted difference§||Reference||0.14 ± 0.04||0.20 ± 0.04||0.32 ± 0.05||<0.001|
|Men (n = 1,081)|| || || || || |
| Unadjusted difference||Reference||0.03 ± 0.06||0.18 ± 0.06||0.33 ± 0.06||<0.001|
| Adjusted difference§||Reference||−0.04 ± 0.06||0.09 ± 0.06||0.16 ± 0.06||<0.001|
|Ages 45–64 years (n = 1,465)|| || || || || |
| Unadjusted difference||Reference||0.22 ± 0.06||0.24 ± 0.05||0.43 ± 0.05||<0.001|
| Adjusted difference¶||Reference||0.08 ± 0.05||0.10 ± 0.05||0.26 ± 0.05||<0.001|
|Ages 65–79 years (n = 1,124)|| || || || || |
| Unadjusted difference||Reference||0.14 ± 0.05||0.32 ± 0.05||0.39 ± 0.08||<0.001|
| Adjusted difference¶||Reference||0.09 ± 0.04||0.24 ± 0.05||0.30 ± 0.07||<0.001|
Prospective data were used to investigate whether a graded relationship between physical activity and functional performance would persist 1 year later. These analyses were restricted to 2,301 patients who were followed up for ≥1 year; of these, 2,013 patients contributed 2 years of followup. Over 1 year, 48% of the patients remained in the same PASE group, 23% improved, and 29% moved to a less active group. The average gait speed was fairly stable over time (year 1 mean ± SD 4.3 ± 0.7 feet/second, year 2 mean ± SD 4.3 ± 0.7 feet/second). The cumulative frequency performance curves (Figure 3) demonstrated a positive graded relationship between physical activity group levels and subsequent gait speed 1 year later. Similar to Figure 2, a graded relationship is graphically depicted by distinct curves, which are ordered by physical activity level groups.
Figure 3. Longitudinal cumulative percentage of patients in each physical activity quartile with the indicated gait speed (feet/second) 1 year after baseline assessment. Patients in physical activity group 1 were the least active, and those in group 4 were the most active. A total of 2,301 patients were assessed. Cumulative percentages are a weighted average of 2,301 observations of year 1 gait speed following baseline physical activity assessment and 2,013 observations of year 2 gait speed following year 1 physical activity assessment.
Download figure to PowerPoint
Statistical analyses that used the full 2 years of longitudinal followup information to evaluate a graded relationship are summarized in Table 3. The average improvement in subsequent functional performance when compared to the lowest group level increased with membership in higher physical activity level groups (mean gait speed after 1 year 4.0, 4.2, 4.3, and 4.5 feet/second, respectively; P for trend < 0.001). These trends persisted in multivariate analyses that simultaneously controlled for demographic and health factors. Further sensitivity analyses (not shown) that additionally adjusted for the baseline gait speed and that modeled gait speed at 2 years as a function of baseline physical activity level groups also confirmed a significant statistical trend. Similarly, subgroup analyses in both men and women as well as by age demonstrated statistically significant trends for a positive graded relationship between physical activity quartiles and subsequent gait speed 1 year later. Confirmatory analyses (not shown) using OAI objective performance data from timed chair stand tests showed the same trends. Taken together, these cross-sectional and longitudinal analyses consistently support a positive graded relationship between physical activity and functional performance.
Table 3. Average differences in gait speed (feet/second) after 1 year, by physical activity quartile*
| ||Quartile 1||Quartile 2||Quartile 3||Quartile 4||P†|
|Entire cohort (n = 2,301)|| || || || || |
| Unadjusted difference||Reference||0.20 ± 0.03||0.28 ± 0.03||0.47 ± 0.03||<0.001|
| Adjusted difference‡||Reference||0.11 ± 0.03||0.16 ± 0.03||0.22 ± 0.03||<0.001|
|Women (n = 1,314)|| || || || || |
| Unadjusted difference||Reference||0.22 ± 0.03||0.32 ± 0.04||0.45 ± 0.05||<0.001|
| Adjusted difference§||Reference||0.14 ± 0.03||0.19 ± 0.04||0.24 ± 0.04||<0.001|
|Men (n = 987)|| || || || || |
| Unadjusted difference||Reference||0.12 ± 0.05||0.18 ± 0.05||0.37 ± 0.05||<0.001|
| Adjusted difference§||Reference||0.04 ± 0.05||0.09 ± 0.05||0.18 ± 0.05||<0.001|
|Ages 45–64 years (n = 1,303)|| || || || || |
| Unadjusted difference||Reference||0.18 ± 0.05||0.16 ± 0.05||0.34 ± 0.05||<0.001|
| Adjusted difference¶||Reference||0.07 ± 0.04||0.07 ± 0.04||0.19 ± 0.04||<0.001|
|Ages 65–79 years (n = 998)|| || || || || |
| Unadjusted difference||Reference||0.21 ± 0.04||0.35 ± 0.05||0.43 ± 0.06||<0.001|
| Adjusted difference¶||Reference||0.15 ± 0.04||0.27 ± 0.04||0.36 ± 0.06||<0.001|
- Top of page
- PATIENTS AND METHODS
- AUTHOR CONTRIBUTIONS
These prospective data from a cohort of adults with confirmed radiographic knee OA indicate that greater levels of physical activity are significantly associated with higher functional performance objectively measured by gait speed. Moreover, we found a graded relationship between physical activity and functional performance, which persisted after controlling for demographic and health factors. These findings add to the literature on the beneficial relationship of physical activity to function by systematically demonstrating that higher performance was associated with greater levels of physical activity. These findings also provide encouragement for persons with arthritis who do not attain recommended physical activity levels; there was evidence of potential benefit of greater physical activity regardless of the level achieved. For example, even among people with low levels of physical activity (i.e., below the median), patients in the level 2 physical activity group had better function than patients in the bottom level 1 group (73% versus 41% with a gait speed faster than the minimum 4 feet/second walking speed needed to safely cross a street).
Strengths of the study included prospective data collection, the large sample size, the objective assessment of performance, radiographic verification of knee OA, the detailed systematic assessment of physical activity based on a broad spectrum of activities, and the age and sex diversity of this OA cohort. The significant graded relationship observed between the physical activity levels and the higher levels of objective performance, as well as the consistency of the findings across strata of age and sex, both cross-sectionally and longitudinally, lend further credence to the notion of an underlying dose-response relationship between physical activity and function for adults with arthritis. Other strengths of the study include detailed information about potential confounding variables.
The literature contains compelling evidence of beneficial relationships between physical activity and cardiovascular outcomes in adults. Early evidence of a graded relationship between greater levels of physical activity and a lower risk of cardiovascular disease and premature mortality was an important factor that motivated the early 1995 federal physical activity recommendations (15, 43). A large body of evidence obtained since 1995 affirms that greater levels of physical activity are related to a reduced risk of cardiovascular disease and early mortality (38–44).
Health, however, is multifaceted, and physical activity relationships can differ across health outcomes. A benchmark RCT demonstrated a dose-response relationship between greater levels of physical activity and aerobic fitness, quantified as peak absolute oxygen consumption, among 464 sedentary, postmenopausal overweight or obese women (10). However, that same study found no relationship between physical activity and physiologic parameters related to changes in blood pressure, lipid profile, or weight loss. A threshold relationship was described between health-related quality of life (HRQOL) and physical activity using epidemiologic data on >175,000 adults who participated in the Behavioral Risk Factor Surveillance System. Although persons who participated in some regular physical activity demonstrated better HRQOL, based on fewer reported unhealthy (physical or mental) days compared to persons who were essentially inactive (beyond a relatively low threshold), higher frequency or duration of activity was not associated with greater HRQOL benefit (45). Clearly, the relationship between physical activity and health can vary depending on the outcome of interest.
A prime objective for many older adults is to maintain function, which is basic to independent living (46). It is well established that physical activity improves objective functional performance in the general population and among older adults (16, 47). Similarly, for patients with chronic diseases such as arthritis, physical activity has a beneficial effect on function (48). For patients with knee OA, randomized clinical trials of exercise programs were shown to improve timed performance tests including chair stands, walking, and stair climbing (17, 49). However, these clinical trial data do not allow us to determine if there is a threshold of physical activity required to attain functional benefits or if the benefits increase with greater accumulation of physical activity irrespective of baseline physical activity levels. Our epidemiologic findings in the knee OA cohort in the present study support a graded response between physical activity and functional performance that is consistent with a dose-response relationship.
There are limitations to acknowledge related to these findings. Our physical activity measure is based on self report, which makes it difficult to determine the actual amount of physical activity performed by subjects. Our physical activity measure assessed by self report using the PASE does not directly translate to a measure that can inform physical activity guideline attainment (e.g., minutes per week of physical activity). The mean reported PASE scores of the OAI participants are higher than those in the studies used to develop the PASE, which likely reflects the younger ages of subjects included in the OAI cohort (ages 45–70) than in the original studies (ages 55 and older). However, the validity of the PASE is established for adults with knee pain and physical disability as a reasonable tool to discriminate those who are more physically active from those who are not (29). Causality cannot be inferred from these observational data; the establishment of a dose-response relationship requires a controlled clinical trial. Also, the generalizability of this cohort merits consideration. It is recognized the OAI is not a probability sample, and its participants met many exclusion criteria. However, OAI participants are recruited from multiple geographic sites using recruitment targets balanced for age and sex groups and represent a broad spectrum of radiographic knee OA.
In conclusion, these prospective data from a large study of a diverse cohort of adults with knee OA showed a strong relationship between greater levels of physical activity and better functional performance. A graded relationship was observed in both men and women and across age groups. These findings lend support to current federal guidelines for patients with arthritis that encourage physical activity goals. These findings are also informative for patients with knee OA who perceive that these goals are not attainable, possibly due to pain or stiffness. There may be a benefit to increasing physical activity even if the recommended levels are not achieved.
- Top of page
- PATIENTS AND METHODS
- AUTHOR CONTRIBUTIONS
All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published. Dr. Dunlop 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 conception and design. Dunlop, Song, Semanik, Chang.
Acquisition of data. Dunlop, Song, Semanik.
Analysis and interpretation of data. Dunlop, Song, Sharma, Chang.