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Abstract

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

Objective

Physical activity improves function in adults with arthritis, but it is unknown if there is a graded relationship between physical activity and functional benefit. This study was undertaken to examine the cross-sectional and longitudinal relationship between self-reported physical activity and observed functional performance in adults with knee osteoarthritis (OA).

Methods

The Osteoarthritis Initiative cohort included 2,589 patients with knee OA (2,301 with longitudinal followup data) who were ages 45–79 years at baseline. Prospective annual functional performance was assessed for 2 years using timed 20-meter walk tests. We used linear regression to estimate differences across physical activity quartiles in subsequent function (baseline and 1-year activity predicts 1-year and 2-year function, respectively) adjusted for demographic factors (age, sex, race/ethnicity, education level, and marital status) and health factors (OA severity, knee symptoms, knee pain, knee injury, body mass index, comorbidity, depression, smoking, alcohol use, and other joint pain).

Results

Increasing physical activity levels had a significant graded relationship to functional performance. Adults in physical activity quartile groups from least active to most active had an average gait speed of 4.0, 4.2, 4.3, and 4.5 feet/second, respectively, at baseline (P for trend < 0.001) and 4.0, 4.2, 4.3, and 4.5 feet/second, respectively, after 1 year (P for trend < 0.001); increasing trends remained significant after adjusting for covariates. Findings were similar within sex and age groups.

Conclusion

These prospective data indicate a consistent graded relationship between physical activity level and better performance in adults with knee OA. These findings support guidelines that encourage patients with arthritis who cannot attain minimum recommended physical activity to be as active as possible.

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.

PATIENTS AND METHODS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

Data and study population.

This study analyzed public data from the Osteoarthritis Initiative (OAI), a prospective natural history study investigating the development and progression of knee OA in men and women ages 45–79 years at enrollment. Annual OAI interviews began in 2004 at 4 clinical sites, Baltimore, MD; Columbus, OH; Pittsburgh, PA, and Pawtucket, RI, and are currently ongoing. The first 2 years of assessments have been completed, and those data have been publicly released (22). Institutional review board approval was obtained at the participating sites, and written informed consent was obtained from each participating subject.

At the baseline OAI visit, 2,678 participants with radiographic knee OA (i.e., a Kellgren/Lawrence grade of ≥2 [23] in 1 or both knees calculated from separate scores for osteophytes and joint space narrowing) were identified from the total OAI enrollment of 4,796 patients. Patients were excluded if they had rheumatoid arthritis or inflammatory arthritis; had severe joint space narrowing in both knees on the baseline knee radiograph or unilateral total knee replacement and severe joint space narrowing in the other knee; had undergone bilateral total knee replacement or planned to undergo bilateral knee replacement in the next 3 years; were unable to undergo a 3.0T magnetic resonance imaging examination of the knee because of contraindications (including the presence of a pacemaker, artificial valve, aneurysm clip or shunt, stent, implanted device, or ocular metallic fragment) or inability to fit into the scanner or the knee coil (including men who weighed >285 pounds and women who weighed >250 pounds); had a positive pregnancy test; were unable to provide a blood sample for any reason; used ambulatory aids other than a single straight cane for >50% of the time in ambulation; had comorbid conditions that might interfere with the ability to participate in a 4-year study; or were currently participating in a randomized double-blind trial.

Assessment of functional performance.

Functional performance was assessed by a timed 20-meter walk test at the baseline, year 1, and year 2 evaluations. The timed 20-meter walk is used in many epidemiologic studies and is a standard outcome measure for OA (24, 25). The test was performed by certified OAI site assessors according to a standard protocol common to all sites that detailed the course setup, measurement procedures, and scripted instructions. A calculated gait speed measured in feet/second was based on the average speed over two 20-meter trials.

Assessment of physical activity.

Self-reported physical activity was measured using the Physical Activity Scale for the Elderly (PASE) (26). The self-administered 26-question PASE systematically assesses a broad spectrum of activities during the previous 7-day period, including lifestyle activity (housework, home repair, gardening, yard work, and paid or volunteer work-related activity), purposeful exercise/sports (light, moderate, or strenuous sport/recreation and muscular strength and endurance), and walking outdoors (27). Questionnaire responses are summarized into a continuous PASE score in which higher scores represent higher levels of activity. The OAI public data set includes the calculated PASE score based on the Washburn algorithm (28). The PASE score has been validated based on a variety of objective measures (26, 27, 29), including objective accelerometer measurement in community-dwelling older adults and in chronic disease populations (30, 31). PASE scores from the baseline OAI radiographic knee OA cohort were used to classify patients into the following physical activity quartiles from lowest to highest activity level: quartile 1 (PASE score ≤93), quartile 2 (PASE score 94–146), quartile 3 (PASE score 147–206), and quartile 4 (PASE score >206). For the purpose of analysis, PASE scores from the baseline and year 1 evaluations for each person in the analysis sample were categorized into one of these physical activity level groups.

Covariates.

All covariates were assessed at baseline and annual visits except where noted otherwise. Baseline demographic factors included race/ethnicity, age, sex, marital status, and education level. Individuals were classified as African American, white, or other racial/ethnic group based on self report. Education level was dichotomized as post high school versus less education.

Knee health was assessed radiographically and by self report. All radiographs were acquired using a fixed flexion knee radiography protocol (32), including bilateral, standing, and posteroanterior knee radiographs with knees flexed to 20–30° and feet internally rotated 10° using a Plexiglas positioning frame (SynaFlex). Right and left knees were imaged together on 14 × 17–inch film using a focus-to-film distance of 72 inches (33). To assess OA disease severity within each tibiofemoral compartment, joint space narrowing was graded in the medial and lateral compartments separately using an adaptation of the OA Research Society International (OARSI) atlas approach (34), in which 0 = none (OARSI grade 0), 1 = narrowed (OARSI grade 1 or 2), and 2 = severely narrowed (OARSI grade 3). For the purposes of analysis, disease severity was based on the maximum (worst) joint space grade from the 2 knees.

Self-reported knee pain was measured using a 5-point Likert scale from the Western Ontario and McMaster Universities OA Index (WOMAC; Likert version 3.1) modified to ask about symptoms during the last 7 days in the right and left knee separately (35). The WOMAC pain score range was 0–20; a higher number represents worse symptoms. For analysis, person-level scores were calculated from knee-specific scores (i.e., WOMAC pain score) using the maximum value of the 2 knees. The presence of knee symptoms was ascertained from a positive response to “During the past 12 months, have you had pain, aching, or stiffness in or around either knee on most days for at least one month?” A history of knee injury was also ascertained (i.e., an injury “… so bad that it was difficult to walk for at least one week”).

General health parameters included hip pain, ankle pain, foot pain, current smoking (baseline), current alcohol consumption (baseline), comorbidity (baseline), depressive symptoms, and body mass index (BMI). Comorbidity was ascertained using the Charlson index (36), and its presence was defined as a score >0. Patients were considered to have evidence of a high level of depressive symptoms if they had a score of ≥16 on the full 20-item Center for Epidemiologic Studies Depression Scale (37). BMI was calculated from measured height and weight (weight [kg]/height [m2]).

Statistical analysis.

Baseline analyses included the entire analysis sample (n = 2,589). Longitudinal analyses were restricted to individuals who also participated in the year 1 (n = 2,301) and/or year 2 (n = 2,013) OAI evaluations. Univariate analyses of baseline trend effects were evaluated by a Mantel-Haenszel test for ordinal categories; a chi-square test for overall differences was applied to nominal variables, and analysis of variance was applied to continuous variables. Standard epidemiologic methods were used to investigate a graded response between a risk factor (physical activity) and the outcome (gait speed) based on risk factor quartile groups (38). Physical activity groups based on quartiles represent a parsimonious balance between the number of risk factor levels examined, interpretability, and sufficient sample size in each group. Descriptive statistics characterized the analysis sample by physical activity group membership. Cumulative frequency curves of functional performance displayed the proportion in each physical activity group with performance below the specific value on the horizontal axis. A graded response would be visually demonstrated by nonoverlapping cumulative frequency curves. Multiple linear regression was used to statistically evaluate the baseline relationship between greater physical activity group levels and performance outcomes. A graded relationship was evaluated by a linear trend test across physical activity group levels.

Longitudinal analyses evaluated the relationship between physical activity groups and subsequent 1-year functional performance using information from the full 2-year followup. For this purpose, we simultaneously modeled year 1 performance (n = 2,301 patients representing 89% of the baseline cohort) as a function of baseline physical activity level groups and covariates, and year 2 performance (n = 2,013 patients, representing 87% of the year 1 cohort) as a function of year 1 physical activity level groups and covariates. (If a covariate was not reassessed at year 1, the baseline value was used.) Graphical (unadjusted) cumulative frequency curves display a weighted average of the 1-year followup subsequent to the baseline and year 1 visits. Multiple linear regression evaluated the relationship between physical activity group levels and subsequent 1-year functional performance using generalized estimating equations to validly account for potentially correlated observations from the same individual. Recognizing that systematic differences between persons included and excluded from the analysis sample could influence our findings, we performed weighted analyses recommended by Hogan et al (39) and Robins et al (40). Results and statistical significance were very similar for weighted and unweighted analyses. For simplicity, unweighted analyses were reported. Analyses were performed using SAS software, version 9.2 (41).

RESULTS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

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.

thumbnail image

Figure 1. Disposition of the patients with radiographic knee osteoarthritis (OA).

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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
  • *

    Except where indicated otherwise, values are the percent of patients. OA = osteoarthritis; OAI = Osteoarthritis Initiative; PASE = Physical Activity Scale for the Elderly; JSN = joint space narrowing.

  • Determined by Mantel-Haenszel chi-square test for trend (with 1 df), except for comparisons of race, sex, and marital status, which were determined by chi-square test for overall differences, and knee pain severity, which was determined by analysis of variance.

  • Western Ontario and McMaster Universities OA Index pain score in the more symptomatic (painful) knee.

  • §

    Presence of pain, aching, or stiffness on most days in at least 1 month during the past 12 months.

  • Charlson index score >0.

  • #

    Center for Epidemiologic Studies Depression Scale score ≥16.

Age, years    <0.001
 45−5414.6016.6920.2844.34 
 55−6427.4828.2234.1740.78 
 65 or older57.9255.0845.5514.88 
Race    <0.001
 White72.6783.1681.4480.00 
 African American24.0715.0215.9116.90 
 Other3.261.822.653.10 
Education level    <0.001
 Less than high school5.594.103.592.79 
 High school graduate42.3941.7337.7537.36 
 College52.0254.1758.6658.66 
Female67.2460.5560.2244.96<0.001
Married57.7669.3568.4968.99<0.001
Knee OA severity    0.54
 JSN grade 030.5928.9832.7629.30 
 JSN grade 146.8950.8346.3351.01 
 JSN grade 222.5220.1820.9016.69 
Mean ± SD knee pain severity score4.26 ± 4.023.86 ± 3.783.87 ± 3.874.05 ± 3.880.22
Presence of knee symptoms§50.1650.6850.2356.280.04
Prior knee injury41.3043.1046.8058.60<0.001
Any hip pain57.1457.0656.0154.730.34
Any ankle pain12.8911.9910.769.770.06
Any foot pain13.5111.0812.6411.470.43
Body mass index, kg/m2    0.21
 <2516.4617.6020.2815.81 
 25–29.935.2540.5241.0340.00 
 ≥3048.2941.8838.6944.19 
Presence of comorbidity30.4329.5926.2120.47<0.001
High depressive symptoms#12.7310.177.4911.630.27
Current smoker6.524.706.087.290.38
Alcohol use    <0.001
 0 drinks/day24.6920.0317.0016.74 
 <1 drink/day37.8935.6638.0736.12 
 ≥1 drinks/day37.4244.3144.9347.13 
Mean ± SD PASE score62.29 ± 21.83120.47 ± 15.55173.15 ± 17.08265.47 ± 47.65<0.001
Gait speed, mean ± SD feet/second4.02 ± 0.734.20 ± 0.694.32 ± 0.624.51 ± 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.

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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.

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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 1Quartile 2Quartile 3Quartile 4P
  • *

    Values are the mean ± SEM difference in average gait speed compared to the lowest physical activity level quartile (quartile 1), as determined by multiple linear regression.

  • By linear trend test.

  • Adjusted for age, sex, race/ethnicity, marital status, education level, knee osteoarthritis (OA) severity, knee pain, knee symptoms, knee injury, hip pain, ankle pain, foot pain, current smoking, current alcohol consumption, comorbidity, high depressive symptoms, and body mass index (BMI).

  • §

    Adjusted for age, race/ethnicity, marital status, education level, knee OA severity, knee pain, knee symptoms, knee injury, hip pain, ankle pain, foot pain, current smoking, current alcohol consumption, comorbidity, high depressive symptoms, and BMI.

  • Adjusted for sex, race/ethnicity, marital status, education level, knee OA severity, knee pain, knee symptoms, knee injury, hip pain, ankle pain, foot pain, current smoking, current alcohol consumption, comorbidity, high depressive symptoms, and BMI.

Entire cohort (n = 2,589)     
 Unadjusted differenceReference0.18 ± 0.040.29 ± 0.040.49 ± 0.04<0.001
 Adjusted differenceReference0.09 ± 0.030.16 ± 0.030.25 ± 0.04<0.001
Women (n = 1,508)     
 Unadjusted differenceReference0.24 ± 0.050.33 ± 0.050.51 ± 0.05<0.001
 Adjusted difference§Reference0.14 ± 0.040.20 ± 0.040.32 ± 0.05<0.001
Men (n = 1,081)     
 Unadjusted differenceReference0.03 ± 0.060.18 ± 0.060.33 ± 0.06<0.001
 Adjusted difference§Reference−0.04 ± 0.060.09 ± 0.060.16 ± 0.06<0.001
Ages 45–64 years (n = 1,465)     
 Unadjusted differenceReference0.22 ± 0.060.24 ± 0.050.43 ± 0.05<0.001
 Adjusted differenceReference0.08 ± 0.050.10 ± 0.050.26 ± 0.05<0.001
Ages 65–79 years (n = 1,124)     
 Unadjusted differenceReference0.14 ± 0.050.32 ± 0.050.39 ± 0.08<0.001
 Adjusted differenceReference0.09 ± 0.040.24 ± 0.050.30 ± 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.

thumbnail image

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.

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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 1Quartile 2Quartile 3Quartile 4P
  • *

    Values are the mean ± SEM difference in average gait speed compared to the lowest physical activity quartile (quartile 1). Multiple regression using generalized estimating equations was performed using performance information from all 2-year followup data (available on 2,301 individuals) by modeling year 1 gait speed (2,301 observations) from baseline physical activity quartile groups and covariate values and year 2 gait speed (2,013 observations) from year 1 physical activity quartile groups and covariate values.

  • By linear trend test.

  • Adjusted for age, sex, race/ethnicity, marital status, education level, knee osteoarthritis (OA) severity, knee pain, knee symptoms, knee injury, hip pain, ankle pain, foot pain, current smoking, current alcohol consumption, comorbidity, high depressive symptoms, and body mass index (BMI).

  • §

    Adjusted for baseline age, race/ethnicity, marital status, education level, knee OA severity, knee pain, knee symptoms, knee injury, hip pain, ankle pain, foot pain, current smoking, current alcohol consumption, comorbidity, high depressive symptoms, and BMI.

  • Adjusted for sex, race/ethnicity, marital status, education level, knee OA severity, knee pain, knee symptoms, knee injury, hip pain, ankle pain, foot pain, current smoking, current alcohol consumption, comorbidity, high depressive symptoms, and BMI.

Entire cohort (n = 2,301)     
 Unadjusted differenceReference0.20 ± 0.030.28 ± 0.030.47 ± 0.03<0.001
 Adjusted differenceReference0.11 ± 0.030.16 ± 0.030.22 ± 0.03<0.001
Women (n = 1,314)     
 Unadjusted differenceReference0.22 ± 0.030.32 ± 0.040.45 ± 0.05<0.001
 Adjusted difference§Reference0.14 ± 0.030.19 ± 0.040.24 ± 0.04<0.001
Men (n = 987)     
 Unadjusted differenceReference0.12 ± 0.050.18 ± 0.050.37 ± 0.05<0.001
 Adjusted difference§Reference0.04 ± 0.050.09 ± 0.050.18 ± 0.05<0.001
Ages 45–64 years (n = 1,303)     
 Unadjusted differenceReference0.18 ± 0.050.16 ± 0.050.34 ± 0.05<0.001
 Adjusted differenceReference0.07 ± 0.040.07 ± 0.040.19 ± 0.04<0.001
Ages 65–79 years (n = 998)     
 Unadjusted differenceReference0.21 ± 0.040.35 ± 0.050.43 ± 0.06<0.001
 Adjusted differenceReference0.15 ± 0.040.27 ± 0.040.36 ± 0.06<0.001

DISCUSSION

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

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.

AUTHOR CONTRIBUTIONS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

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.

Acknowledgements

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

We gratefully acknowledge Leilani Lacson for diligent research support.

REFERENCES

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES
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