Institute for Health Services Research, Feinberg School of Medicine, Northwestern University, 750 N Lake Shore Dr, Chicago, IL 60611
Email : firstname.lastname@example.org
To investigate the association between baseline physical activity and 1-year functional performance in adults with knee osteoarthritis (OA).
Prospective cohort study of knee OA development and progression with 1-year follow-up.
Osteoarthritis Initiative public data on adults with knee OA (n=2274; age, 45-79y) who participated in functional performance assessments (timed 20-m walk and chair stand test) at baseline and 1-year follow-up.
MAIN OUTCOME MEASURE:
A good 1-year performance outcome (separately defined for walk time and chair stand measures) was improvement from baseline quintile or maintenance in the best quintile.
Almost 2 in 5 persons with radiographic knee OA improved or maintained high performance at 1 year. Physical activity measured by the Physical Activity Scale for the Elderly (PASE) was significantly associated with good walk rate and chair stand outcomes (odds ratio per 40 units PASE [95% confidence interval]=1.13 [1.13, 1.17] and 1.10 [1.05, 1.15], respectively), as were participation in sports/recreational activities (1.45 [1.23, 1.71] and 1.29 [1.09, 1.51], respectively) and lifestyle activities (1.11 [1.06, 1.16] and 1.09 [1.04, 1.14], respectively). An independent protective relationship for these physical activity measures approached significance after adjusting for sociodemographic and health factors. Older adults reported the least baseline physical activity and least frequent good 1-year outcomes.
These findings support public health recommendations to be physically active in order to preserve function for persons with knee OA. Physical activity messages should specifically target older adults whose low activity levels may jeopardize their ability to maintain functional performance.
ARTHRITIS IS A COSTLY disease to society and to the person. Costs as a result of arthritis exceed $128 billion annually.1 More than 46 million Americans (more than 1 in every 5 adults) report arthritis and other rheumatic conditions, and 19 million (1 in every 11 adults) report arthritis-attributable activity limitations.2 Knee osteoarthritis is a highly prevalent condition in adults and is a leading cause of arthritis-related activity limitations.3, 4
While some reports examine risk factors predicting functional decline, the research on maintaining high physical function or improving physical function among adults in the general population is limited.5, 6, 7, 8, 9, 10 Among persons with knee osteoarthritis, such studies are particularly scarce. Physical activity is widely recognized to be beneficial to cardiovascular health in the general population and is an attractive mutable risk factor to improve health outcomes for persons with arthritis.11 Recently released updated physical activity guidelines include a section specifically for persons with osteoarthritis.12
This article examines factors that may preserve or improve function over time for persons with knee OA. A good outcome is defined as maintaining high function or improving functional performance over 1 year. Longitudinal data from the OAI that included 2274 persons with radiographic knee osteoarthritis at the baseline interview were used to answer the following research questions related to functional performance 1 year later:
What proportion demonstrated good 1-year outcomes (ie, improved performance by at least 1 quintile compared with baseline or maintenance in the highest performance quintile)?
Is there a type of physical activity that is particularly associated with good outcomes?
What are the significant sociodemographic and health determinants of good outcomes?
Data and Study Sample
This study analyzed public data from the OAI, a prospective study of the development and progression of knee OA in men and women age 45 to 79 years at enrollment. Annual OAI interviews began in 2004 and continue at 4 clinical sites: Baltimore, MD; Columbus, OH; Pittsburgh, PA; and Pawtucket, RI. The baseline OAI visit identified 2678 participants with radiographic knee OA (ie, radiographic evidence based on Kellgren-Lawrence grade of 2 or more calculated from separate scores for osteophytes and joint space narrowing in a knee) in 1 or both knees, of whom 1403 had knee symptoms (pain, aching, and/or stiffness on most days of a month during the past year) from the total OAI enrollment of 4796 persons. Excluded were persons with bilateral total knee replacement, planned knee replacement, inflammatory arthritis, contraindications to 3.0 tesla magnetic resonance imaging (men>130kg and women>114kg), comorbid conditions likely to interfere with participation, plans to relocate, and other clinical trial participation, and persons who were nonambulatory. To investigate factors related to good functional performance over 1 year, we merged public data from OAI baseline (OA1 public data version) and 1-year (V1.2.1) interviews. A total of 2274 persons with baseline radiographic knee OA who participated in functional performance assessments (timed 20-m walk and chair stand test) at both interviews composed the analysis sample. An additional 8 decedents, 163 not returning for the OAI 1-year evaluation, 69 with incomplete baseline data, and 164 persons with incomplete 1-year performance data were excluded from analysis.
Functional Performance Outcome From Baseline to 1 Year
Functional performance was assessed by a timed 20-m walk and a chair stand test. The timed 20-m walk is a standard outcome measure for osteoarthritis.13 Walk rates based on 2 trials measured in meters per minute from the entire baseline OAI radiographic knee OA cohort were classified by quintile from worst to best performance: first quintile, Q1 less than or equal to 67.4; second quintile, Q2 67.4 to 74.9; third quintile, Q3 74.9 to 80.9; fourth quintile, Q4 80.9 to 87.8; and fifth quintile, Q5 greater than 88. Walk rates from the baseline and 1-year evaluations were categorized into one of these quintiles.
Chair stand testing (time required for 5 repetitions to rise from a chair and sit down) depends heavily on knee function and assesses strength, balance, coordination, and flexibility. The OAI chair stand protocol standardizes the chair, its position, the stopwatch, floor surface footwear, arm position (crossed over the chest), and scripted instructions. Chair stand rates based on 2 trials measured in repetitions per minute from the entire baseline OAI radiographic knee OA cohort were classified by quintile from worst to best performance: first quintile, Q1 less than or equal to 21.0; second quintile, Q2 21.0 to 25.8; third quintile, Q3 25.8 to 30.0; fourth quintile, Q4 30.0 to 35.4; fifth quintile, Q5 greater than 35.4. Chair stand rates from the baseline and 1-year evaluations were categorized into one of these quintiles.
The baseline to 1-year functional experience was characterized separately for each performance test using a quintile grid shown in figure 1. This approach captures practical, meaningful changes in functional performance over time in osteoarthritis populations.14, 15, 16 Good outcomes based on improved/high performance are defined by transition to an improved functional performance quintile over 1 year (ie, moving from Q1 to Q4 at baseline to a better quintile at the 1-year evaluation) or maintaining function in the best performance quintile (ie, Q5 at both baseline and 1-year evaluations) as shown by the shaded portion of figure 1. Boxes that are not shaded represent a poor functional performance outcome.
Self-reported physical activity was measured using the PASE.17 The self-administered 26-question PASE assesses a broad spectrum of activities during the previous 7-day period.18 The calculated PASE score has demonstrated reliability and validity.19, 20 PASE scores increase with greater moderate intensity activity time. For the purpose of analysis, we divided the total PASE score into portions corresponding to questions that assess lifestyle activity (housework, home repair, gardening, yard work, paid or volunteer work-related activity), purposeful exercise/sports (light, moderate, strenuous sport/recreation; muscular strength and endurance), and walking outdoors (outside the home or in the yard for any reason).
Baseline sociodemographic factors included race, age, sex, marital status, and education. Individuals were classified as black, white, or other race based on self-report. Education was dichotomized as post–high school versus less education.
Knee health was assessed at baseline. The presence of knee symptoms was ascertained from a positive response to the question, “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?” Disease severity for each knee was based on a Kellgren-Lawrence grade calculated by the OAI from the baseline scores for osteophyte and joint space narrowing as grade 1 (doubtful), possible osteophyte of doubtful significance or isolated mild-moderate joint space narrowing; grade 2 (minimal), definite osteophytes and unimpaired joint space or isolated severe joint space narrowing; grade 3 (moderate), definite osteophytes and moderate diminution of joint space; and grade 4 (severe), definite osteophytes, joint space greatly impaired. Self-reported knee pain was measured by a 5-point Likert scale from the WOMAC (Likert version, 3.1) modified to ask about the right and left knee symptoms separately21 in the past 7 days. The WOMAC pain score range is 0 to 20; a higher number represents worse symptoms. Person-level scores were calculated from knee-specific scores (ie, disease severity grade and WOMAC pain score) using the maximum value of the 2 knees.
General health factors assessed included the self-report of hip pain, ankle pain, foot pain, current smoking, current alcohol consumption, comorbidity, high depressive symptoms, and BMI. The presence of comorbidity was ascertained from a Charlson index22 score greater than 0. Evidence of high depressive symptoms was based on a score greater than or equal to 16 from the full 20-item CESD scale.23 BMI was calculated from measured height and weight [weight (kg)/height (m)2].
The protocol for the OAI was approved by the institutional review boards at each of the participating sites.
Analyses were restricted to individuals who participated in both the baseline and 1-year OAI visits. Descriptive statistics characterized the baseline sample by physical activity (above or below median baseline PASE score). Logistic regression evaluated the relationship between baseline physical activity and 1-year performance outcomes; those results are reported as ORs. Physical activity score ORs are expressed per 40 units; this increment equals 0.5 SD in the PASE score to reflect a potentially meaningful change in physical activity as measured by PASE. Recognizing that systematic differences between persons included (n=2274) and excluded (n=404) from the analysis sample could influence our findings, we performed weighted analyses recommended by Hogan,24 Robins,25 and colleagues. Results and statistical significance were very similar for weighted and unweighted analyses. For simplicity, unweighted analyses were reported. All analyses were performed using SAS software version 9.2.
A total of 2274 persons with radiographic knee OA (age, 45–79y) participated in performance tests at the baseline and 1-year OAI evaluations. In this sample, 60% had bilateral involvement, and half had knee symptoms based on the report of pain, aching, or stiffness on most days in at least one month during the previous year. This analytic sample was predominantly white (81.6%) and female (56.8%), and the average baseline age was 62.5 years. The group of 404 persons with radiographic knee OA who did not return for the 1-year evaluation or had incomplete data were similar in age (mean, 62.0 vs 62.5y) but were more likely to be black (34% vs 17%) or female (67% vs 57%) than those included in the analysis sample.
Baseline physical activity measured by the total PASE score ranged from 0 to 465, with a mean ± SD of 157±79, indicating this radiographic knee OA cohort had substantial variability in physical activity. The total PASE was divided into lifestyle activity, purposeful exercise/sports, and walking outdoors which had mean ± SD values of 130±72, 12±21, and 14±17, respectively.
For descriptive purposes, this radiographic knee OA cohort was stratified by baseline levels of physical activity (table 1). Persons with baseline PASE scores above the median tended to be younger and male, reported more education, and were less likely to report comorbidity or prior knee injuries compared with their less active counterparts. There were no notable differences in disease severity or knee pain related to physical activity.
|Baseline Characteristics||Overall (n=2274)||PASE score<median (147) (n=1132)||PASE score≥median (147) (n=1142)|
|OAI radiograph OA sample||100||100||100|
|Education, post–high school||83.5||80.5||86.4|
|Knee OA severity|
|OA grade 2||31.5||30.7||32.3|
|OA grade 3||47.7||47.5||47.9|
|OA grade 4||20.8||21.8||19.8|
|Knee symptoms: report of pain, aching, or stiffness most days for at least one month during the past 12mo||50.0||47.7||52.4|
|Knee pain severity⁎||3.8±3.8||3.8±3.8||3.8±3.8|
|Prior knee injury||45.4||40.7||50.1|
|Any hip pain||55.2||56.4||54.0|
|Any ankle pain||10.7||11.2||10.2|
|Any foot pain||11.4||11.3||11.5|
|High depressive symptoms††||9.6||9.9||9.3|
|Alcohol use, 0 drinks/d||18.7||21.1||16.3|
NOTE. Values are percentages or mean ± SD.
The cross-sectional relationship between baseline physical activity levels and functional performance for this knee OA cohort is shown in figure 2. Average physical activity measured by the total PASE score increased with membership in higher (ie, better) performance quintiles for both walk rate and chair stand rate. Each PASE score bar is divided into those portions contributed by lifestyle activities, exercise/sports, and walking outdoors questions. It is notable that lifestyle activities contributed the largest portion to the total PASE score. The portion of the total PASE score contributed by walking outdoors is relatively constant across all performance quintiles. Increased physical activity among persons in high performing quintiles is largely a result of more participation in lifestyle and exercise/sports activities.
Analyses addressing the first research question examined the frequency of good functional performance outcomes. The average cohort performance was unchanged at baseline and 12 months (at both evaluations walk rate averaged 78m/min; chair stand rate averaged 28 repetitions/min). However, many individuals demonstrated improvements in performance or maintenance in the highest performance quintile (table 2). More than 1 out of 4 persons improved performance (ie, moved to a better quintile compared with baseline). Another 11% to 15% remained in the best performing quintile. Together, approximately 40% of this knee OA cohort had good (improved/high) outcomes after 1 year.
|Functional Performance Assessment||Good Outcome Based on Maintaining Top Quintile Performance Level||Good Outcome Based on Improved Performance by at Least 1 Quintile||Total Good Outcomes|
Analyses to address the second research question examined the relationship of the total PASE score and that of the partitioned PASE scores (exercise/sport, lifestyle activities, walking outdoors) with good functional performance outcomes (table 3). Good outcomes were significantly related to higher values of the total PASE score for walk rate (OR/40 units=1.13) and chair stand performance (OR/40 units=1.10). Separate analyses entered the partitioned PASE scores into logistic regression models in place of the total score. Lifestyle activity had a significant beneficial relationship to subsequent high/maintained performance (OR/40 units: walk rate=1.11; chair stand rate=1.09) as did purposeful exercise/sports (OR/40 units: walk rate=1.45; chair stand rate=1.29). Walking outdoors as ascertained by PASE was not significantly related to good outcomes (OR/40 units: walk rate=.99; chair stand=1.00). These analyses demonstrated positive relationships between baseline physical activity and good performance outcomes 1 year later; that relationship was strongest for exercise/sports, followed by lifestyle activities. Further analyses that controlled for sociodemographics and health factors demonstrated a positive but attenuated relationship between physical activity and good outcomes. In sensitivity analyses (not shown) that additionally controlled for 1-year changes from baseline in PASE and WOMAC pain; the OR of baseline PASE on good outcomes was identical to the table 4 results.
|Good Walk Rate Outcome Over 1 Year (n=2274)||Good Chair Stand Outcome Over 1 Year (n=2274)|
|Adjustment Factors||PASE Total Activity OR (per 40 Units††)||PASE Lifestyle Activity OR (per 40 Units)||PASE Exercise/Sports Activity OR (per 40 Units)||PASE Walking Activity OR (per 40 Units)||PASE Total Activity OR (per 40 Units)||PASE Lifestyle Activity OR (per 40 Units)||PASE Exercise/Sports Activity OR (per 40 Units)||PASE Walking Activity OR (per 40 Units)|
|Unadjusted||1.13 (1.08–1.17)⁎||1.11 (1.06–1.16)||1.45 (1.23–1.71)||0.99 (0.81–1.20)||1.10 (1.05–1.15)||1.09 (1.04–1.14)||1.29 (1.09–1.51)||1.00 (0.82–1.22)|
|Sociodemographic‡‡||1.05 (1.01–1.10)||1.04 (0.99–1.09)||1.27 (1.07–1.51)||1.03 (0.84–1.26)||1.04 (0.99–1.09)||1.03 (0.97–1.08)||1.15 (0.97–1.36)||1.05 (0.86–1.29)|
|Sociodemographic‡‡ + health factors§||1.05 (1.00–1.10)||1.04 (0.98–1.09)||1.22 (1.03–1.44)||1.00 (0.82–1.23)||1.04 (0.99–1.09)||1.03 (0.98–1.08)||1.12 (0.94–1.32)||1.09 (0.88–1.34)|
|Baseline Characteristics||OR for Good Outcome|
|20-m Walk Rate||Chair Stand Performance|
|OR (95% CI)||OR (95% CI)|
|Total PASE (per 40-unit increase)||1.05 (1.00–1.10)||1.04 (0.99–1.09)|
|Age per 5y||0.88 (0.83–0.93)||0.90 (0.85–0.95)|
|Black||1.14 (0.87–1.49)||0.92 (0.70–1.21)|
|Female||0.70 (0.58–0.84)||0.83 (0.69–1.01)|
|Married||1.32 (1.08–1.62)||0.99 (0.81–1.21)|
|13+||1.45 (1.13–1.86)||1.66 (1.28–2.16)|
|Knee OA severity|
|OA grade 2||Reference||Reference|
|OA grade 3||0.91 (0.75–1.11)||1.00 (0.82–1.22)|
|OA grade 4||0.79 (0.61–1.02)||0.94 (0.72–1.22)|
|Knee symptoms||0.95 (0.78–1.17)||1.10 (0.90–1.34)|
|Knee pain severity††||0.98 (0.95–1.00)||0.94 (0.91–0.97)|
|Prior knee injury||1.09 (0.91–1.31)||0.91 (0.76–1.09)|
|Any hip pain||0.92 (0.77–1.11)||0.91 (0.76–1.08)|
|Any ankle pain||1.16 (0.86–1.57)||0.80 (0.58–1.08)|
|Any foot pain||0.82 (0.62–1.10)||0.85 (0.63–1.14)|
|Charlson index>0||0.84 (0.68–1.03)||0.96 (0.78–1.18)|
|High depressive symptoms‡‡||0.89 (0.65–1.21)||0.91 (0.66–1.25)|
|BMI per unit||0.96 (0.94–0.98)||0.99 (0.97–1.01)|
|Current smoker||0.78 (0.53–1.15)||1.23 (0.84–1.80)|
|Uses alcohol||0.92 (0.73–1.16)||1.06 (0.84–1.34)|
Analyses to address the third research question investigated the relationship of baseline sociodemographic and health factors to subsequent improved/high performance controlling for physical activity (see table 4 for multiple logistic regression results). Sociodemographic factors that significantly increased the odds of good outcomes 1 year later adjusting for all factors were higher education (OR: walk rate=1.45; chair stand=1.66) and being married (OR: walk rate=1.32), while the odds of good outcomes were significantly decreased by older age (OR/5 years: walk rate=.88; chair stand=.90) and female sex (OR: walk rate=.70). Health factors that significantly decreased the adjusted odds of good outcomes were greater BMI (OR/unit: walk rate=.96) and greater WOMAC knee pain (OR/unit: chair stand=.94). It is notable that disease severity based on radiographic evidence was not associated with subsequent good performance outcomes.
The strong inverse relationship of older age to less frequent good outcomes and the attenuated relationship of physical activity with good outcomes after accounting for sociodemographic factors motivated further analyses. We examined the frequency of good outcomes in persons with high (above the median PASE score) versus low physical activity levels stratified on age group (45–54, 55–64, 65–75y). The stratified ORs of good walk rate outcomes for persons in the high versus lower physical activity groups ranged from 1.01 to 1.14, and the stratified odds of good chair stand outcomes ranged from 1.02 to 1.07 with adjustment for other sociodemographic and health factors. These analyses showed that within each age group, good outcomes were most frequent among those persons with higher levels of baseline physical activity compared with their less active counterparts.
Further analyses examined for the potential of an effect modification related to knee symptoms by adding an interaction term between knee symptoms and physical activity to our model. That interaction term was not significant, which indicates that these findings do not depend on the presence or absence of knee symptoms.
This article adds to the literature by examining factors that may preserve function over time for persons with knee OA. This study provides evidence of the positive benefit from physical activity in relation to good outcomes in functional performance over 1 year from a cohort of 2274 persons with radiographic knee OA. Almost 1 in 4 persons in this cohort improved functional performance at 1 year compared with baseline. Among persons performing in top quintiles at baseline on the timed 20-m walk or chair stand tests, over half maintained that high function 1 year later. Greater physical activity measured by the total PASE was significantly associated with improved/high performance over 1 year. Physical activity from lifestyle activities and exercise/sports had strong associations with subsequent improved/high performance.
Our findings that exercise/sports activities (those performed to achieve a training effect) are related to subsequent good performance outcomes are consistent with other studies. Randomized controlled trials demonstrate that exercise, muscle strengthening regimens, and aquatic therapy improve performance in walking, stair stepping, and getting in/out of a car.26, 27, 28, 29 However, the value of lifestyle activities are not addressed in those studies. Recent interest in lifestyle activity is spawned by poor adherence rates26, 30, 31, 32, 33 to structured exercise regimens, which reduces potential benefit.34, 35 Our findings suggest that lifestyle activities are related to subsequent improved/high performance among persons with knee OA.
Sociodemographic and health factors attenuated (often to nonsignificance) the positive relationship of physical activity to good outcomes in walk rate and chair stand performance. Although sociodemographic factors are recognized confounders, some health factors may be on the causal pathway. For example, physical activity is beneficial to reduce pain, depressive symptoms, and body weight. These health factors also influence function and may partially be a result of the beneficial effect of physical activity on function. In addition, more precise objective measures of physical activity may demonstrate stronger relationships in the presence of confounders.
Predictors of good functional outcomes in persons with radiographic knee OA are in harmony with other studies in the general population. Sociodemographic factors related to good functional outcomes in this osteoarthritis cohort were younger age, more education, and male sex, consistent with literature on older adults.8, 10, 36, 37 Health factors in the present study that were significantly detrimental to good outcomes were knee pain and greater BMI. Pain in the leg or knees is also negatively associated with improved function in the general population.6, 8 Greater BMI has been identified as both protective10 and detrimental5, 9 to good function in the general population. Our finding that the severity of knee OA did not affect subsequent improved/high performance is consistent with a knee pain study in older adults that concluded the severity of radiographic knee OA had limited value to predict which persons experienced progressive functional difficulties.16
The value of lifestyle activities and exercise/sports to improve or maintain high function is a valuable message for clinicians to convey to their patients with knee OA. Advice from a health care provider can positively influence the physical activity behavior of patients.38 However, advice to exercise may not resonate with some persons having knee OA because of a lack of resources or limited interest in exercise regimens. This study suggests that a broad spectrum of activities is beneficial. The positive relationship of lifestyle activities and exercise to sustained or improved functional performance strengthens the appeal of a clinician's advice.
Findings from this study have important public health implications. First, the substantial frequency of subsequent improved performance is an encouraging indication that functional decline is reversible in adults with knee OA. Second, results demonstrating a relationship of physical activity to good subsequent performance strengthen the arthritis public health message from the Centers for Disease Control and Prevention to keep moving to preserve function.39 Finally, this study indicates that an important target for this public health message is older adults with arthritis. Low levels of baseline physical activity were common among older persons with knee OA. In turn, older persons with lower levels of physical activity were less likely to have good outcomes in functional performance. Taken together, this information spotlights older adults with knee OA as an important target for the message to keep moving.
Some limitations related to these findings should be considered. Our physical activity measure is based on self-report. However, the validity of PASE is established for adults with knee pain and physical disability.20 While the PASE score was not designed to be partitioned, in general our findings related to exercise/sports, lifestyle, and walking portions of the PASE are consonant with other physical activity studies in adults, including persons with arthritis.14, 26, 27, 28, 29, 40 The weak relationship between walking outdoors and good performance outcomes may be related to the aggregation of various walking intensities within the single posed PASE walking question. We were unable to adjust for OA grade at follow-up. However, the proportion with worsening of grade by 1 year is likely to be extremely low.41 The generalizability of this cohort merits consideration. While all persons in this analysis sample had radiographic disease, some did not have knee symptoms. However, there was no evidence of an effect modification of these results based on knee symptoms. Also, causation cannot be inferred from these observational data.
Findings based on over 2200 persons with radiographic knee OA support a positive relationship of physical activity with improved/high performance over 1 year. These findings undergird clinical advice to patients to engage in physical activity including lifestyle activity and exercise to sustain or improve their functional performance. Public health physical activity messages should specifically target older adults whose low activity levels may jeopardize their functional performance, a critical component of independent community living.