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 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.

Fig 1

Good and poor functional performance outcomes from baseline to 1 year. *Good outcome: improved performance from Q1 to Q4 at baseline to higher quintile at 1-year evaluation or maintained performance in best Q5 quintile at both baseline and 1-year evaluation. ^†Walk rate quintiles based on all n=2664 baseline OAI participants with radiographic knee OA evaluated by timed 20-m walk test. ^‡Chair stand quintiles based on all n=2667 baseline OAI participants with radiographic knee OA evaluated by chair stand test.

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.

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.

Table 1Baseline Characteristics of Persons (n=2274) With Radiographic Knee OA Participating in OAI Baseline and 1-Year Evaluations

Baseline Characteristics	Overall (n=2274)	PASE score<median (147) (n=1132)	PASE score≥median (147) (n=1142)
OAI radiograph OA sample	100	100	100
Sociodemographics
Age (y)	62.5±9.1	65.1±8.9	59.9±8.5
Race
White	81.2	80.4	82.1
Black	16.6	17.6	15.6
Other	2.2	2.0	2.4
Sex, female	57.0	62.5	51.4
Married, yes	66.9	64.3	69.5
Education, post–high school	83.5	80.5	86.4
Health factors
Knee health
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
General health
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
Charlson index>0	25.6	28.6	22.7
High depressive symptoms††	9.6	9.9	9.3
BMI	29.4±4.8	29.6±4.9	29.3±4.7
Current smoker	5.6	5.1	6.1
Alcohol use, 0 drinks/d	18.7	21.1	16.3
<1 drinks/d	36.9	37.2	36.7
1+ drinks/d	44.4	41.7	47.0

NOTE. Values are percentages or mean ± SD.

WOMAC pain score in the more symptomatic (painful) knee.

†CESD>16.

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.

Fig 2

Baseline relationship between physical activity and function (n=2274).

 

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.

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.

Table 3Baseline Physical Activity ORs in Relation to Good Walk Rate Outcomes and Good Chair Stand Performance Outcomes Over 1 Year^⁎

	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)

Boldface values indicate statistical significance based on 95% OR CIs that exclude 1.

†An OR per 40 units is given to reflect a potentially meaningful change of 0.5 SDs in the PASE score.

‡Sociodemographic factors: race, age, sex, marital status, and education.

§Health factors: knee OA severity, knee symptoms, knee pain severity, hip pain, ankle pain, foot pain, presence of comorbidity, presence of high depressive symptoms, BMI, current smoking, and current alcohol consumption.

Table 4Multivariate ORs for Good Outcomes in Walk Rate and Chair Stand Performance From Baseline to 1 Year (n=2274)^⁎

Baseline Characteristics	OR for Good Outcome
	20-m Walk Rate	Chair Stand Performance
	OR (95% CI)	OR (95% CI)
Physical activity
Total PASE (per 40-unit increase)	1.05 (1.00–1.10)	1.04 (0.99–1.09)
Sociodemographics
Age per 5y	0.88 (0.83–0.93)	0.90 (0.85–0.95)
Race
White/other	Reference	Reference
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)
Education (y)
0-12	Reference	Reference
13+	1.45 (1.13–1.86)	1.66 (1.28–2.16)
Health factors
Knee health
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)
General health
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)

Boldface values indicate statistical significance based on 95% OR CIs that exclude 1.

†Per unit of WOMAC pain score in the more symptomatic (painful) knee.

‡CESD>16.

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.