Of 2678 OAI participants with baseline radiographic knee OA, 89 were excluded from analysis: 14 persons with incomplete baseline performance data, and 75 having incomplete baseline interview data, leaving 2589 available for baseline analyses. As shown in , 2301 completed Year 1 assessments and 2013 completed Year 2 assessments.
Analytical sample of radiographic knee OA participants
A total of 2589 persons with radiographic knee OA at the baseline OAI visit (aged 45–79 years) participated in functional performance tests. This analytic sample was predominantly white (79%), female (58%) with an average age of 62.4 years. The 89 persons 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 non-white (62%).
Baseline physical activity measured by the total PASE score ranged from 0 to 465, with mean of 155 with a standard deviation (SD) of 80 which indicates 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 . Adults with radiographic knee OA in the lowest physical activity group (level 1) compared to the more active levels tended to be older, non-white, female, less educated, and 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.
Baseline Characteristics n=2589 persons with radiographic knee OA participating in OAI baseline
Baseline functional performance measured by gait speed ranged from 0.80 to 6.83 with a mean of 4.26 (SD=0.69). Gait speed had a moderate correlation=0.27 with raw PASE scores A positive relationship between graded baseline physical activity level groups and baseline functional performance is shown graphically by cumulative gait speed frequency curves in . For example, at a 4 feet/second corresponding to the minimum walking speed to safely cross a street for which many pedestrian traffic lights are timed39
, more than half (51%) of the lowest physical activity (level 1) group did not meet this threshold compared to only 37%, 29%, and 19% of the higher and more active group levels 2, 3, and 4, respectively. Notably the four lines 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.
Statistical analyses evaluating a cross-sectional graded relationship are summarized in . The average improvement in baseline functional performance when compared to the lowest group level increased with membership in higher baseline physical activity levels (baseline gait speeds: 4.0, 4.18, 4.29, 4.49 feet/second, respectively; P for trend <0.001). These trends remained significant in multivariate analyses that simultaneously controlled for baseline demographic (age, gender, race/ethnicity, marital status, education) 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, high depressive symptoms. To provide perspective on the magnitude of these group differences, if each person in this cohort increased his/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 and 65–79 years).
Average Differences in baseline gait speed (feet/second) among physical activity groups
Prospective data were used to investigate if a graded physical activity relationship would persist with functional performance measured one year later. These analyses were restricted to 2301 persons with at least one year of follow-up; of these 2013 persons contributed two years of follow-up. Over one year, 48% of persons 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 =4.3 ft/second [SD=0.7], year 2 mean =4.3 ft/second [SD=0.7]). The cumulative frequency performance curves () demonstrated a positive graded relationship between physical activity group levels with subsequent gait speed one year later. Similar to , a graded relationship is graphically depicted by distinct curves, which are ordered by physical activity group levels. Statistical analyses that utilized the full two years of longitudinal follow-up information to evaluate a graded relationship are summarized in . The average improvement in subsequent functional performance when compared to the lowest group level increased with membership in higher level physical activity groups (gait speed after one year: 4.0, 4.2, 4.3, 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 two 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 one 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.
Figure 3 Longitudinal cumulative percentage of subsequent (one year) gait speed (feet/second) by physical activity quartile groups (n= 2301 persons). Cumulative percentages are a weighted average of 2301 observations on Year 1 gait speed following baseline physical (more ...)
Cross-sectional cumulative percentage of gait speed by physical activity quartile groups assessed at baseline (n=2589)
Longitudinal Analyses a: Average differences in subsequent (one year) gait speed (feet/second) among physical activity groups