Background/Purpose: Adopting a physically active lifestyle provides major health benefits to people with knee osteoarthritis (OA). Yet, most people with knee OA are sedentary. Difficulty with physical function, such as stair climbing and walking, may be one barrier to physical activity participation for people with knee OA. Little is known about what minimum level of physical function is necessary to engage in physical activity. Establishing minimum thresholds will help clinicians prioritize prescribing intervention for physical activity or physical function in knee OA. The purpose of the study was to identify minimum thresholds of performance on clinical tests of physical function required to target a goal of 6000 steps/day, an important benchmark of activity in knee OA. 

Methods: We used publically available data from the Osteoarthritis Initiative (OAI). Physical activity was measured with an accelerometer (Actigraph GT1M) worn during waking hours during the 48-month follow-up visit, and quantified as steps/day. Physical function was quantified with three clinical tests: timed 5 repetition sit-to-stand test (STS), timed 400-meter walk, and walking speed (calculated from a 20-meter walk). We calculated Pearson correlation coefficients to examine the association between steps/day and physical function.  To identify a minimum threshold for each, we calculated cut-points at 80% to 95% specificity for walking 6,000 steps/day, i.e., the proportion classified with good physical function and ≥ 6000 steps/day divided by all with ≥ 6000 steps/day.

Results: Participants who wore the monitor for ≥ 3 days (n=1790, age [mean (± sd)] 65.0 ± 8.8 years, BMI 28.4 ± 4.9 kg/m², 55% female), had 6319 ± 2920 steps/day with 47% walking ≥ 6000 steps/day. The mean STS was 10.2 ± 3.2 seconds, 400-meter walk was 305.3±51.4 seconds, and walking speed was 1.3 ± 0.2 meters/sec.  Steps/day had a small negative correlation with STS (r=-0.2, p<0.001) and 400-meter walk (r=-0.4, p<0.001) and a small to moderate positive correlation with walking speed (r=0.4, p<0.001). Thresholds of high specificity (80-95%) of physical performance for walking ≥ 6000 steps/day ranged from 11 to 14 seconds for STS, 315 to 350 seconds for the 400-meter walk, and 1.10 to 1.25 meters/sec for walking speed (Table).

Conclusion: Physical function at or worse than stated thresholds may represent insufficient physical function for attaining an important benchmark of physical activity. Intervention targeting physical function may be more appropriate than directly targeting physical activity for those below these physical function thresholds with knee OA. Table: Physical function measures that reflect 80, 85, 90 and 95% specificity*.