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Objective: The aim of the study was to investigate (1) trajectories of physical activity (PA) over 96 months and (2) study to what extent knee pain, muscle strength, physical function, and radiographic disease were associated with PA trajectories in adults with or at risk of knee osteoarthritis (KOA). Methods: Using the Osteoarthritis Initiative (OAI) database, we described PA trajectories with the Physical Activity Scale for the Elderly (PASE) over 96 months. Knee pain was categorized into three groups: "no pain" [visual numeric pain rating scale (VAS = 0)], "little to some pain" (VAS = 1-3), or "moderate to severe pain" (VAS = 4). Knee extensor strength was classified into high [>16.21 (men) and >10.82 (women) N/kg/m2] and low [<16.21 (men) and <10.82 (women) N/kg/m2] groups. Gait speed was classified into slow (<1.22 m/s) and fast (=1.22 m/s) groups. Chair stand time was classified into slow (>12 seconds) and fast (<12 seconds) groups. Radiographic disease was classified as present [Kellgren-Lawrence (KL) =2] or absent (KL grade <2) of KOA. Results: Among 3755 participants (age 61.0 ± 9.0 years, body mass index 28.5±4.8 kg/m2, 58% female), we identified three trajectories: sedentary PA with slow decline (44.3%), low PA with slow decline (41.3%), and high PA with slow decline (14.4%). Poorer gait speed (OR: 2.32; 95% CI: 1.71-3.16), chair stand time (OR: 1.45; 95% CI: 1.07-1.96), and knee extensor strength (OR: 1.35; 95% CI: 1.03-1.76), but not pain or radiographic disease, were associated with PA trajectory of sedentary PA with slow decline. Conclusion: Physical function and strength, but not pain and radiographic disease, were associated with a trajectory of decline in PA among adults with or at risk of KOA. [ABSTRACT FROM AUTHOR]