Physical activity is protective against osteoarthritic development and is among the best approaches to manage frailty, which can be characterized as the presence of health deficits. It is unclear whether overall health of a person influences the relation between physical activity and knee-joint health.

Test the hypothesis that physical activity is associated with knee osteoarthritis symptoms and investigate frailty as a moderator.

This cohort observational study included participants (n = 1351; 728 females) from the Osteoarthritis Initiative, aged 45–79 (60±9) years. Hip-worn accelerometers were used to quantify free-living stationary time, light (LPA) and moderate-to-vigorous-physical-activity (MVPA). The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) defined symptom progression. Baseline frailty was determined via a 31-item Frailty Index and participants grouped into Non-Frail (0–0.09), Very-Mild Frailty (0.1–0.19), and Mild Frailty+ (>0.2). Accelerometry and WOMAC were determined at 72-month follow-up.

MVPA (18±19mins/day) was negatively related to WOMAC outcomes (β<-0.0155, p < 0.0022), while LPA (274±79mins/day) was not (β<0.0005, p > 0.3061). Stationary time (606±88mins/day) was positively associated with WOMAC stiffness (β=0.0009, p = 0.0147). Frailty (0.134±0.077) did not moderate LPA and WOMAC relations (p > 0.308). A stronger negative relation between MVPA and WOMAC pain (β=-0.0092, p = 0.041) was observed in the Mild Frailty+ group compared to the Very-Mild Frailty and Non-Frail groups. A stronger positive relation between Stationary time and WOMAC stiffness (β=0.0013, p = 0.012) was observed in the Mild Frailty+ groups compared to the Very-Mild Frailty and Non-Frail groups.

Engaging in MVPA and limiting stationary time may be more beneficial on knee osteoarthritis pain and stiffness among frailer older adults.