Effects of interaction between varus thrust and ambulatory physical activity on knee pain in individuals with knee osteoarthritis: an exploratory study with 12-month follow-up

Introduction: This study aimed to examine the interaction effect between ambulatory physical activity (PA) and varus thrust on knee pain in individuals with knee osteoarthritis (OA). Method: Subjects (n = 207; mean age: 73.1 years, 71.5% women) in orthopedic clinics with diagnosed knee OA (Kellgren/Lawrence grade ≥ 1) were enrolled in this 12-month observational cohort study. Participants underwent gait observation for varus thrust assessment and pedometer-based ambulatory PA measurements at baseline and 12-month follow-up. Knee pain intensity was assessed using the Japanese Knee Osteoarthritis Measure pain subscale as a primary outcome measure. Multiple linear regression analyses were performed to evaluate ambulatory PA–thrust interaction on knee pain intensity. Results: Ninety-two subjects (mean age, 73.4 years; 68.5% women) completed the 12-month follow-up assessment. Baseline ambulatory PA–thrust interaction was significant (P = 0.017) in the cross-sectional analysis, adjusting for covariates, which yielded R² = 0.310. Subgroup analysis showed that varus thrust was significantly associated with worse knee pain in subjects walking ≥ 5000 steps/day adjusting for covariates (beta: 7.94; 95% CI: 3.82, 12.1; P < 0.001) with a higher predictive ability (R² = 0.664). In contrast, ambulatory PA–thrust interaction in the longitudinal analysis showed no significant association with knee pain changes. Conclusions: Ambulatory PA interacted with varus thrust in the association with knee pain, as coexisting high ambulatory PA and varus thrust had the strongest association with higher knee pain. Maximal pain relief effects might be achieved when both ambulatory PA and varus thrust are treated simultaneously, rather than treating each separately.