Sit-to-Stand Performance, Moderate-to-Vigorous Physical Activity, and Health-Related Quality of Life in Knee Osteoarthritis: A Prospective Mediation Analysis

Introduction: Knee osteoarthritis (KOA) is the most common form of arthritis, leading to illness, decreased physical function, and reduced health-related quality of life (HR-QoL). Sit-to-stand (STS) testing is a commonly used measure of physical capacity in KOA. This study examined whether STS performance and moderate-to-vigorous physical activity (MVPA) helped account for the association between KOA pain and subsequent HR-QoL. Methods: Data across 2 years (at year 6 and year 8 visits) were used from the Osteoarthritis Initiative (OAI), a multisite, longitudinal study of people with or at risk for KOA. We used serial mediation analysis to examine whether year 6 STS performance and year 8 MVPA accounted for the association between year 6 KOA pain and year 8 HR-QoL, adjusting for age, sex, and race. Results: Significant correlations among all model variables supported model testing. The serial mediation model revealed that an increase in KOA pain predicted worse STS performance (β = .15; P < .001). MVPA was reduced by an increase in KOA pain (β = -.13; P < .001) but not STS (β = -.02; P = .46). MVPA subsequently predicted HR-QoL (β = .12; P < .001). The overall indirect pathway was significant (β = -0.02; 95% CI, -0.040 to -0.009), but the direct pathway also remained significant (β = -0.47; 95% CI, -0.536 to -0.410), denoting a partial mediation effect. Conclusion: STS performance and MVPA partially mediated the relationship between KOA pain and HR-QoL, highlighting mechanisms to address in future interventions for improving HR-QoL. These results support STS as a complementary tool to pain for understanding MVPA participation and HR-QoL in KOA patients. The findings also underscore the importance of integrating self-management strategies to address pain in behavior-change interventions.