Distinct Roles of Muscle Strength and Postural Stability in Objective and Subjective Function in Women with Bilateral Knee Osteoarthritis

Background/Objectives: Knee osteoarthritis (OA) significantly impairs physical function and quality of life, particularly in women. Although muscle strength and postural stability are known to influence functional outcomes, their independent contributions after controlling for age, body mass index (BMI), and pain remain unclear. This study aimed to investigate the differential roles of lower extremity muscle strength and postural stability on functional status, evaluated through both performance-based tests and patient-reported outcomes, in women with bilateral knee OA. Methods: Sixty-four women with bilateral knee OA (mean age: 55.71 ± 5.99 years) were included in this study. Lower extremity muscle strength was assessed using the five-times sit-to-stand test, and postural stability was evaluated with the Biodex Balance System. Performance-based function was measured using the Six-Minute Walk Test (6MWT) and the stair climbing test (SCT), while self-reported function was assessed using the WOMAC function subscale (WOMAC-F). Hierarchical linear regression analyses were conducted, controlling for age, BMI, and pain. Results: Muscle strength emerged as the strongest independent predictor of performance-based outcomes, significantly contributing to both SCT (β = 0.330, p = 0.005) and 6MWT (β = −0.409, p = 0.001). In the 6MWT model, the effects of age and BMI became non-significant after the inclusion of muscle strength, indicating a mediating role. In contrast, self-reported function (WOMAC-F) was primarily associated with pain (β = 0.385, p = 0.001) and postural stability (β = 0.243, p = 0.040), while muscle strength showed no significant contribution. Conclusions: Muscle strength is the primary determinant of objective functional performance, whereas pain and postural stability are more influential in shaping perceived functional limitations. These findings highlight a dissociation between actual performance and patient-reported function. Rehabilitation strategies should prioritize strength training to improve physical performance, while also addressing pain and balance to enhance patients’ perceived function.