To determine whether lower thigh muscle specific strength increases risk of incident radiographic knee osteoarthritis (RKOA), and whether there exists a sex-specific relationship between thigh muscle specific strength and BMI.
161 Osteoarthritis Initiative participants (62% female) with incident RKOA (Kellgren-Lawrence grade 0/1 at baseline, developing an osteophyte and joint space narrowing grade ≥1 by year 4) were matched to 186 controls (58% female) without incident RKOA. Thigh muscle anatomical cross-sectional areas (ACSAs) were determined at baseline using axial MRI scans. Isometric extensor and flexor muscle strength were measured at baseline and specific strength (strength÷ACSA) calculated. Logistic regression assessed risk of incident RKOA associated with muscle specific strength (with and without adjustment for BMI).
Lower knee extensor and flexor specific strength significantly increased the risk of incident RKOA in women (OR 1.47 [95%CI 1.10, 1.96] and 1.41 [1.06, 1.89], respectively) but not in men. The significant relationship in women was lost after adjustment for BMI. Lower specific strength was associated with higher BMI in women (r=−0.29, p<0.001), but not in men; whereas (absolute) strength was associated with BMI in men (r=0.28, p=0.001), but not in women.
Lower thigh muscle specific strength predicts incident RKOA in women, with this relationship being confounded by BMI. The sex-specific relationship between muscle specific strength and BMI provides a possible explanation why women with muscle strength deficits typically have a poorer prognosis than men with similar strength deficits.