To explore whether femoral cartilage thickness is related (and changes) with muscle strength in subjects with knee osteoarthritis (OA). Forty patients (27 F, 13 M) with knee OA-who were under quadriceps muscle strengthening program-were enrolled in the study. Isokinetic/isometric knee muscle strength measurements (at 30-60A degrees angles and 60-180A degrees velocity) were performed at baseline, end of the muscle strengthening program, and third month control visit using a biodex dynamometer. Femoral cartilage thicknesses (at medial/lateral condyle and intercondylar area) were measured using ultrasonography. Seventy-nine knees of 40 patients (27 F, 13 M) aged 52.03 A +/- 11.72 years (range, 26-71) were analyzed. Mean VAS scores on the first and third months were significantly lower than the initial values (p < 0.001, p = 0.049). Isometric peak torque and total work values at 180 A degrees/s were significantly higher than the baseline measurements at first and third month controls (all p < 0.05). Cartilage thicknesses (at three sites) were significantly higher than the baseline measurements (all p < 0.05) on the third month but not on the first month (all p > 0.05). Femoral cartilage thicknesses were positively correlated with isometric strength values at baseline and third month. We propose that femoral cartilage thicknesses increase on the third month of strengthening therapy. Since this late-phase thickening parallels the earlier increase in muscle strength (starting, on the first month), we speculate that regeneration rather than edema might be the primary underlying cause.