Quadriceps femoris muscle fatigue in patients with knee osteoarthritis.

The purpose of this study was to characterize quadriceps femoris muscle fatigue of both lower extremities in patients with knee osteoarthritis (OA). Sixty-two subjects (mean age 68.2 years, standard deviation [SD] ± 7.9 years) with knee OA participated in the study. Significantly higher knee pain was reported in the involved knee than in the contralateral knee, as determined by a visual analog scale. Significant differences were demonstrated between the lower extremities in terms of maximal voluntary isometric contraction, in favor of the less involved leg (P = 0.0001). In contrast, the degree of fatigue of the quadriceps femoris muscle, as measured by the decrement in force production following ten repeated contractions, was significantly higher in the contralateral leg (P = 0.0002). Furthermore, normalization of the fatigue results to the first contraction yielded a similar result (P < 0.0001). Similar results were noted when analysis was performed separately for subjects whose involvement was unilateral or bilateral. The results indicate that, irrespective of the initial strength of contraction, the rate of muscle fatigue in the contralateral leg is significantly higher than in the involved leg. Hypotheses for these unexpected results are suggested. Rehabilitation of patients with knee OA should focus on increasing quadriceps muscle strength and endurance for both lower extremities.