Rehabilitation of patients with PFPS requires strengthening of the knee muscles7, 9). Accordingly, many researchers have recommended VMO and VL muscle activation in squat exercises5, 8, 10). This study investigated the activation of the VMO and VL muscles, to enable more precise treatment. We analyzed and compared the magnitude of the VMO and VL muscle activities in squat exercises with four different hip adduction loads. We found that the VMO muscle activity increased in squat exercises with hip adduction compared to the conventional squat exercise. This result agrees with the findings of previous studies, in which the VMO muscle showed significantly greater electrical activity during a double leg semisquat exercise associated with hip adduction5, 11). Some researchers also reported that squat exercises with maximal adduction of the hip showed an increase in the myoelectrical activity of VMO7, 11). These previous studies used squat exercises with hip adduction to establish higher EMG activity levels in VMO and VL. However, in all of these studies, maximal voluntary hip adduction was used. The present study used squat exercise with for different hip adduction loads, and found significantly higher activities at 80% and 40% hip adduction loads than in the conventional squat exercise. Much effort is required to maintain maximum loading, and it is actually difficult to maintain it using biofeedback. Consequently, compensatory movement or irregular activation of muscles might occur. Therefore, we suggest using 40%–80% hip adduction load during squat exercises for VMO strengthening in the clinical setting. Further EMG investigation is necessary to determine
the most appropriate hip adduction load, as well as to clarify the differences between PFPS and healthy persons, in order to compare the effects of muscle training programs utilizing
various hip adduction loads in squat exercises.