The purpose of this study was to investigate the effect of isometric hip adduction and abduction on the muscle activities of vastus medialis oblique(VMO) and vastus lateralis(VL) during leg squat exercises. This study consisted of 21 healthy subjects who had no medical history of anterior knee pain or lower extremity disorders. The ball and belt were used to isometrically adduct and abduct the hip joint during the leg squat exercise, respectively. The surface electromyograms of VMO and VL were analyzed, and the findings were used to calculate the VMO:VL ratio during 3 different quadriceps-strengthening exercises(leg squat, LS leg squat with isometric hip adduction, LSHD leg squat with isometric hip abduction, LSHB). The muscle activities of VMO and VL and the VMO:VL ratios were compared using the paired t-test with Bonferroni adjustment. The results showed that the muscle activities of VMO and VL during LSHD were greater than those during LSHB. The VMO:VL ratio was the highest during LSHD. This finding suggests that LSHD using a ball is more effective than LS and LSHB in selectively increasing the muscle activities of VMO. Therefore, we suggest that leg squat exercise with isometric hip adduction using a ball would be useful for maintaining correct patella tracking and for selectively strengthening VMO.