Using surface electromyography, the myoelectric activity of the vastus medialis obliques (VMO) and vastus lateralis (VL) were recorded under sixteen different isometric knee extension conditions in 27 healthy female subjects. The purpose of the study was to identify the optimal angle of knee flexion for selectively facilitating the activity of VMO relative to the VL. Additionally, the influence of weight bearing and hip adduction was investigated. It was found that the angle of knee flexion and weight-bearing had an effect on the ratio of the VMO/VL. At 60˚ to 90˚ of knee flexion and weight bearing situation, the ratio of the VMO/VL was greater than other situations. Hip adduction did not have significant influence, except in weight-bearing condition. The results of this study suggest that the VMO may be selectively activated on flexed knee in weight-bearing condition with the assistance of hip adduction.