The purposes of this study were to investigate of the effective hip joint angle during quadriceps isometric exercise by electromyophic analysis. All participants performed quadriceps isometric exercise with knee flexion 60°by Cybex7000. The EMG activity of three knee extensor, vastus lateralis(VL), rectus femoris(RF), and vastus medialis(VM), in the subjects isometric exercise(maximal voluntary isometric contraction, MVIC) was monitored using bipolar surface electrodes. The magnitude of the average peak iEMG for each muscle monitored during the stactic motion is calculated as a percentage of the normalized peak iEMG during the MVIC. And the results were follows