The purpose of this study was to examine the effects of knee flexion angles during maximum isometric hip extension. Ten healthy men performed maximum isometric hip extension in prone position at 15° and 90° knee flexion. Then, the hip extension torque was measured, and electromyographic (EMG) data were obtained from the biceps femoris long head, semitendinosus, semimembranosus, adductor magnus, and gluteus maximus muscles. The EMG data were full-wave rectified and integrated (IEMG). The IEMG values obtained during the measurement of isometric hip extension were normalized with the values collected at 90° knee flexion (normalized IEMG [NIEMG]). The hip extension torque at 15° knee flexion was significantly greater than that at 90° knee flexion. The NIEMG values from the hamstrings at 15° knee flexion significantly increased compared with those at 90° knee flexion. Meanwhile, the NIEMG values from the gluteus maximus at 90° knee flexion were significantly greater than those at 15° knee flexion. However, the NIEMG values from the adductor magnus did not significantly differ between 15° and 90° knee flexion. These results indicate that the hamstrings effectively generate contracting force during isometric hip extension and at knee extended position because its fiber length was close to the optimal length.