Effect of hip joint angle on concentric knee extension torque.

This study tested the hypothesis that the effect of hip joint angle on concentric knee extension torque depends on knee joint angle during a single knee extension task. Twelve men performed concentric knee extensions in fully extended and 80° flexed hip positions with maximal effort. The angular velocities were set at 30° s and 180° s. The peak torque and torques attained at 30°, 50°, 70° and 90° (anatomical position = 0°) of the knee joint were compared between the two hip positions. Muscle activations of the vastus lateralis, medialis, rectus femoris and biceps femoris were determined using surface electromyography. The peak torque was significantly greater in the flexed than in the extended hip position irrespective of angular velocity. The torques at 70° and 90° of the knee joint at both angular velocities and at 50° at 180° s were significantly greater in the flexed than in the extended hip position, whereas corresponding differences were not found at 30° (at either angular velocity) and 50° (at 30° s) of the knee joint. No effect of hip position on muscle activation was observed in any muscle. These results supported our hypothesis and may be related to the force-length and force-velocity characteristics of the rectus femoris.