This study was to determine how the angular velocity affects muscle activity during the knee joint isokinetic contraction. Twelve male university students were recruited as the subjects. Four pairs of surface electrodes were attached to the right-hand side of the body to monitor the rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF). Subjects performed the knee joint isokinetic contraction for 3 minutes in each angular velocity (30 °/sec, 120 °/sec, 240 °/sec). In each angular velocity, the peak torque of a knee joint extensor muscle according to the angular velocity to the exercise duration decreased more significantly in the angular velocity 240 °/sec than in the 30 °/sec and 120 °/sec in each exercise duration and also decreased more significantly in the velocity 120 °/sec than in the 30 °/sec in two min. and three. The average torque of a knee joint extensor muscle in the 240 °/sec decreased more significantly in two min. and three than at the first time check. In the velocity 120 °/sec and 240 °/sec, the peak and average torque of a knee joint flexor muscle according to the exercise duration decreased more significantly in all exercise duration than at the first time check. When it comes to the change of the muscle activity according to the angular velocity, the peak IEMG (Integrated electromyography) of the RF, VL, VM, and BF decreased as the angular velocity increased, whereas the average IEMG increased with the angular velocity increasing. In the angular velocity 240 °/sec, the average torque of the extensor muscle significantly decreased, whereas the average IEMG of RF and VM significantly increased. During the constant velocity movement, the increase of the angular velocity makes the average IEMG increase. During the high velocity movement, the torque decreased while the average IEMG of RF and VM increased.