Tension produced by isometric contraction of the rectus femoris muscle for rapid knee extension was examined at different levels of force with five normal subjects, using an on-line method to measure the timing and the rate of development of tension. Tension began to rise about 50 msec. after onset of electromyographic activity of the rectus femoris muscle, increased most sharply at about 60 msec. after its rise, and attained maximum at about 100 msec. The rate of tension development was determined solely by the maximum tension from first 20 msec. up to 100 msec., while timing remained approximately constant regardless of the force. The result confirmed well a force-control hypothesis of maximal speed ballistic movement of a body segment that the motor centre controls the amplitude of force output when time to develop tension is invariant.