Electromyogram premotion silent period and tension development in human muscle.

The EMG silent period has been observed frequently just before rapid movement from a slightly sustained contraction of the muscle. Our experiments were designed to show the relation between the tension developed in an intact human muscle and the occurrence of this silent period. Tension in the knee extensor was obtained from the floor reaction force of a living subject in a squatting position on the basis of a lever ratio-joint angle diagram obtained directly from measurements on an isolated knee extensor muscle of a human cadaver. The muscle tension was calculated under the condition that the floor reaction force in the squatting position passed through the anklebone, and decreased from 8.7 to 0.8 kN with an increase in the knee joint angle from 1.13 to 2.74 rad. When the center of pressure was not assumed, the increased knee extensor muscle torque with a decrease in the knee angle was estimated biomechanically from the free body diagram. Consistently, the EMG recordings confirmed an increased muscle activity with a reduction in the joint angle. The premotion silent period for the knee extensors and their antagonist with movement from the squatting position appeared to be limited to a knee joint angle of 2.44 to 3.07 rad, where the requirement for tension in those muscles was low. This silent period could be interpreted as an electromyographic transitional phase attributable to a switching mechanism in central nervous activity. The premotion silent period was most likely to appear prior to a swift, well coordinated movement.