On the relation between force and shortening during muscle twitch.

During the contraction phase of isotonic twitch there must be the change of muscle force. When initial acceleration was augmented by using heavy load or inertia lever 1,2,3), the muscle tension developed far bigger than the load and the condition of twitch was not 'isotonic' in real meaning. As far as a single twitch was concerned, the change of force should not be neglected. In the previous study 1), it was concluded that the initial shortening, which means the shortening performed during the initial acceleration, was independent of inertia or equivalent mass of the lever system within a considerably wide range. On the contrary, the secondary shortening (or excess shortening), which means the shortening performed during the slowing phase of contraction, was decreased with the increase of equivalent mass. In the present paper further investigations on the initial and secondary shortenings especially under after-load condition or under the control of shortening velocity were carried out.