Changes in contractile speed of cat motor units during activity.

Experiments were conducted to measure the extent of contractile changes during phasic activity of different motor units. Motor units of cat medial gastrocnemius were isolated and classified by their mechanical properties as fast and fatigable (FF), fast and fatigue resistant (FR), or slow (S). Single stimuli interpolated between stimulus trains of the fatigue test produced twitches whose shapes were measured at different times during this test. After 30 seconds of fatigue testing, twitch contraction times of 33% of FF units fell into the slow range, i.e., 5 of 17 units had become slower than the fastest slow units. Mean twitch contraction time of FF units increased by 11.8 msec, whereas that of S units decreased by 16.2 msec. We conclude that the mechanical properties of rested motor units change markedly with use and are a poor index for determining the contractile speed of active muscles.