Properties of motor units in the first deep lumbrical muscle of the cat's foot.

Isometric contractions of single motor units were studied in the first deep lumbrical muscle of the cat's hind-foot. Motor units with short twitch contraction times (15-20 msec) generally differed from those with longer ones (23-50 msec; contraction time measured in unpotentiated twitches) in showing (1) a greater maximum tetanic tension, (2) a smaller resistance to fatigue, (3) more post-tetanic potentiation of twitch tension, and (4) no post-tetanic occurrence of repetitive activity in response to single nerve stimuli (such "post-tetanic repetitive activity" was seen in several of the slower units). The ratio between unpotentiated twitch tension and maximum tetanic tension was similar for units with brief and long contraction times. The peak-to-peak amplitude of a single motor unit spike, recorded with gross electrodes, tended to be directly proportional to the maximum tetanic tension of the same motor unit.