Maximum shortening speed of motor units of various types in cat lumbrical muscles.

1. Isotonic shortening of cat superficial lumbrical muscles was studied during maximal tetanic contractions of single motor units of identified types. For each motor unit, the maximal speed of contraction, Vmax, was determined by extrapolating to zero the hyperbolic relation between applied tension and speed of shortening. 2. The maximal speeds of shortening of motor units formed a continuum with the highest velocities observed for the fast fatigable motor units and the lowest for the slow motor units. 3. On average, the maximum speed of shortening increased with the tetanic tension developed by the motor units. 4. In motor units with isometric twitch contraction times less than 35 ms, these times showed a significant inverse correlation with Vmax. Progressively longer contraction times were associated with rather small changes in Vmax. 5. The implications of these findings on the speed of muscle shortening during motor-unit recruitment are discussed.