The mechanical properties of human motor units with special reference to their fatiguability and recruitment threshold.

The recruitment thresholds, twitch tension, twitch contraction time and fatiguability of human motor units have been studied in the first dorsal interosseous of the hand. Units recruited at contraction strengths less than 50 g had relatively low twitch tensions (0.18-1.9 g), long contraction times (59-146 msec) and were non-fatiguable. Units recruited at higher contraction strengths (less than 200 g) had higher twitch tensions (15-26g), faster contraction times (33-57 msec) and were highly fatiguable. It is concluded that during graded voluntary muscle contractions motor units are recruited in order of increasing contraction strength and diminishing fatigue resistance.