Fast motor units are not preferentially activated in rapid voluntary contractions in man.

Abstract

MOST mammalian muscles include muscle fibres of different histochemical types which are organised in separate motor units (a motor unit is a group of muscle fibres innervated by a single motoneurone) with different physiological characteristics1. When a muscle carries out a slowly graded contraction, the motor units of the slow type are activated before the motor units with a faster contraction time which are also the ones developing more force since they include a larger number of muscle fibres2. This orderly recruitment can also be shown in intact man in whom the spike-triggered averaging method further allows the extraction of the mechanical twitch of a single motor unit from the whole muscle force3. In view of the versatility of the human hand movements and considering recent inferential views about variations in the recruitment order of motor units4,5, we had anticipated that the fast motor units could be preferentially recruited in brick voluntary contractions. This expectation is not borne out by the results presented. On the contrary we found the faster motor units to be recruited after the slow motor units, even in fast contractions.