Motor unit categorization on basis of contractile properties: an experimental analysis of the composition of the cat's m. peroneus longus.

Recordings were made of isometric contractions of single motor units of the cat's m. peroneus longus (PerL). The units were activated by stimulation of dissected filaments of ventral roots. In accordance with the general principles introduced by Burke et al. (1973), the 80 isolated PerL units were classified into three or four type-categories according to their contractile speed and endurance. Three currently used varieties of a "fatigue index" were calculated and found to give equivalent results. Units with a high, intermediate and low resistance to fatigue were responsible for about 22.5, 25.4 and 52.1% respectively of the total muscle force. Two alternative methods for fast/slow categorization were compared: (i) classifying all units as slow that failed to show a "sag" in partly fused contractions ("sag-criterion", Burke et al. 1973) and (ii) classifying all units as slow that had a more prolonged twitch contraction time than that of fatigue-sensitive units ('FF vs. S-criterion'). The relative contribution of slow units to total muscle force was about 2.8 times as great (14 versus 5%) for a classification by the FF vs. S-criterion than for a subdivision according to sagging behaviour. When compared to equivalent data from previously published studies of feline hindlimb muscles, peroneus longus was found to resemble gastrocnemius medialis in relative motor unit composition. The maximum force of individual PerL units was, however, on average less than or equal to 50% of that reported for corresponding types of gastrocnemius units.