The motor units of cat medial gastrocnemius: speed-size relations and their significance for the recruitment order of mortor units.

The interrelationships between axonal conduction velocity, tetanic tension, twitch contraction time and rate of force development during a tetanus have been studied in 126 motor units from 12 cat medial gastrocnemius muscles. While the range of axonal conduction velocities for slow- (contraction time greater than 45 msec) and fast-twitch units overlapped, the mean conduction velocity for the slow-twitch group was significantly lower. No difference could be found between the axonal conduction velocities of the fast non-fatiguing and fast fatiguing units. Within individual experiments, few significant correlations were found between conduction velocity and tetanic tension or contraction time for the fast- and slow-twitch units. Some correlations did appear when data from these populations were pooled, but such results are shown to be misleading. Some weak correlations were found between motor unit contraction strength and twitch contraction time. The rate of rise of isometric force development was found to be most strongly related to tetanic tension and only weakly related to contraction time. The ordering of motor units according to contraction strength reveals the association of motor unit mechnical properties to be ideally suited for the dual role of medial gastrocnemius as a postural and powerful phasic muscle.