[The recruitment of motor units during elbow flexions in man]

The density of spikes of spike output (SO) recorded by means of three bifilar semimicroelectrodes simultaneously implanted in the biceps brachii has been studied during elbow flexions performed against inertia and with a variable velocity (anisometric, anisotonic contractions). The total number of motor unit (MU) potentials as well as the force were sequentially measured during intervals of 30 msec. SO obtained during movements of very similar mechanical characteristics was different from one movement to another. However, when a small number of identical movements was summed an averaged SO was obtained that was representative of the instantaneous global activity of the biceps. Taking into account the respective values of the bin duration and of the interval between two successive discharges of a single MU, the averaged SO represents the number of active MU during the acceleration phase of the movement. Since in this type of movement the frequency discharge of each MU decreases during the acceleration phase, it appears that the increase of averaged SO represents the recruitment of MU during the movement. From the relation established between the instantaneous values of So and force it can be concluded that the shape of force increase in anisometric, anisotonic conditions is entirely related to that of MU recruitment, although the absolute value of force depends on both the firing frequency and the number of active MUs.