Discharge properties of primate forearm motor units during isometric muscle activity

Activity of single motor units (MUs) was recorded in forelimb muscles of rhesus macaques while they generated isometric ramp-and-hold torques about the wrist. Multiunit electromyographic (EMG) activity was recorded from 10-12 identified flexor and extensor muscles of the wrist and digits with implanted EMG wire electrodes. Single MUs from these muscles were recorded with a remotely controlled tripolar microelectrode array. The parent muscle of each MU was determined by compiling MU-triggered averages of multiunit EMGs. The MU firing patterns during the isometric task were determined from response histograms aligned with change in torque. At moderate torque levels, MUs (n = 86) exhibited four types of discharge patterns during the ramp-and-hold trajectory: phasic-tonic (23%), tonic (33%), decrementing (39%), and phasic (5%). Phasic-tonic MUs exhibited a phasic burst of activity during the torque ramp which exceeded the firing rate during the static hold period. Both phasic-tonic and tonic MUs exhibited a constant mean firing rate during the hold period; the discharge of decrementing MUs gradually decreased during the static hold. Phasic MUs fired only during the change in force. The relation between MU firing rate and torque was investigated as the monkeys generated responses of different levels of static torque during the hold period. Mean firing rate during the hold was found to be proportional to static torque up to moderate torque levels, where it reached a maximum. In the linear range, the mean rate-torque slope was 3.4 +/- 1.9 imp/s per 10(5) dyn . cm (n = 9).