Single motor unit activity in relation to pulsatile motor output in human finger movements

1. Forty-six single motor units in the common finger extensor, superficial finger flexor, and first dorsal interosseus muscles were recorded with intr amuscular wire electrodes while subjects made voluntary flexion and extensi on finger movements at a single metacarpo-phalangeal joint. 2. Motor unit firing was analysed in relation to the 8-10 Hz discontinuitie s which previously have been shown to characterize these movements. Statist ical time- and frequency-domain analyses of the activity of individual moto r units in relation to the discontinuities slow ed that when the muscle was the agonist, all motor units in the common finger extensor muscle, and all units except one in the flexor muscles exhibited significant frequency mod ulation of their discharge in close temporal association with the joint acc eleration. On the other hand, motor unit firing rate was not related to the frequency of the discontinuities. When the muscle recorded from was the an tagonist, 21 of the 25 active units: exhibited a similar frequency modulati on. 3. When angular movement velocity was increased from 4 to 25 deg s(-1), the strength of motor unit frequency modulation increased. Peak coherence betw een motor unit activity and acceleration increased by 74%, on average, in t he common finger extensor units. 4. The findings rule out a tentative mechanism attributing the discontinuit ies to newly recruited motor units firing at circa 8-10 Hz. Instead, a cohe rent 8-10 Hz input to the agonist and antagonist motoneurone pools is impli ed.