1. Forty-six single motor units in the common finger extensor, superficial finger flexor, and first dorsal interosseus muscles were recorded with intramuscular wire electrodes while subjects made voluntary flexion and extension finger movements at a single metacarpo-phalangeal joint. 2. Motor unit firing was analysed in relation to the 8-10 Hz discontinuities which previously have been shown to characterize these movements. Statistical time- and frequency-domain analyses of the activity of individual motor units in relation to the discontinuities showed 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 modulation of their discharge in close temporal association with the joint acceleration. On the other hand, motor unit firing rate was not related to the frequency of the discontinuities. When the muscle recorded from was the antagonist, 21 of the 25 active units exhibited a similar frequency modulation. 3. When angular movement velocity was increased from 4 to 25 deg s-1, the strength of motor unit frequency modulation increased. Peak coherence between motor unit activity and acceleration increased by 74 %, on average, in the common finger extensor units. 4. The findings rule out a tentative mechanism attributing the discontinuities to newly recruited motor units firing at circa 8-10 Hz. Instead, a coherent 8-10 Hz input to the agonist and antagonist motoneurone pools is implied.