Common modulation of motor unit pairs during slow wrist movement in man

1. The activity of 36 pairs of single motor units were recorded with intram uscular wire electrodes from m. extensor carpi radialis while subjects perf ormed slow wrist extension and flexion movements. Periods of steady positio n holding were interposed between movements. 2. The discharge trains from pairs of motor units were analysed statistical ly in the time and frequency domains. During extension movements, when the muscle recorded from was the agonist, coherence between motor units was sig nificant below 12 Hz, with a peak at 6-12 Hz in 30 of 36 pairs (83 %). The magnitude of coherence decreased during position holding compared to moveme nts in 26 pairs, while the difference in average firing rate was small. 3. During movements, but not during position holding, coherence estimates b etween single motor units and acceleration showed a significant peak at 6-1 2 Hz in 56 out of 62 motor units, suggesting that a modulation of motor uni t discharge contributed to angular acceleration at these frequencies. Commo n motor unit modulation was present at 3 Hz as well, although the coupling between motor unit activity was weaker than at 6-12 Hz. 4. It is concluded that a 6-12 Hz common modulation of agonist motor units- is a distinguishing feature of slow voluntary wrist movements, extending th e previously established notion of an 8-10 Hz rhythmic organization of slow finger movements to more proximal limb segments. It is suggested that the 6-12 Hz input is specific for movements and is normally absent or much weak er during steady maintenance of position or force.