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

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Abstract
1. The activity of 36 pairs of single motor units were recorded with intramuscular wire electrodes from m. extensor carpi radialis while subjects performed slow wrist extension and flexion movements. Periods of steady position holding were interposed between movements. 2. The discharge trains from pairs of motor units were analysed statistically in the time and frequency domains. During extension movements, when the muscle recorded from was the agonist, coherence between motor units was significant 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 movements in 26 pairs, while the difference in average firing rate was small. 3. During movements, but not during position holding, coherence estimates between single motor units and acceleration showed a significant peak at 6-12 Hz in 56 out of 62 motor units, suggesting that a modulation of motor unit discharge contributed to angular acceleration at these frequencies. Common 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 the 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 weaker during steady maintenance of position or force.