Short-term synchronization of motor units in human extensor digitorum communis muscle : relation to contractile properties and voluntary control

Synchronous activity was studied in relation to the contractile properties of pairs of motor units (MUs) recorded with independent microelectrodes in the right extensor digitorum communis muscle (EDC) of human subjects during isometric finger extension. MU contractile properties were characterized in terms of the rise time and amplitude of twitch tensions extracted by spike-triggered averages of the extension force. Synchronization of MU discharges appeared in the form of narrow central peaks in the cross-correlograms of 35 of 50 pairs of MUs, suggesting the contribution of common last-order neurons. Synchronization peaks tended to be briefer and higher among pairs of MUs with slower and smaller twitches than among pairs of MUs with faster and larger twitches. The higher peaks of slow-contracting MUs suggest a greater effectiveness of the common synaptic inputs. The broader peaks of fast-contracting MUs might reflect an additional synchronization of the inputs to fast MUs at high force levels. The areas of the cross-correlogram peaks were similar for both groups and suggest that under our conditions, about three motoneurons would discharge synchronously for a given motoneuron spike. To test whether the amount of MU synchronization could be altered voluntarily, four subjects attempted to increase or decrease synchrony, using as feedback clicks triggered by coincident firings of the recorded MUs. In nine of 15 conditioning sessions, the magnitudes of the synchronization peaks showed significant changes in the intended direction. These results imply that supraspinal centers can control the relative amount of inputs that contribute to the synchronization of motoneuron discharges during voluntary contraction of EDC.