Rhythmicity associated with a high degree of short‐term synchrony of motor unit discharge in man

A rhythmic modulation of motor unit discharge at 17–24 Hz has been observed in the tibialis anterior muscle of subjects who display a high degree of short-term synchrony. Motor unit discharges were recorded both from extensor digitorum communis in the arm and tibialis anterior in the leg during voluntary isometric contractions. Time domain analysis of motor unit discharges in both muscles revealed a prominent central peak in the cross-correlograms that was interpreted as short-term synchrony. In tibialis anterior, but not in extensor digitorum communis, the primary, central peak was accompanied by secondary peaks. These were symmetrically or unilaterally present at an average time lag of 52 ms (range 37–68 ms) with respect to the primary peak. In both muscles other peaks occurred at time lags equivalent to the mean interspike interval of motor unit discharge (range 80-150 ms). In all subjects frequency domain analysis of the discharges of pairs of motor units revealed prominent peaks of coherence in the range 17–24 Hz. The actual frequency matched closely the reciprocal of the time lag interval between primary and secondary peaks of the cross-correlogram of the same motor units. Coherence was less evident for motor unit discharges in extensor digitorum communis and no prominent peaks occurred in the range 17–24 Hz. We propose that the secondary peaks and the rhythmicity may involve recurrent inhibition activated by the synchronous discharge of motor units. The presence of this pattern of discharge in tibilias anterior but not extensor digitorum communis would fit with recent animal and human work suggesting that recurrent inhibition is poorly represented in certain distal muscles of the forelimb.