Synchronous discharges in pairs of steadily firing motor units tend to form clusters.

Discharges of several motor units (MU) were simultaneously recorded during slight isometric contractions of the first dorsal interosseus muscle using bipolar needle electrodes. Correlograms constructed by counting the relative discharge intervals (1-ms binwidth) between two MU frequently showed narrow central peaks reflecting the occurrence of more synchronies than expected by chance. Diagrams of the temporal distribution of these synchronies revealed that they tend to form clusters consisting of several subsequent events associated with an adjustment of the firing pattern of the two MU. The synchronization described here may be explained by similar mechanisms as the so-called 'short-term synchronization'.