Motor unit synchronization in FDI and biceps brachii muscles of strength-trained males

Motor unit (MU) synchronization is the simultaneous or near-simultaneous firing of two MUs which occurs more often than would be expected by chance. The present study sought to investigate the effects of exercise training, muscle group, and force level, by comparing the magnitude of synchronization in the biceps brachii (BB) and first dorsal interosseous (FDI) muscles of untrained and strength-trained college-aged males at two force levels, 30% of maximal voluntary contraction (MVC) and 80% MVC. MU action potentials were recorded directly via an intramuscular needle electrode. The magnitude of synchronization was assessed using previously-reported synchronization indices: k', E, and CIS. Synchronization was significantly higher in the FDI than in the BB. Greater synchronization was observed in the strength-trained group with CIS, but not with E or k'. Also, synchronization was significantly greater at 80% MVC than at 30% MVC with E, but only moderately greater with CIS and there was no force difference with k'. Synchronization prevalence was found to be greater in the BB (80.1%) than in the FDI (71.5%). Thus, although the evidence is a bit equivocal, it appears that MU synchronization is greater at higher forces, and greater in strength-trained individuals than in untrained subjects.