Synchronization of low‐ and high‐threshold motor units

: We examined the degree of synchronization for both low‐ and high‐threshold motor unit (MU) pairs at high force levels. Methods : MU spike trains were recorded from the quadriceps during high‐force isometric leg extensions. Short‐term synchronization (between −6 and 6 ms) was calculated for every unique MU pair for each contraction. Results : At high force levels, earlier recruited motor unit pairs (low‐threshold) demonstrated relatively low levels of short‐term synchronization (approximately 7.3% extra firings than would have been expected by chance). However, the magnitude of synchronization increased significantly and linearly with mean recruitment threshold (reaching 22.1% extra firings for motor unit pairs recruited above 70% MVC). Conclusions : Three potential mechanisms that could explain the observed differences in synchronization across motor unit types are proposed and discussed. Muscle Nerve 49:575–583, 2014