Synchronization of low- and high-threshold motor units

INTRODUCTION: 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.