Synchronization of low- and high-threshold motor units
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Updated: May 9, 2026

Functional Isolation of Single Motor Units of Rat Medial Gastrocnemius Muscle
Published on: December 26, 2020
Jason M Defreitas1, Travis W Beck, Xin Ye
1University of Oklahoma, 1401 Asp Avenue, Norman, Oklahoma, 73019.
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.
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