Fatigue-related changes in discharge patterns of motor units in the inferior head of the lateral pterygoid muscle in humans

Objective: To investigate single motor unit (SMU) firing patterns of the inferior head of the human lateral pterygoid muscle (IHLP) during sustained isometric contractions, and to examine IHLP fatigability. Design: In 5 subjects, EMG was recorded from the right IHLP during generation of horizontal isometric mandibular force towards left side. Subjects tracked a target on an oscilloscope, slowly increasing the force to 25% of maximum voluntary contraction, and then maintained that force for 120 s (endurance task). Subjects rested for 30 s after each test, and three 10 s recovery tests were performed every 30 s (recovery task). Fatigue data were obtained from 23 SMUs. Results: Twelve of 23 SMUs (52.2%) continued to fire throughout the 120 s, and each subject had at least one of these units. These units were classified as tonic pattern (T-unit). The firing rate of 6 of the 12 T-units (50.0%) decreased gradually with time. Comparison of the mean firing rate and coefficient of variance (CV) of interspike interval (ISI) of the 12 T-units between pre-fatigue, post-fatigue and the three recovery tasks showed that the mean firing rate decreased and CV of ISI increased after fatigue. Mean firing rate and CV of ISI recovered after 30 s of rest after the endurance task. Conclusions: The firing patterns of IHLP units changed with the passing of time during sustained isometric contractions leading to fatigue. (C) 2005 Elsevier Ltd. All rights reserved.