Mechanomyogram and electromyogram analyses for investigating human masseter muscle fatigue

The purpose of this study was to examine the validation of mechanomyogram (MMG) for assessing the masseter muscle fatigue. Data were obtained from 16 healthy Japanese males (aged 25.6 ± 2.3 years). The measuring device for the MMG consisted of an amorphous sensor and a small magnet. The bite-force, the MMG, and the electromyogram (EMG) signals were recorded simultaneously. The subjects were instructed to perform 0%, 5%, 10%, and 20% maximal voluntary contraction (MVC) for 20 s as the pre-fatigue trials. A recovery period of 5 min was allowed after each pre-fatigue trial. As the fatigue task, the subjects were instructed to sustain 40% level of MVC until they feel intolerable discomfort. The subjects were asked to perform 0%, 5%, 10%, and 20% MVC for 20 s as the post-fatigue trials. No recovery period was allowed after each post-fatigue trial. The mean values of the average rectified value for EMG of the post-fatigue trials were significantly higher than those of the pre-fatigue trials at 10% (p < 0.05) and 20% MVC (p < 0.05). The mean values of the electromechanical efficiencies of the post-fatigue trials were significantly lower than those of the pre-fatigue trials at 10% (p < 0.01) and 20% MVC (p < 0.05). These findings suggest that the MMG analysis combined with the EMG may be a more useful method for evaluating the masseter muscle status.