Effect of a repeated jaw motor task on masseter muscle performance

The aim of this study was to investigate the effects of 5-days repeated jaw-motor tasks on masseter muscle accuracy performance.

Sixteen healthy participants performed a tooth-clenching task (TCT) of 58-minutes on five consecutive days. During measurements, electromyography (EMG) of both masseter muscles was recorded. Each day, the 100% maximum voluntary contraction (MVC) level was determined before the TCT. In the first and third TCT series, participants were instructed to target force levels without visual feedback. During the second TCT series, visual feedback on muscle activity level was displayed. One series consisted of three force levels (10%, 20%, and 40% MVC). In the series, participants alternated between a 30-s rest-block and a 30-s task-block for 360 s. In the task-block, participants alternated between a 5-s rest-block and a 5-s task-block. EMG activity during epochs of 5-s was quantified by calculation of the root-mean-square (RMS) values. To evaluate the accuracy of the performance, the coefficient of determination (CD) of the target force level-EMG curve was calculated from all series.

No significant day-to-day differences in EMG RMS amplitudes were observed during MVC. CDs differed significantly between the five days (P<0.001). CDs in the first series on day 1 were significantly lower than CDs in the first series on days 4 and 5 (P<0.05).

The findings suggest that a rigorous training paradigm may improve the performance of masseter muscles in terms of accuracy but not MVC. This might have implications for understanding the rehabilitation of patients with functional disorders in the stomatognathic system.