Effect of repeated jaw motor task on masticatory muscles

Method: Sixteen participants with normal stomatognathic function participated. All subjects performed three series of tooth-clenching tasks (TCT) on five consecutive days. During all measurements, the EMG activities from the left masseter (LM) and right masseter (RM) muscles were recorded using surface electrodes. Each day, the 100% maximum voluntary contraction (MVC) level was determined before the TCT. In the first and third TCT series, participants received no visual feedback but were simply instructed to target different force levels. During the second TCT series, visual feedback of the muscle activity level, via the electromyographic (EMG) data, was displayed on a monitor. One series consisted of three measurements (10, 20, and 40% MVC), and one measurement consisted of one force level in randomized order within 1-h. During all measurements, 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. The EMG activity during epochs of 5-s was quantified by calculation of the root-mean-square (RMS) amplitude from both muscles. To evaluate the accuracy of the performance on each day, the coefficient of determination (CD) of the target force level–EMG curve from both muscles was calculated from all series.

Result: There were no significant day-to-day differences in the EMG RMS values during MVC (P=0.326). The CDs were significantly different between the five days (P<0.001). The CDs in the first series at day-1 were significantly lower than the CDs in the first series at day-4 and -5 (P<0.05).

Conclusion: Our findings suggest that the masticatory muscles are prone to improve, not the performance of MVC, but the accuracy of the performance due to a training effect, i.e, motor learning of the masticatory muscles.