Abstract
[Purpose] Masticatory muscle activities are often expressed by means of the relative value against the burst at maximum voluntary contraction (MVC), i.e., %MVC ratio. The purpose of this study was to clarify the relation between %MVC ratio of electromyographic bursts of the masseter muscle during daily mastication and maximum bite force.
[Methods] Nineteen females who were clinically diagnosed as bruxism patients participated in this study. A wearable ultraminiature electromyographic device was attached to the unilateral masseteric part of the face on the dominant mastication side, and an electromyogram (EMG) was recorded during daily life. EMG data during meal time on the second day of measurements were analyzed, and EMG bursts longer than 0.08 seconds, with more than three times the baseline amplitude and with an interval to the neighboring burst of 0.08 seconds or more were selected. %MVC ratio, i.e., the value of maximum amplitude of each selected burst divided by the amplitude of MVC, was calculated. The mean %MVC ratio for each subject was calculated. Maximum bite force was measured twice for each subject using Dental Prescale®, and the mean of the two measurements was used as a representative value for each subject.
[Results] Mean and standard deviation of maximum bite force for the 19 subjects were 518.8 and 327.1 N, respectively. Mean and standard deviation of %MVC ratios during mastication were 64.3 and 27.4 %, respectively. A significant negative correlation was found between maximum bite force and %MVC ratio during mastication (RS=-0.71, p=0.0025).
[Conclusions] The relation between %MVC ratio of electromyographic bursts of the masseter muscle during daily mastication and maximum bite force was elucidated. It was demonstrated that masseteric activity during mastication tended to be expressed as EMG %MVC ratio being higher than the actual force in cases with less maximum bite force. The results suggested that we should carefully interpret EMG results expressed using %MVC ratios in consideration of maximum bite force when we assess the masticatory muscle activity during mastication.
