Alteration of masticatory muscle EMG activities during chewing after a reversible bite-raising in guinea pigs

Previous studies have investigated the effects of increasing the occlusal vertical dimension (OVD) with an oral appliance on masticatory muscle EMG activity during oral behaviours in humans and animals. The present study investigated whether a short-term and reversible increase in OVD, followed by a reduction in OVD to the normal level, resulted in a time-correlated change in the EMG activities of the masseter and digastric muscles during chewing. To do this, a guinea pig model in which an increased OVD was established with natural tooth contacts was used. In the control group, in which no bite-raising treatment was applied, OVD gradually increased with a natural growth during the experimental period whilst the masseter and digastric EMG activities, burst duration, and chewing rhythm were unchanged. When the increase in OVD was established in the bite-raised group, the EMG activities of the masseter and digastric muscles were significantly increased by 88.6 and 55.2% from those before bite-raising treatment, respectively. However, during the following 11 days, the increased EMG activities of both muscles did not show changes associated with the subsequent decrease in the OVD to a normal level. The burst durations of both muscles and chewing rhythm were not significantly affected by the change in OVD during the experimental period. Within the limited recording period of the study, the return of OVD from increased to normal levels did not reverse the increased chewing-related masticatory muscle EMG activity that was induced by the bite-raising treatment.