Electromyographic Study of the Horizontal Force Exertion Mechanism during the Final Phase of Closure during Mastication

Abstract

Using an electromyograph, I experimentally examined the engagement of the bilateral medial pterygoid muscles (MPt), posterior temporal muscles (Tp) and masseter muscles (Mm) with exertion of horizontal forces during the final phase of closure during mastication. Measurements were carried out using an experimental intraoral apparatus on 5 male subjects with normal jaw function. The myoelectric discharges were measured during mandibular isometric vertical and lateral retrusive contractions at 4 different force levels (1/4, 1/2, 3/4 and maximum) at 4 different jaw positions (habitual closing position and three eccentric positions).
The results were as follows :
1. EMG activities increased significantly in the ipsilateral MPt and contralateral Tp during lateral retrusive contraction for each of the 4 forces and each of the 4 positions.
2. The mean EMG amplitudes for the ipsilateral MPt during lateral retrusive contraction were lower than those during vertical contraction, although there was no statistical difference between the two. This implies the ipsilateral MPt strongly engages in both vertical and lateral retrusive occlusion.
3. Lateral retrusive forces were always associated with vertical force.
From these findings it was concluded that the ipsilateral medial pterygoid muscle plays an important role in horizontal force exertion during the final phase of closure during mastication.