[Electromyographic studies on the exhibition of isometric protrusive and lateral protrusive mandibular forces]

To study the mechanism of exertion of horizontal mandibular force, the myoelectric effect of mandibular isometric protrusive and lateral protrusive contraction under four forces (1/4, 2/4, 3/4, or 4/4 of the maximum voluntary contraction) at three different jaw positions (O: centric occlusion, half: middle position between O and max, and max: maximum eccentric protrusive or lateral protrusive mandibular positions) was measured in six male subjects with normal jaw function. Electromyographic recordings of the inferior part of the lateral pterygoid (LPt), posterior temporalis (Tp) and masseter (Mm) muscles were taken bilaterally, and forces were measured with force transducer. EMG activities in the bilateral LPt during isometric protrusive contraction and in the contralateral LPt during lateral protrusive contraction increased with load intensity. Mm bilaterally during protrusive contraction, ipsilateral Tp and contralateral Mm during lateral protrusive contraction played an auxiliary role in exertion of horizontal mandibular force. EMG activities in the contralateral LPt during lateral protrusive contraction with every force and in the bilateral LPt during protrusive contraction with 1/4, 2/4, 3/4 force, were significantly increased under the eccentric mandibular position. However, EMG activities in the bilateral LPt during protrusive contraction with 4/4 force did not increase under the eccentric mandibular position. EMG activity in the LPt during protrusive contraction were higher than that during lateral protrusive contraction at every mandibular position and with every force. It was concluded that the LPt was predominantly concerned with exertion of horizontal mandibular force.