The purpose of this study was to investigate the functional property of the inferior head of lateral pterygoid muscle (ILP) in jaw movements. Electromyographic recordings (EMG) of bilateral ILP were obtained intraorally by using bipolar fine wire electrodes. In addition, surface electrodes were used on the posterior temporal muscles and the other 4 muscles. Jaw movement data were taken by the 6 degree of freedom digital jaw tracking device (MM-JI). Both EMG and jaw movement were recorded simultaneously in 8 subjects. The results were as follows.
1. Strong correlations existed between the activity of ILP and both distance and velocity of condylar translation during opening, protrusion and lateral movement in the contralateral direction.
2. During the slowest opening movement of mandible, the onset of ILP activity often occurred after the beginning of the forward translation of the condyle.
3. Restriction of the lateral border movement was due to earlier activity of the ipsilateral ILP and decreased activity of the ipsilateral posterior temporal muscle.
4. The relationships among the activity of ILP, chewing path and condylar movement during mastication were elucidated.
