[An electromyographic study of the inferior heads of the lateral pterygoid muscles during displacements of mandible].

The purpose of this study was to investigate electromyography of the inferior heads of lateral pterygoid muscles in experimental displacement of mandible, and its clinical application for reproduction and diagnosis of mandibular position. In the experiments, the mandibular position was changed in vertical and horizontal directions and muscle activity of the inferior heads was analyzed quantitatively. The results obtained were as follows: 1. Muscle activity of inferior heads was increased with amount of the displacement in anterior direction, and reached to great increase in 1.0 mm and 2.0 mm displacement of mandible. 2. No definite tendency of ipsilateral inferior heads was shown in lateral displacement of mandible. 3. Muscle activity of contralateral inferior heads in lateral displacement was increased with amount of the displacement, and 3 muscles showed great increase in 1.0-2.0 mm displacement of mandible. 4. As vertical increase was less than 2.0 mm, muscle activity of the inferior heads was decreased as to that of intercuspal contact position. 5. Muscle activity of the inferior heads in retruded position showed a little increased, but evidently as to that of intercuspal contact position. 6. Increasing patterns of muscle activities in anterior displacement were different from in contralateral displacement. 7. In anterior mandibular position an ability of presuming a mandible displacement over the limits of 1.0 mm from intercuspal contact position was suggested using muscle activity of inferior head as an index, and in lateral mandibular position that over the limits of 1.5 mm suggested.