[The changes of the orofacial muscle activity induced by the lip forces in subjects with normal occlusion]

This study was intended to investigate whether lip exercise affect the activities of the lips, tongue, digastric and masseter muscles and to understand the functional interrelationship between them. As the forces of maximum lip grimace was reduced to slight lip seal gradually, the electromyograph was taken from 9 conscious human adults with normal occlusion in the upper and lower lips, tongue, digastric and masseter muscle using surface electrodes, especially, newly designed subminiature surface electrodes which was attached directly to the tongue. The results were as follows. 1. During induced maximum lip grimace in the mandibular rest position, there're prominent increase of EMG activity in the upper and lower lips, tongue and digastric muscle 1714 mu v, 2787 mu v, 2142 mu v and 623 mu v, respectively, but there was little activity in the masseter muscle, 129 mu v. The force level induced by maximum grimace of the lips was about 110g when the force transducer was positioned between upper and lower lips in the incisor area. 2. As the lip forces were reduced gradually, EMG activities of the lips, tongue and digastric muscles were decreased definitely with certain tendency, but there was no change in the masseteric EMG activity. Above mentioned results suggest that (1) lip exercise can promote the activities of the lips, tongue and digastric muscles but cannot change the activity of the masseter muscle and (2) masseteric muscle activity seems to be independent of the lips and the tongue but digastric muscle activity is related more closely to them through lip exercise in the subjects with normal occlusion.