Tracking of mandinular and cooperative head movements during mastication

Masticatory movements are rhythmical movements carried out by the coordinated activity of masticatory muscles, head and neck muscles, lingual muscles and intra-oral soft tissues. It has been reported that changes in head posture affect stomatognathic functions. Therefore it is hypothesized that the head is moving rhythmically simultaneous with mandibular movements during mastication. The purpose of this study was to observe the head movement and to investigate the coordination between movements of the head and the mandible. We used a system that tracks head and mandibular movements at the same time, and simultaneously recorded bilateral electromyographic signals from Masseter and Sternocleidomastoid muscles. During the experiment the subjects' head was not restrained. Three male adults who have no craniomandibular dysfunctions were selected. The task was to chew unilaterally a piece of chewing gum. Results were as follows; 1. During tapping movements, the maxillary incisor point moved vertically and rhythmically. This movement occured at the same time with vertical movement of the mandibular incisor point. The lateral movement of the maxillary incisor point was very slight. 2. During masticatory movements, the head made lateral cyclic movements cooperatively to the lateral component of the mandibular movement. 3. This cooperative movements were observed frequently in the early stage of mastication. 4. Lateral cyclic movements of the head showed a time delay of approximately 170-190msec from those of the mandible. So we conclude that the head produces vertical and lateral cyclic movements cooperative to those of the mandible during mastication. It is possibly brought about by the coordinated activity of masticatory and neck muscles.