[Influence of posture on the sternocleidomastoid muscle activity in occlusal function]

The purpose of this research was to investigate influence on the sternocleidomastoid muscle (SCM) activity in clenching depending on body and head positions. Seven male subjects without jaw-dysfunction were asked to exert maximum voluntary contraction in three body positions; sitting, supine and prone. As for the head positions, those of normal and full rotations (right and left) in each body position were required. In addition, in the sitting position, those of half rotation (right and left) were performed. EMG activity was recorded by means of bipolar surface electrodes. The results were as follows: 1. In full rotated head positions, EMG activity of the contralateral SCM varied about twice as much as in normal head positions. 2. In half rotated head positions, EMG activity of the contralateral SCM varied 1.20 times as much as in normal head positions. 3. In rotated head positions, EMG activity of the ipsilateral SCM varied from 0.81 to 1.22 times as much as in normal head positions. 4. EMG activity of SCM without clenching (background activity) had little influence on that with clenching. 5. In normal head positions, EMG activity in the supine position varied 0.86 times as much as in the sitting position. In conclusion, the head positions have much more influence on EMG activity of SCM in clenching than the body positions.