This study was conducted to investigate the muscle activity of the medial pterygoid, the superficial masseter and the anterior temporal muscles when two bite forces (49 N and 147 N) were exerted on different teeth. The subjects were instructed to perform isometric muscle contractions on either the right or left first premolar or second molar. Simultaneously, the EMG activity of the muscles was registered using subcutaneous needle electrodes.
The results were as follows:
1. The change of the bite-point location affected the EMG activity of all the muscles. Although the occlusal bite force was altered from 49 N to 147 N, various patterns in each muscle activity at each bite location were almost consistent.
2. The largest activity was observed in the medial pterygoid and masseter muscles on the contralateral first premolar, while that of the temporal muscles was on the ipsilateral first premolar. Clenching on one first premolar compared with the other first premolar resulted in a significant increase in the activity of the working-side temporal muscle.
3. The bite-point location being changed from right second molar to various other teeth, at a 49 N bite force the right medial pterygoid muscle had a significant correlation between the right or left masseter and right temporal muscles. At a 147 N bite force the medial pterygoid muscle significantly correlated to ipsilateral masseter muscle.