Electromyographic Heterogeneity in the Human Masseter Muscle

Abstract

The complex, pennate architecture of the human masseter muscle points to a functional division into more than the commonly distinguished deep and superficial parts. In this study, the possible existence of regional differences in activation was examined. EMG activity was registered in three deep and three superficial regions with the use of bipolar fine-wire electrodes. Recordings were made during different static bite tasks, in specified directions, and with a specified bite-force magnitude. A linear bite-force/EMG relationship was observed. Furthermore, it appeared that muscle regions showed a different pattern of change in activity as a function of bite-force direction. Heterogeneity was nearly absent in anteriorly-, anteriomedially-, and medially-directed bites, but became increasingly obvious in the other bite-force directions. The posterior deep region showed the most aberrant activation pattern, which was almost opposite that from the other regions. This part was fully active in posterolaterally-directed bites. The posterior superficial region showed the largest variability in activity as a function of bite-force direction. The results point to a functional partition of the masseter muscle in at least three parts: anterior deep, posterior deep, and superficial. A further subdivision of the superficial portion might be present, but was not as obvious as the division of the deep masseter.