RESEARCH ARTICLE Functional Compartmentalization of the Human Superficial Masseter Muscle

Some muscles have demonstrated a differential recruitment of their motor units in relation to their location and the nature of the motor task performed; this involves functional compart-mentalization. There is little evidence that demonstrates the presence of a compartmentali-zation of the superficial masseter muscle during biting. The aim of this study was to describe the topographic distribution of the activity of the superficial masseter (SM) mus-cle’s motor units using high-density surface electromyography (EMGs) at different bite force levels. Twenty healthy natural dentate participants (men: 4; women: 16; age 20±2 years; mass: 60±12 kg, height: 163±7 cm) were selected from 316 volunteers and included in this study. Using a gnathodynamometer, bites from 20 to 100%maximum voluntary bite force (MVBF) were randomly requested. Using a two-dimensional grid (four columns, six elec-trodes) located on the dominant SM, EMGs in the anterior, middle-anterior, middle-posterior and posterior portions were simultaneously recorded. In bite ranges from 20 to 60%MVBF, the EMG activity was higher in the anterior than in the posterior portion (p-value = 0.001). The center of mass of the EMG activity was displaced towards the posterior part when bite force increased (p-value = 0.001). The topographic distribution of EMGs was more homoge-neous at high levels of MVBF (p-value = 0.001). The results of this study show that the su-perficial masseter is organized into three functional compartments: an anterior, a middle and a posterior compartment. However, this compartmentalization is only seen at low levels of bite force (20–60%MVBF)