Functional Compartmentalization of the Human Superficial Masseter Muscle

Abstract

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 compartmentalization. There is little evidence that demonstrates the presence of a compartmentalization 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) muscle’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 electrodes) 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 homogeneous at high levels of MVBF (p-value = 0.001). The results of this study show that the superficial 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).

Expression of MyHC isoforms mRNA transcripts in different regions of the masseter and medial pterygoid muscles in chimpanzees

“…In summary, we found no differences in the expression of the MyHC isoforms mRNA transcripts between the two portions of the masseter and the medial pterygoid in Pan troglodytes, indicating that the two muscles work together to elevate the mandible as has been seen in electromyography. Nevertheless, we did find slight differences between the superficial and deep portions of the masseter, perhaps related to the different functions of the portions reported in electromyographic studies in humans and other primate species (Basmajian & de Luca, 1985;Blanksma et al, 1997;Guzmán-Venegas et al, 2015).…”

“…These findings are in line with those of Taylor and Vinyard (2013), who found that the superficial portion constituted 82.8% and the deep portion 17.2% of the total mass of the masseter. Nevertheless, functional differences between the anterior and posterior regions of the masseter observed in electromyographic studies in humans (Blanksma et al, 1997;Guzmán-Venegas et al, 2015) have not been borne out by our findings on the percentage of expression of MyHC isoforms mRNA transcripts in chimpanzees ( Table 1).…”

“…Moreover, the authors did not report information on hybrid fibers that can express different types of MyHC isoforms. In other species of primates, however, electromyographic studies have identified functional differences between the superficial and deep portions of the masseter (Basmajian & de Luca, 1985;Blanksma et al, 1997;Guzmán-Venegas et al, 2015;Hylander et al, 2000;Vinyard et al, 2008) and between the anterior and posterior regions of the two portions in humans (Basmajian & de Luca, 1985;Blanksma et al, 1997;Guzmán-Venegas et al, 2015). In contrast, no functional differences were detected between the masseter and the medial pterygoid muscleseither in humans (Basmajian & de Luca, 1985;Schindler et al, 2006) or in non-hominid primates (Hylander et al, 1987;Wall et al, 2006;Williams et al, 2011).…”

“…The medial pterygoid arises from the pterygoid fossa and the lateral pterygoid plate and inserts in the medial surface of the ramus and angle of the mandible (Williams & Warwick, 1980). Although both the masseter and the medial pterygoid act on the two halves of the ramus of the mandible, several electromyographic studies in humans and other primates have identified slight functional differences between the two muscles, as well as between the two portions of the masseter, during the chewing cycle (Basmajian & de Luca, 1985;Blanksma, van Eijden, van Ruijven, & Weijs, 1997;Guzmán-Venegas, Biotti Picand, & de la Rosa, 2015;Hylander, Johnson, & Crompton, 1987;Hylander, Ravosa, Ross, Wall, & Johnson, 2000;Schindler, Rues, Türp, & Lenz, 2006;Vinyard, Wall, Williams, & Hylander, 2008;Wall, Vinyard, Johnson, Williams, & Hylander, 2006;Williams et al, 2011).…”

Statistical parametric mapping of three-dimensional local activity distribution of skeletal muscle using magnetic resonance imaging (MRI)

“…Masticatory muscles are fractionated by aponeurosis, and their internal structures vary greatly depending on the person 24 , and different local activity distributions are thought to exist within the muscles depending on the subject. Against this background, some recent studies have reported that the local activity of the masseter muscle was measured two-dimensionally using array electrodes 13 . Testa et al analyzed the local activity distribution in the masseter muscle during tooth clenching with maximum occlusal force using array electrodes arranged in 13 rows and 5 columns at 8-mm intervals, and such distribution was compared between patients with chronic neck pain and healthy subjects 12 .…”