Surface electromyographic evaluation of the asymptomatic human masseter muscle with turns and amplitude analysis

Electromyographic turns, or reversals of direction, per second (TS) and mean amplitude per turn (AT) are variables that have been widely used to assist and support an electrodiagnosis in neuromuscular disorders. The purpose of this study was to explore the feasibility of utilizing the TS and AT in conjunction with surface electrodes as a method for assessing masticatory muscle function. Ten normal subjects were instructed to exert brief isometric contractions at various levels, and the TS and AT were calculated from the surface electromyographic signals of the masseter muscle. Our results showed that the AT increased with contraction at all contraction levels, demonstrating a linear relationship, while the TS showed a nonlinear increase characterized by an initial steep rise followed by a plateau, which was simulated with a third polynomial function. It is considered that the TS and AT may provide valuable physiological information about the underlying mechanisms of recruitment and the firing of motor units.