Cumulative electromyography of the human masseter muscle during fatiguing isometric contractions.

Six adult males performed maximum voluntary tooth clenching (MVC) for 10, 20, 30, 40 s and, after 15 min, for 40, 30, 20, 10 s. During the isometric exercises the electrical currents of the masseter muscle were sampled by integrated and cumulative surface electromyography. Subjective masseter fatigue was present after 30 and 40 s of MVC clenching, accompanied by changes in myoelectrical activity. Strength testing of the masseter muscle, before and after endurance testing, showed that the strength increased by a significant 16% following two endurance tests. This observation was explained by a post-tetanic potentiation and/or a differentiated use of motor units in the fatigued muscle. It is concluded that brief MVC isometric activity, or strength testing, is not a reliable measure of fatigue in the masseter muscle when cumulative electromyography is used. Prolonged MVC isometric activity, or progressive endurance testing, monitors reliably the onset and progression of masseter fatigue.