Contractile activity of the masseter muscle in experimental clenching and grinding of the teeth in man.

Six human subjects exercised maximum voluntary tooth clenching and right-sided tooth grinding to determine the onset of fatigue in the right and left masseter muscle. Static and dynamic contractile activity of the two muscles was determined by surface electromyography. Muscle fatigue appeared after about 30 s of isometric contractions (clenching), while 30 s of combined concentric and eccentric contractions (grinding) induced no fatigue. In the right muscle the contractile activity of negative work (eccentric contractions of mandibular laterotrusion) was about 50% of that of positive work (concentric contractions of mandibular mediotrusion). During clenching an increased number of contacting teeth might have facilitated the contractile activity of the two muscles. During grinding the height of the cusps of the working side teeth might have contributed to a decrease of tension production by the right masseter muscle. Non-working side tooth contacts and peripheral receptors might have facilitated the contractile activity of the left masseter muscle during tooth grinding. Static contractile activity of the mandibular elevator muscles produced high levels of isometric tension and led to masseter muscle fatigue in about 30 s. The same duration of dynamic contractile activity, resulting in low levels of tension during positive and negative work, did not induce fatigue.