The effect of fatigue on the silent period of the masseter electromyogram.

Abstract The silent period of the masseter electromyogram (EMG) is of an abnormally long duration in patients suffering from temporomandibular joint pain dysfunction syndrome (TMJPDS). Increased or abnormal patterns of muscular activity are generally accepted as being involved in the aetiology of TMJPDS and muscle fatigue may account for many of its key characteristics. This study demonstrated that the silent period was significantly lengthened after experimental fatigue induced by sustained masseteric clenching. EMG amplitude and mean power frequency (MPF) were monitored during the fatigue process. Overall, amplitude declined by 24 per cent and MPF shifted down by 33 Hz. These changes showed a different shaped time-course, exponential for amplitude and linear for MPF. The rate of change of MPF was related to endurance time. The findings are interpreted in terms of underlying changes in the fibre composition of neuromuscular activity during fatigue. They are consistent with a central role for fatigue and hyperactivity in the pathogenesis of TMJPDS, although it is argued that experimental tooth grinding may provide a better aetiological model.

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