Electromyographic study of human jaw-closing muscle endurance, fatigue and recovery at various isometric force levels.

The effect of a sustained isometric clenching, at various force levels or maximum voluntary bite-force levels, was evaluated on normal jaw-function subjects. Surface electromyographic (EMG) recordings of the masseter and temporalis were taken and bite force was measured using an intra-oral force transducer placed unilaterally in the 2nd premolar-1st molar region. Subjects sustained isometric force at the 25, 50, 75 and 100 per cent force level. Maximal bite force and EMG contraction levels were measured before, during and after these sustained isometric tasks. Subjects showed no change in their brief maximal contraction or force levels during or after various fatigue-inducing isometric tasks. The findings suggested a lack of contractile failure in the jaw-closing muscles. Pain intolerance rather than neuromuscular fatigue is the limiting factor of a sustained submaximal or even maximal clenching effort.