Retrusive endurance, fatigue and recovery of human jaw muscles at various isometric force levels.

The effect of a sustained isometric retrusive contraction on maximum voluntary retrusive force levels was measured in normal jaw function. Surface electromyographic recordings of the suprahyoids, masseter and posterior temporalis were taken and force was measured with a force transducer. Subjects sustained isometric force at the 25, 50, 75 and 100 per cent level, and measurements made before, during and after these sustained isometric tasks. There was no change in the brief maximum voluntary contraction levels of the retruder muscles during or after such tasks, which suggests a lack of contractile or electrical failure in these muscles. Pain intolerance, rather than demonstrable neuromuscular fatigue, was the limiting factor for sustained submaximal or even maximal contraction effort.