Quantitative observations on maximum static work efforts and associated pain of the human masseter muscle.

The single case experimental design was used to study day-to-day variations in the onset (PL), tolerance (PT), and intensity (VAS) of masseter muscle pain. Pain was induced by maximum voluntary teeth clenching, with no artificial feedback-control of the level of isometric activity, and static work efforts were quantified by cumulative electromyography. A continual effort to produce maximum static work, about 40% increase from baseline work, elicited in 30-40 s an initial sensation of muscular pain that had an intensity of about 25% of maximum possible score. A further increase in maximum static work effort, about 60% increase from baseline work, caused in about 2 min an intolerable increase in the intensity of muscular pain, about 50% of maximum possible score. Whereas PL showed day-to-day variation, PT and the ratio PL:PT did not. Intensity (VAS) scores and maximum static work efforts showed no day-to-day variations. Onset (PL) and tolerance (PT) showed no linear associations with VAS scores, and VAS scores showed no linear associations with maximum static work efforts; if anything, the latter associations tended to resemble a cubic parabola. Two different central neural processes, not associated with maximum static work efforts in a simple linear manner, might have been instrumental in: (i) the establishment of criteria for onset and tolerance of pain, and (ii) the discrimination of variable levels of pain intensity.