Delayed onset of masseter muscle pain in experimental tooth clenching.

At 100% and about 50% of maximum voluntary isometric contraction strength of the masseter muscle, six healthy human subjects exercised tooth clenching until there was onset of muscle pain in 42 and 146 sec, respectively. Onset of pain, at maximum contraction strength, might have been due to the cumulative amount of activity in relatively large motor units. The delayed onset of pain might have been associated with a decline in the quantity and quality of contracting large motor units. The Clinical implications of this study suggest that an increase of the vertical dimension of the face will delay the onset of muscle pain in patients suffering from tooth clenching.