Lateral preference in mastication: relation to pain.

Ten subjects performed deliberate right and left-sided chewing of gum, each exercise with a duration of 60 s, followed by maximum voluntary teeth clenching (MVC) until onset of pains in the masseter muscles (pain threshold). Contractile activities of the right and left masseter muscles were assessed by integrated and cumulative surface electromyography (EMG). When the right and left muscles functioned as chewing side muscles only, they generated practically identical levels of chewing force; the same applied when they functioned as non-chewing side muscles only. During identical functions, EMGs suggested identical motor unit recruitment and firing patterns in the two paired muscles; i.e., units producing relatively high tensions for chewing side activity, and other units producing relatively low tensions for non-chewing side activity. During isometric MVC activity, until onset of pains, the right and left muscles generated practically identical levels of bite force, but right and left motor unit activity patterns seemed to differ. Right and left motor units might have fired at different rates or begun to drop out at different times. In the latter case, rapidly before slowly fatigued units. There was a tendency for a low pain threshold with high chewing side activity in both masseter muscles. Motor units producing high levels of tension, but easily fatigued, might have caused early onset of muscle pains. No relationships could be established between preferred chewing side and side of initial muscle pains, nor between MVC activity and side of pain onset and preferred chewing side.