WEAKNESS IN MOUSE MASTICATORY MUSCLES BY REPETITIVE CONTRACTIONS WITHFORCED LENGTHENING

The etiology of myofascial tenderness and pain of masticatory muscles in humans is difficult to understand. Parafunctional oral habits such as tooth grinding or vigorous chewing are thought to be factors. The o bjective of this study was to determine if masticatory muscles are sus ceptible to weakness and injury induced by repetitive, dynamic, forced -lengthening contractions. Results would support the hypothesis that c ontraction-induced injuries could occur in hyperactive masticatory mus cles of humans in response to parafunctional oral habits. Mice were an esthetized and randomly assigned to three groups: nontreated controls, treated by repetitive passive jaw opening, or treated by repetitive i sometric tetanic contractions with lengthening by jaw opening. In each treatment group, masticatory muscle injury was evaluated by contracti le tension, plasma creatine kinase, and muscle glycogen. Contractile t ension was determined at different stimulation frequencies and was sig nificantly decreased 5 min, 4 h, and 72 h after repetitive contraction /lengthening. Plasma creatine kinase was significantly elevated at 4 b ut not at 72 h post-treatment in mice subjected to repetitive contract ion/lengthening. Masticatory muscle glycogen was not significantly dif ferent in any groups at 4 or 72 h posttreatment. These results indicat e that contraction injuries can be induced in masticatory muscle of mi ce by forced lengthening contractions which simulate eccentric contrac tions.