Abstract
The aim of this study was to elucidate the fatigue characteristics of the masticatory muscles. In five healthy, fully dentate male subjects, maximal voluntary isometric bite force and elbow flexion and knee extension isometric strength were registered with the Dental Prescale System and a load cell with weight indicator respectively. Simultaneous to the tasks, the electromyographic (EMG) activity from the masseter, temporal, biceps brachial, and rectus femoral muscles was recorded with the Multi Telemeter System. A protocol involving three 5-second and three 30-second long isometric contractions with 10-second rest periods in between was designed to induce fatigue in the muscles studied. The 5-second tasks were made five times during the fatigue recovery period at different rest intervals. The 5-second isometric contractions were compared to a control 5-second one that was recorded at the beginning of the protocol. The EMG signals of each muscle during the tasks were integrated and their peak frequency of power spectrum was calculated. As a parameter of muscle fatigue, the ratio of muscle force and integrated EMG was used to observe the efficiency with which the muscle action potentials were transformed into mechanical work.
The results are as follows:
1. The maximum voluntary isometric bite force, elbow flexion and knee extension strength decreased gradually with repeated trials, and increased gradually in the fatigue recovery stage.
2. The onset of muscle fatigue and of recovery of the masseter and temporal muscles occurred earlier than those of the biceps brachial and rectus femoral muscles.
3. In all the muscles, a shift in the peak frequency of the EMG power spectrum was observed according to the state of the muscle; lower values in the fatigue phase, and higher values in the recovery stage.
4. In the masticatory muscles the integrated EMG decreased with muscle fatigue.
5. The muscle force/integrated EMG ratio of the masseter and temporal muscles showed higher values than the biceps brachial and rectus femoral muscles.
From these results, we suggest, that the masticatory muscles have fatigue resistant characteristics that differ form those of the limbs.
