A Comparison of the Frontalis Muscle and Masseter Muscle with Regard to Reactivity to Stress and Stress-Recovery

An increasing number of clinicians are utilizing electromyogram (EMG) biofeedback as a technique for teaching general relaxation in the treatment of certain stress-related disorders. The muscle most widely accepted for training in this technique has been the frontalis. The choice of the frontalis, or any other muscle, is based upon several assumptions regarding its properties, with one very important one being the muscle\u27s capacity to react (to exhibit increased muscle tension) to stress. Another muscle, the masseter, has also shown itself to be reactive to stress. One purpose of this study was to compare the frontalis and masseter muscles for their respective reactivity to stress and to establish if one is significantly more reactive to induced stress than the other. A second assumption underlying the choice of a muscle is if, and to what extent, the trained muscle facilitates a transfer of training (of relaxation) outside of the training situation. The recently developed concept of stress-recovery has been utilized as a method for determining a muscle\u27s ability to produce a transfer of training effect. A second purpose of this study was to compare the frontalis and masseter muscles as to their respective stress-recovery rates. Subjects in the study were 48 undergraduate males and females (24 of each). Each subject participated in a 50-minute experimental session consisting of a baseline period, experimental (induced) stress period, and a stress-recovery period. EMG readings were taken from each muscle during the criterion periods. These readings were averaged for the baseline and experimental stress periods and taken at two-minute intervals over the eleven-minute stress-recovery period. Results showed that, while both the frontalis and masseter muscles reacted significantly to the experimental stressor, the frontalis reacted at a statistically significantly higher level than the masseter. Results from comparing the two muscles for differences in stress-recovery rate revealed no significant differences. In fact, their respective recovery rates were almost identical. A post hoc analysis of possible sex differences revealed some differences for both hypotheses along this variable when compared to the combined sample. However, the results did not allow for firm conclusions as to the real significance of these differences..