This paper describes facial muscular activities in drowsy expression. We measured facial images and facial electromyograms (F-EMGs) in a tracking task conducted with 17 participants. We investigated the mechanism of drowsy expression through analysis of the F-EMGs. Contraction of the frontalis muscle and relaxation of the zygomaticus major muscle were found to be the two common features of muscular activities in drowsy expression. The former muscular activity appeared in a medium drowsy condition, but not in arousal or dozing states. This result indicates a driver's resistance against drowsiness before reaching a dozing state, in that contraction of the frontalis muscle causes the driver's eyebrows to rise. The latter muscular activity became more obvious with an increase in the driver's drowsiness. This result indicates a decline in the driver's physical activity, in that relaxation of the zygomaticus major muscle causes the driver's cheeks to slack. Detecting these two features is considered to be useful in estimating a driver's drowsiness level by means of facial image processing. The activities of the orbicularis oris muscle, mentalis muscle, and masseter muscle, which causes the driver's "lips part (slightly open mouth)", showed a variety of tendencies in respect of the drowsiness levels among different individuals. Consideration of the difference in muscular activities among individuals is necessary when the "lips part" feature is applied to estimation of a driver's drowsiness level through facial image processing.

This paper describes an experiment designed to verify whether contraction of the frontalis muscle indicates resistance to drowsiness. Electromyograms (EMG) of the frontalis muscle were monitored while the subject operated a driving simulator under the following two conditions : (1) Dozing not permitted, (2) Dozing permitted. We measured the resulting data of the 15 tested subjects and analyzed the contraction of the frontalis muscle by comparing the results taken from the two conditions listed above. The results showed that the contraction of frontalis muscle observed in condition (2) was smaller than that which was observed in condition (1) by a significant degree. It was concluded that contraction of the frontalis muscle indicates a resistance to drowsiness. There were significant differences in frontalis activity between condition (1) and (2) during the early stage of the test before dozing occurred. We have concluded that frontalis muscle activity is a viable method to detect early stage drowsiness.