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Frontal plane mandibular movements during mastication and the associated electromyographic (EMG) activity for left and right superficial masseter, posterior temporalis, anterior temporalis, and anterior belly of the digastric (ABD) were studied for two adult male Macaca mulatta by the new technique of "contour" analysis. Contour analysis allowed graphic and quantitative portrayal of multiple chew cycle patterns of mandibular movement and EMG activity during active mastication. A series of computer programs (ATS, ATSED, ATSXYZ) facilitated the collection, editing and definition, and finally processing of these masticatory data into contour plots. These preliminary data indicated the essential symmetry of mandibular movement patterns, high chew cycle variability inferior to occlusion, multiple centers of intense EMG activity for balancing-side superficial masseter, and no difference between working-side anterior and posterior temporalis EMG patterns. Maximum EMG amplitude was found in the area of buccal phase power stroke (BPS). Maximum EMG amplitude for ABD was located medial and inferior to occlusion; all other muscle maximum amplitudes were buccal and inferior to occlusion. The location of maximum EMG amplitudes for superficial masseter and ABD were closer to occlusion (more superior) during mastication of carrot than were maximum amplitudes during biscuit mastication. The absence of any detectable shift of EMG maximum amplitude location between biscuit and carrot for posterior and anterior temporalis suggested, along with the continuous EMG activity of working-side posterior temporalis, a secondary role for the temporalis (compensation for superficial masseter activity) during active mastication.The problem of correlating electromyographic (EMG) activity with patterns of mandibular movement during the masticatory act has been a constant companion to students of mammalian mastication. As noted by previous investigators, masticatory EMG patterns vary from chew cycle to chew cycle during a single chewing sequence (Thexton and Hiiemae, 1975; Hiiemae, 1976Hiiemae, , 1978. It should prove helpful, then, to (1) consider both patterns of mandibular movement and EMG activity for multiple chew cycles in a fashion which depicts both central tendency and variability of the data, and (2) present such information in a graphic and quantitative fashion.In this paper we describe a technique for analyzing patterns of mandibular movement and EMG activity for multiple chew cycles. We additionally present some initial findings and interpretations made possible by these "contour" analyses of rhesus monkey mastication. MATERIALS AND METHODSTwo young adult male Macaca mulatta (5-6 kg) were used for this study. Data were collected by means of the optoelectronic method of mandibular tracking (Luschei and Goodwin, 1974; Byrd e t al., 1978). EMG leads (bipolar, fine-wire electrode) were surgically implanted in the left and right superficial masseter, anterior belly of the digastric (ABD), posterior temporalis, and anterior tempora...