Association between surface electromyography of human jaw-closing muscle and quantified food breakdown.

Simultaneous recordings of the activity of the masseter and anterior temporalis and of jaw movement were made on 10 healthy dentate volunteers while they chewed roasted peanuts in their habitual manner. The quantity of food taken was altered by varying (a) its total weight (mouthful) between 8 and 1 g, but with a fixed initial particle size and (b) by varying its initial particle size between median sizes of 9.2 and 2.4 mm, but keeping the mouthful constant. Significant differences were found in the peak and mean (r.m.s.) estimates of muscle activity (particularly in the masseter), and the dimensions of jaw movements. These differences were associated with the variation in both the size of the mouthful and of the initial particles but were much greater with changes in the mouthful. A dimensional argument treating both the weight of food in the mouth and initial particle size as volumes showed that initial particle volume had been varied by approximately 50 times as much as mouthful volume and therefore that the mouthful was a far more critical factor in masticatory physiology than was the particle size of the food. The reasons for this probably lie in an understanding of mechanisms of food comminution.