Masticatory motion is controlled in humans by a limited set of muscle synergies

The masticatory motion, whereby food introduced into the mouth is processed into a bolus suitable for swallowing, can be divided into successive masticatory cycles, each comprising downward and subsequent upward movements of the mandible. The present study deals with the problem of the existence of muscle synergies in mastication, that is whether some of the muscles involved in mastication receive common motor drives, rather than controlled individually. Evidence for muscle synergy during mastication is scarce, partly due to the difficulties in simultaneous recording of the electromyographic (EMG) activities from all the muscles involved. Thus, we analyzed the variability of the mandibular motion during mastication rather than to examine the EMG patterns, based on the hypothesis that a motion elicited by a limited set of muscle synergies can be approximated as a superposition of the same number of independent motions. Mandibular motion paths were recorded from 8 healthy males (25-31 years), who chewed gum or gummy candy. A morphometric technique, which describes the shape of a closed curve by using normalized elliptic Fourier descriptors and reduces the variance of the shape by using principal component analysis, was applied to analyze the variability of the mandibular motion paths. We found three independent variations of the motion paths, whose linear combinations accounted for an average of 93% (range, 88-96%) of the total variance. The extracted variations were similar among the subjects. These findings provide indirect evidence for the existence of a limited set of muscle synergies for mastication in humans.