Synchronous Muscle Synergy Evaluation of Jaw Muscle Activities during Chewing at Different Speeds, a Preliminary Study

Abstract

Human mastication is a complex and rhythmic biomechanical process regulated by the central nervous system (CNS). Muscle synergies are a group of motor primitives that the CNS may combine to simplify motor control in human movement. This study aimed to apply the non-negative matrix factorization approach to examine the coordination of the masticatory muscles on both sides during chewing. Ten healthy individuals were asked to chew gum at different speeds while their muscle activity was measured using surface electromyography of the right and left masseter and temporalis muscles. Regardless of the chewing speed, two main muscle synergies explained most of the muscle activity variation, accounting for over 98% of the changes in muscle patterns (variance accounted for >98%). The first synergy contained the chewing side masseter muscle information, and the second synergy provided information on bilateral temporalis muscles during the jaw closing. Furthermore, there was robust consistency and high degrees of similarity among the sets of muscle synergy information across different rate conditions and participants. These novel findings in healthy participants supported the hypothesis that all participants in various chewing speed conditions apply the same motor control strategies for chewing. Furthermore, these outcomes can be utilized to design rehabilitation approaches such as biofeedback therapy for mastication disorders.

Keywords: chewing; masticatory muscles; muscle synergy; surface electromyography.

Figure 1

The framework of the proposed muscle synergy analysis.

Figure 2

The timeline of chewing at different speeds.

Figure 3

Synchronous averaging of temporalis and masseter muscle activation at each speed during chewing by right (blue) and left side (green) for one participant. The solid line and shaded area indicate the mean and standard deviation of muscle activation for all sequential chewing cycles. Each muscle activity was normalized by maximum activation among all chewing speeds.

Figure 4

Mean and standard deviation across all participants and speed of VAF. Two synergies were for all participants and speeds.

Figure 5

Muscle synergy vectors (W) and synergy activation coefficients (C) over all participants for two extracted synergy at different speeds. The error bar indicates the standard deviation of the muscle synergy vectors of all participants.

Figure 6

Individual illustration of the reconstructed muscle activation patterns (black solid line) at a speed of 50 cycles/min by combining two muscle synergies (red dotted line).

Figure 7

Mean and standard deviation of the VAF for all participants at each speed.

Figure 8

Mean and standard deviation of the VAF for each participant at each speed.

Figure 9

Error bar of inter-subject variability of similarity between two participants in each extracted synergy during right and left-side chewing (upper row). Error bar of similarity between two speeds (lower row).