Temporal profile and amplitude of human masseter muscle activity is adapted to food properties during individual chewing cycles

Jaw actions adapt to the changing properties of food that occur during a masticatory sequence. In the present study, we investigated how the time-varying activation profile of the masseter muscle changes during natural chewing in humans and how food hardness affects the profile. We recorded surface electromyography (EMG) of the masseter muscle together with the movement of the lower jaw in 14 healthy young adults (mean age 22) when chewing gelatin-based model food of two different hardness. The muscle activity and the jaw kinematics were analysed for different phases of the chewing cycles. The increase in the excitatory drive of the masseter muscle was biphasic during the jaw-closing phase showing early and late components. The transition between these components occurred approximately at the time of tooth-food contact. During the masticatory sequence, when the food was particularised, the size of the early component as well as the peak amplitude of the EMG significantly decreased along with a reduction in the duration of the jaw-closing phase. Except for amplitude scaling, food hardness did not appreciably affect the muscle's activation profile. In conclusion, when chewing food during natural conditions, masseter muscle activation adapted throughout the masticatory sequence, principally during the jaw-closing phase and influenced both early and late muscle activation components. Furthermore, the adaptation of jaw actions to food hardness was affected by amplitude scaling of the magnitude of the muscle activity throughout the masticatory sequence.