Purpose: Neck pain is one of the main symptoms of temporomandibular disorder. Muscle activity of the sternocleidomastoid muscle during occlusion has been clarified in recent years. We reported that when healthy individuals were instructed to chew rapidly, the activity of the sternocleidomastoid muscle responded to activity of the masseter muscle, however, during voluntary jaw opening, activity of the sternocleidomastoid muscle did not respond, but worked actively due to motor programming. The objective of the present study was to investigate the learning effects of repetitive training, that is, changes in activity mode of the neuromuscular system.
Materials and Methods: The sternocleidomastoid and the anterior belly of digastric muscles in 8 healthy male adults were analyzed. In response to acoustic stimulation, each subject was instructed to open their mouth as quickly and widely as possible a total of 30 times with a break between measurements. EMG-reaction times (RT) of the sternocleidomastoid and anterior belly of digastric muscles were measured, and the length of time from the start of EMG activity of agonist to the start of actual movement was measured.
Results: In all subjects, at first measurement, EMG-RT of the sternocleidomastoid muscle did not precede that of the anterior belly of digastric muscle. With each measurement, the difference in EMG-RT between the sternocleidomastoid and the anterior belly of digastric muscles decreased, and in 6 of the 8 subjects, EMG-RT of the sternocleidomastoid muscle preceded that of the anterior belly of digastric muscle.
Conclusion: Repetitive task movement alters the start times of muscular activities, and from the perspective of EMG kinesiology, motor learning effects were confirmed with maximum ballistic voluntary jaw opening.