Abstract
Purpose: To clarify the time course of the coordination pattern in the jaw and hyoid muscle activities during swallowing following mastication.
Methods: Nine healthy dentate subjects were directed to chew a test food, consisting of sphere-shaped gelatin, on the preferred chewing side, and to swallow it. The test food was standardized in diameter, hardness and viscosity. Bilateral EMG activities of masseter, anterior temporal, suprahyoid, and infrahyoid muscles were recorded simultaneously with the mandibular movement, detected by a mandibular kinesiograph, and the laryngeal movement was detected by a handmade device with a highly sensitive force transducer.
Results: After rhythmical chewing strokes, while masseter and anterior temporal muscles were pausing, suprahyoid and infrahyoid muscle activities were seen to coincide with jaw opening and closing strokes, different from chewing strokes. The onset of infrahyoid muscle activities was significantly delayed from that of suprahyoid muscle activities. These activities finished at the same time. Subsequently, masseter and anterior temporal muscles were activated, while the mandible moved to the jaw-closing position. The suprahyoid and infrahyoid muscles were then activated again. The onset of suprahyoid and infrahyoid muscle activities was significantly delayed from that of masseter and anterior temporal muscle activities. The offset of infrahyoid muscle activities was significantly delayed from that of masseter, anterior temporal, and suprahyoid muscle activities. Rapid laryngeal movement occurred during this stage. The first activities of the suprahyoid and infrahyoid muscles may be equivalent to the transport stroke. The masseter and anterior temporal muscle activities, and the second activities of suprahyoid and infrahyoid muscles, may have a role in stabilizing the mandible during oral propulsive and pharyngeal stages of swallowing.
Conclusions: These results clarify the time course of the coordination pattern of jaw and hyoid muscle activities during swallowing following mastication.
