ELECTROMYOGRAPHIC ANALYSIS OF THE STERNOHYOID MUSCLE AN ANTERIOR BELLY OF THE DIGASTRIC MUSCLE IN JAW MOVEMENTS

The sternohyoid muscle and the anterior belly of the left digastric mu scle were studied electromyographically in 20 young adult volunteer in dividuals. A surface monopolar electrode and a needle monopolar electr ode, inserted into the muscle mass 1.0 cm apart were employed. The mos t significant action of the two muscles was found in the opening of th e jaw, during which the sternohyoid muscle presented an isotonic contr action, allowing for displacements of the hyoid bone. They also acted on those movements that included one of the jaw depression components, such as protrusion, lateral movements to either side, and retrusion. They were inactive when the jaw was in the resting position. Both musc les operated simultaneously most of the time, but a synchronization of their actions could not be demonstrated.