Electromyographic analysis of the sternohyoid muscle and anterior belly of the digastric muscle in jaw movements.

The sternohyoid muscle and the anterior belly of the left digastric muscle were studied electromyographically in 20 young adult volunteer individuals. A surface monopolar electrode and a needle monopolar electrode, inserted into the muscle mass 1.0 cm apart were employed. The most significant action of the two muscles was found in the opening of the jaw, during which the sternohyoid muscle presented an isotonic contraction, 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 muscles operated simultaneously most of the time, but a synchronization of their actions could not be demonstrated.