Dynamics of the human masticatory muscles during a jaw open-close movement.

The movements of the human jaw are controlled by the forces produced by the masticatory muscles. As the jaw moves, these muscles change in length and their force producing units, the sarcomeres, change in length simultaneously. The lengths and length changes of the sarcomeres are determinants for the forces they are able to produce. Hence, masticatory muscle force and jaw movement influence each other which makes it difficult to study their mutual relationship. In this paper, lengths and contraction velocities of the sarcomeres of the human jaw-opening and jaw-closing muscles are presented as well as the consequences for force production during jaw open-close movements simulated with a biomechanical model. Jaw-opening muscles acted almost synchronic in terms of sarcomere length, contraction velocity and force production. They were able to produce the largest isometric forces at relatively small jaw openings at the cost of reduced force production capabilities in wide open positions. In contrast, the jaw-closing muscles acted more differently. They were able to sustain active muscle force throughout a large range of the closing movement. Within this group the masseter and medial pterygoid contracted excentrically during a short time. The lateral pterygoid muscle portions behaved differently with respect to both groups. The jaw-opening muscles produced negligible passive forces during jaw closing. The passive forces of the jaw-closing muscles, however, contributed significantly to a limitation of the jaw-opening movement.