DYNAMICS OF THE HUMAN MASTICATORY MUSCLES DURING A JAW OPEN-CLOSE MOVEMENT

The movements of the human jaw are controlled by the forces produced b y the masticatory muscles. As the jaw moves, these muscles change in l ength and their force producing units, the sarcomeres, change in lengt h simultaneously. The lengths and length changes of the sarcomeres are determinants for the forces they are able to produce. Hence, masticat ory muscle force and jaw movement influence each other which makes it difficult to study their mutual relationship. In this paper, lengths a nd contraction velocities of the sarcomeres of the human jaw-opening a nd jaw-closing muscles are presented as well as the consequences for f orce production during jaw open-close movements simulated with a biome chanical model. Jaw-opening muscles acted almost synchronic in terms o f sarcomere length, contraction velocity and force production. They we re able to produce the largest isometric forces al relatively small ja w openings at the cost of reduced force production capabilities in wid e open positions. In contrast, the jaw-closing muscles acted more diff erently. They were able to sustain active muscle force throughout a la rge range of the closing movement. Within this group the masseter and medial pterygoid contracted excentrically during a short time. The lat eral pterygoid muscle portions behaved differently with respect to bot h groups. The jaw-opening muscles produced negligible passive forces d uring jaw closing. The passive forces of the jaw-closing muscles, howe ver, contributed significantly to a limitation of the jaw-opening move ment. (C) 1997 Elsevier Science Ltd.