The riddle of the large loss in bite force after fast jaw-closing movements.

Abstract

In unloading experiments (in which the resistance to a forceful static bite is suddenly removed), it is shown that the residual bite force (when the jaw system is arrested shortly after the unloading) is remarkably small. For example, of a 100-N initial bite force, only 18 N is left after a jaw travel distance of 5.0 mm. The present experiments were designed to study whether the magnitude of the low residual bite force is dependent on the initial bite force, the initial degree of mouth opening, and the distance of jaw travel. Furthermore, we analyzed whether the low magnitude of the residual force can be attributed to reflex events of the jaw muscles or to the force-length properties of the jaw-closing muscles. It was found that the residual forces are largely dependent on the distance of jaw travel and are barely sensitive to variations in initial mouth-opening. The relative residual forces are independent of the magnitude of the initial bite force. The maximum residual forces are on the order of 25% of the initial bite force after a jaw travel of 4.5 mm. The low values of the residual forces cannot be attributed to reflex events, because it took about 80 ms for the masseter muscles to decrease their force to a 50% level after their excitation was switched off. Furthermore, it was shown that the force-length properties of the jaw-closing muscles are not responsible for the small values of the residual forces, since over the trajectories used in the present experiments, the sarcomere lengths of the jaw-closing muscles were beyond their optimum. It is suggested that the low residual forces are brought about by (1) a non-uniform sarcomere behavior of the jaw-closing muscles when contracting, or (2) a long-lasting change in the myofilament system of the closing muscles induced by the sudden shortening of muscle fibers.