The effects of digastric muscle tenotomy on jaw opening in the rabbit.

The digastric and geniohyoid muscles of the rabbit both produce jaw-opening torque. Anatomic and biomechanical analysis, and electromyography of normal chewing, are not wholly adequate in determining the roles of these two synergists. Cinematographic and electromyographic records of pellet and carrot chewing were obtained before and after tenotomy of both digastric muscles. After tenotomy, jaw opening occurred more slowly and maximum gape was reduced for both foods. However, the overall frequency of chewing was unchanged, and the jaw muscles did not change their contraction patterns. Changes in opening speed and amount of gape result from loss of functional digastric muscles, not fully compensated for by the synergistic geniohyoids. The changes in opening speed and maximum gape are consistent with a biomechanical analysis which predicts a maximal contribution to jaw-opening torque by the geniohyoid muscle of about 25 per cent at the start of opening, and a substantial reduction of this torque in the course of the opening movement.