A model for mylohyoid muscle mechanics

The purpose of the present study was to develop a mathematical model of the mylohyoid muscle allowing to analyze the complex mechanics of the muscle during jaw movement. The model was based on muscle morphology and physiological properties. Bending of fibers was incorporated into the model by pulleys located along the upper lateral border of the anterior belly of the digastric muscle. The dynamical properties of the muscle portions, i.e. force-length and force-velocity relationships, were related to sarcomere length changes. In addition, the effective force component produced by each portion in the sagittal plane was calculated. The model provided information on the geometrical changes of the muscle portions and the concomitant effect on sarcomere length, dynamical properties and effective force component as a function of jaw opening angle. Muscle configuration changed drastically and non-uniformly during jaw opening. However, sarcomere length changes were relatively small and differed but slightly between the muscle portions. The muscle portions all operated near optimum length regarding to their force- length relationship. In all muscle portions effective muscle force was the smallest (10-20% of maximum isometric force) in the beginning of the opening movement. With an increase of jaw angle effective muscle force gradually increased to 60-70% of maximum isometric force in the posteriormost muscle portion and to 20% in the anteriormost portion. Muscle fiber bending appeared to increase the sagittal plane component of the muscle force substantially.