[Methods proposed for the determination of external forces applied to the mandible during mastication. Initial results].

The purpose of this study is to determine the external forces that are brought to bear on the mandible during various masticatory exercises. An analysis is presented of a static two-dimensional mandibular model taking into account projections in the sagittal plane of the forces exerted by six muscle groups and the joint reaction. In calculating the external forces, two working hypotheses are adopted: the existence of a linear relationship between cross-sectional surface of the muscle and its maximum force of contraction; the existence of a linear relationship between the electrical signal emitted by a muscle and the force the muscle then develops. It follows that, in order to calculate, the muscle forces, there is a need to measure the intensity of masticatory effort, the cross-sectional surface and the electromyographic activity of the different muscles involved. These values are then used for working out the equations of equilibrium. Results are provided for 14 masticatory exercises effected in three different mandibular positions and compared with data from literature. Individual variations recorded in the measurement of the different parameters involved in the equilibrium equations are such that they must be taken into account for the calculation of the forces. Moreover, the functional value of the muscles appears to be closely dependent on the mandibular position with which they are associated, which is not brought out in earlier studies. Finally, intra-joint action proves to be intense, although its direction varies according not only to the mandibular position but also the type of exercise performed.