A 3D muscle element nased on Equilibrium Point Hypothesis (EPH) and its application in a biomechanical face model

In our opinion, speech motor control cannot be fully understood without accounting as realistically as possible for the physical properties of soft tissue articulators, and in particular of muscle soft tissues. The present work is a new step toward the design of increasingly realistic muscle models and their integration in biomechanical models of speech articulators. A finite element muscle model has been designed with the UserElem functionality of ANSYS®. This new element is very flexible and it can be used in all kinds of muscle modelling problems. Due to the intrinsic tensile force generation, the tissue stiffness increases in the transversal direction with increasing activation. Another important feature of the muscle model is the fiber direction definition. These fibers can be defined independently of the element orientation and muscles can be thus implemented independently of the mesh geometry. This element has been tested in an existing finite element model of the human face, which has been designed to study interactions between motor control and physical properties of the face in speech production. Two models are proposed for the muscle activation function, a Hill type model and a model based on the Equilibrium Point Hypothesis. A comparison of these models is provided.