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Biomechanical Face Modeling: Control of Orofacial Gestures for Speech Production

Abstract : To address motor control issues in speech production a 3D finite element model of the face has been constructed. This model is made of a mesh that consists of hexahedral and wedge elements. The mesh has three distinctive layers and is symmetrical about the mid-sagittal plane. Face muscles are anatomically represented in the mesh as subsets of contiguous elements. The elements of the mesh have elastic properties described by an isotropic nearly incompressible hyperelastic constitutive law. In order to study the global effects of muscles on facial mimics and lips gestures, and more specifically on speech gestures like protrusion and rounding, a simple linear muscle model has been first designed. The impact on facial gestures of stiffness changes in soft tissues is studied. Stiffening in soft tissues is indeed concomitant with muscle activation due to stress stiffening effect. This effect is accounted for in the muscle model through a variation of the hyperelastic constitutive law. Special attention is also devoted to the production of protruded and rounded lips which are required for the production of rounded vowels particularly in French. It is shown that stiffening helps the achievement of an accurate protrusion/rounding gesture thanks to the existence of a saturation effect in the relation between the muscle activations and the acoustically relevant geometrical characteristics of the lips. The result shows the importance of the dynamical properties of the articulators in the achievement of speech production gestures. Having been incited to improve the modeling of the main source of the force in speech movements, namely the muscles, a more realistic muscle model including a new constitutive law corresponding to a transversely isotropic nearly incompressible hyperelastic material and a Hill-type muscle model is designed in the ANSYS® finite element software thanks to the USERMAT programming facilities of this software. To account for a full Hill-type muscle model a force-velocity characteristic is then included in the new muscle element, thanks to the USERELEM facilities of ANSYS®. The implementation of this force-velocity characteristic introduces a damping effect on muscle movement due to a decrease of the muscle force when muscle compression velocity increases. The designed structure of the muscle element is general enough to enable studying other muscle models. Hence, Feldman's muscle model, which has been extensively used in former modelling works at Gipsa-lab, is implemented. In a bid to integrate the Feldman's model in a finite element structure a distributed formulation of this model has been proposed. The Hill-type and the Feldman-type muscle element are included in the face model to replace the first simple linear muscle model. The first simulations of lips protrusion/rounding gesture show realistic results. A comparison of the results obtained with the Hill-type model with those obtained with the Feldman's model is also conducted which shows that the final face shapes are very similar to those of these two models.
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Mohammad Ali Nazari. Biomechanical Face Modeling: Control of Orofacial Gestures for Speech Production. Biomechanics [physics.med-ph]. Université de Grenoble, 2011. English. ⟨tel-00665373⟩

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