Physically based models attempt to model the shape and dynamic changes of the face by modeling the underlying properties of facial tissue and muscle action [149][148][133][147][120][124][122]. Most of these models are based on mass-spring meshes and spring lattices, with muscle actions approximated by a variety of force functions. These models are computationally expensive and difficult to control with force-based functions. Research has also been reported on modeling the physical properties of skin using a finite element approach [121][78]