This paper describes an improvement of a classical energy-based model to simulate elastically deformable solids. The classical model lacks the ability to prevent the collapsing of solids under influence of external forces, such as user interactions and collision. A thorough explanation is given for the origins of instabilities, and extensions that solve the issues are proposed to the physical model. Within the original framework of the classical model a complete restoration of area and volume is introduced. The improved model is suitable for interactive simulation and can recover from volumetric collapsing, in particular upon large deformation.