We propose a method to simulate muscle deformation in real-time, still aiming at satisfying visual results; that is, we are not attempting perfect simulation but building a useful tool for interactive applications. Muscles are represented at 2 levels: the action lines and the muscle shape. The action line represents the force produced by a muscle on the bones, while the muscle shapes used in the simulation consist of a surface based model fitted to the boundary of medical image data. The algorithm to model muscle shapes is described. To physically simulate deformations, we used a mass-spring system with a new kind of springs called “angular springs” which were developed to control the muscle volume during simulation. Results are presented as examples at the end of the paper