We have developped a physically-based model where an object is represented by a set of mass points on its contour. Each object may be defined locally and physically using surface regions. Physical properties (such as elasticity, motor functioning, or rigidity) can be assigned to the regions. Physical constraints, like incompressibility (i.e. constant volume deformation), can be set. Moreover, additional constraints can be used to control the animation and the object behaviour (e.g. nailed point, pre-defined trajectory). Depending on this physical properties and constraints, forces and movements are deduced.Using this model, we can handle bio-mechanical movements. Our simulation provides an anatomical and functional model of the evolutions of the human trunk structures during respiration.