1. A computer-implemented method for simulating a shape of a musculoskeletal system comprising a skeletal subsystem and a soft-tissue subsystem, the method comprising: saving, in a memory accessible to a computer system, a skeletal subsystem model comprising one or more rigid bodies and rigid body variables representative of shapes of the one or more rigid bodies in a physical space at at least one specific time; saving, in the memory accessible to the computer system, a soft-tissue subsystem model representative of the soft-tissue subsystem, the soft-tissue subsystem model comprising one or more deformable soft-tissue objects and, for each of the one or more soft-tissue objects, a soft-tissue object representation of the soft-tissue object in an Eulerian space comprising a plurality of vertices, the soft-tissue object representation comprising an associated Eulerian representation for each vertex, the Eulerian representation specifying which one or more material space coordinates from among a set of material space coordinates for the soft-tissue object correspond to the vertex at the at least one specific time; simulating, by the computer system, an evolution of the skeletal subsystem model over a series of time steps to thereby determine the rigid body variables representative of the shapes of the one or more rigid bodies in the physical space at each time step of the series of time steps; simulating, by the computer system, an evolution of the soft-tissue subsystem model over the series of time steps to thereby determine, for each of the one or more soft tissue objects, the soft-tissue object representation and the associated Eulerian representation for each vertex in the Eulerian space at each time step in the series of time steps, the associated Eulerian representation, for each vertex and for each time step in the series of time steps, specifying which one or more material space coordinates for the soft-tissue object correspond to the vertex at the time step; and saving, in the memory accessible to the computer system, the rigid body variables and the soft-tissue object representation for each of the one or more soft-tissue objects at each time step of the series of time steps; wherein simulating the evolution of the skeletal subsystem model and the soft-tissue subsystem model over the series of time steps comprise coupling the skeletal subsystem model and the soft-tissue subsystem model to one another using one or more coupling constraints, the one or more coupling constraints constraining available solutions to the skeletal subsystem model and the soft-tissue subsystem model over the series of time steps.