Human muscle modelling from a user's perspective.

Methods for developing mathematical models representing entire human muscles are briefly reviewed, with special emphasis on aspects of modelling velocity dependence using cross-bridge dynamics, and isometric force-length properties from myofilament lengths and muscle architecture. For each of these components, mechanistic (using basic contraction mechanisms) and phenomenological ("black-box") models are available. Experiments on constant-velocity lengthening at different velocities were simulated using (a) a cross-bridge based model and (b) a Hill-based model. The Hill model was superior in its ability to predict muscle forces under different conditions with the same model parameters. Regarding force-length properties, myofilament overlap and muscle architecture did not correctly predict maximal isometric joint moments over the entire functional range of motion. The width of the force-length relationship of all contractile elements in a lower extremity model may be optimized to fit measured isometric moment-angle relationships. The resulting increase in width suggests that for some short-fibered muscles with complex architecture, the "effective" muscle fibre length is increased because muscle fibres may be partly connected in series as well as in parallel. It is concluded that a hybrid phenomenological/mechanistic muscle model is most likely to be practical (i.e. parameters can be estimated for human muscle) as well as accurate (i.e. correct forces are predicted for a wide range of conditions.