A physiological model for the evaluation of muscular forces in human locomotion: theoretical aspects

In this paper a model capable of predicting individual muscle forces in the right lower limb is formulated. The approach developed is a physiological hierarchical model the lowest level of which concerns itself with the data necessary to define the musculoskeletal system. These include the muscle's moment arms, lengths, and velocities. Data describing the muscle anatomy and bone structure are also included. The middle level, the muscle model, defines the force-length-velocity-activation relationship of a single muscle unit. These data, with those from the anatomical model, are used to estimate the maximum and minimum force a muscle can exert given its previous force history, current kinematics, and previous stimulation level. The stimulation needed to generate any intermediate force is also defined. At the top of the hierarchy the control model provides a singular solution of the muscle forces, within the constraints defined in the muscle and anatomical models, by stimulating the muscles. The control model selects the muscle stimuli, based on a simple neurophysiological model of pattern generators, to satisfy the measured net muscle moments about those axes for which a large range of movement exist. It is assumed that one pattern generator exists for each major degree of freedom in the lower extremity. The discussion is focused upon the limitations of the model.