Paraplegic standing controlled by functional neuromuscular stimulation. I. Computer model and control-system design

A planar computer model for investigating paraplegic standing induced by functional neuromuscular stimulation is discussed. The model consists of nonlinear musculotendon dynamics (pulse train activation dynamics and musculotendon actuator dynamics), nonlinear body-segmented dynamics, and a linear output-feedback control law. The model of activation dynamics is an analytic expression that characterizes the relation between the stimulus parameters and the muscle activation. Hill´s classic two-element muscle model was modified into a musculotendon actuator model in order to account for the effects of submaximal activation and tendon elasticity on development force by the actuator. The three body-segmental, multijoint model accounts for the anterior-posterior movements of the head and trunk, the thigh, and the shank. Arm movements was modeled as an external disturbance, and the disturbance to the body-segmented dynamics was imposed by means of quasistatic analysis.