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Using functional electrical stimulation (FES), muscles of spinal-cord injured patients can be activated by externally generated electrical currents in order to restore function. As for gait, the question arises when during the gait cycle and two what extent individual muscles should be stimulated. Computer simulation provides the designer with a tool to evaluate the performance of different muscle stimulation patterns without the need to test patients at every stage of system development. The goals of this paper are: first, to identify, using computer simulation, multi-channel stimulation patterns that are capable of reproducing normal gait kinematics for a full gait cycle, without relying on sensory feedback (open-loop control); second, to briefly assess the stability of the gait obtained. A two-dimensional musculo-skeletal model was developed, based on mathematical representations of muscle properties (including force-length and force velocity characteristics and muscle activation dynamics). A visco-elastic model, including non-linear heel-pad properties, was used to describe the foot-ground interaction. A seven segment skeletal model was actuated by 8 major muscle groups in each leg. Rectangular muscle stimulation patterns were defined by 3 parameters: onset, termination and level of stimulation. Thus, the minimization of the differences between simulated and measured normal gait kinematics was a 24 (3 by 8) parameter optimization problem. Although a good agrement was found between simulated and measured kinematics (rms difference equals 6.5 degrees), stable cyclic locomotion was not achieved. At this point it is concluded that muscle properties do not provide sufficient stability to permit cyclic locomotion with sixteen channels of muscle stimulation, and that incorporation of sensory feedback control will be necessary to achieve this goal.
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