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Compensating for changing muscle geometry of the biceps brachii during neuromuscular electrical stimulation: A switched systems approach

Functional electrical stimulation (FES) is commonly used in rehabilitation therapy for people with injuries or various neurological disorders. Noninvasive treatments use surface electrodes to provide a potential field across the muscle and induce contractions/output force. The placement of the electrodes has a significant impact on the induced force output. As the muscle geometry changes (i.e., muscle lengthening or shortening), the force induced by the static electrode placement may also change. In this paper, an array of electrodes is placed across the biceps brachii and the electric field is switched across the electrodes (i.e., channels) to maximize the induced muscle force throughout the arm's range of motion, despite changes in the muscle geometry. To yield this outcome, a switched systems approach is used to develop a position-based switching law for the uncertain nonlinear system. Specifically, a switched robust sliding mode controller is developed to track a desired angular trajectory about the elbow. Lyapunov-based methods for switched systems are used to prove global exponential tracking and experimental results demonstrate the performance of the switched control system.