Switched tracking control of a human limb during asynchronous neuromuscular electrical stimulation

Neuromuscular electrical stimulation (NMES) is a method commonly used for rehabilitation whereby an applied electrical stimulus induces muscle contractions. NMES can also evoke functional movements; however, a fundamental limitation is the early onset of fatigue. Asynchronous stimulation is a method that can reduce fatigue by utilizing multiple stimulation channels to segregate and switch between different sets of motor units. However, one limitation is that switching introduces discontinuities since each set of motor units responds differently to stimulation. Therefore, there is a need to design a controller which considers the switching dynamics and differing muscle response. In preliminary work, a control law was developed for asynchronous stimulation. However, the previous control design required there to be a finite window of time where the control voltage is transitioned from one channel to another. Since a transition period is undesirable in practice (as it will lead to increased fatigue), a switched systems analysis is used in the present work to design a controller that allows for instantaneous switching. The developed controller yields semiglobal exponential tracking of a desired angular trajectory for a person´s knee-shank complex. The result of the work is promising for the implementation of asynchronous stimulation for closed-loop rehabilitative procedures and in assistive devices as an approach to limit NMES-induced fatigue while tracking a desired trajectory.