Tracking control of a human limb during asynchronous neuromuscular electrical stimulation

Neuromuscular electrical stimulation (NMES) is defined as the use of an electrical stimulus to elicit muscle contractions and is commonly used in rehabilitative settings. NMES is also used for assistive purposes to create functional movements where it is termed functional electrical stimulation (FES). One limitation of NMES/FES is early onset of fatigue due to the nonselective, spatially fixed, synchronous activation of motor units. Asynchronous stimulation can reduce NMES-induced fatigue; however, one limitation of asynchronous stimulation is that switching between stimulation channels may introduce discontinuities due to a differing response to stimulation by each group of recruited motor units. Thus, there is a need to design a controller which considers the switching dynamics and muscle response to stimulation during asynchronous stimulation. A closed-loop feedback controller is developed in this paper to yield semi-global asymptotic tracking of a desired trajectory for a person´s knee-shank complex during asynchronous stimulation. The result is promising for the implementation of asynchronous stimulation in assistive devices as a method to reduce fatigue while tracking a desired trajectory.