Nonlinear Control of NMES: Incorporating Fatigue and Calcium Dynamics

Neuromuscular electrical stimulation (NMES) is a promising technique that has the potential to restore functional tasks in persons with movement disorders. Clinical and commercial NMES products exist for this purpose, but a pervasive problem with current technology is that overstimulation of the muscle (among other factors) leads to muscle fatigue. The objective of the current effort is to develop a NMES controller that incorporates the effects of muscle fatigue through an uncertain function of the calcium dynamics. A neural network-based estimate of the fatigue model mismatch is incorporated in a nonlinear controller through a backstepping based method to control the human quadriceps femoris muscle undergoing non-isometric contractions. The developed controller is proven to yield uniformly ultimately bounded stability for an uncertain nonlinear muscle model in the presence of bounded nonlinear disturbances (e.g., spasticity, delays, changing load dynamics).