Isometric Torque Control for Neuromuscular Electrical Stimulation With Time-Varying Input Delay.

Previous results have shown experimental evidence that the muscle response to neuromuscular electrical stimulation (NMES) is delayed; the time lag is often referred to as electromechanical delay. NMES closed-loop control methods have been developed to compensate for a known constant input delay. However, as a muscle fatigues, this delay increases. This paper develops a feedback controller that robustly compensates for the time-varying delay of an uncertain muscle model during isometric contractions. The controller is proven to yield global uniformly ultimately bounded torque tracking error. Experimental results illustrate the effectiveness of the developed controller and the time-varying nature of the delayed response.