Model-based control of FES-induced single joint movements

A crucial issue of functional electrical stimulation (FES) is the control of motor function by the artificial activation of paralyzed muscles. Major problems that limit the success of current FES systems are the nonlinearity of the target system and the rapid change of muscle properties due to fatigue. In this study, four different strategies, including an adaptive algorithm, to control the movement of the freely swinging shank were developed on the basis of computer simulations and experimentally evaluated on two subjects with paraplegia due to a complete thoracic spinal cord injury. After developing a nonlinear, physiologically based model describing the dynamic behavior of the knee joint and muscles, an open-loop approach, a closed-loop approach, and a combination of both were tested. In order to automate the individual adjustments cited, we further evaluated the performance of an adaptive feedforward controller. The two parameters chosen for the adaptation were the threshold pulse width and the scaling factor for adjusting the active moment produced by the stimulated muscle to the fitness of the muscle. These parameters have been chosen because of their significant time variability. The first three controllers with fixed parameters yielded satisfactory result. An additional improvement was achieved by applying the adaptive algorithm that could cope with problems due to muscle fatigue, thus permitting on-line identification of critical parameters of the plant. Although the present study is limited to a simplified experimental setup, its applicability to more complex and functional movements can be expected.