Controlling FES-assisted paraplegic standing-a case study

Using electrical stimulation of lower limb muscles to assist paraplegics to stand has many advantages over conventional passive standing techniques. However, stability and balance needs to be maintained, in response to muscle spasms and fatigue, postural changes and external perturbations, if the system is to be safe for a subject to take one hand off to perform tasks. The authors´ system monitors the stand by measuring the knee angles and uses a closed-loop design, based on a PID (proportional-integral-derivative) type, to respond to deviations by adjusting the stimulation to the quadriceps. With this system the authors have stood 24 paraplegics, with 14 using it at home. The authors report their experience from one of their subjects who started retraining his quadriceps with electrical stimulation in October 1998 and first stood three months later. During sessions over the next three months, he was instructed in standing techniques and the closed-loop algorithm was implemented. The authors found that during standing, if the stimulation increased too quickly then strong stomach spasms resulted. This was improved by removing the controller´s derivative term and low-level stimulation of the abdominal muscles as a daily exercise routine