Functional electrical stimulation control of standing and stepping after spinal cord injury: a review of technical characteristics.

Objectives. To investigate the different approaches in the field of functional electrical stimulation (FES) control of gait and address fundamental perquisites to enable FES walking systems to become safer, more practical, and therefore clinically efficacious. Design. Systematic review was conducted from electronic data bases up to March 2008. Studies with innovative control strategies were highlighted for analysis, but all relevant literatures were described to deliver a broad viewpoint. Study Selection. FES studies applying 1) open and closed-loop controllers; 2) control algorithm techniques; or 3) feedback information to the control unit of neuromuscular stimulators via biological signals or artificial sensors. These studies were mostly associated to FES gait. Results. By far, more spinal cord-injured users have benefited from open-loop FES walking systems because they have had an easier and faster setup. However, because of their limitations over the control of knee extension, closed-loop control of gait may be a superior approach. The use of electromyogram to quantify quadriceps fatigue was not considered sufficiently appropriate to predict knee-buckle events; instead, the use of motion sensors for such purposes is recommended. Finite state controllers based on a set of deterministic rules to process feedback signals seemed more suitable to provide accurate command-and-control compared with dynamic or neural network controllers. Conclusions. Progress in the development of closed-loop FES walking systems has been impeded by their lack of practicality. In the near future, this obstacle could be overcome via implanted systems, especially if using controllers based on deterministic rule sets derived from motion sensor feedback.