FES systems typically consist of three major components. First, there is a control mechanism. Second, there is an electronic stimulator, which may be external or implanted. Third, there are electrodes that allow for interface between the stimulator and the nervous system. Cabling or other technologies such as radio-frequency coupled telemetry allow for communication between these three components.
Electronic stimulators are usually battery powered. Stimulators range in sophistication and number of channels. Most portable FES neuroprostheses are built around a microprocessor controller, which may allow for software modification of the stimulator output. The number of stimulator channels can vary with the application. Channels can be activated sequentially or in unison to allow for orchestration of complex movements.
Control systems for therapist-operated FES systems consist of dials and switches. For subject-controlled FES systems (i.e., neuroprostheses), control can come in the form of switches, buttons, joysticks, joint position sensors, electromyography (EMG) electrodes, voice activated controls, sip and puffdevices, switches, etc. Subject control of an FES system can be either "open loop" or "closed loop". In open loop control, the electrical output of the FES system is not dependent on the muscular force or joint movement produced by the stimulation. In closed loop control, real time information on muscle force and/or joint position or movement is fed back into the FES system to allow for modification of its output (Crago et al., 1991).
Electrodes provide the interface between the electrical stimulator and the nervous system. Different types that have been used include surface, percutaneous intramuscular, implantable intramuscular, epimysial, and nerve cuff electrodes. These electrodes will be discussed separately.
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