Introduction

Functional electrical stimulation (FES), also known as functional neuromuscular stimulation (FNS) or neuromuscular electrical stimulation (NMES) is the method of applying safe levels of electric current to activate the damaged or disabled neuromuscular system. The terms FES, FNS, and NMES are often used interchangeably, although NMES typically refers to using surface electrodes. The term neuroprosthesis refers to devices that use electrical stimulation to activate the nervous system in order to perform a specific functional task. This chapter will examine both the therapeutic aspects of FES as well as the direct functional applications as it is applied to individuals with neurologic injury.

Mechanism of FES activation of nervous tissue

Peripheral effects

When a sufficiently strong external electric field is applied to a nerve via a pair of electrodes, depolarization of the axon will occur. If depolarization occurs with sufficient intensity and speed, the membrane will reach threshold and an action potential will fire and propagate bidirectionally. The number of nerve fibers activated during applied stimulation will be related to the amount of phase charge delivered with each pulse (Adams et al., 1993).

Motor units are activated electrically by depolarization of motor axons, or terminal motor nerve branches. Electrical current can directly depolarize muscle fibers, but the amount of current necessary for this to occur is considerably greater than that for depolarization of nerve axons (Crago et al., 1974).