Excessive Stimulation of Muscle
Until closed-loop feedback control is developed with respect to timing of events in the gait cycle, overtaxation of muscles will be a major contributing factor to muscle fatigue. Openloop stimulation is a one-way delivery of electrical signals from a stimulator to a muscle. An open-loop stimulator is unable to turn on and turn off muscle contractions at precisely the right moment during the gait cycle; nor can it exactly balance the body against gravity or efficiently propel it forward because information about forces and moments are not sent to the computer. For example, in normal walking the quadriceps are active during the first 15% of stance, but they are inactive in the remaining 30%; thus, the duty cycle for the quadriceps in stance phase is 33%.22 In FES open-loop walking, the duty cycle for the quadriceps may be as high as 75%-100% of stance phase.
In closed-loop stimulation, information about the condition of the body during standing or walking is taken from sensors mounted on the body and processed in the computer so that the muscles may be made to respond more appropriately to the environment and to perturbations. Closed-loop feedback control constantly analyzes the body much in the same way that the nervous system monitors the musculoskeletal system, i.e., proprioception, tendon-stretch reflex, and attempts to safely lower stimulation levels to the minimum necessary.
