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Closed-Loop Control of Force During Electrical Stimulation of Muscle

The control of contractions elicited by electrical stimulation of muscle could be improved if there was a linear repeatable input-output relationship. The input is the command to the controlled stimulator and the output is the evoked contraction. Systems employing closed-loop force feedback to provide regulation of contractions were investigated in these studies. Force was modulated by both recruitment and temporal summation during intramuscular stimulation. Closed-loop systems with combined proportional and integral control were found to be stable and linear and to have good compensation for variations in muscle properties. A low proportional loop gain (approximately unity) was found necessary to prevent oscillation when modulating recruitment. Ratios of integral to proportional gain of about 10 gave the fastest response without compromising stability. The response time of the closed-loop system was as fast as or faster than the open-loop system.