Electromyographic feedback: effects on voluntary muscle contractions in normal subjects.

To evaluate the efficacy and function of EMG feedback (FB) in muscle reeducation, voluntary muscle contractions with and without EMG FB were compared under controlled experimental conditions in normal human subjects (n=10). Each subject was instructed to produce 12 sustained, 30-second contractions of the left abductor hallucis muscle, 6 contractions in each of 2 sessions. For all subjects, EMG FB was provided in half of the trials, alternating with nonfeedback (NFB) trials. A ranking task was included to measure ability to discriminate contractions in the target muscle. EMG activity was significantly greater during muscle contractions attempted with EMG FB; this was due to increased motor unit recruitment early in the course of the trials, a positive response in normal subjects similar to that in paretic subjects in our previous study. Performance on the ranking task also indicated that subjects had little perceptual awareness of their muscle contractions. Our findings suggest that the positive response to EMG FB was due to the precise information it provided concerning small, poorly discriminated muscle contractions.