EFFECTS OF ELECTRICAL STIMULATION ON THE RECRUITMENT ORDER OF MOTOR UNITS IN MAN: INDIRECT EXAMINATION BY ELECTRICALLY EVOKED MUSCLE RESPONSES

Although the neural mechanisms responsible for the orderly recruitment of motor units have been investigated extensively, the flexibility of the underlying neural circuitry remains unclear. For example, the effects of electrical stimulation on the recruitment order of motor units is not well understood. This project was designed to study the recruitment order of motor units in man during different stimulation protocols. Examination of the compound-twitch characteristics of electrically evoked responses allowed an indirect determination of motor-unit recruitment order. The results demonstrate that the recruitment order of quadriceps femoris and triceps surae motor units differs according to the stimulation protocols used. Analysis of the compound-twitch characteristics indicated that the recruitment order of motor units during Hoffmann reflexes is similar to that of volitional muscle contractions but effectively the reverse of that during direct-motor responses. Moreover, the results suggest that cutaneous-afferent stimulation alters the recruitment thresholds of different motor unit types during the Hoffman reflex