Electrical stimulation of the ventral lumbo-sacral spinal cord results in mixed recruitment order of quadriceps and tibialis anterior motor units

Pulse width modulated, twitch recruitment curves for quadriceps and tibialis anterior were obtained by stimulating their respective motor activation pools in the ventral lumbo-sacral portion of the spinal cord. The duration of the twitch responses were analyzed as a function of stimulus strength to determine the order of motor unit recruitment. It was found that activating the quadriceps or tibialis anterior by stimulating the spinal cord resulted in a predominantly mixed recruitment order of its motor units. These results suggest that spinal cord stimulation could have application in future neuromuscular stimulation systems where normal recruitment order of muscle fibers is desired for producing fine control of muscle activity and enhancing fatigue resistance.