Using Forelimb EMG to Control an Electronic Spinal Bridge to Facilitate Hindlimb Stepping After Complete Spinal Cord Lesion

Abstract

A complete spinal cord transection results in loss of all supraspinal motor control below the level of the injury. The neural circuitry in the lumbosacral spinal cord, however, can generate locomotor patterns in the hindlimbs of rats and cats with the aid of epidural stimulation and administration of serotoninergic agonists. We hypothesized that there are patterns of EMG signals from the forelimbs during quadrupedal locomotion that uniquely represent a signal for the “intent” to step with the hindlimbs. These observations led us to determine whether “indirect” volitional control of stepping can be restored after a complete spinal cord injury. We developed an electronic bridge that can trigger specific patterns of EMG activity from the forelimbs to enable quadrupedal stepping after a complete spinal cord transection in rats. We found dominant frequencies of 180–220 Hz in the EMG of forelimb muscles during active periods, whereas the frequencies were between 0–10 Hz when the muscles were inactive. A moving window detection algorithm was implemented in a small microprocessor to detect bilateral activity in the biceps brachii that then was used to initiate and terminate epidural stimulation. This detection algorithm was successful in detecting stepping under different pharmacological conditions and at various treadmill speeds and in facilitating quadrupedal stepping after a complete mid-thoracic spinal cord transection.