Prolonged electrical stimulation over hip flexors increases locomotor output in human SCI

Objectives: The objective of this study was to determine whether enhanced feedback from thigh afferents improves locomotor output in human spinal cord injury (SCI). Methods: The effects of afferent feedback originating from the upper thigh muscles on locomotion was examined using electrical stimulation in 10 subjects with incomplete SCI and three neurologically intact controls during robotic-assisted treadmill walking. Electrical stimulation consisted of 20 pulses at 30 Hz, applied bilaterally to the skin of the medial thigh, approximately over the sartorius muscle. The stimulation was applied at four different phases of the gait cycle. Torque responses of hip and knee joints and electromyograms of both legs were recorded during baseline with no stimulation, stimulation, and post-stimulation. Results: During stimulation, enhanced hip and knee extension and flexion torque responses were observed during the stance and swing phases, respectively, for all four different stimulation conditions. Larger hip extension torque was observed when the stimulation was applied during the stance phase and the transition from stance to swing. Conclusions: Enhanced afferent feedback produced by electrical stimulation may increase the excitability of the spinal cord locomotor circuits in human SCI. Significance: Results from this study emphasize the contribution of sensory information from thigh muscles, particularly the sartorius muscle afferents, to locomotor control in human SCI during treadmill walking.