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Background: The human lumbosacral spinal circuitry can generate rhythmic motor output in response to different types of inputs after motor-complete spinal cord injury., Objective: To explore spinal rhythm generating mechanisms recruited by phasic step-related sensory feedback and tonic posterior root stimulation when provided alone or in combination., Methods: We studied stepping in 4 individuals with chronic, clinically complete spinal cord injury using a robotic-driven gait orthosis with body weight support over a treadmill. Electromyographic data were collected from thigh and lower leg muscles during stepping with 2 hip-movement conditions and 2 step frequencies, first without and then with tonic 30-Hz transcutaneous spinal cord stimulation (tSCS) over the lumbar posterior roots., Results: Robotic-driven stepping alone generated rhythmic activity in a small number of muscles, mostly in hamstrings, coinciding with the stretch applied to the muscle, and in tibialis anterior as stance-phase synchronized clonus. Adding tonic 30-Hz tSCS increased the number of rhythmically responding muscles, augmented thigh muscle activity, and suppressed clonus. tSCS could also produce rhythmic activity without or independent of step-specific peripheral feedback. Changing stepping parameters could change the amount of activity generated but not the multimuscle activation patterns., Conclusions: The data suggest that the rhythmic motor patterns generated by the imposed stepping were responses of spinal reflex circuits to the cyclic sensory feedback. Tonic 30-Hz tSCS provided for additional excitation and engaged spinal rhythm-generating networks. The synergistic effects of these rhythm-generating mechanisms suggest that tSCS in combination with treadmill training might augment rehabilitation outcomes after severe spinal cord injury., (© The Author(s) 2015.)