People with spinal cord injury (SCI) have motor dysfunction that results in substantial social, personal, and economic costs. Robotic gait training is often used with the aim to improve walking ability in these individuals. Investigators recently reported that robotic gait training reorganizes spinal neuronal circuits, improves motor activity, and contributes substantially to recovery of walking ability in people with motor incomplete SCI. However, pathological muscle tone and abnormal muscle activation patterns during assisted stepping were still evident after multiple sessions of robotic gait training. Locomotor training alone may thus be insufficient to strengthen weak neuronal synapses connecting the brain with the spinal cord or to fully optimize spinal neural circuits. On the other hand, spinal cord stimulation increases sprouting and plasticity of axons and dendrites in spinalized animals. Furthermore, transcutaneous spinal cord stimulation (termed here transspinal stimulation) in people with SCI can evoke rhythmic leg muscle activity when gravity is eliminated. A fundamental knowledge gap still exists on induction of functional neuroplasticity and recovery of leg motor function after repetitive thoracolumbar transspinal stimulation during body weight supported (BWS) assisted stepping in people with SCI. The central working hypothesis in this study is that transspinal stimulation delivered during BWS-assisted stepping provides a tonic excitatory input increasing the overall responsiveness of the spinal cord and improving motor output. The investigators will address 3 specific aims: Establish induction of neuroplasticity and improvements in leg sensorimotor function in people with motor incomplete SCI when transspinal stimulation is delivered during BWS-assisted stepping at low frequencies (0.3 Hz; Specific Aim 1) and at high frequencies (30 Hz; Specific Aim 2), and when BWS-assisted step training is administered without transspinal stimulation (Specific Aim 3). In all groups, outcomes after 20 sessions will be measured via state-of-the-art neurophysiological methods. Corticospinal circuit excitability will be measured via transcranial magnetic stimulation motor evoked potentials in seated subjects (Aims 1A, 2A, 3A). Soleus H-reflex and tibialis anterior flexor reflex excitability patterns will be measured during assisted stepping (Aims 1B, 2B, 3B). Sensorimotor function will be evaluated via standardized clinical tests of gait and strength (Aims 1C, 2C, 3C). Additionally, poly-electromyographic analysis of coordinated muscle activation will be measured in detail. It is hypothesized that transspinal stimulation at 30 Hz during assisted stepping improves leg motor function and decreases ankle spasticity more compared to 0.3 Hz. It is further hypothesized that transspinal stimulation at 30 Hz normalizes the abnormal phase-dependent soleus H-reflex and flexor reflex modulation commonly observed during stepping in people with motor incomplete SCI. To test the project hypotheses, 45 people with motor incomplete SCI will be randomly assigned to receive 20 sessions of transspinal stimulation at 0.3 or 30 Hz during BWS-assisted stepping or 20 sessions of BWS-assisted stepping without transspinal stimulation (15 subjects per group). Results from this research project will advance considerably the field of spinal cord research and change the standard of care because there is great potential for development of novel and effective rehabilitation strategies to improve leg motor function after motor incomplete SCI in humans.