Motor dysfunction after SCI results in substantial personal, social, and economic costs. Secondary complications related to muscle spasticity and motor dysfunction significantly reduce quality of life. Rehabilitation efforts are compromised because most therapeutic interventions rely heavily on sensory afferent feedback that is interpreted by malfunctioned neuronal networks. On the other hand, electrical signals delivered over the spine regenerate neurons of the spinal cord in injured animals, and paired associative stimulation produces enduring neuronal plasticity in healthy and injured humans and animals. A fundamental knowledge gap still exists on induction of functional neuroplasticity and recovery of leg motor function following repetitive pairing of transspinal cord stimulation with transcortical stimulation in people with motor incomplete SCI. Our central working hypothesis is that transspinal cord stimulation paired with transcortical stimulation strengthens corticospinal neuronal connections, decreases ankle spasticity, and improves leg motor function. This notion is based on the concept tested in the applicant's laboratory that this novel stimulation paradigm potentiates synaptic actions and activity of spared but compromised axons providing the necessary environment for functional neuroplasticity. The rationale of this research study is that neuromodulation methods that can modify effectively the input-output relations of cortical and spinal neuronal pathways in people with motor incomplete SCI are needed. Based on strong preliminary data, two specific aims will be addressed: Specific Aim 1: Establish induction of transspinal-transcortical paired associative stimulation (PAS) neuroplasticity and improvements in leg sensorimotor function in people with motor incomplete SCI when PAS is administered during robotic-assisted gait training. Cortical and corticospinal neuronal circuits via state-of-the-art neurophysiological methods in seated (Aim 1A), soleus H-reflex excitability during assisted stepping (Aim 1B), and sensorimotor function evaluated via standardized clinical tests (Aim 1C) will be assessed before and after 15 sessions of transspinal-transcortical PAS delivered with subjects supine. Specific Aim 2: Establish induction of transcortical-transspinal PAS neuroplasticity and improvements in leg sensorimotor function in people with motor incomplete SCI when PAS is administered during assisted stepping. Cortical and corticospinal neuroplasticity via state-of-the-art neurophysiological methods in seated (Aim 2A), soleus H-reflex excitability during assisted stepping (Aim 2B), and leg sensorimotor function evaluated via standardized clinical tests (Aim 2C) will be assessed before and after 15 sessions of transspinal-transcortical PAS delivered during assisted stepping. It is hypothesized that transspinal-transcortical PAS delivered at rest or during assisted stepping strengthens corticospinal connections, increases spinal inhibition, decreases ankle spasticity, and improves leg motor function. Further, transspinal-transcortical PAS delivered during assisted stepping normalizes the abnormal phase-dependent soleus H-reflex modulation commonly observed during stepping in people with motor incomplete SCI. To test the project hypotheses, 12 people with motor incomplete SCI will receive 20 sessions of transspinal-transcortical or transcortical-transspinal PAS during assisted stepping. In people with SCI, and assess improvements in leg motor function with standardized clinical tests. In all subjects, assessments of neuroplasticity in cortical, corticospinal, and spinal neuronal networks will be administered. These results 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 manage spasticity and improve motor function after SCI in humans.