Locomotor training (LT) facilitates recovery of spinal locomotor networks after incomplete spinal cord injury (ISCI), but walking impairments persist. A limitation of LT is insufficient excitation of spinal locomotor circuits to induce neuroplastic recovery. Transcutaneous spinal direct current stimulation (tsDCS) is a non-invasive approach to increase spinal excitation and modulate spinal reflexes. The study will examine if tsDCS combined with LT enhances locomotor rehabilitation after ISCI.

Transcutaneous spinal direct current stimulation (tsDCS) is a mild, non-invasive approach to increase spinal excitation and modulate spinal reflexes. Although ample evidence suggests tsDCS could enhance the effect of locomotor rehabilitation, the efficacy of this combinatorial strategy has not been investigated in individuals with ISCI. Based on supportive evidence from our prior feasibility study, the investigators will examine initial efficacy and mechanistic changes from combined tsDCS and locomotor training (tsDCS+LT).

Specific aim 1: To determine the effect size and variance of response to combined tsDCS and LT to enable power analysis for a larger clinical trial. Participants (chronic, motor ISCI) will complete a double-blind, randomized parallel-group design study to examine the effects of tsDCS (20 sessions, 30 minutes, anodal, 2.5 mA) delivered concurrently with LT. Walking outcomes will be measured pre- and post-intervention to compare the effects of tsDCS+LT versus sham+LT. Primary outcomes will be changes in overground walking speed and endurance.

Specific aim 2: To test the hypothesis that tsDCS+LT increases spinal motor excitability and reflex modulation. Spinal excitability will be assessed by Hoffman reflex amplitudes. Reflex modulation will be assessed by normalized measures of reflex post-activation depression as well as by gait phase-dependent modulation.

This proposed clinical trial will utilize a double-blind, randomized parallel-group experimental design. Once screening is complete, individuals will be enrolled for approximately two months to complete all study procedures. Participants will be randomized to receive either the higher or lower (sham) dose of tsDCS. The intervention will consist of 20 sessions (5 weeks x 4 sessions/week) of LT with the assigned tsDCS dose.

All participants will complete assessments at three time points. Clinical assessments to characterize injury severity and baseline characteristics, biomechanical, electromyographic (EMG) and clinical assessments of walking function, and reflex testing will be completed pre-intervention and one to three days post-intervention. A follow-up assessment consisting of clinical walking and mobility function outcomes only will be reassessed two weeks post-intervention.

Inclusion Criteria:

• Adults 18 to 65 years of age
• Single chronic SCI for ≥ 1 year classified as neurologic level T12 or above based on the International Standards for the Neurological Classification of SCI67 and classified according to the American Spinal Injury Association Impairment Scale (AIS) as motor incomplete (AIS C or D) at initial screening
• Medical stability with clearance from physician
• Ability to walk 3 meters with or without devices, braces, or assistance of one person
• Ability to provide informed consent.

Exclusion Criteria:

• Current diagnosis of an additional neurologic condition such as multiple sclerosis, Parkinson's disease, stroke, or brain injury
• Presence of unstable or uncontrolled medical conditions such as cardiovascular disease, myocardial infarction (<1 year prior), pulmonary infection or illness, renal disease, autonomic dysreflexia, infections, pain, heterotopic ossification
• Cognitive or communication impairments limiting communication with study staff or ability to provide informed consent
• Lower extremity joint contractures limiting the ability to stand upright and practice walking
• Skin lesions or wounds affecting participation in walking rehabilitation
• Acute or unstable fracture, diagnosis of osteoarthritis or bone impairments affecting safe participation in walking rehabilitation
• Severe spasticity or uncontrolled movements limiting participation in walking rehabilitation
• Body weight or height that is incompatible with safe use of a support harness and body weight support system
• Pain that limits walking or participation in walking rehabilitation
• Current participation in rehabilitation to address walking function
• Botox injections in lower extremity muscles affecting walking function within 4 months of study enrollment
• Legal blindness or severe visual impairment
• Known pregnancy
• Implanted metal hardware of the spine below the 8th thoracic vertebrae or in region of electrode placement
• Implanted cardiac pacemaker or baclofen pump

Stratified randomization will be used to allocate participants to their treatment group (higher or lower dose of tsDCS)