This study will help the investigators better understand the changes in short-term excitability and long-term plasticity of corticospinal, reticulospinal and spinal neural circuits and how the changes impact the improvements of spinal cord stimulation (SCS) mediated motor function.

The goal of this project is to determine the changes in short-term excitability and long-term plasticity of corticospinal, reticulospinal, and spinal neural circuits that are involved in SCS-mediated motor function improvements in individuals with spinal cord injury (SCI). The study will: (1) Determine the short-term effects in neural excitability induced by SCS and activity-based training. (2) Determine the effect of motor training on short-term changes in neural excitability enabled by SCS. (3) Determine the long-term changes in motor control and neural plasticity induced by combined SCS and activity-based training in individuals with chronic SCI. Having a better understanding of the neural mechanisms that are enhanced by SCS can allow the development of therapies that directly target the excitability and plasticity states of these structures towards improved and accelerated recovery.

Inclusion Criteria:

Healthy Volunteers

• Age between 16 and 65 years old
• Healthy people with no major comorbidities of any organ system

Exclusion Criteria:

Healthy Volunteers

• Subjects younger than 16 or older than 65 years old
• Subjects not providing consent or not able to consent
• Subjects with any acute or chronic pain condition
• Subjects with any acute or chronic disease of a major organ system
• Use of analgesics within 24 hours prior to study period
• Use of caffeine with 3 hours of study appointment

Inclusion criteria:

Participants with spinal cord injury (SCI)

• Age between 16-65 years old
• Traumatic SCI C4-T9 level, incomplete (ASIA C or D)
• at least 1 year post injury
• Stable medical condition
• difficulty independently performing leg movements in routine activities
• able to follow simple commands
• able to speak and respond to questions

Exclusion criteria:

Participants with spinal cord injury (SCI)

• Subjects younger than 16 or older than 65 years old
• Not willing or able to provide consent
• Any acute or chronic pain condition
• Any acute or chronic disease of a major organ system
• Use of analgesics within 24 hours prior to study appointment
• Use of caffeine with 3 hours of study appointment
• Presence of tremors, spasms and other significant involuntary movements
• Etiology of SCI other than trauma
• Concomitant neurologic disease traumatic brain injury (TBI) that will significantly impact the ability to follow through on study directions, multiple sclerosis (MS), stroke or peripheral neuropathy)
• History of significant medical illness (cardiovascular disease, uncontrolled diabetes, uncontrolled hypertension, osteoporosis, cancer, chronic obstructive pulmonary disease, severe asthma requiring hospitalization for treatment, renal insufficiency requiring dialysis, autonomic dysreflexia, etc).
• Severe joint contractures disabling or restricting lower limb movements.
• Unhealed fracture, contracture, pressure sore, urinary tract infection or other uncontrolled infections, other illnesses that might interfere with lower extremity exercises or testing activities
• Depression, anxiety, or cognitive impairment
• Deficit of visuospatial orientation
• Sitting tolerance less than 1 hour
• Severe hearing or visual deficiency
• Miss more than 3 appointments without notification
• Unable to comply with any of the procedures in the protocol
• Botulinum toxin injection in lower extremity muscles in the prior six months
• Any passive implants (osteosynthesis material, metallic plates or screws) below T9.
• Any implanted stimulator in the body (pacemaker, vagus nerve stimulator, etc.)
• History of alcoholism or another drug abuse
• Pregnancy (or possible pregnancy)
• Having an Intrathecal Baclofen Therapy Pump (ITB pump)
• History of epilepsy