Chronic Transcutaneous Stimulation to Promote Motor Function and Recovery in Individuals With Paralysis or Paresis

Healthy Volunteer Inclusion Criteria: * Individuals between 18 and 75 years of age * Individuals without physical disabilities or conditions/diseases that may make them incapable of completing the physical study tasks or otherwise places them at a greater risk of harm * Individuals that are considered English Proficient due to the study requirements to follow verbal commands during sessions * Individuals that are able to comprehend the study goals and procedures, and are able to provide informed consent for participation * Individuals that are willing and able to visit the study center for study procedures, which may be up to 5 sessions a week for up to 1 year (at least 2 months) at up to 4 hours per session Stroke and Spinal Cord Injury Participant Inclusion Criteria: * Individuals between 18 and 75 years of age * Individuals with a limited ability to use at least one hand due to a spinal cord injury or stroke * Individuals that are at least six months from their initial stroke or one year from their initial spinal cord injury * Individuals that are considered English Proficient due to the study requirements to follow verbal commands during testing sessions * Individuals that are able to comprehend the study goals and procedures, and are able to provide informed consent for participation * Individuals that are willing and able to visit the study center for study procedures, which may be up to 5 sessions a week for up to 1 year (at least 2 months) at up to 4 hours per session Brain or Nerve Injury Participant Inclusion Criteria: * Individuals between 18 and 75 years of age * Individuals with a limited ability or no ability to use at least one limb due to a brain or nerve injury such as a traumatic brain injury, brachial plexus injury, peripheral nerve damage, or Parkinson's Disease. * Individuals that are considered English Proficient due to the study requirements to follow verbal commands during testing sessions * Individuals that are able to comprehend the study goals and procedures, and are able to provide informed consent for participation * Individuals that are willing and able to visit the study center for study procedures, which may be up to 5 sessions a week for up to 1 year (at least 2 months) at up to 4 hours per session Exclusion Criteria for All Study Participants: * Individuals participating in another study that may affect the conduct or results of this study * Individuals having or exhibiting any of the following, as specified by self-report: * Stage III-IV pressure ulcers that adversely interfere with study involvement (i.e., pressure ulcer on the extremity of interest or prohibits extended bouts of sitting) * History of epilepsy * Chronically-implanted electronic medical device (e.g. deep brain stimulator, epidural stimulator, cardiac pacemaker, vagus nerve stimulator, or other) * Abnormalities of the arms/hands, legs/feet, or spinal column that would prevent electrical stimulation * Ventilator dependence * History of serious mood or thought disorder * Uncontrolled autonomic dysreflexia * Severe spasticity in extremities of interest that are uncontrolled by pharmacological methods or prevents electrical stimulation * Botulinum toxin injections to the extremity of interest within 3 months * Presence of active cancer at or near the areas of transcutaneous stimulation * History of heart or kidney disease * History of additional neurologic diseases (e.g., multiple strokes, multiple sclerosis, etc.) * Presence of other serious infections, diseases, or disorders that could affect ability to participate in this study (e.g., uncontrolled hypertension, uncontrolled diabetes, rheumatic disease, peripheral neuropathy, HIV or AIDS, etc.) * Individuals with a substance abuse (alcoholism or other) problem * Pregnant women * Prisoners