This research study will investigate the use of smart lower limb robotic exoskeleton (developed by the CSIC, Spain) in rehabilitation after stroke. It will compare robotic-assisted rehabilitation with supervised motor practice. Additionally, it will also examine the use of noninvasive scalp electroencephalography (EEG) to learn specific brain wave patterns associated with learning to walk on the powered lower limb exoskeleton. The findings will be used to understand human-robot interaction and to design smart orthotic devices that can be controlled by thought activity and assist those that have lost all or part of their walking abilities.

Stroke is the leading cause of neurological disability in the United States and accounts for the poor physical health and the social dysfunction evident in survivors. Gait impairment is a large contributor to long-term disability and ambulatory function in daily living. Many patients, however, lose the ability to walk independently, and furthermore, a large proportion does not regain their normal walking speeds following a stroke. In this context, newer robotic-aided therapeutic tools such as "wearable" lower-limb robotic exoskeletons have been developed, which allow for the user to be augmented by mechanically actuated lower limb joints that can either completely or partially assist movements of the lower limb segments depending on the patient needs.

The H2 exoskeleton (developed by Technaid S.L., Spain) is an example of one such system that has hip, knee and ankle joints actuated for both lower limbs. These devices are very new, and therefore, systematic investigations of therapeutic benefits of these devices are lacking in the field. Further, the nature of plasticity in the brain triggered by wearing and training such exoskeletons is unknown. In this exploratory research study, the investigators aim to compare robotic-assisted rehabilitation using the H2 exoskeleton with supervised motor practice particularly in terms of functional recovery. Additionally, this study will also examine brain plasticity associated with robotic-assisted training using non-invasive scalp electroencephalography (EEG) and changes in lower limb joint kinematics during robotic-assisted training. Taken together, the findings from this research will be used to understand human-robot interaction and to design smart powered orthotic devices that can be controlled directly by brain activity and assist those that have lost all or part of their walking abilities due to neurological disease or injury. Moreover, this study will systematically track neuroplasticity associated with functional recovery after stroke, which will help determine optimal windows for treatment that would maximize therapeutic benefit. Lastly, it will also help characterize markers of learning to use these new devices, which will be important in the clinical setting for modifying and adapting rehabilitation protocols to suit changing needs of the patient (user).

Inclusion Criteria:

• Sub-acute or chronic stroke i.e., interval of at least 3 months or interval of at least 6 months from stroke to time of enrollment, respectively;
• Cognitive ability to assimilate and participate actively in the treatment protocol (Mini Mental State Examination score > 24 points, out of a total 30 indicating normal cognitive ability);
• Modified Rankin scale scores 2-4 (Mild-Moderate functional disability post-stroke);
• Modified Ashworth Scale of Spasticity score <= 2 (ranges from 0-4 with 4 reflecting maximum spasticity);
• Have no skin integrity issues;
• Sufficient passive range of motion at the hip (at least 90 deg flexion, 15-20 deg extension), knee (90 deg flexion, complete extension) and ankle (15 deg dorsiflexion, 15 deg plantarflexion);
• Have no contraindications to standing or walking; able to stand with assistive device for at least 5 minutes, and able to walk with assistive device for 10 m.

Exclusion Criteria:

• Severe cognitive and/or visual deficit;
• Hemineglect (determined based on medical record or initial clinical assessment);
• Severe sensory deficit;
• Joint contractures of any extremity that limits normal range of motion during ambulation with assistive devices;
• Skin lesions that may hinder or prevent the application of exoskeleton;
• Uncontrolled angina;
• Severe chronic obstructive pulmonary disease;
• Other medical contraindications; any medical co-morbidities that would prevent standard rehabilitation.

Inclusion criteria for healthy, able-bodied participants:

• Able to understand and sign the consent form
• Age 18-75 years

Exclusion criteria for healthy, able-bodied participants:

- History of neurological, neuromuscular or physical disability.