Inclusion Criteria:

• - Healthy adults have no ongoing neurological, musculoskeletal issues.

• - Individuals with chronic stroke had medical history of a unilateral stroke occurring ≥ 6 months prior to enrollment.

• - Individuals have multiple sclerosis disease diagnosed by his/her physician.

• - MRI or CT evidence from the imaging report shown that the stroke and multiple sclerosis disease primarily involve cortical and subcortical regions.

• - Individuals with chronic stroke have hemiparesis involving the lower extremity.

• - Individuals with chronic stroke or multiple sclerosis have no passive range of motion limitation in bilateral hips and knees.

Limitation of ankle passive range of motion to 10 degrees of dorsiflexion or less.

• - Visual acuity can be corrected by glasses or contact lens to 20/20.

• - Able to walk independently with/without assistant devices for 10 meters.

• - Able to maintain standing position without any assistance for more than 30 sec.

• - Evaluation of cognitive status: Mini-mental status examination (MMSE) score ≥ 24.

Exclusion Criteria:

• - Pregnant women.

• - MRI or CT evidence of involvement of the basal ganglia or cerebellum, or evidence of any other brain damage or malignant neoplasm or tumors.

• - Have any metal implants, cardiac pacemakers, or history of seizures.

• - Ongoing orthopedic or other neuromuscular disorders that will restrict exercise training.

• - Any vestibular dysfunction or unstable angina.

- Significant cognitive deficits (inability to follow a 2-step command) or severe receptive or global aphasia*

The walking after stroke called "hemiparetic gait" is characterized by slow and asymmetrical steps with poor motor control on the paretic leg while paradoxically increasing the cost of energy expenditure. Biomechanical evidence shows that impaired gait performance for people with chronic stroke is not solely the result of the loss of muscle strength, but involves complicated movement discoordination across multiple joints in the affected leg. This has been taken to indicate a persistent motor control deficit in the paretic leg post stroke. Recent imaging studies suggest that the persistent motor control deficit after stroke may be the result of the disruption of motor memory consolidation, a process by which a newly-learned motor skill is transformed from a fragile state to a stable state and is "saved" in our brain afterward. This indicates that the same brain area responsible for controlling motor activity is also involved in memorizing newly-learned skills during the early stage of motor learning. Presence of persistent motor control deficits in the chronic stage may be attributed to the fact that damage to the brain cortex significantly impacts the ability of acquiring motor skills and consequentially defers the improvement of motor function, including gait.

Arms

: Chronic Stroke

Participants who suffered a single event of cerebral vascular accident at least six months ago before study enrollment.

: Multiple Sclerosis

Participants who has been diagnosed with multiple sclerosis

: Healthy control

Age-matched healthy adults who are self-reported healthy and has no known musculoskeletal, neuromuscular, and cardiovascular diseases.

Interventions

Behavioral: - Visuomotor leg reaching task

Similar to a hand reaching task in which participants were asked to reach to a tea-cup, in a visuomotor leg reaching task, participants will be seated and given real-time visual feedback about their leg movements via a cursor display on a computer screen. The task is to control a foot mouse/marker attached to the foot and move the cursor from a start location to the target displayed on a wall screen. Three different targets, equidistant from the start location at top, top-left, and top-right screen positions, will be used for leg reaching. In each trial, one of three targets will be randomly presented and subjects will be instructed to make forward, or rightward, or leftward foot reaches to guide the cursor to one of the targets. Throughout the entire experiment, subjects are blocked from viewing leg movements by a cardboard.