This study will take place at Texas Woman's University (TWU)-Houston campus and include two phases: Phase I involves a single-intervention study and Phase II involves a four-week intervention study. After enrolling to the study, participants can choose to only participate in the Phase I study or choose to participate in both Phase I and Phase II studies.

Phase I Study - participants will be asked to complete two visits, 1-14 days apart. It will take about a total of 6.5 hours of their time to complete all tests (3.5 hours for the first visit and 3 hours for the second visit, two visits). Participants will need to wear short-sleeve t-shirt, shorts, and a pair of sneakers for testing. During the first visit, the investigators will measure how well the nervous system works and how well the individual walks (see details as follows). If participants have had a stroke before, on the first visit the investigators will do some tests on to make sure participants have good feeling, balance, and motor responses in their legs. If all tests are good, participants will fill out a form asking about their medical history. If participants have not had a stroke, the investigators will ask a few questions to make sure participants do not have any medical issues.

Next, the investigators will ask participants to learn a leg movement task at each visit. The task is to control and move a computer cursor from a start location to one of three targets displayed on the computer monitor. Participants will need to make forward, rightward or leftward foot movements to guide the cursor to one of the targets. The task itself is similar to the daily computer task performed by a hand mouse. In each visit, it will take approximately a total of 45 minutes to complete a set of leg reaching task. Several one-minute rest breaks will be provided as needed during test. Participants will then be asked to come back for the second visit within 1-14 days later to repeat the same task. Throughout practice, participants will learn how to control the cursor using their feet. So the investigators can compare the learning capacity over two visits to best indicate the change in the learning capacity over time.

The investigators also want to know if participants can learn this motor skill faster with cutaneous brain stimulation. Participants will receive weak electrical brain stimulation for 20 minutes during leg skill learning. The brain stimulation device that the investigators used is similar to the commercial product seen in the TV/magazine, called "transcutaneous electrical Nerve stimulation" in which electrical current is delivered by electrodes for pain relief and muscle stimulation. In this study, two electrodes, one placed on the top of the scalp and one placed on the forehead, will deliver weak electrical current for 20 minutes. Because the investigators set the current intensity at a very low level, it will not cause any muscle twitches and participants may only feel little tingling sensation in the first 10 seconds. Most of the time, participants likely become accustomed to the stimulation and no longer feel the stimulation throughout the rest of the session.

Phase II Study - This study is an expanded version of Phase I study. Participants will be asked to complete 12 visits of exercise training paired with brain stimulation over a four-week period. It will take about a total of 18 hours of the time to complete all training sessions (1.5 hour for each visit/training for a total of 12 visits). Participants will need to wear short-sleeve t-shirt, shorts, and a pair of sneakers for training. Before and after four-week training, the investigators will measure how well the nervous system works and how well the participant walks (see details as follows). One week after completion of four-week training, participants will be asked to come back to repeat the same tests. The investigators will compare changes in the learning capacity and motor function before and after four-week training, and at one week after completion of training to best indicate permanent changes in the learning capacity and motor function over time.

During each visit, the investigators will ask participants to continue learning a leg movement task same as the Phase I study. The investigators will also ask participants to learn a similar leg task, call stepping task during standing. This stepping task is very similar to the leg task in the Phase I study, but is a more advanced task that requires a good standing balance. The task is to control a computerized marker attached to the foot and move a computer cursor from a start location to one of three targets displayed on the computer monitor. Participants will need to make forward, rightward or leftward stepping movements to guide the cursor to one of the targets. The task itself is similar to stepping motion during walking. In each visit, it will take approximately a total of 1.5 hour to complete two sets of leg task training during sitting and standing. Same brain stimulation used in the Phase I study (see above) will be delivered during the middle of the exercise training. Several one-minute rest breaks will be provided as needed during training. Participants will then be asked to come to the laboratory three times per week for four weeks in order to complete 12 training sessions.

Outcome measures - For the measurement of brain activity, the investigators will put a recording electrode on the calf muscle in one leg. Five low-intensity brain stimulation will be delivered to the scalp to trigger the motor responses. Most of the time, participants will feel single muscle twitch due to the stimulation. It would take a total of 10 minutes to finish data collection. For the measurement of nerve activity, the investigators will put a recording electrode on the calf muscle in one leg. Low-intensity electrical stimulation will be delivered to a nerve behind the knee to trigger motor responses. A series of small tendon vibration stimuli will be delivered during electrical stimulation to determine the sensitivity of the nerve. Most of the time, participants will feel nothing or just light tingling sensation in the stimulated area. It would take a total of 40 minutes to finish data collection. Several one-minute rest breaks will be provided as needed during test. For the measurement of walking, the investigators will put sticky markers on both legs and ask participants to walk normally across a 10-meter walkway for five trials. It would take approximately a total of 30 minutes to finish data collection. Several one-minute rest breaks will be provided as needed during test. For the stepping reaction time test, subjects will stand on the force plates and will be instructed to step forward onto a target marked on the floor as soon as they sense electrical stimulation delivered to the posterior of the leg. It would take approximately a total of 40 minutes to finish data collection. Several one-minute rest breaks will be provided as needed during test.