Experimental: Gait Training

The gait training in the experimental group will use a treadmill with a partial body-weight support system and our gait training device. A gait training session will include 2-minute warming up at the beginning and 2-minute cooling down at the end, at a treadmill speed of 0.6 mph. After the warming up, the treadmill speed will increase at every two minutes by an increment of 0.2 mph. Participant's heart rate will be monitored throughout the training session by using a Polar OH1 optical HR sensor placed in the participant's left forearm. Reseachers will stop the increase and maintain the same treadmill speed after the HR reaching the target heart rate zone of 50% - 60% of age-predicted heart rate reserve.

Active_comparator: Leg Cycling Exercise

The leg cycling exercise will use a stationary bicycle and the similar protocol as in the experimental group. The cycling exercise will include 2-minute warming up in the beginning and 2-minute cooling down at the end with a speed of 10 rpm and resistance of level 1 of 4. After warming up, the speed will increase at every two minutes by an increment of 5 rpm until reaching the target heart rate zone of 50% - 60% of age-predicted maximum heart rate. If the participant cannot tolerate any more increase in cycling speed before reaching the target heart rate, the resistance will be increased. The training duration will be individualized at the beginning and increase weekly by 5 minutes, depending on the tolerance of the participant, up to the maximum of 30 minutes. Both heart rate and blood pressure will be monitored before, during, and after each session to ensure safety. Each session including setting up will be about one hour.