This study will examine the role of the spinal cord in improving leg movements after physical training in healthy people. The results of this study may be helpful in developing new rehabilitation therapies for people with impaired leg function.

Healthy volunteers between 21 and 65 years of age may be eligible for this study. Candidates will be screened with a brief medical history and physical examination.

Participants will undergo EMG-H reflex measurement while pedaling on a stationary bicycle. For this test, the subject is given an electrical stimulation at the nerve at the back of the knee while pedaling on a stationary bicycle. The shock produces a jerky movement called the "H reflex." The muscular activity causing these reflexes is measured using electrodes (small metal disks) filled with a conductive gel and taped to the skin over the calf muscle. The nerve is stimulated at four different frequencies during the experiment. While cycling, pedal resistance changes, making it harder or easier to pedal.

On study day 1, participants have EMG-H reflex measurement to determine baseline performance, then a training session to prepare for the actual test, and then a post-training test to measure performance again. The test is repeated on days 2, then between days 5 and 8, and again between days 11 and 14. The screening and first visit last 2 hours; the remaining test visits are 1 hour each. The bicycling time is only 15 minutes during training and 5 minutes at each test session.