Nervous System Function in Normal Volunteers During Cycling Training

This study will identify changes that occur in the nervous system of healthy normal volunteers while they are learning a cycling task. Healthy subjects 18 years of age and older who can cycle for 16 minutes may be eligible for this study. Candidates will be screened with a brief medical history and physical examination. Participants will be assigned to one of three groups: * Complex training: Cycling for 16 minutes at a constant speed of 60 rpm with frequent changes in pedal resistance. * Simple training: Cycling for 16 minutes at a constant speed of 60 rpm with no changes in pedal resistance. * No training: Remaining seated on the bike for 16 minutes performing no exercise. Participants will cycle for 16 minutes for the training task, plus 4 minutes before and after the training. Reflexes will be measured with nerve conduction studies and somatosensory evoked potential (SSEP) recordings before and after training. Nerve conduction studies measure the speed with which nerves conduct electrical impulses and the strength of the connection between the nerve and the muscle. For these studies, a probe is placed on the skin over the calf muscles and the knee to deliver a small electrical stimulus, and wires are taped to the skin to record the impulses. SSEP recordings, which measure of the excitability of the brain to sensory stimuli, are collected from electrodes placed on the scalp. After the training period, multiple train stimulation (MTS) is applied for 10 minutes. For MTS, weak electrical currents are delivered for 10 seconds every 10 seconds. These stimuli produce a buzzing sensation without pain, discomfort, or muscle twitching. The MTS is followed by another 4 minutes of cycling. Participants also undergo transcranial magnetic stimulation (TMS). For this procedure, an insulated wire coil is held over the scalp. A brief electrical current is passed through the coil, creating a magnetic pulse that travels through the scalp and skull and causes small electrical