This study will compare the effectiveness of unilateral versus bilateral exercise training for recovering movement abilities in chronic stroke patients. After a stroke, some people have improved their movement ability by exercising the arm affected by the stroke. Others have improved by exercising both arms together. This study will compare these two kinds of exercise to see which might work best.

Healthy volunteers and people who have had a stroke more than 6 months ago may be eligible for this study. Stroke patients must have some weakness in their stroke-affected arm. All participants must be 18 years of age or older.

Candidates are screened with a clinical and neurological examination. Women who can become pregnant have a pregnancy test. Stroke patients have a magnetic resonance imaging (MRI) study of the brain if they have not had one in the last year.

Participants undergo the following tests and procedures:

MRI: This procedure uses a magnetic field and radio waves to produce images of body tissues and organs. The subject lies on a table that slides into the scanner (a narrow cylinder), wearing earplugs to muffle loud knocking sounds that occur during the scanning process. The procedure lasts about 45 minutes, during which time the subject must lie still for up to 30 minutes.

Testing sessions: There are six test sessions - two to familiarize the subject with the reaching test, two to do the reaching test plus transcranial magnetic stimulation (TMS) before and after an exercise session, and two to do the reaching test plus TMS one day after the exercise session. The sessions are on consecutive days, with a

1-week break after session 3. The procedures involved are:

• Reaching test: The subject responds as quickly as possible to a "GO" signal presented on a computer monitor by performing a reaching movement with his or her arm. The activity from the arm muscles is recorded using surface electrodes. During this test, a questionnaire is used to determine the subject's attention, fatigue, and mood.
• TMS: For this test, a wire coil is held on the scalp, and a brief electrical current is passed through the coil, creating a magnetic pulse that stimulates the brain. During the stimulation, the subject may be asked to tense certain muscles slightly or perform other simple actions. The stimulation may cause a twitch in muscles of the face, arm, or leg, and the subject may hear a click and feel a pulling sensation on the skin under the coil. The effect of TMS on the muscles is detected with small metal disk electrodes taped onto the skin of the arms. The subject performs the reaching test while TMS is delivered at various times after getting the "GO" signal.
• Arm exercises: The subject is seated in front of an exercise device that has two handles that slide forward and backward along a set of tracks. The subject is asked to slide either one or both handles back and forth repeatedly, alternating 5 minutes of arm exercise with 5 minutes of rest for 30 minutes.