Adult patients with suspected neurological disorders of muscle control and weakness may be eligible for this study. Normal volunteers may also participate. For the electromyography procedure, a special needle is inserted into a muscle. The patient will slightly tense the muscle and maintain the tension while electrical signals from the muscle fibers are being recorded. The electrical signals are played through a loudspeaker, providing feedback to help the patient tense the muscle the appropriate amount. The test, which is usually done for only one muscle, takes 1 to 2 hours. If needed, short breaks can be taken.