Background: \- Electrical impedance myography (EIM) is a new technique being studied to see if it is helpful in evaluating muscle disorders and nerve disorders. EIM looks at how a mild, painless electrical current travels through muscles. Researchers want to gain experience in using the EIM device. They will collect information on the results of using it on people with and without nerve and muscle diseases, and compare that with information from other standard tests. First, they will test the device on healthy people. Then they will test people with a variety of neuromuscular diseases. Because the test is noninvasive and not painful, researchers will test both children and adults.

Objectives: \- To gain experience using the EIM muscle testing device.

Eligibility: * Healthy volunteers at least 2 years old. * Individuals at least 2 years old who have neuromuscular disease.

Design: * Participants will be screened with a medical history and physical exam. * Participants will have one 2-3 hour clinic visit. Researchers may request follow-up visits. * Participants will be tested with the EIM device. The device and small electrodes will be placed on their skin. An electric current will pass through the device, but the participants will not feel this. * Participants may have an ultrasound test. A gel will be put on their skin, and a device will be moved over the skin. * Participants may have a nerve test. Electrodes will be placed on their skin, and they will feel a small shock. * Participants may have a test where a thin needle is inserted in their muscle.