Nerves control the muscles in the body using electrical impulses. Whenever a muscles contract (tenses up) in response to a signal from the brain, it also produce an electrical discharge of its own. Electromyography (EMG) measures the electrical discharges made by the muscles. Nerve conduction studies (nerve conduction velocity) measure how well individual nerves can transmit electrical signals.
Measuring the electrical activity in muscles and nerves can help detect the presence, location, and extent of diseases that can damage muscle tissue (such as muscular dystrophy) or nerves (such as amyotrophic lateral sclerosis). In the case of nerve injury, the actual site of nerve damage can often be located. EMG and nerve conduction studies are often done together to provide more complete information.