Electromyography (EMG) is a measurement of the electrical activity of muscles and is usually recorded by a small needle electrode, which is inserted through the skin and into the muscle. The first insertion activity into a muscle may show a short-lived burst of electrical signals, but once the muscle settles down, it should produce no electrical activity at all if it is fully relaxed.
However, if the muscle is diseased or its nerve supply is impaired, electrical activity may be seen when the muscle is at rest. The first stage of the EMG examination therefore is to record muscle at rest. The next step is to contract the muscle. When a muscle is contracted, there should be a steady stream of electrical activity from it. If the nerve supply is abnormal, this pattern of activity is reduced. There may also be other changes in the electrical signals that can give useful information about what has been happening to the nerves.
In muscle diseases, different but quite characteristic changes in the electrical signals are seen when the muscle is being contracted. This test can therefore help determine whether a person has a disorder affecting the nerve supply to the muscles or the actual muscle tissue itself. The electrical activity of the muscles is displayed on a computer screen. These also produce sound, which is heard simultaneously through a loudspeaker.
EMG can be useful in testing for a wide range of conditions including muscular dystrophy and motor neurone disease.