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What is an electromyography (EMG)?
Also called a myogram, an electromyography (EMG) measures muscle response or electrical activity in response to a nerve’s stimulation of the muscle. The test is used to help detect neuromuscular abnormalities.
During the test, one or more small needles (also called electrodes) are inserted through the skin into the muscle. The electrical activity picked up by the electrodes is then displayed on an oscilloscope (a monitor that displays electrical activity in the form of waves). An audio-amplifier is used so the activity can be heard.
EMG measures the electrical activity of muscle during rest, slight contraction, and forceful contraction. Muscle tissue does not normally produce electrical signals during rest. When an electrode is inserted, a brief period of activity can be seen on the oscilloscope, but after that, no signal should be present.
After all of the electrodes have been inserted, you may be asked to contract the muscle, for example, by lifting or bending your leg. The action potential (size and shape of the wave) that this creates on the oscilloscope provides information about the ability of the muscle to respond when the nerves are stimulated. As the muscle is contracted more forcefully, more and more muscle fibers are activated, producing action potentials.
A related procedure that may be performed is nerve conduction velocity (NCV). NCV is a measurement of the speed of conduction of an electrical impulse through a nerve. NCV can determine nerve damage and destruction, and is often performed at the same time as EMG. Both procedures help to detect the presence, location, and extent of diseases that damage the nerves and muscles.
What happens during an EMG?
An EMG procedure may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician's practices.
The EMG is performed by a neurologist, although a technologist may also perform some portions of the test.
The EMG is usually performed immediately following a nerve conduction study (a test that measures the flow of current through a nerve before it reaches the muscle rather than the response of muscle itself).
Generally, an EMG procedure follows this process:
- You will be asked to remove any clothing, jewelry, hairpins, eyeglasses, hearing aids, or other metal objects that may interfere with the procedure.
- If you are asked to remove clothing, you will be given a gown to wear.
- You will be asked to sit or lie down for the test.
- A neurologist will locate the muscle(s) to be studied.
- The skin will be cleansed with an antiseptic solution. Next, a fine, sterile needle will be inserted into the muscle. A metal plate will be positioned under you.
- Ten or more needle insertions may be necessary for the test. You may experience slight pain with the insertion of the electrode, but it is usually painless.
- If the test is painful you must tell your examiner because this can interfere with the results.
- You will be asked to relax and then perform slight or full-strength muscle contractions.
- The electrical activity from your working muscle will be measured and displayed on the oscilloscope.
- An audio-amplifier may also be used so that both the appearance and sound of the electrical potentials can be evaluated. If the recorder is attached to an audio-amplifier, you may hear a sound like hail on a tin roof when you contract your muscle.
Procedures where an EMG is used:
Treatment for the following neurological conditions may need an EMG:
- muscular dystrophy
- neuromuscular diseases, such as myasthenia gravis
- nerve compression or injury, such as carpal tunnel syndrome
- nerve root injury, such as sciatica