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What is an electromyography?
Electromyography (EMG) measures muscle response or electrical activity in response to a nerve’s stimulation of your muscle. The test is used to help detect nerve and muscle problems.
During the test, your doctor will insert one or more small needles (also called electrodes) through your skin into your muscle. The electrical activity picked up by the electrodes is then displayed on a monitor in the form of waves. An audio-amplifier is used so the activity can be heard.
EMG measures the electrical activity of your muscle during rest, slight contraction, and forceful contraction. Muscle tissue does not normally make electrical signals during rest. When an electrode is inserted, a brief period of activity can be seen, but after that, no signal should be present.
After all of the electrodes have been inserted, you may be asked to contract your muscle, for example, by lifting or bending your leg. The action potential (size and shape of the wave) that this creates on the monitor give information about the ability of your muscle to respond to nerve stimulation. As your contract your muscle more forcefully, more and more muscle fibers are activated, producing action potentials.
A healthy muscle will show no electrical activity (no signs of action potential) during rest, only when it contracts. However, if your muscle is damaged or has lost input from nerves, it may have electrical activity during rest. When it contracts, its electrical activity may produce abnormal patterns.
An abnormal EMG result may be a sign of a variety of muscle or nerve disorders, including:
- Polymyositis (an inflammatory muscle disease that causes decreased muscle power)
- Muscular dystrophy (a chronic genetic disease that progressively affects muscle function)
- Myasthenia gravis (a genetic or immune disorder that occurs at the point where the nerve connects with the muscle)
- Myotonic (stiff) muscles
A related test that may be done is a nerve conduction velocity (NCV) test. This is also called a nerve conduction study (NCS). 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 done at the same time as EMG. Both tests help detect the presence, location, and extent of diseases that damage the nerves and muscles.
Why might I need an EMG?
EMG is often used along with nerve conduction velocity (NCV) to tell the difference between a muscle problem and a nerve problem. NCV detects a problem with the nerve, whereas EMG can detect a problem with the muscle.
EMG may be done to find the cause of symptoms, such as muscle weakness, deformity, stiffness, and shrinkage. It may be used to detect whether someone is having true muscle weakness or weakness because of pain or psychological reasons.
EMG may be used to evaluate many problems or disorders, including:
- Neuromuscular diseases, such as myasthenia gravis
- Motor problems, such as involuntary muscle twitching
- Nerve compression or injury, such as carpal tunnel syndrome
- Nerve root injury, such as sciatica
- Muscle degeneration, such as muscular dystrophy