What is EMG?
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. Please see this procedure for additional information.
Interpretation of the test results:
A healthy muscle will show no electrical activity (no signs of action potential) during rest, only when it contracts. However, if the 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), and myotonic (stiff) muscles.
Reasons for the Procedure
EMG is often used along with nerve conduction velocity (NCV) to differentiate a muscle disorder from a nerve disorder. NCV detects a problem with the nerve, whereas EMG can detect diseases stemming from problems with the muscle itself, as well as other problems that result from influences on the muscle from other systems, such as nerves.
EMG may be done to identify the cause of symptoms, such as muscle weakness, deformity, spasticity, atrophy, and stiffness. It may be used to detect whether someone is experiencing true muscle weakness or weakness because of pain or psychological reasons.
EMG may be used to evaluate many problems/disorders including, but not limited to, the following:
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
There may be other reasons for your physician to recommend EMG.
Risks of the Procedure
Some discomfort, similar to the feeling of an injection or an acupuncture needle, may be felt when the needle electrodes are inserted into the muscle. Afterwards, the muscle may feel sore for a few days and a bruise may appear at the needlestick site.
The insertion of the electrodes may also cause false results on a muscle biopsy or during blood tests in which muscle enzymes are measured.