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.
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:
There may be other reasons for your healthcare provider to recommend EMG.
You may feel some discomfort when the needle electrodes are inserted into your muscle. It may feel similar to that of an acupuncture needle. Afterwards, your 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.
EMG should not be done in people on anticoagulant therapy (blood thinning medicine such as warfarin) because the needle electrodes may cause bleeding within the muscle.
There may be other risks depending on your specific medical condition. Be sure to discuss any concerns with your healthcare provider before the procedure.
Ask your healthcare provider to tell you what you should do before your test. Below is a list of common steps that you may be asked to do:
An EMG may be done on an outpatient basis or as part of your stay in a hospital. Steps may vary depending on your condition and your healthcare provider's practices. Talk with your healthcare provider about what you will experience during your test.
The EMG is done by a neurologist (a doctor who specializes in brain and nerve disorders), although a technologist may also do some parts of the test.
The EMG is usually done right after a nerve conduction velocity 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 test follows this process:
After the test, the electrodes will be removed. You may be given pain medicine and warm compresses may be applied to the affected area immediately after the test.
Some muscle soreness may persist for a day or so after the test. Tell your healthcare provider if you have increasing pain, tenderness, swelling, or pus at the needle insertion sites.
Your healthcare provider may give you other instructions after the test, depending on your particular situation.
Before you agree to the test or the procedure make sure you know: