Small needle sensors (electrodes) are used to record the electrical activity in selected muscles and peripheral nerves during rest and as they contract during movement. The electrodes transmit this information to a machine that provides a graphic representation of the muscles’ electrical discharge; the resulting data show how well the muscles work. Electroneurography, which evaluates the function of nerves in a similar manner, is often performed at the same time as EMG in order to fully evaluate people experiencing weakness or paralysis.
Purpose of the EMG
- To aid in the diagnosis of primary muscle disorders, such as muscular dystrophy; degenerative nerve diseases such as amyotrophic lateral sclerosis (Lou Gehrig’s disease); and neuromuscular diseases (which affect both nerves and muscles), such as myasthenia gravis
- To diagnose disorders that affect the neuromuscular junction (connection between a nerve fiber and the muscle it supplies)
- To diagnose disorders of the nerves or nerve roots caused by nerve damage or ongoing nerve injury
- To detect blockages or slowed responses to nerve stimulation
- To help diagnose pain, numbness, tingling, weakness or cramping in the arms or legs
Who Performs EMG
- A doctor, a physical therapist, or a trained technician
Special Concerns about EMG
- People with a widespread skin infection should not take this test because the infection may pass into the muscle via the needle electrodes.
- Because the needles may cause intramuscular bleeding, people with bleeding disorders or those who take anticoagulants should not undergo this procedure.
- Obesity or edema (swelling) may make it difficult to insert the electrodes into the muscle.
- A variety of medications may interfere with the results of this test.
Before the EMG
- Tell your doctor if you regularly take any medications, herbs, or supplements—particularly anticoagulants, nonsteroidal anti-inflammatory drugs (such as aspirin, ibuprofen, or naproxen), muscle relaxants, and anticholinergic medications. You will be instructed to discontinue certain agents before the test.
- Fasting is not usually required. However, your doctor may ask you to refrain from smoking or drinking caffeinated beverages for 2 to 3 hours prior to the test.
- Do not use creams or lotions on the day of the test.
- Wear loose clothing that permits access to the muscle being studied. In some cases, you may be instructed to undress and put on a hospital gown.
What You Experience
- You will either lie down on a bed or sit in a chair, depending on which muscles are being tested. A metal plate is placed underneath you to serve as a reference electrode.
- The skin over the selected muscle is cleansed with an antiseptic and a small needle electrode—which is very thin and resembles a stick pin—is inserted into the muscle. A reference electrode is inserted under the skin nearby. You may experience some discomfort due to the needles.
- The electrodes record the electrical activity while the muscle is relaxed. You will then be asked to contract the muscle gradually, with increasing forcefulness. The impulses are transmitted to a machine that amplifies the signals and displays them. Sometimes the signals are converted into audio and played through a speaker.
- The procedure may be repeated at other locations.
- EMG generally takes about 25 to 30 minutes.
Risks and Complications of EMG
- Rarely, blood may collect and clot under the skin (hematoma) at the needle insertion sites, causing swelling and discoloration. This is harmless but may cause some discomfort.
After the EMG
- You may be given pain-relieving medication to allay any soreness around the electrode insertion sites.
- Resume your normal activities and any medications withheld before the test.
- If a large hematoma develops at the needle insertion sites, apply ice initially; after 24 hours, use warm, moist compresses to help dissolve the clotted blood.
Results of EMG
- The function of your muscles is indicated by the strength and pattern of their electrical activity. Normally, there is no electrical activity in a muscle at rest. If a muscle disease is present, there may be electrical activity in the rested state and abnormal activity when the muscle contracts.
- Your doctor will consider the test results—as well as your symptoms, your physical exam, and the results of other tests—in order to diagnose any muscle disease and identify the muscles affected.
- If a definitive diagnosis can be made, appropriate therapy will be initiated.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media