An electromyogram (EMG) measures the electrical activity of muscles at rest and during contraction. Nerve conduction studies measure how well and how fast the nerves can send electrical signals.
Nerves control the muscles in the body with electrical signals called impulses. These impulses make the muscles react in specific ways. Nerve and muscle problems cause the muscles to react in abnormal ways.
If you have leg pain or numbness, you may have these tests to find out how much your nerves are being affected. These tests check how well your spinal nerves and the nerves in your arms and legs are working.
Why It Is Done
An EMG is done to:
- Find diseases that damage muscle tissue, nerves, or the junctions between nerve and muscle. These problems may include a herniated disc, amyotrophic lateral sclerosis (ALS), or myasthenia gravis (MG).
- Find the cause of weakness, paralysis, or muscle twitching. Problems in a muscle, the nerves supplying a muscle, the spinal cord, or the area of the brain that controls a muscle can cause these symptoms. The EMG does not show brain or spinal cord diseases.
A nerve conduction study is done to:
- Find damage to the peripheral nervous system, which includes all the nerves that lead away from the brain and spinal cord and the smaller nerves that branch out from those nerves. This test is often used to help find nerve problems such as carpal tunnel syndrome or Guillain-Barré syndrome.
How To Prepare
Tell your doctors all the medicines, vitamins, supplements, and herbal remedies you take.
- Some medicines can affect the test results. You may need to stop taking some medicines before you have this test.
- If you take blood thinners, such as warfarin (Coumadin), clopidogrel (Plavix), or aspirin, your doctor will tell you if you should stop taking these medicines before your procedure. Make sure that you understand exactly what your doctor wants you to do.
Wear loose-fitting clothing so your muscles and nerves can be tested. You may be given a hospital gown to wear.
Since the electrodes are attached to your skin, make sure it is clean and free of sprays, oils, creams, and lotions.
You may be asked to sign a consent form that says you understand the risks of the test and agree to have it done.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).
How It Is Done
An EMG is done in a hospital, clinic, or doctor's office. It may be done in a room that stops any outside electrical interference. The test may be done by an EMG technologist or a doctor.
You will be asked to lie on a table or bed or sit in a reclining chair so your muscles are relaxed.
The skin over the areas to be tested is cleaned. A needle electrode that is attached by wires to a recording machine is inserted into a muscle.
When the electrodes are in place, the electrical activity in that muscle is recorded while the muscle is at rest. Then the technologist or doctor asks you to tighten (contract) the muscle slowly and steadily. This electrical activity is recorded.
The electrode may be moved a number of times to record the activity in different areas of the muscle or in different muscles.
The electrical activity in the muscle is shown as wavy and spiky lines on a video monitor and may also be heard on a loudspeaker as machine gun-like popping sounds when you contract the muscle. The activity may also be recorded on video.
An EMG may take 30 to 60 minutes. When the test is done, the electrodes are removed and those areas of the skin where a needle was inserted are cleaned.
Nerve conduction studies
In this test, several flat metal disc electrodes are attached to your skin with tape or a paste. A shock-emitting electrode is placed directly over the nerve, and a recording electrode is placed over the muscles controlled by that nerve. Several quick electrical pulses are given to the nerve, and the time it takes for the muscle to contract in response to the electrical pulse is recorded. The speed of the response is called the conduction velocity.
The same nerves on the other side of the body may be studied for comparison. When the test is done, the electrodes are removed.
Nerve conduction studies are done before an EMG if both tests are being done. Nerve conduction tests may take from 15 minutes to 1 hour or more, depending on how many nerves and muscles are studied.
How It Feels
During an EMG test, you may feel a quick, sharp pain when the needle electrode is put into a muscle. After the test, you may be sore and have a tingling feeling in your muscles for up to 2 days. If your pain gets worse or you have swelling, tenderness, or pus at any of the needle sites, call your doctor.
With nerve conduction studies, you will be able to feel the electrical pulses. The tests make some people anxious. Keep in mind that only a very low-voltage electrical current is used, and each electrical pulse is very quick (less than a split-second).
- If you still have pain after the test:
- Put ice or a cold pack on the sore area for 10 to 20 minutes at a time. Put a thin cloth between the ice and your skin.
- Take an over-the-counter pain medicine, such as acetaminophen (Tylenol), ibuprofen (Advil, Motrin), or naproxen (Aleve). Be safe with medicines. Read and follow all instructions on the label.
An EMG is very safe. You may get some small bruises or swelling at some of the needle sites. The needles are sterile, so there is very little chance of getting an infection.
There is no chance of problems with nerve conduction studies. Nothing is put into your skin, so there is no chance of infection. The voltage of electrical pulses is not high enough to cause an injury.
Your doctor may be able to tell you about some of the results of your nerve studies right after the tests. A full report may take 2 to 3 days.
The EMG recording shows no electrical activity when the muscle is at rest. There is a smooth, wavy line on the recording with each muscle contraction.
The nerve conduction studies show that the nerves send electrical impulses to the muscles or along the sensory nerves at normal speeds, or conduction velocities. Sensory nerves allow the brain to feel pain, touch, temperature, and vibration. Different nerves have different normal conduction velocities. Nerve conduction velocities generally get slower as a person gets older.
Electrical activity in a muscle at rest shows that there may be a problem with the nerve supply to the muscle. Abnormal wave lines when a muscle contracts may mean a muscle or nerve problem, such as a herniated disc, amyotrophic lateral sclerosis (ALS), or inflammation.
In nerve conduction studies, the speed of nerve impulses is slower than what is normal for that nerve. Slower speeds may be caused by injury to a nerve or group of nerves. Nerve conduction velocities generally get slower as a person gets older.
The results from EMG and nerve conduction studies are used along with your medical history, symptoms, physical and neurological exams, and the results of other tests to help your doctor find out what the problem is or see how a disease is changing.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Taking medicines, such as muscle relaxants and anticholinergics.
- Having bleeding, swelling, or too much fat under the skin at the site of the nerves or muscles being tested.
- Not being able to do what is asked during the test.
What To Think About
- The levels of some enzymes in the blood, such as aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and creatine phosphokinase (CPK), may rise when muscle tissue is damaged. An electromyogram can cause higher levels of these enzymes for up to 10 days after the EMG, so blood tests for these enzymes should not be done for 5 to 10 days after an EMG.
- Special types of electromyograms (EMG) may also be done. For example:
- Single-fiber EMG is a special type of EMG test. For this test, very small needles are inserted into a muscle to see how a single muscle fiber contracts. It is a useful test for myasthenia gravis, a disease that affects the nerve-muscle (neuromuscular) junctions.
- Repetitive nerve stimulation is a useful test for myasthenia gravis and Lambert-Eaton syndrome. In this test, small, repeated shocks are given to measure how the muscle reacts to repeated nerve stimulation.
- External sphincter electromyogram measures the electrical activity of the external urinary sphincter to help find urination problems. The urinary sphincter is a ring-shaped band of muscles around the urethra that helps control urination. The electrical activity can be measured by skin electrodes, by needle electrodes inserted through the skin, or by electrodes in an anal plug.
Other Works Consulted
Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Primary Medical ReviewerAnne C. Poinier, MD - Internal Medicine
Specialist Medical ReviewerColin Chalk, MD, CM, FRCPC - Neurology
Current as ofMarch 12, 2014