Nerves control the muscles in the body using electrical impulses and these impulses make the muscles react in specific ways. Nerve and muscle disorders cause the muscles to react in abnormal ways.

Measuring the electrical activity in muscles and nerves can help detect the presence, location and extent of disease that can damage muscle tissue or nerves. In the case of nerve injury, the actual site of nerve damage can often be located. EMG and nerve conduction studies are usually performed together to provide more complete information.

Why it is performed
An EMG is performed to:

• Diagnose diseases that damage muscle tissue, nerves or the junctions between nerve and muscle. These disorders include a herniated disc, amyotrophic lateral sclerosis or myasthenia gravis.
• Evaluate the cause of weakness, paralysis, involuntary muscle twitching or other symptoms. Problems in a muscle, the nerves supplying a muscle, the spinal cord or the area of the brain that  controls a muscle can all cause these kinds of symptoms.

How to prepare
Tell your doctor if you:

• Are taking any medications. Certain medications that act on the nervous system can interfere with an EMG results.
• Have had bleeding problems or are taking medications that thin the blood, such as warfarin or heparin.
• Have a pacemaker.

The patient does not need to restrict food or fluids. Do not smoke for at least three hours before the test.

Wear loose-fitting clothing that permits access to the muscles and nerves to be tested. You may be given a hospital gown to wear.

How it is performed
An EMG is performed in a hospital, clinic or doctor's office. A special room that screens out electrical interference may sometimes be used. The test may be performed by an EMG technologist or a doctor specializing in diseases of the nervous system or in physical rehabilitation.

The patient is asked to lie on a table or bed or sit in a reclining chair so the muscles being tested are relaxed and easy to reach.

The skin over the areas to be tested is cleaned with an antiseptic solution. An electrode that combines the reference point and a needle for recording is inserted into the specific muscle to be tested and attached by wires to a recording machine.

Once the electrodes are in place, the electrical activity in that muscle is recorded while the muscle is at rest. The technologist or doctor asks you to tense the muscle with gradually increasing force while the electrical activity in the muscle is being recorded.

The needle may be repositioned a number of times to record the electrical activity in different areas of the muscle or in different muscles.

The electrical activity in the muscle is displayed as wavy and spiky lines on a special video monitor and may be heard on a loudspeaker as machine gun-like popping sounds when you contract the muscle. The activity may be recorded on magnetic tape.

An EMG may take 30 - 60 minutes. When the testing is completed, the needle and skin electrodes are removed and those areas of the skin where a needle was inserted are cleaned. The patient may be given a pain reliever if any of the areas where a needle was inserted are sore.

How it feels
The patient feels a brief, sharp pain each time a needle electrode is inserted into the muscle. Some people find this part of the test very uncomfortable. After EMG testing, some soreness and a tingling sensation may persist for 1 - 2 days.

Risks
EMGs are safe. The patient may develop small bruises or swelling at some of the needle insertion sites. The needles are sterilized, there is very little chance of developing an infection.