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Electromyography (EMG)
What is it?
Active muscles produce an electrical current that is usually proportional to the level of muscle activity; the stronger the current, the stronger the muscle activity. An electromyogram (EMG) measures the electrical activity of muscles at rest and during contraction. The intramuscular EMG is the test most commonly ordered and involves inserting a small needle electrode through the skin into the muscle in question to measure that muscle’s electrical activity. In the vast majority of patients, this study is free of discomfort. A nerve condcuction velocity (NCV) test is often done at the same time as an EMG.
Why should I have the test?
An EMG is often used to evaluate unexplained muscle weakness, twitching or paralysis, and to find the causes of numbness, tingling and pain. EMG testing can differentiate between true weakness and reduced use because of pain or lack of motivation. It can also determine whether a muscle disorder begins in the muscle itself or is caused by a nerve disorder.
How is it performed?
A needle is inserted through the skin into the muscle. The electrical activity is detected by this needle (which serves as an electrode). The activity is displayed visually on an oscilloscope and may also be detected audibly with a speaker.
Since skeletal muscles are often large, several needle electrodes may need to be placed at various locations to obtain an informative EMG.
After placement of the electrode(s), the patient may be asked to contract the muscle (for example, to bend the leg).
The presence, size, and shape of the wave form (the action potential) produced on the oscilloscope provide information about the ability of the muscle to respond to nervous stimulation. Each muscle fiber that contracts produces an action potential. The size of the muscle fiber affects the rate (how frequently an action potential occurs) and the size (the amplitude) of the action potential.
How do I prepare for an EMG?
For adults, no special preparation is needed. For infants and children, the physical and psychological preparation depends on the child's age, behavior, and prior experience.
Does an EMG hurt?
There may be some minor discomfort, similar to an injection, when the needle or needles are inserted. Afterward, the examined muscle may feel tender or sore for a few days, and there may be a small bruise.
Click below to return to the main Electrodiagnostics page:
