Electromyogram
(EMG)
Medical Author: William C.
Shiel Jr., MD, FACP, FACR
Medical Editor: Melissa Conrad Stöppler, MD
What is an electromyogram?
An electromyogram (EMG) is a test that is used to record the electrical activity of muscles.
When muscles are active, they produce an electrical current. This current is usually
proportional to the level of the muscle activity. An EMG is also referred to as a myogram.
EMGs can be used to detect abnormal electrical activity of muscle that can occur in many
diseases and conditions, including muscular dystrophy, inflammation of muscles, pinched nerves,
peripheral nerve damage (damage to nerves in the arms and legs),
amyotrophic lateral sclerosis
(ALS), myasthenia gravis,
disc herniation,
and others.
Why is an EMG test done?
An EMG is often performed when patients have unexplained muscle weakness. The EMG helps to
distinguish between muscle conditions in which the problem begins in the muscle and muscle
weakness due to nerve disorders. The EMG can also be used to detect true weakness, as opposed to
weakness from reduced use because of pain or lack of motivation. EMGs can also
be used to isolate the level of nerve irritation or injury.
How is an intramuscular EMG done?
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.
Next: How do you prepare for an intramuscular EMG? »
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