What is electromyography?
Electromyography (EMG) measures
muscle response or electrical activity in response to a nerve’s stimulation of your
muscle. The test is used to help find nerve and muscle problems.
During the test, your healthcare
provider will put a small needle (also called electrodes) through your skin into a
muscle. The electrical activity picked up by the electrodes is then displayed on a
monitor in the form of waves. An audio-amplifier is used so the activity can be heard.
Often, several muscles are tested one at a time. The number and location of muscles
depends on what the healthcare provider is looking for.
EMG measures the electrical
activity of your muscle during rest, slight contraction, and forceful contraction.
Muscle tissue does not normally make electrical signals during rest. When an electrode
is put in, a brief period of activity can be seen. But after that, there should be no
After the electrode is put in, you
may be asked to contract your muscle, such as by lifting or bending your leg. The action
potential (size and shape of the wave) that this makes on the monitor gives information
about how your muscle responds to nerve stimulation. As you contract your muscle more
forcefully, more and more muscle fibers are activated, making more action
A healthy muscle will show no
electrical activity (no signs of action potential) during rest. It will only show it
when it contracts. But if your muscle is damaged or has lost input from nerves, it may
have abnormal electrical activity during rest. When it contracts, its electrical
activity may make abnormal patterns.
An abnormal EMG result may be a sign of a muscle or nerve disorder,
- Polymyositis. This is an inflammatory
muscle disease that causes decreased muscle power.
- Muscular dystrophy. This is a chronic
genetic disease that slowly impairs how muscles work.
- Myasthenia gravis. This is a genetic
or immune disorder that occurs at the point where the nerve connects with the
- Myotonic (stiff) muscles
- Damage or disease of the motor nerve, such as can be seen with
- Pinched nerve such as carpal tunnel syndrome or a pinched nerve
in the spine (radiculopathy)
A related test that is often done is a nerve conduction study (NCS).
NCV measures the speed of conduction of an electrical impulse through a nerve. NCV can
determine if your nerves are damaged. It is often done at the same time as EMG. Both
tests help find diseases that damage the nerves and muscles.
Why might I need an EMG?
EMG is often used along with NCS to
tell between a muscle problem and a nerve problem. NCS and EMG are best used at the same
time. Together, these tests can spot disorders of the nerves, muscles, or both.
EMG may be done to find the cause
of symptoms, such as muscle weakness, deformity, stiffness, and shrinkage.
EMG may be used to test for many
problems, such as:
- Motor problems, like involuntary
- Nerve compression or injury, such as
carpal tunnel syndrome
- Nerve root injury, such as sciatica
- Muscle degeneration, such as muscular dystrophy
There may be other reasons for your healthcare provider to recommend EMG.
What are the risks of EMG?
You may feel some discomfort when
the needle electrodes are put into your muscles. It may feel like an acupuncture needle.
Afterwards, your muscle may feel sore for a few days. A bruise may appear at the
The insertion of the electrodes may also cause false results on a muscle biopsy or during blood tests in which muscle enzymes are measured.
EMG may call for special
precautions when done in people on anticoagulant therapy (blood thinning medicine such
as warfarin). That’s because the needle electrodes may cause bleeding within the
There may be other risks based on
your specific health problem. Be sure to talk about any concerns with your healthcare
provider before the test.
How do I get ready for an EMG?
Ask your healthcare provider to tell you what you should do before your test. Below is a list of common steps that you may be asked to do:
- Your healthcare provider will explain the test to you and you can ask questions.
- You will be asked to sign a consent form that gives your permission to do the test. Read the form carefully and ask questions if something is not clear.
- Generally, fasting is not needed
before the test. In some cases, cigarettes and caffeinated drinks, such as coffee,
tea, and cola may be restricted 2 to 3 hours before testing.
- Tell your healthcare provider of all medicines (prescribed and over-the-counter) and herbal supplements that you are taking.
- Tell your healthcare provider if you have a pacemaker.
- Dress in clothes that permit access to the area to be tested or that are easily removed.
- Stop using lotions or oils on your skin for a few days before your test, or at least stop using them on the day of the exam.
- A sedative or pain reliever may be prescribed before the test. If so, you may need to have someone drive you home afterwards.
- Based on your health problem,
your healthcare provider may request other specific preparations.
What happens during an EMG?
An EMG may be done on an outpatient
basis or as part of your stay in a hospital. Steps may vary based on your health problem
and your healthcare provider’s practices. Talk with your healthcare provider about what
you will go through during your test.
The EMG is done by a healthcare
provider who specializes in nerve disorders. This is often a neurologist or physiatrist.
The EMG is often done right after a
nerve conduction study.
Generally, an EMG test follows this process:
- You will be asked to remove any clothing, jewelry, hairpins, eyeglasses, hearing aids, or other metal objects that may interfere with the test.
- If you are asked to remove clothing, you will be given a gown to wear.
- You will be asked to sit or lie down for the test.
- A neurologist will find the muscle(s)
to be studied.
- The skin will be cleansed with an
antiseptic solution. Next, a fine, sterile needle will be put into the muscle. A
metal plate will be placed under you.
- Several needle insertions may be
needed for the test. You may have slight pain with the placing of the electrode. But
it is often painless.
- If the test is painful, tell your
examiner because it can interfere with the results.
- You will be asked to relax and then do slight or full-strength muscle contractions.
- The electrical activity from your
working muscle will be measured and displayed on the monitor. An audio amplifier may
also be used so that both the look and sound of the electrical potentials can be
checked. If the recorder is attached to an audio amplifier, you may hear a sound like
hail on a tin roof when you contract your muscle.