Understanding NCS and EMG Test Results

Electromyography and nerve conduction studies help diagnose nerve and muscle disorders

Electromyography (EMG) tests use electrodes to detect and record electrical signals in your muscles and nerve cells while they're active and at rest. EMG results can help determine if muscle weakness or numbness is caused by a problem that affects the nerves, muscles, or the connection between the two.

Negative or positive EMG results aren't really meaningful ways to describe the data. Rather, it is used by neurologists in a broader diagnostic process to evaluate diseases that affect your muscles and nerves, as well as assess how much damage there is. An EMG is often done during the same visit as nerve conduction studies (NCS) and both are valuable diagnostic tools.

This article discusses why EMG and nerve conduction studies are done, and what they can reveal about your neuromuscular function and health.

electromyography test

Illustration by Cindy Chung for Verywell Health

Purpose of the EMG Test

Your healthcare provider may opt for EMG when you have signs and symptoms of nerve or muscle damage, and abnormal sensations that suggest damage. These can include:

EMG can detect if muscle weakness or numbness is due to a nervous system disorder or injury to a nerve that's attached to the muscle.

Testing for Nerve and Muscle Disorders

The EMG, along with the nerve conduction study (NCS), can differentiate if you have a muscle disorder or a nerve disorder. Some common examples include:

Monitoring Medical Conditions

The EMG can also be used to monitor specific medical conditions in which nerve or muscle symptoms emerge. These conditions can include:

Nerve Conduction Study and EMG

A nerve conduction study uses artificially generated electrical stimulation to measure how fast electrical signals move through your nerves. An EMG is usually done along with an NCS. The two tests yield complementary information and, in most cases, offer a more comprehensive picture than one test alone.

Risks and Contraindications

Both EMG and NCS are low-risk. With EMG, there is a slight risk of getting an infection or bleeding where the electrodes were inserted.

Let the neurologist who's performing the procedure know ahead of time if you're taking anticoagulants (blood thinners) or have hemophilia, a disorder that inhibits blood clotting.

If the muscles in your chest are being tested with EMG, there's a tiny risk that air could get into the space between your chest wall and your lungs and cause pneumothorax (lung collapse).

If you're having an NCS, let the neurologist know ahead of time if you have a pacemaker or cardiac defibrillator, since you're going to be getting small electric shocks. Precautions may need to be taken.

Before the Test

When your healthcare provider orders EMG and NCS, they will talk to you about what these tests are looking for and what's going to happen. Be prepared with clean, dry skin to ensure the electrodes adhere well; avoid lotions, sunscreen, perfume, and oils ahead of your appointment.

  • Timing: EMG can take 30 to 60 minutes. NCS may take from 15 minutes to more than an hour, depending on how many nerves your healthcare provider wants to test. In the likely event that you have both tests, plan for the whole thing to take from one to three hours.
  • Location: These tests are performed on an outpatient basis at an office, lab, or clinic, or you may have them while you're already in the hospital. They're conducted in an examination room with a table or bed for you to lie down on, or a reclining chair to sit in so you're relaxed. The EMG machine will be close by.
  • What to Wear: Whether or not you can keep your clothes on will depend on what part of your body is being tested. Wearing loose clothing that can be pushed aside is helpful, and if you are asked to take your clothes off, you'll be given a gown.
  • Food and Drink: For most people, fasting is not required before either of these tests. Your healthcare provider will let you know if this isn't the case in your situation.
  • Medication: Ask your healthcare provider if you need to stop taking any of your prescription or over-the-counter medications before the test, especially if you're on Mestinon (pyridostigmine).
  • What to Bring: Bring along your insurance card in case the neurologist's team needs your information.

EMG Cost and Health Insurance

EMG test costs vary by location and by what you need to have tested. It can run up to several thousands of dollars. Be sure you have pre-authorization if required for your insurance carrier to pay the claim. You may have to pay a co-pay and/or co-insurance. If you're paying out of pocket, it's worth comparing different provider costs and asking about income-based discounts.

During the Test

A neurologist (who specializes in conditions that affect the nervous system), will likely perform most or all of your tests. There may be a technician to help attach the surface electrodes as well. The NCS usually comes first when both are done.

Pre-Test

You will likely need to fill out a consent form before you have this test. Next steps include:

  • Changing out of your clothes into a gown, if needed
  • Removing jewelry, glasses, hairpins, or any other metal that may interfere with the test
  • Positioning on the table or chair for the test

The neurologist will find the nerve(s) that are going to be tested. A stimulating electrode is then pasted onto the skin overlying the nerve of interest, while a recording electrode is pasted over the muscle that the nerve controls.

Preparation for an EMG will occur after NCS is complete.

nerve conduction study
Illustration by Cindy Chung, Verywell

Throughout the Test

Once everything is set for the nerve conduction study, a low-level electrical shock is applied through the stimulating electrode, and the speed of the electrical impulse is recorded by the recording electrode. The shock is mild and quick, but will probably be uncomfortable for a few seconds.

The procedure is then repeated for each area your healthcare provider wants to test. Each electrical impulse shows up on a monitor in what looks like waves that the neurologist will be able to interpret.

When the NCS is complete, the electrodes will be removed and any leftover paste will be cleaned off of your skin. EMG is normally performed right after the NCS. In this case, you'll remain where you are while the neurologist finds the muscle(s) they want to test.

  • The area is cleaned with antiseptic.
  • A fine, thin needle electrode is inserted into your muscle. This may feel like a brief, sharp pain or sting, followed by mild discomfort or pressure.
  • The neurologist will give you instructions on what to do to contract (tighten) and rest the muscle at the right times, such as bending or lifting your arm.
  • The neurologist may then move the needle electrode to another muscle or another part of the muscle and repeat the procedure, but the number of times this happens will depend on the areas being tested.

The electrode doesn't emit any electrical impulses; it simply records your muscle activity at rest and as you move. Activity is measured and translated as waves on a screen, and static-like noises are played on a speaker. The neurologist who does the test listens to these sounds and watches the monitor in order to detect abnormalities.

When your test is complete, the electrode will be removed. You should be able to get dressed, if necessary, and go home right away. Your healthcare provider may give you specific instructions, depending on your medical condition.

Most people get through the EMG and NCS tests just fine, but if you find it to be too uncomfortable or painful, let the neurologist know as this can affect your test results.

After the Test

Once your test is finished, you can usually go about your regular activities as normal, unless your healthcare provider instructs you otherwise.

The muscles that were tested may feel sore for a few days, and you may notice some bruising, tingling, or swelling in the area. This should go away within a few days, but you can apply ice or a cold pack for 10 to 20 minutes at a time and take Tylenol (acetaminophen) or Motrin/Advil (ibuprofen) for pain.

If you notice increasing pain, swelling, tenderness, or drainage from any of the areas in which the needles were inserted, let your healthcare provider know. These may be signs of an infection.

NCS and EMG Test Results

Positive EMG test results are just one part of data interpretation. The EMG and NCS are not always straightforward and may not always lead to just one possible diagnosis. However, the tests can reduce the number of diagnostic possibilities. The neurologist will interpret the tests and send the report to your healthcare provider, which may take a day or two.

Since these tests don't have a simple negative or positive result, your healthcare provider will discuss the results with you at a follow-up appointment that you may have scheduled in advance.

If you do see your EMG or NCS report, here are some terms you may encounter:

  • Amplitude: The electrical signal is represented as a wave, and the amplitude is its height.
  • Conduction velocity (CV): The conduction velocity describes the speed at which the electrical impulse travels along the nerve.
  • Duration: This describes the width of an electrical wave.
  • Conduction block: This is the reduction of a signal across an anatomical region, such as the wrist. This suggests nerve entrapment as with carpal tunnel syndrome.
  • F reflex: The F wave is a kind of electrical echo in which the impulse travels up to the spine and then back down along the same fiber. It thereby gives a sense of the conduction along the entire length of a motor nerve.
  • H reflex: The H wave is the electrical equivalent of a reflex in the leg. An impulse travels to the spinal cord via a sensory nerve, then back along a motor nerve.

These measures give information about both motor and sensory components of the peripheral nervous system. They also suggest whether the axon or myelin sheath, which is the exterior layer of nerves that helps electrical impulses travel faster, is more damaged by neuropathy.

Myelin and Axon Function

Myelin helps action potentials travel faster. With problems related to myelin (myelinopathy), conduction velocity is decreased. In problems with the axon (axonopathy), fibers that are intact can conduct signals at normal speeds, but there are fewer fibers, which leads to a weaker signal and decreased amplitude.

The findings of the EMG can include:

  • Normal EMG: When a nerve stimulates a muscle to contract, the result is a brief burst of electrical activity called a motor unit action potential (MUP). Normally, the more you contract the muscle, the more electrical activity occurs. This activity appears smooth in a normal EMG.
  • Activity at rest: In diseases like carpal tunnel syndrome and peripheral neuropathy, muscles sometimes start having spontaneous activity on their own. This shows that there's an issue with the nerve supply to that muscle. Resting electrical activity can also point to inflammation or a muscle disease. Sometimes the abnormality causes visible muscle twitches called fasciculations, which are especially important when diagnosing ALS (and may be better diagnosed using newer ultrasound techniques).
  • Abnormal activity during contraction: As your muscle is contracted, nerve fibers signal more and more bits of muscle (called motor units) to join in and help. There are changes in this so-called recruitment pattern with peripheral nerve diseases as the nerve is unable to connect to as many units. In muscle diseases, the number of these motor units is normal, but the amplitude is smaller. The patterns help with a diagnosis.

Abnormal results on an NCS almost always occur from nerve damage that's due to conduction block, axonopathy (damaged nerve fiber), or demyelination (damage to or loss of the outer insulating layer of nerves). There are a number of conditions that can lead to nerve damage and, thus, an abnormal NCS result.

Your test may be normal if the nerves that are affected are small and don't have a myelin sheath, or if you have a disorder that only affects your brain, spinal cord, spinal nerve roots, or muscle. In other words, even though you may have some nerve damage, your NCS result may still be normal.

Follow-Up

Your healthcare provider may need to do additional testing to pinpoint a diagnosis, or one may be determined by your results and a treatment plan started. Next steps will vary greatly, depending on the cause or your symptoms. Your healthcare provider will let you know how to proceed. If you want a second opinion, ask for a referral.

If you had an NCS or EMG to monitor a condition you already have, the test will likely show your healthcare provider how much nerve damage or change in muscle activity there has been since your last test. This can help them adjust your treatment plan accordingly, if needed. You may have these tests periodically to check disease progression.

Summary

The electromyography (EMG) test is used to evaluate nerve and muscle function. It relies on artificially generated electrical impulses delivered to the body to test how well the impulses travel through the neuromuscular network. An EMG often is used with a nerve conduction study.

Remember that while EMG and NCS are helpful diagnostic tools, they are only one piece of the puzzle. The results of your test(s) may not be able to pinpoint a specific diagnosis, but they can help narrow it down. Talk with your healthcare provider about your results and treatment plan.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Peter Pressman, MD
Peter Pressman, MD, is a board-certified neurologist developing new ways to diagnose and care for people with neurocognitive disorders.