What is an EMG test?
An EMG records the electrical activity of your muscles. Paired with a nerve conduction study, it shows whether a neck or low back injury has damaged the nerves that run to your arms or legs. Timing matters. The test is most helpful several weeks after the injury. And a normal result does not rule out nerve damage.
When muscles work, they make an electrical current. Most often, an EMG is done at the same time as a nerve conduction study (NCS). These tests are used to diagnose nerve injuries and muscle damage. Together they are called electrodiagnostic testing (EDX). They are very often given to people with low back or neck injuries. The goal is to find nerve damage which may be causing symptoms in the upper or lower extremities (arms or legs).
The EMG checks the electrical path from the spinal cord to the muscle. It is often normal at first, even with severe spinal cord injury.
The NCV will measure how fast and how fully a person’s arm or leg nerve passes along “information”, or how fast and fully they respond. If there is nerve damage, the response is very often slow or incomplete. NCS will show conduction changes in carpal tunnel and tarsal tunnel syndromes. Most EMG/NCV tests are not 100% accurate, and have at least a 10-30% margin of error. Very often people with nerve damage will have normal EMG/NCV tests even though they have severe nerve damage. One key part of using an EMG /NCS is when the test is done compared to when the injury happened.
Electrodiagnostic studies (EMG and NCS) may be most helpful when done several weeks after the injury. Still, NCSs are often useful acutely, right after a nerve injury. One example is when a nerve may have been severed. In fact, if testing waits too long, the chance to pin down the injured area or to step in may be lost. In some cases, even needle EMG testing done right after a nerve injury may show abnormal motor unit action potential (MUAP) recruitment. It can also give baseline data. That data can help record conditions that were already there, date the injury, or serve as a starting point to compare with later tests.
Related reading: repetitive strain injury.
Frequently asked questions
Why would a doctor order an EMG test?
Most often it is ordered after a neck or low back injury, when pain, numbness or weakness spreads into the arms or legs. The EMG is usually done with a nerve conduction study. It shows whether the injury has damaged the nerves that run to those limbs. Together the two tests are called electrodiagnostic testing. They are used to diagnose nerve injuries and muscle damage.
What can an EMG and nerve conduction study diagnose?
These tests diagnose nerve injuries and muscle damage. That includes nerve damage from a neck or low back injury that causes symptoms in the arms or legs. The nerve conduction study measures how fast and how fully a nerve carries its signal. It shows the conduction changes of carpal tunnel and tarsal tunnel syndromes. A slow or incomplete response points to nerve damage.
Can an EMG miss nerve damage?
Yes. Most EMG and nerve conduction tests are not 100% accurate and carry at least a 10-30% margin of error. People with severe nerve damage often have normal results. The EMG is often normal at first, even with a severe spinal cord injury. A normal test is one piece of the picture, not proof that the nerves are healthy.
How soon after an injury should you get an EMG?
The EMG and nerve conduction study are usually most helpful several weeks after the injury. A nerve conduction study can still be useful right away, for example when a nerve may have been cut. Waiting too long can cost the chance to pinpoint the injury. An early needle EMG can also record a baseline. It can help date the injury and record conditions that were already there.
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Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP — Board Certified in Anesthesiology, Pain Medicine, Interventional Pain Management, Addiction Medicine, and Obesity Medicine.
