An electromyogram (EMG) is a procedure that tests the health of skeletal muscles (muscles attached to bone that control posture and movement) and the nerves that control them. EMGs measure the electrical activity of a muscle in response to nerve stimulation.

The nature and speed of impulse conduction along a nerve may also be measured during what is typically a companion test to the EMG, a nerve conduction velocity (NCV) test. According to the American Academy of Neurology (AAN), EMGs performed with needle electrodes, together with NCVs, represent the gold standard for diagnosing neuromuscular disorders.

Nerves and muscles work together to allow the body to perform everyday tasks. Motor nerves send signals that stimulate the muscles. This generates electrical activity inside the muscle, causing it to contract. This tightening of the muscle generates electrical activity of its own.

In most cases, an EMG involves inserting a thin needle electrode into skeletal muscles. Patients are asked to contract a muscle, and the electrical activity of the muscle in response is recorded by the electrode and transmitted to a receiving device. This muscle activity is then graphically displayed on a special monitor (oscilloscope). It may also be heard and monitored through a speaker.

EMGs are most often used to help diagnose the cause of certain symptoms, such as pain or weakness in the muscles or numbness in parts of the body. Through this test, a physician often is able to establish whether a patient’s muscle weakness is a result of a problem with the muscle itself, or a byproduct of a neurological disorder. An EMG and NCV can help a physician determine whether the source of the problem is with a muscle, nerve or the neuromuscular junction (connection between a nerve fiber and the muscle it supplies).  

The accuracy of EMGs may be limited by certain factors. If a patient’s hands or feet are cool, this may affect EMG results and result in the wrong diagnosis. EMGs may also be difficult to perform on patients who are obese. Furthermore, professionals may have limited training or experience in performing EMGs.

If a patient’s symptoms are related to a recent injury, a physician may schedule the EMG to take place several weeks later. Muscle activity affected by an injury may not show up in an EMG for up to three weeks after the injury.

Aside from the standard EMG, there are various other types of EMG. Two of the most common types are the single-fiber EMG and the surface EMG (SEMG).

A single-fiber EMG is a type of EMG that concentrates on a specific muscle fiber, rather than the entire muscle. It uses very small needle electrodes and its results are generally more sensitive than the standard EMG. This type of EMG may be used when certain disorders are suspected (e.g., myasthenia gravis) that may not be identified with a standard EMG. Single-fiber EMGs may only be available in specialized laboratories.

A noninvasive type of EMG called a surface EMG (SEMG) may sometimes be performed. No needles are used in this type of EMG. Instead, surface electrodes are placed on the skin overlying the muscle to record muscular activity. SEMG results are not as reliable as those of a needle EMG, and the clinical value of the SEMG has not been well established. For example, SEMGs are not considered an acceptable method of diagnosing neuromuscular disorders, according to the AAN and American Association of Neuromuscular and Electrodiagnostic Medicine. SEMGs are more likely to be used in rehabilitation centers and similar healthcare settings to measure muscle activity (i.e., whether a muscle is being used in exercise). SEMGs may also be used in biofeedback.

Depending on the results of the EMG and NCV tests, additional tests (e.g., blood tests, muscle biopsy) may be recommended by a physician.