An electromyogram (EMG) is a procedure that tests the electrical activity of a muscle in response to nerve stimulation. The results of this test help physicians to determine the health of a patient’s muscles and nerves. An EMG is most often used to diagnose the source of symptoms such as pain or weakness in the muscles or numbness in parts of the body.

An EMG may be performed in a hospital, clinic or physician’s office. Preparation for the test is usually minimal, and patients are urged to follow the recommendations of their physician. In general, patients may be advised not to consume caffeine or use tobacco products for up to three hours prior to the EMG. In addition, certain information should be provided to a physician prior to the EMG.

In most cases, an EMG involves inserting a needle electrode into the muscle to be tested. Patients will be asked to contract a muscle (i.e., bend an arm or leg, or make a fist). When this occurs, nerves in the area send signals to the muscles to move. Each muscle fiber that contracts produces electrical activity of its own. This activity is detected and recorded by the needle electrode. The electrical activity of a muscle at rest will also be recorded by the needle electrode (although no activity should occur in a normal muscle at rest).

Needle electrodes may be repositioned during an EMG to different areas of a certain muscle, or into different nearby muscles.

Muscle activity during the test will appear as wavy lines on a special monitor. The presence, size and shape of the waves reveals information about the muscle’s response to nerve stimulation. Sometimes this activity is measured audibly through a speaker. In these cases, rapid-fire popping sounds may be heard when a muscle contracts.  

In addition to an EMG, a nerve conduction velocity (NCV) test may also be performed. A NCV records the speed of impulses through a nerve. This test identifies how well certain nerves pass electrical signals. A NCV may help identify whether a patient’s symptoms may be due to problems with a particular nerve.

EMG results are typically available immediately, but they usually need to be interpreted and analyzed by a specialist (e.g., a neurologist) before a proper diagnosis can be made. As a result, it may take a day or two before a patient receives the test results.