Electromyography, or EMG, is a diagnostic test that measures the electrical activity inside your muscles. Because muscles produce electrical signals when they contract, electromyography can help your doctor see if they’re working correctly.

You might have an EMG if an imaging test such as an MRI doesn’t adequately explain your symptoms or if the doctor thinks more than one health problem could be causing your symptoms. An EMG can also help identify nerve problems such as polyneuropathies, or damage to several nerves, that can be caused by conditions such as diabetes.

The procedure

It takes about 30 minutes to test one arm or leg. Your doctor does a brief history and physical exam to identify the muscles that need to be tested. Next, the doctor places very thin needles, about the size of acupuncture needles, in the muscles. The needles go about one-eighth to one-quarter inch under the skin. The doctor can test three to seven muscles at a time.

The needles are connected to a machine that produces graphs, sounds or numbers to indicate how well your muscles are working. Your doctor will ask you to relax and then gently contract the muscles. The needles pick up electrical activity in the muscles and send the signals to the machine. The results show the doctor which nerves or muscles, if any, are damaged.

Your doctor might send a mild electrical current to the muscle through a needle or small patch (electrode). This feels like a small static electricity shock. Some people describe the feeling as a strong tap. Another needle or electrode picks up the signal and sends it to the machine.

When this test is used

Doctors use electromyography to help diagnose the causes of: