An electromyogram (EMG) is a test that measures the electrical discharges made by the muscles. It can determine whether muscle weakness or paralysis is due to a nerve problem or to a problem with the muscle itself.
During an electromyogram, a thin needle is placed in a muscle to measure its electrical activity at rest and during use. The needle is attached to a machine with a viewing screen (like a TV) that displays electrical activity in the muscles as waves on the screen.
WebMD Medical Reference from Healthwise