Electromyography or Needle EMG is a test that measures muscle response to nervous stimulation. A needle electrode is inserted through the skin into the muscle. Each muscle fiber that contracts will produce an action potential. The presence, size, and shape of the wave form of the action potential produced on the oscilloscope, provides information about the ability of the muscle to respond to nervous stimulation. A Needle EMG is most often used when people have symptoms of weakness and examination shows impaired muscle strength. A Needle EMG can help to differentiate primary muscle conditions from muscle weakness caused by neurological disorders.