An electromyogram (EMG) is an extension of the history and physical examination. It uses sensory and motor nerve conduction studies (NCS) and qualitative or quantitative needle EMG to diagnose a variety of nervous system disorders along the motor unit, including damage to anterior horn cells, peripheral nerves, the neuromuscular junction, and muscle fibers. Sensory and motor NCS provide information on large myelinated nerves by stimulating the nerve and recording its associated action potential with electrodes placed on a patient’s skin. NCS are usually normal with exception of a possible decrement in motor NCS amplitude. Neuromuscular junction disorders like myasthenia gravis can be evaluated using a special NCS technique known as repetitive stimulation.