Electromyography (EMG) is a medical technique for evaluating and recording physiologic properties of muscles at rest and while contracting. For nerve conduction studies (NCS) studies, a noninvasive stimulator applies brief electrical impulses to a peripheral nerve transcutaneous, the nerve then transmits the impulse and a response is recorded by electrodes at some distance away.
The time it takes for the stimulus to reach the recording electrodes is called as latency. It can be accurately measured and a velocity of transmission calculated. Healthy nerves will transmit the electrical impulse faster than diseased ones.