The EMG, which looks like a computer, measures muscle response to nerve stimulation as the patient contracts and relaxes the muscle, and the NCV, which is done with electrodes attached to the skin, measures the speed of electrical impulses zipping along nerves.
The EMG and NCV help us see when spinal nerves are being compressed by disc herniations. They also help in diagnosing carpal tunnel syndrome, diabetes, nerve damage, muscular dystrophy, and Lou Gehrig’s disease, among others. With the NCV, for example, a slower electrical impulse than normal can signify nerve damage because of direct trauma, diabetic or peripheral neuropathy, or a viral infection. Such tests aren’t wholly free of discomfort – with the EMG, needle electrodes are inserted in the body and the muscle may feel sore afterward; with the NCV, the electrical impulses may startle you – but they’re not intensely painful.