Electromyography (EMG) and nerve conduction velocity (NCV) tests measure muscle response or electrical activity in response to a nerve’s stimulation of the muscle. These tests help detect neuromuscular abnormalities.
Nerve Conduction Velocity (NCV)
A Nerve conduction velocity (NCV) test—also called a nerve conduction study (NCS)— is often performed in conjunction with a EMG. NCV is a measurement of the speed of conduction of an electrical impulse through a nerve. NCV can determine nerve damage and destruction. During the test, the nerve is stimulated, usually with surface electrode patches attached to the skin. Two electrodes are placed on the skin over the nerve.
One electrode stimulates the nerve with a very mild electrical impulse and the other electrode records it. The resulting electrical activity is recorded by another electrode. This is repeated for each nerve being tested. The nerve conduction velocity (speed) is then calculated by measuring the distance between electrodes and the time it takes for electrical impulses to travel between electrodes.
During the test, one or more small needles (also called electrodes) are inserted through the skin into the muscle. The electrical activity picked up by the electrodes is then displayed on an oscilloscope (a monitor that displays electrical activity in the form of waves). An audio-amplifier is used so the activity can be heard.
EMG measures the electrical activity of muscle during rest, slight contraction, and forceful contraction. Muscle tissue does not normally produce electrical signals during rest. When an electrode is inserted, a brief period of activity can be seen on the oscilloscope, but after that, no signal should be present.
After all of the electrodes have been inserted, patients are asked to contract the muscle, for example, by lifting or bending their leg. The action potential (size and shape of the wave) that this creates on the oscilloscope provides information about the ability of the muscle to respond when the nerves are stimulated. As the muscle is contracted more forcefully, more and more muscle fibers are activated, producing action potentials.
Reasons for the procedures
EMG is often used along with nerve conduction velocity (NCV) to differentiate a muscle disorder from a nerve disorder. NCV detects a problem with the nerve, whereas EMG can detect diseases stemming from problems with the muscle itself, as well as other problems that result from influences on the muscle from other systems, such as nerves.
EMG may be done to identify the cause of symptoms, such as muscle weakness, deformity, spasticity, atrophy, and stiffness. It may be used to detect whether someone is experiencing true muscle weakness or weakness because of pain or psychological reasons.
EMG may be used to evaluate many problems or disorders including, but not limited to, the following:
- Neuromuscular diseases, such as myasthenia gravis
- Motor problems, such as involuntary muscle twitching
- Nerve compression or injury, such as carpal tunnel syndrome
- Nerve root injury, such as sciatica
- Muscle degeneration, such as muscular dystrophy
- There may be other reasons for your doctor to recommend EMG.