Electromyography (EMG) is a test of a muscle’s electrical activity. It is used to test how a muscle responds to signals from the nerves responsible for muscle movement, called motor nerves. An EMG may also include a test of how fast the motor nerve conducts impulses. This is called a nerve conduction study (NCS) or nerve conduction velocity (NCV) test.
An EMG is ordered when patients have symptoms such as muscle weakness, stiffness, or atrophy. This test is often used to diagnose muscle and nerve disorders such as amyotrophic lateral sclerosis (ALS), myasthenia gravis, muscular dystrophy, myotonia, neuropathies, and myopathies. NCV is used to help make a diagnosis when the primary symptoms involve sensations such as pain, tingling, and numbness, rather than weakness.
A Electromyography (EMG) measures muscle response or electrical activity in response to a nerve's stimulation of the muscle. The test is used to help detect neuromuscular abnormalities. 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. An 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 the electrodes have been inserted, you may be asked to contract the muscle, for example, by lifting or bending your 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.
A Nerve Conduction Velocity (NCV) test is a measurement of the speed of conduction of an electrical impulse through a nerve. An 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. A related procedure that may be performed is Electromyography (EMG). An EMG measures the electrical activity in muscles, and is often performed at the same time as an NCV. Both procedures help to detect the presence, location and extent of diseases that damage the nerves and muscles. Please see this procedure for additional information.
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