An EMG tests muscle response to electrical activity

Electromyography (EMG) measures muscle response or electrical activity in response to a nerve’s stimulation of your muscle. The test is used to help detect neuromuscular abnormalities.

During the test, one or more small needles (also called electrodes) are inserted through your skin into your 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.

Damaged muscle shows electrical activity even when muscle is at rest

EMG measures the electrical activity of your 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, you may be asked to contract your 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 your muscle to respond when your nerves are stimulated. As you contract your muscle more forcefully, more and more muscle fibers are activated, producing action potentials.

A healthy muscle will show no electrical activity (no signs of action potential) during rest, only when it contracts. However, if your muscle is damaged or has lost input from nerves, it may have electrical activity during rest. When it contracts, its electrical activity may produce abnormal patterns.

An abnormal EMG result may be a sign of a variety of muscle or nerve disorders, including:

  • Polymyositis (an inflammatory muscle disease that causes decreased muscle power)
  • Muscular dystrophy (a chronic genetic disease that progressively affects muscle function)
  • Myasthenia gravis (a genetic or immune disorder that occurs at the point where the nerve connects with the muscle)
  • Myotonic (stiff) muscles

A related procedure that may be done is nerve conduction velocity (NCV). NCV is a measurement of the speed of conduction of an electrical impulse through a nerve. NCV can determine nerve damage and destruction, and is often done at the same time as EMG. Both procedures help to detect the presence, location, and extent of diseases that damage the nerves and muscles.

Reasons for getting an EMG

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 health care provider to recommend EMG.

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