Neurophysiology

A guide for patients and carers

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EMG in measuring electrical activity of the muscles

As explained earlier, strictly speaking EMG is the study of the electrical activity of the muscles, rather than the electrical function of nerves in the limbs, and it is this type of EMG that we shall look at here.

We have already seen that if a nerve in a limb is stimulated with an electrical pulse, a signal can be recorded from a muscle which that particular nerve supplies. Electrical activity can also be recorded from a muscle when it is contracted, or used.

This activity is usually recorded by a small needle electrode, which is inserted through the skin and into the muscle. The electrodes used for EMG are slender with finely pointed tips, which means that they can pass through the skin and muscle easily and produce only a small and short-lived pinprick sensation.

When an electrode is first inserted into a muscle there may be a short-lived burst of electrical signals, but once the muscle settles down, it should produce no electrical activity at all if it is fully relaxed. However, if the muscle is diseased or its nerve supply is impaired, electrical activity may be seen when the muscle is at rest. The first stage of the EMG examination therefore is to sit quietly with the muscle at rest while the medical staff look for any sign of electrical activity.

The next step is to contract the muscle. When a muscle is contracted, there should be a steady stream of electrical activity from it. If the nerve supply is abnormal, this pattern of activity is reduced. There may also be other changes in the electrical signals that can give useful information about what has been happening to the nerves. In muscle diseases, different but quite characteristic changes in the electrical signals are seen when the muscle is being contracted. This test can therefore help determine whether a person has a disorder affecting the nerve supply to the muscles or the actual muscle tissue itself. The electrical activity of the muscles is displayed on a screen on the EMG machine as a moving train of spiky signals. These also produce sound, which is heard simultaneously through a loudspeaker.

EMG can be useful in testing for a wide range of conditions including muscular dystrophy and motor neurone disease.

Can EMG and nerve conduction studies be done on children?
These investigations can all be carried out on babies and children if a calm and gentle approach is adopted. Children often don’t mind nerve conduction testing, but they usually dislike needles, so this part of the test is usually shorter and better carried out at the end of the session. Although they may be a bit tearful at first, children are often fascinated by the fact that when they move a muscle there is a roar of sound from the loudspeaker and a flurry of signals on the screen.

Babies who are felt to be unusually ‘floppy’ may be referred for EMG testing to see if there is any sign of one of the rare neuromuscular disorders that can affect young children. They are often surprisingly unperturbed by the procedure. Although they cannot move their muscles to order they can usually be induced to do so. A gentle tickle on the sole of the foot, for example, may provoke movement of the leg muscles and give a very good quality EMG recording.

How long does it take to do EMG and nerve conduction studies?
This depends on the person being tested and on the aim of the tests, but an examination on a relaxed adult for a condition such as carpal tunnel syndrome can usually be completed within about half an hour. Different neurophysiologists have their own style of doing things and some may take longer and others a shorter time.

For more complicated conditions - such as injuries to the brachial plexus or evaluation for possible motor neurone disease - testing may involve both nerve conduction studies and EMG on a number of muscles. This can take as long as one-and-a-half hours or more - particularly if the person is elderly and unwell or in discomfort from the underlying condition.

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