What is surface electromyography?

The word electromyography looks dauntingly long but it is made up of three parts: “electro” + “-myo” + “graphy.” Myo is Greek for muscle and graphy comes from the Greek “grapho” meaning to write. So electromyography literally is the writing (recording) of muscle electricity.

As muscles contract, very small level electrical signals (microvolt) are created within the muscle that may be measured on the surface of the body with specialized equipment. A procedure that measures muscle activity from the skin is referred to as surface electromyography.

This is different than an Intramuscular EMG (the most commonly used type). An EMG involves inserting a needle electrode through the skin into the muscle whose electrical activity is to be measured.

Many different forms of clinicians use SEMG to evaluate the functional status of skeletal muscles and assist in neuromuscular training and rehabilitation. Many SEMG machines can be used clinically as a type of biofeedback mechanism.

The small electrical current, or signal, which comes from active muscles, is detected by sensors placed on the skin directly above the muscles. The strength and pattern of the signal is displayed onto a computer screen and the data is collected in a software program that is able to run various analyses of the data to create useful reports regarding muscle function.

Because SEMG signals are small, their measurement is susceptible to interference – for example, from electrical equipment, lights or movement of cables that carry signals from the body to the measuring instrument.

In surface electromyography, the electrical activity of individual muscles or muscle groups is detected, amplified, and analyzed by a computer. The most basic information obtainable from an SEMG signal is whether the tested muscle was used during a period of exertion. It is not used to determine the health, or strength of signal from the nerve that controls that muscle. It is possible to measure electrical nerve activity, but not with this test.

There are types of SEMGs that are designed to measure the activity of the muscles on either side of the spine. Initially it would appear that such a device would be of great benefit to chiropractors, physiotherapists, orthopedists, and other professionals that work on the spine.

Unfortunately these devices can not provide an objective measurement of overall spinal health by detecting electrical activity in the muscles along the spine. A SEMG study is also not an accepted way to screen patients or follow their progress with treatment. There is simply too much variability in the information produced with SMEG for it to have clinical use beyond biofeedback. A SEMG device can not measure the health or activity of the spinal nerves.

In the year 2000, after reviewing more than 2,500 original articles, reviews, and books, an American Academy of Neurology subcommittee concluded that SEMG was “unacceptable as a clinical tool” for diagnosing low back pain or neuromuscular disease. In 2001, the Massachusetts Board of Registration of Chiropractors issued a policy guideline that criticized routine use of paraspinal electromyography. It is the position of the American Association of Electrodiagnostic Medicine that there is no clinical indication for the use of SEMG in the diagnosis and treatment of disorders of nerve or muscle.

For these reasons, use of SEMG as a means of clinical measurement is not possible. This is not to suggest that electromyogram and nerve conduction studies are not of great clinical value. A nerve conduction study (which is performed with needle probes) measures how well the nerves can send electrical signals. These electrical signals are how the body controls how the muscles react. Measuring the electrical activity in muscles and nerves can help find diseases that damage muscle tissue or nerves.

A nerve conduction velocity test is often done at the same time as an EMG. In this test, the nerve is electrically stimulated while a second electrode detects the electrical impulse ‘down stream’ from the first. The distance between electrodes and the time it takes for electrical impulses to travel between electrodes are used to calculate the speed of impulse transmission (nerve conduction velocity). A decreased speed of transmission indicates nerve disease.