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Electromyography or EMG is a diagnostic test used to evaluate and record the electrical activity of skeletal muscles. The instrument is an electromyograph and the result is an electromyogram. The test can be run either by stimulating the muscles electrically, or by asking the patient to voluntarily move the muscle. There are two general methods, one which requires only that electrodes be placed on the skin, and a more invasive procedure where fine needles are inserted directly into the muscles.
The technique dates to 1849, although the theory that electrical stimulus drives muscles in the body dates back to the 17th century. However, the work of Dubios-Raymond showed at this time that electrical activity in a muscle when it was being used was possible. The first actual recording was made in 1890. At that time, the recording was quite rough and showed little detail. It was not until the 1960s that recording methods and the use of electrodes allowed the technique to be used to detect muscle pathologies that could differentiate between healthy and abnormal muscle activity that would be useful to a diagnostician.
Electromyography is now primarily used to diagnose neurological and neuro-musclular illnesses, although it is also useful in the study of ergonomics to overcome muscle strain and repetitive injury.
In a normal patient, there should be no electrical activity in a muscle at rest after the needle has been inserted, except at the junction between the nerve and muscle where spontaneous electrical activity occurs normally. When the patient is asked to move the muscle, there should be a steady increase in electrical activity until the muscle is fully contracted, at which point the electrical signals will normally be chaotic. The following results are abnormal:
- The increase in electric activity is double the normal rate, generally because muscles are re-firing too early.
- The electrical activity for each individual cell contraction goes on too long.
- The number of electrical impulses is lower than normal.
- The electrical impulses occur for too short a time.
Abnormal results may be the result of a number of diseases, but the type of abnormality can be useful for the differential diagnosis.