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Electromyography (EMG) is a medical test in which electrodes are inserted into a muscle to test for neuromuscular abnormalities. This is typically done to determine why a patient is experiencing muscle weakness or twitches, or a loss of feeling due to nerve compression. The results of EMG can be very difficult to interpret on your own. The results should be discussed with a neurologist, but understanding the basics can help you understand the doctor when he is explaining the EMG results.
Any muscle can be tested with an EMG. Each muscle has a normal range of electrical activity, also referred to as action potential, when it is in motion. This range usually depends on the size of the muscle and what it does. The EMG results are generally based around the normal range of the specific muscle being tested so, without knowing what is normal for that area of the body, deciphering the results on your own can be almost impossible.
For example, when you move, muscles fibers are activated to make that unique motion. Slight movements — like wiggling the fingers — activate fewer muscles fibers than stronger motions — such as clenching the hand into a fist. The more muscles fibers used, the more electrical activity the electromyography should record. The analysis of the entire test is then based on such motions and results.
A muscle usually has a slight increase in electrical activity as the electrodes are being put into place. After that stimulation subsides, the electromyograph should not record any electrical activity coming from the muscle if it is not moving. If the EMG results show any recordable measurements while the muscles are not moving, it could be a sign of a problem.
Not only does each muscle have a normal range of electrical activity during movement, there is also a normal range for how long each one took to stop showing electrical activity once it ceased moving. Damaged nerves, neuromuscular junction disease, or degenerative muscle disease can affect the EMG results in different ways. For this reason, a doctor must carefully examine the outcome of the test.
Nerve damage or neuromuscular junction disease can be indicated if the EMG results show that the test recorded electrical activity when the muscle was at rest. Also, nerve damage can cause a muscle in motion to use double the normal range of electrical activity, and take longer for it to subside when the muscle is relaxed. Degenerative muscle disease can show the opposite, as in muscles that never reach the normal range of electrical activity per movement or display no electrical activity at all.
Nerve damage can occur due to carpal tunnel disease, a ruptured disc in the back, and pinching of the sciatic nerve. Degenerative muscles disease can include a condition called polumysitis, which causes inflammation in and weakening of the muscles. Some genetic disorders, such as muscular dystrophy, can also cause muscle degeneration.