Electromyography (EMG) is a medical test in which electrodes are inserted into a muscle to detect neuromuscular abnormalities. This is usually done to determine why a patient is experiencing muscle weakness or twitching, or a loss of sensation due to nerve compression. EMG results 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. Every muscle has a normal range of electrical activity, also known as an action potential , when in motion. This range generally depends on the size of the muscle and what it does. EMG results are usually based on 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 nearly impossible.
For example, when you move, the muscle fibers are activated to make that unique movement. Mild movements, such as wiggling your fingers, activate fewer muscle fibers than stronger movements, such as clenching your hand into a fist. The more muscle fibers used, the more electrical activity the EMG must record. The analysis of the entire test is then based on such movements and results.
A muscle usually has a slight increase in electrical activity as the electrodes are placed. After the stimulation subsides, the electromyograph should not record any electrical activity coming from the muscle if it is not moving. If the EMG results show 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 it took for each muscle to stop showing electrical activity once it stopped moving. Damaged nerves, neuromuscular junction disease, or degenerative muscle disease can affect EMG results in different ways. For this reason, a doctor must carefully examine the test result.
Nerve damage or disease of the neuromuscular junction may 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 moving muscle to use twice the normal range of electrical activity, and take longer to decrease 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 show 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. Wasting disease of the muscles can include a condition called polymysitis, which causes inflammation and weakening of the muscles. Some genetic disorders, such as muscular dystrophy, can also cause muscle wasting.