An electromyograph (EMG) is an instrument for observing the electric activity of muscle fibers and the motor neurons that excite them. It diagnoses neuromuscular disorders, and it can provide insight into the causes of muscle weakness and nerve damage. The electric signal transmitted from the muscle to the electromyograph is measured in millivolts and translated into both a visual and an auditory record. The visual record will usually appear as a graph of a wave function on a television monitor, while the auditory record will be a static, crackling sound.
The two major categories of electromyography are intramuscular EMG and surface EMG. During an intramuscular — or needle — EMG procedure, a thin, solid needle electrode is inserted into the muscle to measure its activity. Upon insertion, there will be a brief burst of electric activity as the needle moves through the muscle, and then the signal will fade. Resting muscle usually produces no electrical signal after the initial needle insertion, so any irregular resting activities, such as fibrillation potentials, detected by the electromyograph are signs of damage or disease.
When the muscle is contracted, an intramuscular EMG can measure the strength of the contraction, the frequency at which the motor neurons are firing, and the density of motor units in a particular section of muscle. Motor units are made of individual neurons and the fibers of muscle they innervate and control. With extensive nerve damage, a few motor neurons will sometimes grow new branches and take over muscle fibers that originally belonged to the damaged nerves. These neurons fire at a higher frequency to compensate for the loss of the neurons that were damaged. This will result in fewer large-amplitude motor-unit-action potentials detected by the electromyograph.
Diagnostic needle EMG can help trained healthcare professionals gather all of this information to form a potential diagnosis. Common neuromuscular diseases diagnosed by EMG include herniated discs in the spine that compress a nerve root, damage to nerves from trauma or pressure from nearby structures, and muscular dystrophy. Multiple tiny insertions are needed to sample enough muscle to make a diagnosis.
Surface EMG involves electrode patches placed on the skin, rather than a needle electrode. While it does not provide a detailed record of individual motor units the way intramuscular EMG does, it can provide general measurements of muscular activity in wider regions of the body. As a result, it is most commonly used during physical therapy with patients who have nerve or muscle damage. The electromyograph becomes a form of biofeedback and allows them to see when and how strong their muscles are contracting.