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Electromyography (EMG) is a medical technique for measuring muscle response to nervous stimulation. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle cells when these cells contract.

Uses

EMG can help to distinguish primary muscle conditions from muscle weakness caused by neurologic disorders. It can be used to differentiate between true weakness and reduced use due to pain or lack of motivation. It is used to find causes of weakness, paralysis, involuntary twitching, and abnormal levels of muscle enzymes. It can help diagnose neuromuscular disorders such as Motor Neurone Disease, neuropathy, nerve damage and muscle damage. See #Abnormal results.

Electromyography is also used in biofeedback studies and training. Electromyography training is a kind of biofeedback in which patients learn to control muscle tension in the face, neck, and shoulders. For example, such training is sometimes given to migraine patients.

Procedure

To perform EMG, needle electrodes are inserted through the skin into the muscle tissue. Each electrode gives only an average picture of the activity of the selected muscle, because skeletal muscles are isolated and often large units. Several electrodes may need to be placed at various locations to obtain an accurate study. Then the patient is asked to contract the muscle. Each muscle fiber that contracts produces an action potential (a change in electrical potential), which is displayed on the electromyogram. The form of the action potential provides information about the ability of the muscle to respond when the nerves are stimulated. The size of the muscle fiber affects the rate (frequency) and size (amplitude) of the action potentials.

A nerve conduction velocity test is often done at the same time as an EMG.

Because of the needle electrodes, EMG may be somewhat painful to the patient, and the muscle may feel tender for a few days. (Though, there's a distinction: there's "needle EMG", and EMG performed using surface electodes, though giving much less accurate results, with a higher level of disturbance from the surrounding environment.)

Normal results

Muscle tissue at rest is normally electrically inactive. After the activity caused by the trauma of needle insertion subsides, the electromyograph should detect no action potential. When the muscle is voluntarily contracted, action potentials begin to appear. As contraction is increased, more and more muscle fibers produce action potentials. When the muscle is fully contracted, there should appear a disorderly group of action potentials of varying rates and amplitudes (a complete recruitment and interference pattern).

Abnormal results

Abnormal results may be caused by the following medical conditions:

• Polymyositis
• Denervation (reduced nervous stimulation)
• Carpal tunnel syndrome
• Motor neurone disease
• Myopathy (muscle degeneration, which may be caused by a number of disorders, including muscular dystrophy)
• Myasthenia gravis
• Alcoholic neuropathy
• Axillary nerve dysfunction
• Becker's muscular dystrophy
• Brachial plexopathy
• Cervical spondylosis
• Common peroneal nerve dysfunction
• Dermatomyositis
• Distal median nerve dysfunction
• Duchenne's muscular dystrophy
• Facioscapulohumeral muscular dystrophy (Landouzy-Dejerine)
• Familial periodic paralysis
• Femoral nerve dysfunction
• Friedreich's ataxia
• Guillain-Barre
• Lambert-Eaton Syndrome
• Mononeuritis multiplex
• Mononeuropathy
• Peripheral neuropathy
• Poliomyelitis
• Radial nerve dysfunction
• Sciatic nerve dysfunction
• Sensorimotor polyneuropathy
• Shy-Drager syndrome
• Thyrotoxic periodic paralysis
• Tibial nerve dysfunction
• Ulnar nerve dysfunction

References

• MedlinePlus entry on EMG describes EMG
• University of Oklahoma Health Sciences Center describes the electromyograph

• Medical tests
• Neurology