Definition

A test that measures muscle response to nervous stimulation (electrical activity within muscle fibers).

Alternative Names

EMG; Myogram

How the test is performed

A needle electrode is inserted through the skin into the muscle . The electrical activity detected by this electrode is displayed on an oscilloscope (and may be displayed audibly through a speaker). Because skeletal muscles are isolated and often large units, each electrode gives only an average picture of the activity of the selected muscle. Several electrodes may need to be placed at various locations to obtain an accurate study. After placement of the electrode(s), you may be asked to contract the muscle (for example, by bending the arm). The presence, size, and shape of the wave form produced on the oscilloscope (the action potential) provide information about the ability of the muscle to respond to nervous stimulation. Each muscle fiber that contracts will produce an action potential, and the size of the muscle fiber affects the rate (how frequently an action potential occurs) and size (amplitude) of the action potential(s). A nerve conduction velocity test is often done at the same time as an EMG.

How to prepare for the test

Adults: No special preparation is usually necessary. Infants and children: The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:

Infant test or procedure preparation

(birth to 1 year)

Toddler test or procedure preparation

(1 to 3 years)

Preschooler test or procedure preparation

(3 to 6 years)

Schoolage test or procedure preparation

(6 to 12 years)

Adolescent test or procedure preparation

(12 to 18 years)

How the test will feel

There may be some discomfort with insertion of the electrodes (similar to an intramuscular injection). Afterward, the examined muscle may feel tender or bruised for a few days.

Why the test is performed

EMG is most often used when people have symptoms of weakness , and examination shows impaired muscle strength. It can help to differentiate primary muscle conditions from muscle weakness caused by neurologic disorders. EMG can be used to differentiate between true weakness and reduced use because of pain or lack of motivation.

Normal Values

Muscle tissue is normally electrically silent at rest. Once the insertion activity (caused by the trauma of needle insertion) quiets down, there should be no action potential on the oscilloscope. When the Muscle.aspx' title='Muscle'>Muscle is voluntarily contracted, action potentials begin to appear. As contraction is increased, more and more Muscle.aspx' title='Muscle'>Muscle fibers produce action potentials until a disorderly group of action potentials of varying rates and amplitudes (complete recruitment and interference pattern) appears with full contraction.

What abnormal results mean

Disorders or conditions that cause abnormal results include:

Polymyositis

Denervation (reduced nervous stimulation)

Carpal tunnel syndrome

ALS

Myopathy (

muscle degeneration, 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

Radial nerve dysfunction

Sciatic nerve dysfunction

Sensorimotor polyneuropathy

Shy-Drager syndrome

Thyrotoxic periodic paralysis

Tibial nerve dysfunction

Ulnar nerve dysfunction

What the risks are

Bleeding

(minimal)

Infection at the electrode sites (minimal risk)

Special considerations

Trauma to the muscle from EMG may cause false results on blood tests (such as creatine kinase ), a muscle biopsy , or other tests.