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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.

Electrical Characteristics

Electrical source is the muscle membran potential, about -70mV. Due to the applied method the resulting measured potentials might range between 50uV and some mV.

Typical repetition rate of muscle unit firing is about 7-20Hz, depending of the size of the muscle (eye muscles versus seat (gluteal) muscles), previous axonal damage and other factors.

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. The neurologist observes the electrical activity while inserting the electrode. The insertion activity provides already valuable information about the state of the muscle and its innervating nerve. Then the electrical activity when the muscle is at rest is studied. Spontaneous activity might indicate some nerve damage. Then the patient is asked to contract the muscle smoothly. The shape, size and frequency of the resulting motor unit potentials is judged. Then the electode is retracted a few millimeters, and again the activity is analyzed until at least 10-20 units have been collected. Each electrode track gives only a very local picture of the activity of the whole muscle. Because skeletal muscles differ in the inner structure, the electrode has to be placed at various locations to obtain an accurate study.

Each action potential running down a single nerve fibre innervates all muscle fibres belonging to that motor unit. After transmission at the neuromuscluar synapse, this will elicit action potentials along all of the innervated muscle fibres. The sum of all this electrical activity is recorded as a motor unit potential. The composition of the motor unit, the number of muscle fibres per motor unit, the metabolic type of muscle fibres and many other factors affect the shape of the motor unit potentials in the myogram.

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. However, so called "needleless EMG"—an EMG performed using surface electrodes—. will not give the information required for neurological diagnosis.

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:

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