Electromyography

Alternative names:

EMG; myogram

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 is voluntarily contracted, action potentials begin to appear. As contraction is increased, more and more 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 and/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