Electromyography (EMG) is a technique for evaluating and recording the activation signal of muscles. 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, and also when the cells are at rest.

EMG is used to diagnose two general categories of disease: neuropathies and myopathies. Neuropathic disease has the following defining EMG characteristics:

• An action potential amplitude that is twice normal due to the increased number of fibers per motor unit because of reinnervation of denervated fibers.
• An increase in duration of the action potential
• A decrease in the number of motor units in the muscle (as found using motor unit number estimation techniques)

Myopathic disease has these defining EMG characteristics:

• A decrease in duration of the action potential
• A reduction in the area to amplitude ratio of the action potential
• A decrease in the number of motor units in the muscle (in extremely severe cases only) Abnormal results may be caused by the following medical conditions (please note this is nowhere near an exhaustive list of conditions that can result in abnormal EMG studies)

List of some diseases in which EMG could be done:

• Alcoholic neuropathy
• Axillary nerve dysfunction
• Becker's muscular dystrophy
• Brachial plexopathy
• Carpal tunnel syndrome
• Centronuclear myopathy
• Cervical spondylosis
• Charcot-Marie-Tooth disease
• Common peroneal nerve dysfunction
• Denervation (reduced nervous stimulation)
• Dermatomyositis
• Distal median nerve dysfunction
• Duchenne muscular dystrophy
• Facioscapulohumeral muscular dystrophy (Landouzy-Dejerine)
• Familial periodic paralysis
• Femoral nerve dysfunction
• Fields condition [1]
• 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)
• Myotubular myopathy
• Neuromyotonia
• Peripheral neuropathy
• Poliomyelitis
• Polymyositis
• Radial nerve dysfunction
• Sciatic nerve dysfunction
• Sensorimotor polyneuropathy