Electromyography (EMG) is a technique for evaluating and recording physiologic properties of muscles at rest and while contracting. 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.
The electrical source is the muscle membrane potential of about -70mV. Due to the applied method, the resulting measured potentials range between less than 50 μV and 20 to 30 mV.
Typical repetition rate of muscle unit firing is about 7–20 Hz, depending on the size of the muscle (eye muscles versus seat (gluteal) muscles), previous axonal damage and other factors. Damage to motor units can be expected at ranges between 450 and 780 mV.
To perform intramuscular EMG, a needle electrode is inserted through the skin into the muscle tissue. A trained medical professional (most often a physiatrist, neurologist, or physical therapist) observes the electrical activity while inserting the electrode. The insertional activity provides valuable information about the state of the muscle and its innervating nerve. Normal muscles at rest make certain, normal electrical sounds when the needle is inserted into them. Then the electrical activity when the muscle is at rest is studied. Abnormal spontaneous activity might indicate some nerve and/or muscle 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 electrode 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.
Intramuscular EMG may be considered too invasive or too specific in some cases. A surface electrode may be used to monitor the general picture of muscle activation, as opposed to the activity of only a few fibres as observed using a needle. This technique is used in a number of settings; for example, in the physiotherapy clinic, muscle activation is monitored using surface EMG and patients have an auditory or visual stimulus to help them know when they are activating the muscle (biofeedback).
A motor unit is defined as one motor neuron and all of the muscle fibers it innervates. When a motor unit fires, the impulse (called an action potential) is carried down the motor neuron to the muscle. The area where the nerve contacts the muscle is called the neuromuscular junction, or the motor end plate. After the action potential is transmitted across the neuromuscular junction, an action potential is elicited in all of the innervated muscle fibres of that particular motor unit. The sum of all this electrical activity is known as a motor unit action potential (MUAP). This electrophysiologic activity from multiple motor units is typically evaluated during an EMG. 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.
Nerve conduction testing is also often done at the same time as an EMG in order to diagnose neurological diseases.
Muscle tissue at rest is normally electrically inactive. After the electrical activity caused by the irritation of needle insertion subsides, the electromyograph should detect no abnormal spontaneous activity (i.e. a muscle at rest should be electrically silent, with the exception of the area of the neuromuscular junction, which is normally electrically very spontaneously active). When the muscle is voluntarily contracted, action potentials begin to appear. As the strength of the muscle 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).
Neuropathic disease has the following defining EMG characteristics:
- An action potential amplitude that is twice normal due to the increased number of fibres per motor unit because of reinnervation of denervated fibres.
- 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)
Because of the individuality of each patient and disease, some of these characteristics may not appear in every case.
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):
- 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)
- Distal median nerve dysfunction
- Duchenne muscular dystrophy
- Facioscapulohumeral muscular dystrophy (Landouzy-Dejerine)
- Familial periodic paralysis
- Femoral nerve dysfunction
- Fields condition 
- Friedreich's ataxia
- Lambert-Eaton Syndrome
- Mononeuritis multiplex
- Motor neurone disease
- Myasthenia gravis
- Myopathy (muscle degeneration, which may be caused by a number of disorders, including muscular dystrophy)
- Myotubular myopathy
- Peripheral neuropathy
- Radial nerve dysfunction
- Sciatic nerve dysfunction
- Sensorimotor polyneuropathy
- Shy-Drager syndrome
- Sleep bruxism
- Spinal Stenosis
- Thyrotoxic periodic paralysis
- Tibial nerve dysfunction
- Ulnar nerve dysfunction
EMG Signal Decomposition
EMG signals are essentially made up of superimposed motor unit action potentials (MUAPs) from several motor units. For a thorough analysis, the measured EMG signals can be decomposed into their constituent MUAPs. MUAPs from different motor units tend to have different characteristic shapes, while MUAPs recorded by the same electrode from the same motor unit are typically similar. Notably MUAP size and shape depend on where the electrode is located with respect to the fibers and so can appear to be different if the electrode moves position. EMG decomposition is non-trivial, although many methods have been proposed.
- American Association of Neuromuscular and Electrodiagnostic Medicine
- American Board of Electrodiagnostic Medicine
- MedlinePlus entry on EMG describes EMG
- EMG and Nerve Conduction education, training, and expert analysis of NCV reports
- University of Oklahoma Health Sciences Center describes the electromyograph
- A tutorial-style dissertation by Volker Koch that introduces message-passing on factor graphs to decompose EMG signals