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Information about Electromyography

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.

Electrical characteristics

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.

Procedure

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.

Normal results

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).

Abnormal results

EMG is used to diagnose two general categories of disease: neuropathies and myopathies.

Neuropathic disease has the following defining EMG characteristics: Myopathic disease has these defining EMG characteristics: 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):

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.

References

The introduction to this article is vague. To comply with Wikipedia's guidelines, it should be improved.
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Electric potential is the potential energy per unit of charge associated with a static (time-invariant) electric field, also called the electrostatic potential, typically measured in volts. It is a scalar quantity.
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An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte or a vacuum). The word was coined by the scientist Michael Faraday from the Greek words elektron
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Skin layers: epidermis, dermis, and subcutis, showing a hair follicle, sweat gland & sebaceous gland.]] In zootomy and dermatology, skin is the largest organ of the integumentary system made up of multiple layers of epithelial tissues that guard underlying muscles and organs.
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Physical medicine and rehabilitation (PM&R), or physiatry, is a branch of medicine dealing with functional restoration of a person affected by physical disability. A physician who has completed training in this field is referred to as a physiatrist (fizz eye' a trist).
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Neurology is a branch of medicine dealing with disorders of the nervous system. Medical professionals (such as Biomedical Doctors and Physicians) specializing in the field of neurology are called neurologists
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Skeletal muscle is a type of striated muscle, usually attached to the skeleton. Skeletal muscles are used to create movement, by applying force to bones and joints; via contraction.
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An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte or a vacuum). The word was coined by the scientist Michael Faraday from the Greek words elektron
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MUSCLE (multiple sequence comparison by log-expectation) is public domain, multiple sequence alignment software for protein and nucleotide sequences.
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Biofeedback is a form of alternative medicine that involves measuring a subject's bodily processes such as blood pressure, heart rate, skin temperature, galvanic skin response (sweating), and muscle tension and conveying such information to him or her in real-time in order to
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A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates. When a motor unit is activated, all of its fibers contract. Groups of motor units often work together to coordinate the contractions of a single muscle; all of the motor units
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An action potential is a "spike" of electrical discharge that travels along the membrane of a cell. Action potentials are an essential feature of animal life, rapidly carrying information within and between tissues. They also occur in some plants.
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A nerve conduction study (NCS) is a test commonly used to evaluate the function, especially the ability of electrical conduction, of the motor and sensory nerves of the human body. Nerve conduction velocity (NCV) is a common measurement made during this test.
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neuromuscular junction (NMJ) is the synapse or junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately
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Neuropathy
Classification & external resources

ICD-10 G 56. - G 63. ,
G 90.0 , G 99.0
ICD-9 337.0 - 337.1 ,
356 - 357 , 377

eMedicine topic list Neuropathy is usually short for peripheral neuropathy
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MeSH D009135 In medicine, a myopathy is a neuromuscular disease in which the muscle fibers do not function for any one of many reasons, resulting in muscular weakness. "Myopathy" simply means muscle disease (myo- Greek μυσ "muscle" + -pathy Greek "suffering").
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An action potential is a "spike" of electrical discharge that travels along the membrane of a cell. Action potentials are an essential feature of animal life, rapidly carrying information within and between tissues. They also occur in some plants.
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amplitude is a nonnegative scalar measure of a wave's magnitude of oscillation, that is, the magnitude of the maximum disturbance in the medium during one wave cycle.

Sometimes this distance is called the peak amplitude
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For other meanings of fiber/fibre please see Fiber (disambiguation).


Fiber or fibre[1] is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.
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When a motor nerve dies, a nearby nerve can grow a new axon to the affected muscle in order to take over the functionality of the dead nerve.

See Also

  • Reinnervation process and deRe

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For other meanings of duration, see Duration (disambiguation).
A duration is an amount of time or a particular time interval. For example, an event in the common sense has a duration greater than zero (but not very long), but in certain specialized senses
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An action potential is a "spike" of electrical discharge that travels along the membrane of a cell. Action potentials are an essential feature of animal life, rapidly carrying information within and between tissues. They also occur in some plants.
..... Click the link for more information.
A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates. When a motor unit is activated, all of its fibers contract. Groups of motor units often work together to coordinate the contractions of a single muscle; all of the motor units
..... Click the link for more information.
Motor Unit Number Estimation (MUNE) is a technique that uses electromyography to estimate the number of motor units in a muscle.

Principles

A motor unit consists of one alpha motoneuron and all the muscle fibres it innervates.
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An action potential is a "spike" of electrical discharge that travels along the membrane of a cell. Action potentials are an essential feature of animal life, rapidly carrying information within and between tissues. They also occur in some plants.
..... Click the link for more information.
Area is a physical quantity expressing the size of a part of a surface. The term Surface area is the summation of the areas of the exposed sides of an object.

Units

Units for measuring surface area include:
square metre = SI derived unit

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amplitude is a nonnegative scalar measure of a wave's magnitude of oscillation, that is, the magnitude of the maximum disturbance in the medium during one wave cycle.

Sometimes this distance is called the peak amplitude
..... Click the link for more information.
An action potential is a "spike" of electrical discharge that travels along the membrane of a cell. Action potentials are an essential feature of animal life, rapidly carrying information within and between tissues. They also occur in some plants.
..... Click the link for more information.
Neuropathy
Classification & external resources

ICD-10 G 56. - G 63. ,
G 90.0 , G 99.0
ICD-9 337.0 - 337.1 ,
356 - 357 , 377

eMedicine topic list Neuropathy is usually short for peripheral neuropathy
..... Click the link for more information.


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