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. Medicine is the branch of health science and the sector of public life concerned with maintaining human health or restoring it through the treatment of disease and injury. ... Electrical potential is the potential energy per unit charge associated with a static (time-invariant) electric field, also called the electrostatic potential or the electric potential, typically measured in volts. ... Drawing of the structure of cork as it appeared under the microscope to Robert Hooke from Micrographia which is the origin of the word cell being used to describe the smallest unit of a living organism Cells in culture, stained for keratin (red) and DNA (green) The cell is the...

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. For other uses, see Electrode (disambiguation). ... Beyond overall skin structure, refer below to: See-also. ... A physiatrist is a doctor with specialty training in Physical Medicine and Rehabilitation. ... Neurology is the branch of medicine that deals with the nervous system and disorders affecting it. ... A top-down view of skeletal muscle Skeletal muscle is a type of striated muscle, usually attached to the skeleton. ...

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). For other uses, see Electrode (disambiguation). ... For other uses of Muscles, see Muscles (disambiguation). ... Biofeedback mechanism. ...

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 fibers 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. A motor unit is a group of cells under the control of a single motor neuron; groups of motor units work together, as a single muscle. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ...

Nerve conduction testing is also often done at the same time as an EMG in order to diagnose neurological diseases. 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. ...

Patients often find the procedure somewhat or extremely painful.

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). A neuromuscular junction is the 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 muscles surface. ...

Abnormal results

EMG is used to diagnose two general categories of disease: neuropathies and myopathies. Neuropathy strictly speaking is any disease that affects the neurons of the nervous system. ... In medicine, a myopathy is a neuromuscular disease in which the muscle fibers dysfunction for any one of many reasons, resulting in muscular weakness. ...

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. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... It has been suggested that pulse amplitude be merged into this article or section. ... For the meaning of fiber in nutrition, see dietary fiber. ... 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. ... A duration is an amount of time or a particular time interval. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... A motor unit is a group of cells under the control of a single motor neuron; groups of motor units work together, as a single muscle. ... Motor Unit Number Estimation (MUNE) is a technique that uses electromyography to estimate the number of motor units in a muscle. ...

• 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. A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... This article is about the physical quantity. ... It has been suggested that pulse amplitude be merged into this article or section. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ...

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)
• 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
• Shy-Drager syndrome
• Sleep bruxism
• Spinal stenosis
• Thyrotoxic periodic paralysis
• Tibial nerve dysfunction
• Ulnar nerve dysfunction

Neuropathy strictly speaking is any disease that affects the neurons of the nervous system. ... Axillary nerve dysfunction is a general term that refers to any type of dysfunction of the axillary nerve. ... Beckers muscular dystrophy (also known as Benign pseudohypertrophic muscular dystrophy) is an X-linked recessive inherited disorder characterized by slowly progressive muscle weakness of the legs and pelvis. ... This article is about the medical condition. ... Note: Centronuclear myopathy includes Myotubular myopathy, as outlined below. ... Not to be confused with spondylitis, spondylolysis or spondylolisthesis. ... Charcot-Marie-Tooth disease (CMT), known also as Hereditary Motor and Sensory Neuropathy (HMSN) or Peroneal Muscular Atrophy, is a heterogeneous inherited disorder of nerves (neuropathy) that is characterized by loss of muscle tissue and touch sensation, predominantly in the feet and legs but also in the hands and arms... X-Ray of the knee in a patient with dermatomyositis. ... Duchenne muscular dystrophy (DMD) (also known as muscular dystrophy - Duchenne type) is a form of muscular dystrophy that is characterized by decreasing muscle mass and progressive loss of muscle function in male children. ... Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant form of muscular dystrophy that initially affects muscles of the face (facio), scapula (scapulo) and upper arms (humeral). ... Familial periodic paralysis is a form of myopathy that are inherited. ... Friedreichs ataxia is a rare autosomal recessive disorder caused by a mutation in Gene X25 that codes for frataxin, located on chromosome 9. ... Guillain-Barré syndrome (GBS), is an acquired immune-mediated inflammatory disorder of the peripheral nervous system (i. ... Lambert-Eaton myasthenic syndrome (LEMS) is a rare disorder of nerve-muscle (neuromuscular) junction. ... Mononeuritis multiplex is the clinical picture that arises from problems with multiple individual nerves serially or almost simultaneously. ... Mononeuropathy (or mononeuritis) is a type of neuropathy that only affects a single peripheral or cranial nerve. ... The motor neurone diseases (MND) are a group of progressive neurological disorders that destroy motor neurones, the cells that control voluntary muscle activity such as speaking, walking, breathing, and swallowing. ... Myasthenia gravis (sometimes abbreviated MG; from the Greek myastheneia, lit. ... 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. ... Note: Centronuclear myopathy includes Myotubular myopathy, as outlined below. ... Neuromyotonia is spontaneous muscular activity resulting from repetitive motor unit action potentials of peripheral origin. ... Peripheral neuropathy is the term for damage to nerves of the peripheral nervous system, which may be caused either by diseases of the nerve or from the side-effects of systemic illness. ... This article is about the disease. ... {{ }} Polymyositis is a type of inflammatory myopathy, related to dermatomyositis and inclusion body myositis. ... Sciatica is a pain in the leg due to irritation of the sciatic nerve. ... Shy-Drager syndrome is a rare, progressively degenerative disease of the autonomic nervous system. ... Bruxism (from the Greek βρυγμός (brugmós), gnashing of teeth) is the grinding of the teeth, typically accompanied by the clenching of the jaw. ... This article does not cite any references or sources. ...

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. For decomposition of dead organisms, see Decomposition. ...

References

• 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
• Italian website of Electromyography