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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. A medical device is an object which is useful for diagnostic or therapeutic purposes An electromyograph detects the electrical potential generated by muscle cells when these cells contract, and also when the cells are at rest. At a point in space the electric potential is the Potential energy per unit of charge that is associated with a static (time-invariant Electric field The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called

Contents

Electrical characteristics

The electrical source is the muscle membrane potential of about -70mV. Measured EMG potentials range between less than 50 μV and up to 20 to 30 mV, depending on the muscle under observation.

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.

History

The development of EMG started with Francesco Redi’s documentation in 1666. The document informs that highly specialized muscle of the electric ray fish generates electricity. By 1773, Walsh had been able to demonstrate that Eel fish’s muscle tissue could generate a spark of electricity. In 1792, a publication entitled "De Viribus Electricitatis in Motu Musculari Commentarius" appeared, written by Luigi Galvani, where the author showed that electricity could initiate muscle contractions. Luigi Galvani was an Italian Physician and Physicist who lived and died in Bologna. Six decades later, in 1849, Dubios-Raymond discovered that it was also possible to record electrical activity during a voluntary muscle contraction. The first recording of this activity was made by Marey in 1890, who also introduced the term electromyography. Étienne-Jules Marey ( March 5, 1830 – May 21, 1904) was a French scientist and chronophotographer, born in In 1922, Gasser and Erlanger used an oscilloscope to show the electrical signals from muscles. Joseph Erlanger ( January 5, 1874 &ndash December 5, 1965) was an American physiologist. An oscilloscope (commonly abbreviated to scope or O-scope) is a type of Electronic test equipment that allows signal Voltages to be viewed Because of the stochastic nature of the myoelectric signal, only rough information could be obtained from its observation. The capability of detecting electromyographic signals improved steadily from the 1930s through the 1950s and researchers began to use improved electrodes more widely for the study of muscles. Clinical use of surface EMG (sEMG) for the treatment of more specific disorders began in the 1960s. Hardyck and his researchers were the first (1966) practitioners to use sEMG. In the early 1980s, Cram and Steger introduced a clinical method for scanning a variety of muscles using an EMG sensing device.

It is not until the middle of the 1980s that integration techniques in electrodes had sufficiently advanced to allow batch production of the required small and lightweight instrumentation and amplifiers. At present a number of suitable amplifiers are commercially available. In the early 1980s, cables became available which produce artifacts in the desired microvolt range. During the past 15 years, research has resulted in a better understanding of the properties of surface EMG recording. In recent years, surface electromyography is increasingly used for recording from superficial muscles in clinical protocols, where intramuscular electrodes are used for deep muscle only.

There are many applications for the use of EMG. EMG is used clinically for the diagnosis of neurological and neuromuscular problems. It is used diagnostically by gait laboratories and by clinicians trained in the use of biofeedback or ergonomic assessment. EMG is also used in many types of research laboratories, including those involved in biomechanics, motor control, neuromuscular physiology, movement disorders, postural control, and physical therapy.

Procedure

To perform intramuscular EMG, a needle electrode is inserted through the skin into the muscle tissue. An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e The skin is the outer covering of living tissue of an animal (or plant A trained medical professional (most often a physiatrist, neurologist, chiropractor or physical therapist) observes the electrical activity while inserting the electrode. Physical medicine and rehabilitation (PM&R or physiatry, is a branch of Medicine dealing with functional restoration of a person affected by physical disability 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. Skeletal muscle is a type of Striated muscle, which usually attaches to tendons

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. An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e Muscle (from Latin musculus, diminutive of mus "mouse" is contractile tissue of the body and is derived from the 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). Biofeedback is a form of Alternative medicine that involves measuring a subject's quantifiable bodily functions such as Blood pressure, Heart rate,

A motor unit is defined as one motor neuron and all of the muscle fibers it innervates. A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates. When a motor unit fires, the impulse (called an action potential) is carried down the motor neuron to the muscle. In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal 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 the signal 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. 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

Patients can occasionally find the procedure somewhat painful while others experience only a small amount of discomfort when the needle is inserted. The muscle or muscles being tested may be slightly sore for a day or two after the procedure.

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). A neuromuscular junction ( NMJ) is the Synapse or junction of the Axon terminal of a Motoneuron with the motor end plate, the When the muscle is voluntarily contracted, action potentials begin to appear. In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal 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. Neuropathy is a medical term usually short for Peripheral neuropathy. 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

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. A tone may be sustained for varying lengths of time. duration is a property of tone that becomes one of the bases rhythem or an amount of Time or a particular time A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates. Motor Unit Number Estimation (MUNE is a technique that uses Electromyography to estimate the number of Motor units in a muscle Area is a Quantity expressing the two- Dimensional size of a defined part of a Surface, typically a region bounded by a closed Curve. Amplitude is the magnitude of change in the oscillating variable with each Oscillation, within an oscillating system

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. Neuropathy is a medical term usually short for Peripheral neuropathy. Axillary nerve dysfunction is a general term that refers to any type of dysfunction of the Axillary nerve. Becker's muscular dystrophy (also known as Benign pseudohypertrophic Muscular dystrophy) is an X-linked recessive inherited disorder characterized Brachial plexus lesions are classified as either traumatic or obstetric Carpal tunnel syndrome (CTS or median neuropathy at the wrist is a medical condition in which the Median nerve is compressed at the Wrist, leading Centronuclear myopathies (CNM are a group of Congenital Myopathies where cell nuclei are abnormally located in skeletal muscle cells Not to be confused with Spondylitis, Spondylolysis or Spondylolisthesis. Charcot-Marie-Tooth disease ( CMT) known also as Hereditary Motor and Sensory Neuropathy ( HMSN) Hereditary Sensorimotor Neuropathy ( HSMN Dermatomyositis (DM is a connective-tissue disease related to Polymyositis (PM that is characterized by Inflammation of the muscles and the skin Duchenne muscular dystrophy ( DMD) is a severe recessive x-linked form of Muscular dystrophy that is characterized by rapid progression of muscle degeneration eventually Facioscapulohumeral muscular dystrophy (FSHMD FSHD or FSH which is also known as Landouzy-Dejerine is an autosomal dominant form of Muscular dystrophy that initially Periodic paralysis is a group of rare genetic diseases that lead to weakness or paralysis (rarely death from common triggers such as cold heat high Carbohydrate meals not Friedreich's ataxia is an Inherited disease that causes progressive damage to the Nervous system resulting in symptoms ranging from Gait disturbance Guillain-Barré syndrome ( GBS) (in French ɡilɛ̃ baˈʁe in English ˈɡiːlæn ˈbɑreɪ /ɡiːˈæn bəˈreɪ/ etc Lambert-Eaton myasthenic syndrome (LEMS is a rare Autoimmune disorder which affects calcium channels of the nerve-muscle ( neuromuscular) junction Mononeuritis multiplex is the clinical picture that arises from problems with multiple individual nerves serially or almost simultaneously Mononeuropathy (or "mono Neuritis " is a type of Neuropathy that only affects a single Nerve. The motor neurone diseases (or motor neuron diseases) (MND are a group of progressive neurological disorders that destroy Motor neurones the cells that control voluntary Myasthenia gravis (literally "serious muscle-weakness" from Greek μύς "muscle" "weakness" and Latin gravis 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 Centronuclear myopathies (CNM are a group of Congenital Myopathies where cell nuclei are abnormally located in skeletal muscle cells Neuromyotonia, also known as Isaacs' Syndrome, 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 Poliomyelitis, often called polio or infantile paralysis, is an acute viral Infectious disease spread from person to person primarily via Polymyositis is a type of Inflammatory myopathy, related to Dermatomyositis and Inclusion body myositis. Sciatica is a set of symptoms including Pain that may be caused by general compression and/or irritation of one of five nerve roots that give rise to 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 and is typically accompanied by the clenching Spinal stenosis is a medical condition in which the spinal canal narrows and compresses the spinal cord and nerves 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.

Applications of EMG

EMG signals are used in many clinical and biomedical applications. EMG is used as a diagnostics tool for identifying neuromuscular diseases, assessing low back pain, kinesiology and disorders of motor control. Neuromuscular disease is a very broad term that encompasses many Diseases and ailments that either directly via intrinsic Muscle pathology or indirectly via Kinesiology, also known as Human Kinetics, is the science of human movement EMG signals are also used as a control signal for prosthetic devices such as prosthetic hands, arms, and lower limbs. In Medicine, a prosthesis (plural prostheses) is an Artificial extension that replaces a missing Body part.

EMG can be used to sense isometric muscular activity where no movement is produced. This enables definition of a class of subtle motionless gestures to control interfaces without being noticed and without disrupting the surrounding environment. These signals can be used to control a prosthesis or as a control signal for an electronic device such as a mobile phone or PDA.

EMG signals have been targeted as control for flight systems. The Human Senses Group at the NASA Ames Research Center at Moffett Field, CA seeks to advance man-machine interfaces by directly connecting a person to a computer. NASA Ames Research Center (ARC is a NASA facility located at Moffett Federal Airfield, which covers 43 acres at the borders of the cities of Mountain View Moffett Federal Airfield, also known as Moffett Field, is a joint civil-military Airport located 3 miles (5 km north of Mountain View, in Santa In this project, an EMG signal is used to substitute for mechanical joysticks and keyboards. EMG has also been used in research towards a "wearable cockpit" which employs EMG-based gestures to manipulate switches and control sticks necessary for flight in conjunction with a goggle-based display. A gesture is a form of Non-verbal communication made with a part of the body used instead of or in combination with verbal communication.

Unvoiced speech recognition recognizes speech by observing the EMG activity of muscles associated with speech. It is targeted for use in noisy environments, and may be helpful for people without vocal cords and people with aphasia. The vocal folds, also known commonly as vocal cords, are composed of twin infoldings of Mucous membrane stretched horizontally across the Larynx.

EMG has also been used as a control signal for computers and other devices. An interface device based on EMG could be used to control moving objects, such as mobile robots or an electric wheelchair. A Mobile Robot is an automatic machine that is capable of movement in a given environment A wheelchair is a wheeled Mobility device in which the user sits This may be helpful for individuals who cannot operate a joystick-controlled wheelchair. Surface EMG recordings may also be a suitable control signal for some interactive video games.

An EMG sensor switch was developed in 2008 called the Libertas EMG Sensor([2]). This sensor picks up minute EMG signals typically from ALS victims with little remaining muscular activity. Amyotrophic Lateral Sclerosis ( ALS, sometimes called Maladie de Charcot, or in the United States Lou Gehrig's Disease) is a progressive The Libertas transmits the EMG signal through Bluetooth to a computer. Bluetooth is a wireless protocol utilizing short-range communications technology facilitating data transmission over short distances from fixed and/or mobile devices creating wireless When volitional muscle contration is detected, virtual switch closure is then passed to specialized augmentative communication software that allows users to control the computer with any remaining muscles under volitional control. The technique effectively replaces the keyboard and mouse inputs with EMG control.

References

Dictionary

electromyography

-noun

  1. A technique for evaluating and recording the activation signal of muscles.
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