Editing Electromyography (section)

== Technique ==

===Skin preparation and risks===
The first step before insertion of the needle electrode is skin preparation. This typically involves simply cleaning the skin with an alcohol pad.{{citation needed|date=December 2021}}

The actual placement of the needle electrode can be difficult and depends on a number of factors, such as specific muscle selection and the size of that muscle. Proper needle EMG placement is very important for accurate representation of the [[muscle]] of interest, although EMG is more effective on superficial muscles as it is unable to bypass the action potentials of superficial muscles and detect deeper muscles. Also, the more [[body fat]] an individual has, the weaker the EMG signal. When placing the EMG sensor, the ideal location is at the belly of the muscle: the longitudinal midline. The belly of the muscle can also be thought of as in-between the motor point (middle) of the muscle and the tendonus insertion point.{{citation needed|date=December 2021}}

[[Cardiac pacemaker|Cardiac pacemakers]] and implanted [[Defibrillation|cardiac defibrillators]] (ICDs) are used increasingly in clinical practice, and no evidence exists indicating that performing routine electrodiagnostic studies on patients with these devices pose a safety hazard. However, there are theoretical concerns that electrical impulses of [[Nerve conduction study|nerve conduction studies]] (NCS) could be erroneously sensed by devices and result in unintended inhibition or triggering of output or reprogramming of the device. In general, the closer the stimulation site is to the pacemaker and pacing leads, the greater the chance for inducing a voltage of sufficient amplitude to inhibit the pacemaker. Despite such concerns, no immediate or delayed adverse effects have been reported with routine NCS.{{citation needed|date=December 2021}}

No known contraindications exist for performing needle EMG or NCS on pregnant patients. Additionally, no complications from these procedures have been reported in the literature. Evoked potential testing, likewise, has not been reported to cause any problems when it is performed during pregnancy.<ref name="aanem.org">{{cite web |title=Not Found – American Association of Neuromuscular & Electrodiagnostic Medicine |url=http://www.aanem.org/getmedia/2034191e-583b-4c55-b725-fc38ea8262e2/risksinEDX.pdf.aspx. |url-status=dead |archive-url=https://web.archive.org/web/20230225222910/https://www.aanem.org/Not-Found?aspxerrorpath=/getmedia/2034191e-583b-4c55-b725-fc38ea8262e2/risksinEDX.pdf.aspx. |archive-date=2023-02-25 |access-date=2014-12-26 |website=www.aanem.org}}</ref>

Patients with [[lymphedema]] or patients at risk for lymphedema are routinely cautioned to avoid percutaneous procedures in the affected extremity, namely [[venipuncture]], to prevent development or worsening of [[lymphedema]] or [[cellulitis]]. Despite the potential risk, the evidence for such complications subsequent to venipuncture is limited. No published reports exist of cellulitis, infection, or other complications related to EMG performed in the setting of lymphedema or prior lymph node dissection. However, given the unknown risk of cellulitis in patients with lymphedema, reasonable caution should be exercised in performing needle examinations in lymphedematous regions to avoid complications. In patients with gross edema and taut skin, skin puncture by needle electrodes may result in chronic weeping of serous fluid. The potential bacterial media of such serous fluid and the violation of skin integrity may increase the risk of cellulitis. Before proceeding, the physician should weigh the potential risks of performing the study with the need to obtain the information gained.<ref name="aanem.org"/>

===Surface and intramuscular EMG recording electrodes===
There are two kinds of EMG: surface EMG and intramuscular EMG. Surface EMG assesses muscle function by recording muscle activity from the surface above the muscle on the skin. Surface EMG can be recorded by a pair of electrodes or by a more complex array of multiple electrodes. More than one electrode is needed because EMG recordings display the potential difference (voltage difference) between two separate electrodes. Limitations of this approach are the fact that surface electrode recordings are restricted to superficial muscles, are influenced by the depth of the subcutaneous tissue at the site of the recording which can be highly variable depending on the weight of a patient, and cannot reliably discriminate between the discharges of adjacent muscles. Specific electrode placements and functional tests have been developed to minimize this risk, thus providing reliable examinations.{{citation needed|date=December 2021}}

Intramuscular EMG can be performed using a variety of different types of recording electrodes. The simplest approach is a monopolar needle electrode. This can be a fine wire inserted into a muscle with a surface electrode as a reference; or two fine wires inserted into muscle referenced to each other. Most commonly fine wire recordings are for research or [[kinesiology]] studies. Diagnostic monopolar EMG electrodes are typically insulated and stiff enough to penetrate skin, with only the tip exposed using a surface electrode for reference. Needles for injecting therapeutic [[botulinum toxin]] or phenol are typically monopolar electrodes that use a surface reference, in this case, however, the metal shaft of a [[hypodermic needle]], insulated so that only the tip is exposed, is used both to record signals and to inject. Slightly more complex in design is the concentric needle electrode. These needles have a fine wire, embedded in a layer of insulation that fills the barrel of a hypodermic needle, that has an exposed shaft, and the shaft serves as the reference electrode. The exposed tip of the fine wire serves as the active electrode. As a result of this configuration, signals tend to be smaller when recorded from a concentric electrode than when recorded from a monopolar electrode and they are more resistant to electrical artifacts from tissue and measurements tend to be somewhat more reliable. However, because the shaft is exposed throughout its length, superficial muscle activity can contaminate the recording of deeper muscles. Single fiber EMG needle electrodes are designed to have very tiny recording areas, and allow for the discharges of individual muscle fibers to be discriminated.{{citation needed|date=December 2021}}

To perform intramuscular EMG, typically either a monopolar or concentric needle electrode is inserted through the skin into the muscle tissue. The needle is then moved to multiple spots within a relaxed muscle to evaluate both insertional activity and resting activity in the muscle. Normal muscles exhibit a brief burst of muscle fiber activation when stimulated by needle movement, but this rarely lasts more than 100ms. The two most common pathologic types of resting activity in muscle are fasciculation and fibrillation potentials. A fasciculation potential is an involuntary activation of a [[motor unit]] within the muscle, sometimes visible with the naked eye as a muscle twitch or by surface electrodes. Fibrillations, however, are detected only by needle EMG, and represent the isolated activation of individual muscle fibers, usually as the result of nerve or muscle disease. Often, fibrillations are triggered by needle movement (insertional activity) and persist for several seconds or more after the movement ceases.{{citation needed|date=December 2021}}

After assessing resting and insertional activity, the electromyographer assess the activity of muscle during voluntary contraction. The shape, size, and frequency of the resulting electrical signals are judged. Then the electrode is retracted a few millimetres, and again the activity is analyzed. This is repeated, sometimes until data on 10–20 motor units have been collected in order to draw conclusions about motor unit function. 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 the interpretation of EMG study is important to evaluate parameters of tested muscle motor units. This process may well be partially automated using appropriate software.<ref>{{Cite journal |last1=Kędzia |first1=Alicja |last2=Derkowski |first2=Wojciech |title=The Use of Different Methods of Computer Analysis of Motor Potentials in Emg Record |url=https://zenodo.org/records/10615275 |journal=Computer-Aided Scientific Research |volume=17 |date=2010 |pages=161–168 |language=en |doi=10.5281/zenodo.10615275}}</ref>

Single fiber electromyography assesses the delay between the contractions of individual muscle fibers within a motor unit and is a sensitive test for dysfunction of the neuromuscular junction caused by drugs, poisons, or diseases such as myasthenia gravis. The technique is complicated and typically performed only by individuals with special advanced training.

Surface EMG 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 review of the literature on surface EMG published in 2008, concluded that surface EMG may be useful to detect the presence of neuromuscular disease (level C rating, class III data), but there are insufficient data to support its utility for distinguishing between neuropathic and myopathic conditions or for the diagnosis of specific neuromuscular diseases. EMGs may be useful for additional study of fatigue associated with post-poliomyelitis syndrome and electromechanical function in myotonic dystrophy (level C rating, class III data).<ref name="aanem.org"/> Recently, with the rise of technology in sports, sEMG has become an area of focus for coaches to reduce the incidence of soft tissue injury and improve player performance.

Certain US states limit the performance of needle EMG by nonphysicians. New Jersey declared that it cannot be delegated to a [[Physician assistant|physician's assistant]].<ref>Arthur C. Rothman, MD, v. Selective Insurance Company of America, Supreme Court of New Jersey, Jan. 19.</ref><ref>Texas Court of Appeals, Third District, at Austin, Cause No. 03-10-673-CV. April 5, 2012</ref> Michigan has passed legislation saying needle EMG is the practice of medicine.<ref>Section 333.17018 Michigan Compiled Laws http://legislature.mi.gov/doc.aspx?mcl-333-17018.</ref> Special training in diagnosing medical diseases with EMG is required only in residency and fellowship programs in [[neurology]], [[clinical neurophysiology]], [[neuromuscular medicine]], and physical medicine and rehabilitation. There are certain subspecialists in otolaryngology who have had selective training in performing EMG of the laryngeal muscles, and subspecialists in [[urology]], [[Obstetrics and gynaecology|obstetrics and gynecology]] who have had selective training in performing EMG of muscles controlling bowel and bladder function.{{citation needed|date=December 2021}}

===Maximal voluntary contraction===
One basic function of EMG is to see how well a muscle can be activated. The most common way that can be determined is by performing a [[muscle contraction|maximal voluntary contraction]] (MVC) of the muscle that is being tested.<ref>{{Cite journal |last1=Behm |first1=D.G. |last2=Whittle |first2=J. |last3=Button |first3=D. |last4=Power |first4=K. |date=2002-01-28 |title=Intermuscle differences in activation |url=https://onlinelibrary.wiley.com/doi/10.1002/mus.10008 |journal=Muscle & Nerve |language=en |volume=25 |issue=2 |pages=236–243 |doi=10.1002/mus.10008 |pmid=11870692 |s2cid=20430130 |issn=0148-639X|url-access=subscription }}</ref> Each muscle group type has different characteristics, and MVC positions are varied for different muscle group types. Therefore, the researcher should be very careful while choosing the MVC position type to elicit the greater muscle activity level from the subjects.<ref>Peter Konrad, The ABC of EMG, https://www.noraxon.com/wp-content/uploads/2014/12/ABC-EMG-ISBN.pdf.</ref>

The types of MVC positions can vary among muscle types, contingent upon the specific muscle group being considered, including trunk muscles, lower limb muscles, and others.<ref>{{Cite journal |last1=Vera-Garcia |first1=Francisco J. |last2=Moreside |first2=Janice M. |last3=McGill |first3=Stuart M. |date=2010-02-01 |title=MVC techniques to normalize trunk muscle EMG in healthy women |url=https://www.sciencedirect.com/science/article/pii/S1050641109000571 |journal=Journal of Electromyography and Kinesiology |volume=20 |issue=1 |pages=10–16 |doi=10.1016/j.jelekin.2009.03.010 |pmid=19394867 |issn=1050-6411|url-access=subscription }}</ref><ref>{{Cite journal |last1=Avdan |first1=Goksu |last2=Onal |first2=Sinan |last3=Smith |first3=Bryan K. |date=2023-04-01 |title=Normalization of EMG Signals: Optimal MVC Positions for the Lower Limb Muscle Groups in Healthy Subjects |url=https://doi.org/10.1007/s40846-023-00782-3 |journal=Journal of Medical and Biological Engineering |language=en |volume=43 |issue=2 |pages=195–202 |doi=10.1007/s40846-023-00782-3 |s2cid=257966584 |issn=2199-4757|url-access=subscription }}</ref>

Muscle force, which is measured mechanically, typically correlates highly with measures of EMG activation of muscle. Most commonly this is assessed with surface electrodes, but it should be recognized that these typically record only from muscle fibers in close proximity to the surface.

Several analytical methods for determining muscle activation are commonly used depending on the application. The use of mean EMG activation or the peak contraction value is a debated topic. Most studies commonly use the [[muscle contraction|maximal voluntary contraction]] as a means of analyzing peak force and force generated by target muscles. According to the article "Peak and average rectified EMG measures: Which method of data reduction should be used for assessing core training exercises?",<ref>{{Cite journal |last1=Hibbs |first1=A. E. |last2=Thompson |first2=K. G. |last3=French |first3=D. N. |last4=Hodgson |first4=D. |last5=Spears |first5=I. R. |date=February 2011 |title=Peak and average rectified EMG measures: Which method of data reduction should be used for assessing core training exercises? |url=https://linkinghub.elsevier.com/retrieve/pii/S1050641110000891 |journal=Journal of Electromyography and Kinesiology |language=en |volume=21 |issue=1 |pages=102–111 |doi=10.1016/j.jelekin.2010.06.001 |pmid=20655245|url-access=subscription }}</ref> it was concluded that the "average rectified EMG data (ARV) is significantly less variable when measuring the muscle activity of the core musculature compared to the peak EMG variable." Therefore, these researchers would suggest that "ARV EMG data should be recorded alongside the peak EMG measure when assessing core exercises." Providing the reader with both sets of data would result in enhanced validity of the study and potentially eradicate the contradictions within the research.<ref>{{Cite journal |last1=Buchanan |first1=Thomas S. |last2=Lloyd |first2=David G. |last3=Manal |first3=Kurt |last4=Besier |first4=Thor F. |date=November 2004 |title=Neuromusculoskeletal Modeling: Estimation of Muscle Forces and Joint Moments and Movements from Measurements of Neural Command |url=https://journals.humankinetics.com/view/journals/jab/20/4/article-p367.xml |journal=Journal of Applied Biomechanics |volume=20 |issue=4 |pages=367–395 |doi=10.1123/jab.20.4.367 |issn=1065-8483 |pmc=1357215 |pmid=16467928}}</ref><ref>{{Cite journal |last1=Halperin |first1=Israel |last2=Aboodarda |first2=Saied Jalal |last3=Button |first3=Duane C. |last4=Andersen |first4=Lars L. |last5=Behm |first5=David G. |date=February 2014 |title=Roller massager improves range of motion of plantar flexor muscles without subsequent decreases in force parameters |journal=International Journal of Sports Physical Therapy |volume=9 |issue=1 |pages=92–102 |issn=2159-2896 |pmc=3924613 |pmid=24567860}}</ref>

===Other measurements===
EMG can also be used for indicating the amount of fatigue in a muscle. The following changes in the EMG signal can signify [[muscle fatigue]]: an increase in the mean absolute value of the signal, increase in the [[amplitude]] and duration of the muscle action potential and an overall shift to lower frequencies. Monitoring the changes of different frequency changes the most common way of using EMG to determine levels of fatigue. The lower conduction velocities enable the slower [[motor neuron]]s to remain active.<ref>Cifrek, M., Medved, V., Tonković, S., & Ostojić, S. (2009). Surface EMG based muscle fatigue evaluation in biomechanics. Clinical Biomechanics, 24(4), 327-340.</ref>

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 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.{{citation needed|date=December 2021}}

[[Nerve conduction study|Nerve conduction testing]] is also often done at the same time as an EMG to diagnose neurological diseases.<ref>{{Cite web |title=Electromyography (EMG) and Nerve Conduction Studies: MedlinePlus Medical Test |url=https://medlineplus.gov/lab-tests/electromyography-emg-and-nerve-conduction-studies/ |access-date=2023-11-26 |website=medlineplus.gov |language=en}}</ref>

Some patients can find the procedure somewhat painful, whereas 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.<ref>{{Cite web |title=Electromyography (EMG) |url=https://my.clevelandclinic.org/health/diagnostics/4825-emg-electromyography |access-date=2023-11-26 |website=Cleveland Clinic |language=en}}</ref>

=== 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 [[Wavelet transform|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.<ref>{{Cite journal |last=Stashuk |first=Dan |date=June 2001 |title=EMG signal decomposition: how can it be accomplished and used? |url=https://linkinghub.elsevier.com/retrieve/pii/S105064110000050X |journal=Journal of Electromyography and Kinesiology |language=en |volume=11 |issue=3 |pages=151–173 |doi=10.1016/S1050-6411(00)00050-X|pmid=11335147 |url-access=subscription }}</ref>

===EMG signal processing===
Rectification is the translation of the raw EMG signal to a signal with a single [[Electrical polarity|polarity]], usually positive. The purpose of rectifying the signal is to ensure the signal does not average to zero, due to the raw EMG signal having positive and negative components. Two types of rectification are used: full-wave and half-wave rectification.<ref>{{cite journal |last1=Raez |first1=M. B. I. |last2=Hussain |first2=M. S. |last3=Mohd-Yasin |first3=F. |date=Mar 23, 2006 |title=Techniques of EMG signal analysis: detection, processing, classification and applications |journal=Biol. Proced. Online |volume=8 |issue=8 |pages=11–35 |doi=10.1251/bpo115 |pmc=1455479 |pmid=16799694}}</ref> Full-wave rectification adds the EMG signal below the baseline to the signal above the baseline to make a conditioned signal that is all positive. If the baseline is zero, this is equivalent to taking the [[absolute value]] of the signal.<ref>{{cite journal |last1=Weir |first1=J. P. |last2=Wagner |first2=L. L. |last3=Housh |first3=T. J. |year=1992 |title=Linearity and reliability of the IEMG v. torque relationship for the forearm flexors and leg extensors |journal=American Journal of Physical Medicine & Rehabilitation |volume=71 |issue=5 |pages=283–287 |doi=10.1097/00002060-199210000-00006 |pmid=1388975 |s2cid=25136951}}</ref><ref>{{cite journal |last1=Vrendenbregt |first1=J. |last2=Rau |first2=G. |last3=Housh |first3=T. J. |year=1973 |title=Surface eletromyography in relation to force, muscle length and endurance. |journal=New Developments in Electromyography and Clinical Neurophysiology |pages=607–622}}</ref> This is the preferred method of rectification because it conserves all of the signal energy for analysis. Half-wave rectification discards the portion of the EMG signal that is below the baseline. In doing so, the average of the data is no longer zero therefore it can be used in statistical analyses.

=== Limitations ===
Needle EMG used in clinical settings has practical applications such as helping to discover disease. Needle EMG has limitations, however, in that it does involve voluntary activation of muscle, and as such is less informative in patients unwilling or unable to cooperate, children and infants, and in individuals with paralysis. Surface EMG can have limited applications due to inherent problems associated with surface EMG. Adipose tissue (fat) can affect EMG recordings. Studies show that as adipose tissue increased the active muscle directly below the surface decreased. As adipose tissue increased, the amplitude of the surface EMG signal directly above the center of the active muscle decreased. EMG signal recordings are typically more accurate with individuals who have lower body fat, and more compliant skin, such as young people when compared to old. Muscle cross talk occurs when the EMG signal from one muscle interferes with that of another limiting reliability of the signal of the muscle being tested. Surface EMG is limited due to lack of deep muscles reliability. Deep muscles require intramuscular wires that are intrusive and painful in order to achieve an EMG signal. Surface EMG can measure only superficial muscles and even then it is hard to narrow down the signal to a single muscle.<ref>{{cite journal |last=Kuiken |first=T. A. |author2=Lowery, Stoykob |date=April 2003 |title=The Effect of Subcutaneous Fat on myoelectric signal amplitude and cross talk |journal=Prosthetics and Orthotics International |volume=27 |issue=1 |pages=48–54 |doi=10.3109/03093640309167976 |pmid=12812327 |doi-access=free}}</ref>

=== Electrical characteristics ===
The electrical source is the muscle [[membrane potential]] of about –90&nbsp;mV.<ref>Nigg B. M., & Herzog W., 1999. Biomechanics of the Musculo-Skeletal system. Wiley, p. 349.</ref> Measured EMG potentials range between less than 50&nbsp;μV and up to 30&nbsp;mV, depending on the muscle under observation.{{citation needed|date=December 2021}}

Typical repetition rate of muscle [[motor unit]] firing is about 7–20&nbsp;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&nbsp;mV.<ref>{{cite web|last1=Patterson|first1=John R.|title=Fitwise|url=https://www.fitwise.com.au/|website=Castillo|publisher=Brian T.|access-date=24 June 2009}}</ref>