EMG (for its acronym in English) is a diagnostic procedure that assesses the state of health of the muscles and nerve cells that control them.
These neurons are known as motor neurons. They transmit electrical signals that cause the muscles to contract and relax. Electromyography translates these signals into graphs or numbers, helping doctors make a diagnosis.
A doctor will usually order electromyography when someone shows muscle or nervous disorder symptoms. These symptoms may include unexplained tingling, numbness, or weakness in the extremities.
The results of electromyography can help the doctor diagnose muscle disorders, nervous disorders, and disorders that affect the connection between nerves and muscles. Some doctors may refer to electromyography as an electrodiagnostic test.
The electromyography test has a variety of clinical and biomedical applications.
Electromyography is used as a diagnostic tool to identify neuromuscular diseases or as a research tool for studying kinesiology and motor control disorders.
Electromyography signals are sometimes used to guide botulinum toxin or phenol injections in the muscles.
Electromyography and an acceleromyography can be used for neuromuscular monitoring in general anesthesia with neuromuscular blocking drugs to avoid postoperative residual curarization (PORC, for its acronym in English).
Except in the case of some purely primary myopathic conditions, electromyography is usually performed with another electrodiagnostic medicine test that measures the nerve’s conduction function.
Needle electromyography and nerve conduction studies are usually indicated when there is pain in the extremities, weakness of the spinal nerve compression, or concern for some other injury or neurological disorder.
Spinal nerve injury does not cause cervical, lumbar, or lumbar pain. For this reason, the evidence has not shown that electromyography or nerve conduction studies help diagnose causes of axial low back pain, chest pain, or pain in the cervical spine.
Needle electromyography can help diagnose compression or nerve injury (such as carpal tunnel syndrome ), nerve root injury (such as sciatica), and other muscle or nerve problems.
Less common medical conditions include amyotrophic lateral sclerosis, myasthenia gravis, and muscular dystrophy.
Skin preparation and risks
The first step before the insertion of the needle electrode is preparing the skin. This usually involves simply cleaning the skin with an alcohol pad.
The actual placement of the needle electrode can be difficult and depends on several factors, such as the specific muscle selection and the size of that muscle.
Proper placement of needle electromyography is essential for accurately representing the muscle of interest. However, electromyography is more effective in superficial muscles because it cannot bypass the superficial muscles’ action potentials and detect deeper muscles.
In addition, the more body fat an individual has, the weaker the electromyography signal.
When placing the electromyography sensor, the ideal location is in the belly of the muscle: the longitudinal midline. The stomach of the power can also be considered between the motor point (center) of the strength and the end of the tendon’s insertion.
Cardiac pacemakers and implanted cardiac defibrillators (ICDs) are increasingly used in clinical practice. There is no evidence to suggest that carrying out routine electrodiagnostic studies in patients with these devices represents a safety risk.
However, there are theoretical concerns that electrical impulses from nerve conduction studies (NCS) can be mistakenly detected by devices and result in unintended inhibition or triggering of output or reprogramming of the device.
In general, the closer the stimulation site of the pacemaker and the stimulation leads, the greater the possibility of inducing a voltage of sufficient amplitude to inhibit the pacemaker.
No immediate or late adverse effects have been reported with routine nerve conduction studies despite these concerns.
There are no known contraindications to performing needle electromyography or nerve conduction studies in pregnant patients. In addition, no complications of these procedures were reported in the literature.
Neither has it been reported that the evoked potential test causes problems when performed during pregnancy.
Patients with lymphedema or patients at risk of lymphedema are cautioned to avoid percutaneous procedures in the affected limb, such as venipuncture, to prevent the development or worsening of lymphedema or cellulitis.
Despite the potential risk, evidence of such complications after a venous puncture is limited. There are no published reports of cellulitis, infection, or other complications related to electromyography performed in lymphedema or previous dissection of lymph nodes.
However, given the unknown risk of cellulitis in patients with lymphedema, reasonable caution should be exercised when performing needle scans in lymphedematous regions to avoid complications.
In patients with macroscopic edema and tight skin, puncture of the skin with needle electrodes can cause a chronic cry of serous fluid.
The potential bacterial media of said serous fluid and the violation of the integrity of the skin may increase the risk of cellulitis. Before continuing, the doctor must weigh the potential risks of conducting the study with the need to obtain the information obtained.
Surface and intramuscular electromyography electrodes
There are two types of electromyography: surface electromyography and intramuscular electromyography. Surface electromyography evaluates muscle function by recording muscle activity from the surface of the skin’s muscle.
The surface electrodes can provide only a limited assessment of muscle activity. Surface electromyography can be recorded by a pair of electrodes or a more complex array of multiple electrodes.
More than one electrode is needed because the electromyographic recordings show the potential difference (voltage difference) between two separate electrodes.
The limitations of this approach are that the records of surface electrodes are restricted to superficial muscles, are influenced by the depth of the subcutaneous tissue at the registry site, can be very variable depending on the weight of the patient, and can not reliably discriminate discharges of adjacent muscles.
Intramuscular electromyography can be performed using various types of recording electrodes. The most straightforward approach is a monopolar needle electrode.
This can be a thin wire inserted into a muscle with a surface electrode as a reference or two thin threads inserted in power referenced together. Most fine wire recordings are for research studies or kinesiology.
The monopolar diagnostic electromyography electrodes are typically isolated and rigid enough to penetrate the skin, with only the tip exposed using a surface electrode as a reference.
The needles for injecting botulinum toxin or phenol are typically monopolar electrodes using a surface reference; in this case, however, the metallic axis of a hypodermic needle, isolated so that only the tip is exposed, is used both to record signals as to inject.
A slightly more complex design is the concentric needle electrode. These needles have a thin wire embedded in an insulating layer that fills the barrel of a hypodermic needle, which has an exposed axis, 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, the signals tend to be smaller when recorded from a concentric electrode than when recorded from a monopolar electrode and are more resistant to electrical artifacts of the tissue. The measurements tend to be somewhat more reliable.
However, superficial muscle activity can contaminate the deeper muscle register because the axis is exposed throughout its length.
The single-fiber electromyography needle electrodes have tiny recording areas and allow individual muscle fiber discharges to discriminate.
A concentric or monopolar needle electrode is typically inserted through the skin into the muscle tissue to perform intramuscular electromyography.
The needle then moves to multiple points within a relaxed muscle to assess both insertion activity and rest-activity in power.
Normal muscles exhibit a brief burst of muscle fiber activation when stimulated by the movement of the needle, but this rarely lasts more than 100 ms.
The two most common pathological types of resting activity in the muscle are the potential for fasciculation and fibrillation.
A fasciculation potential is the involuntary activation of a motor unit within the muscle, sometimes visible to the naked eye as a muscle contraction or surface electrodes.
However, fibrillations are only detected by needle electromyography and represent the remote activation of individual muscle fibers, usually resulting from nerve or muscle disease.
Often, fibrillations are triggered by the movement of the needle (insertion activity) and persist for several seconds or more after motion ceases.
After evaluating rest and insertion activity, the electromyograph evaluates muscle activity during voluntary contraction. The resulting electrical signals’ shape, size, and frequency are considered.
Then, the electrode retracts a few millimeters, and the activity is analyzed again. This is repeated, sometimes until data of 10-20 motor units are collected to conclude the function of the motor unit.
Single fiber electromyography evaluates the delay between contractions of individual muscle fibers within a motor unit. It is a sensitive test for dysfunction of the neuromuscular junction caused by drugs, poisons, or diseases like myasthenia gravis.
The technique is complicated and usually only performed by individuals with special advanced training.
A review of the literature on surface electromyography published in 2008 concluded that surface electromyography can help detect the presence of neuromuscular disease (classification of level C, class III data). Still, insufficient data supports its usefulness for distinguishing between neuropathic and neuropathic.
Myopathic conditions or for the diagnosis of specific neuromuscular diseases. Electromyography may be helpful for the additional study of fatigue associated with post-poliomyelitis syndrome and electromechanical function in myotonic dystrophy (level C classification, class III data).
Certain states of the USA UU limit the performance of needle electromyography by non-physicians. New Jersey stated that you could not delegate to a medical assistant. Michigan passed legislation that says needle electromyography is the practice of medicine.
Special training is required in diagnosing medical diseases with electromyography only in residency and fraternity programs in neurology, clinical neurophysiology, neuromuscular medicine, and physical medicine and rehabilitation.
Certain subspecialists in otolaryngology have received selective training in the electromyography of the laryngeal muscles, and subspecialists in urology, obstetrics, and gynecology have had demanding training in the electromyography of the muscles that control bowel function and the bladder.
Maximum voluntary contraction
A primary function of electromyography is to see how well a muscle can be activated. The most common form determined is making a maximum voluntary contraction (CVM) of the evaluated power.
Muscle strength, which is measured mechanically, typically correlates highly with measures of electromyographic muscle activation.
Most commonly, this is evaluated with surface electrodes, but it should be recognized that these typically only record the muscle fibers near the surface.
Depending on the application, several analytical methods to determine muscle activation are commonly used. The use of average electromyographic activation or the maximum contraction value is debated.
Most studies commonly use a maximal voluntary contraction to analyze the peak force and force generated by the target muscles.
According to the article, peak and average rectified electromyography measurements: What data reduction method should be used to evaluate core training exercises?
They concluded that the “average rectified electromyographic data (ARV) is significantly less variable when measuring core muscle activity.”
Accordingly, these investigators suggest that “ARV electromyography data should be recorded along with the maximum electromyography measurement when evaluating core exercises.”
Providing the reader with both sets of data would result in greater validity of the study and potentially eradicate the contradictions within the research.
Electromyography can also indicate the amount of fatigue in a muscle.
The following changes in the electromyography signal can mean muscle fatigue: an increase in the mean absolute value of the movement, an increase in the amplitude and duration of the muscle action potential, and a general change at lower frequencies.
Monitoring different frequency changes change the most common way to use electromyography to determine fatigue levels. Lower driving speeds allow the slower motor neurons to remain active.
A motor unit is defined as a motor neuron and all the muscle fibers it innervates. When a motor unit fires, the impulse (called an action potential) is transported by the motor neuron to the muscle.
The nerve conduction test is also often performed simultaneously with electromyography to diagnose neurological diseases.
Decomposition of the electromyography signal
The electromyography signals are composed of several motor units’ overlapping motor unit action potentials (MUAPs).
For a complete analysis, the measured electromyography signals can be decomposed into the action potentials of their constituent motor unit.
The action potentials of the motor unit of different motor units tend to have different characteristic shapes. In contrast, the action potentials of the motor unit recorded by the same electrode of the same motor unit are typically similar.
Notably, the potential and action of the motor unit depend on where the electrode is located concerning the fibers and, therefore, may look different if the electrode moves.
Electromyographic decomposition is not trivial, although many methods have been proposed.
Processing of electromyography signals
Rectification is the translation of the raw electromyography signal to a single polarity frequency (generally positive).
The goal of rectifying a signal is to ensure that the raw signal does not average zero because the raw electromyography signal has both positive and negative components.
The two types of signal rectification refer to what happens with the electromyography wave when it is processed. These types include the frequency of full length and half the size.
The full-length frequency adds the electromyography signal below the baseline (usually negative polarity) to the sign above the baseline, making a conditioned signal that is all positive.
This is the preferred rectification method because it conserves all signal energy for analysis, usually in positive polarity. The half-length rectification removes the electromyography signal below the baseline.
By doing so, the data average is no longer zero. Therefore, it can be used in statistical analysis. The only difference between the two types of rectification is that the full-wave rectification takes the absolute value of the signal matrix of data points.
Needle electromyography used in clinical settings has practical applications, such as helping to discover diseases.
However, needle electromyography has limitations since it involves the voluntary activation of the muscle and, as such, is less informative in patients who do not wish to cooperate, children and babies, and individuals with paralysis.
Surface electromyography may have limited applications due to inherent problems associated with surface electromyography. Adipose tissue (fat) can affect electromyographic recordings.
Studies show that as the adipose tissue increased, the active muscle directly below the surface decreased.
As the fatty tissue increased, the amplitude of the surface electromyography signal directly above the center of the active muscle decreased.
The recordings of electromyography signals are usually more accurate with individuals who have less body fat and more docile skin, such as young people, compared to the elderly.
Muscle cross-communication occurs when the electromyographic signal of a muscle interferes with that of another, limiting the reliability of the muscle signal being tested. Surface electromyography is limited due to the lack of reliability of the deep muscles.
Deep muscles require intramuscular cables that are intrusive and painful to achieve an electromyography signal. Surface electromyography can only measure superficial muscles, and even then, it is difficult to reduce the call to a single power.
The electrical source is the muscle membrane potential of approximately -90mV. Depending on the muscle under observation, the measured electromyography potentials vary between less than 50μV and up to 20 to 30mV.
Results of the procedure
Resting muscle tissue is usually electrically inactive.
After the electrical activity caused by the irritation of the needle insertion decreases, the electromyograph must not detect any abnormal spontaneous activity (i.e., a muscle at rest must be electrically silent, except for the neuromuscular junction area, which is, under normal circumstances, very spontaneously active).
When the muscle contracts voluntarily, the action potentials begin to appear. As the strength of muscle contraction increases, more and more muscle fibers produce action potentials.
When the muscle is fully contracted, a disordered group of action potentials of different rates and amplitudes should appear (a complete pattern of recruitment and interference).
The electromyographic findings vary according to the type of disorder, the duration of the problem, the patient’s age, the degree of cooperation of the patient, the type of needle electrode used to study the patient, and the sampling error in terms of the number of areas studied, within a single muscle and the number of forces studied in general.
The interpretation of electromyographic findings is generally best performed by an individual informed by a focused history and a physical examination of the patient, together with other relevant diagnostic studies conducted, including nerve conduction studies.
But also, as appropriate, imaging studies such as magnetic resonance and ultrasound, muscle and nerve biopsy, muscle enzymes, and serological studies.
The first documented electromyography experiments began with Francesco Redi’s works in 1666. Redi discovered a highly specialized muscle of electricity generated by electric ray fishes (electric eel).
In 1773, Walsh proved that the muscle tissue of the eel fish could generate a spark of electricity.
In 1792, a publication entitled De Viribus Electricitatis appeared in Motu Muscular Commentarius, written by Luigi Galvani, in which the author demonstrated that electricity could initiate muscle contraction.
In 1849, Emil du Bois-Reymond discovered that it was also possible to record electrical activity during a voluntary muscular contraction.
The first actual recording of this activity was made by Marey in 1890, who also introduced the term electromyography. In 1922, Gasser and Erlanger used an oscilloscope to show the muscles’ electrical signals.
Due to the stochastic nature of the myoelectric signal, only approximate information about its observation could be obtained.
The ability to detect electromyographic signals improved steadily from the 1930s to the 1950s, and researchers began using improved electrodes more widely to study muscles.
The American Association of Neuromuscular and Electrodiagnostic Medicine was formed in 1953 as one of several currently active medical societies interested in promoting science and the clinical use of the technique.
At present, several suitable amplifiers are commercially available. In the early eighties, cables appeared that produced signals in the desired microvolt range.
It is used diagnostically by walking laboratories and by doctors trained in biological feedback or ergonomic assessment.