PART 1 Oral cavity, pharynx and esophagus

GI Motility online (2006) doi:10.1038/gimo32
Published 16 May 2006

Electromyography in oral and pharyngeal motor disorders

Adrienne L. Perlman, Ph.D.

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Key Points

  • Electromyography (EMG) is the recording of the electrical phenomena that determine muscle health, muscle excitation and contraction. It is particularly useful in the study of the striated muscles.

  • EMG is very useful in clinical diagnosis, understanding of physiology and pathophysiology, and treatment of oropharyngeal dysphagia

  • EMG studies provide precise timing of activity of muscles and can be used even for very small muscles in vivo. EMG studies have provided very important information on the sequence of activation of muscles involved in oropharyngeal phase of swallowing

  • Diagnostic electromyography (EMG) can determine a site of pathology along the lower motor neuron, and it can be used in the differential diagnosis of various neuromuscular diseases.

  • Clinically, EMG is very helpful in the diagnosis, evaluation and follow up of cases of oropharyngeal dysphasia due to myositis, myasthenia gravis, motor neuron disease [most commonly amyotrophic lateral sclerosis (ALS)], post-polio syndrome, muscle paralysis, and scleroderma.

  • There are many different types of EMG instrumentations that may produce different reports. Care should be taken in interpreting results from different laboratories using different EMG instruments.



In clinical medicine, electromyography is often looked upon as simply one of a variety of electrodiagnostic examinations that are performed in an effort to determine the presence and site of neuromuscular abnormalities. In fact, Stedman's Medical Dictionary1 defines electromyography (EMG) as "The recording of electrical activity generated in muscle for diagnostic purposes...." Because of the semantic implications associated with the word diagnostic, this definition has restrictive boundaries.

Although electrodiagnosis is likely the most common application of EMG, it is used in both clinical and bench research, with both humans and animals, to study the contribution of healthy nerves and muscles to the execution of voluntary and involuntary physiologic events. Additionally, it is used by a variety of disciplines for purposes of biofeedback.2, 3

Therefore, a broader definition in which EMG is defined as the recording of the electrical phenomena that are associated with the first stage in the sequence of events that link muscle excitation with muscle contraction4 may be more appropriate. For our purposes, this broader definition covers a range of applications, particularly as they relate to efforts to better understand healthy as well as diseased muscle function and the effects of medical and behavioral treatment techniques associated with normal and disordered swallowing.


Principles of Electromyography

Electromyography is the only technique that can directly display activity from a specific muscle. The functional unit of muscle contraction is the motor unit. The motor unit is composed of a single motor neuron with the cell body and dendrites that reside in either the brainstem or the spinal cord, the axon traveling from the motor neuron, the myoneural junction, and the muscle fibers innervated by that motor neuron.

At rest, a muscle fiber maintains a steady potential across the membrane. When an impulse travels along a nerve and arrives at the myoneural junction, acetylcholine is emitted from the motor end plate. This results in depolarization of the muscle fiber and a subsequent muscle contraction. The depolarization generates an electromagnetic field and the action potential is measured as a voltage. It is important not to confuse muscle contraction with muscle shortening. If a muscle antagonist is also contracting, the degree of antagonistic contraction limits the ability of a contracting muscle to shorten.

The motor unit action potential is the spatiotemporal summation of the individual muscle action potentials for all the fibers in the vicinity of a given electrode or electrode pair. Consequently, the EMG signal is the summation of the motor unit action potentials within the pickup field of a particular electrode or electrode pair. Figure 1 is expanded in the time domain in order to display a single motor unit action potential (MUAP) from the medial thyroarytenoid muscle of a healthy adult larynx. Figure 2 shows a train (MUAPT) of single MUAPs from that muscle with time more compressed.

Figure 1: Single motor unit action potential (MUAP).
Figure 1 : Single motor unit action potential (MUAP). Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact

This MUAP is from the medial thyroarytenoid muscle of a healthy adult larynx. The time domain is expanded.

Any portion of the muscle may contain fibers belonging to as many as 20 to 50 motor units and a single motor neuron can innervate as few as three or as many as 2000 muscle fibers.5 Muscles controlling fine movements, such as those of the eye, human tensor tympani, larynx and pharynx,5 have smaller numbers of muscle fibers per motor unit, whereas those controlling gross movements, such as the gastrocnemius m., have a large number of fibers per motor unit. Usually the muscle fibers of different motor units are intermingled throughout a muscle, in which case, the pickup field of an electrode will include more than one motor unit. Figure 3 shows the firing pattern from bipolar hooked wire electrodes recording a collection of multiple MUAPs from the medial thyroarytenoid muscle.

Thorough explanation of the principles of EMG as are presented in classic textbooks, such as those by Lahoda et al.6 and by Basmajian and DeLuca,5 and muscle physiology has been well described by Aidley.4 Computer ability was just burgeoning at the time of these publications; thus references to digital recording and computer analysis are minimal or absent. Although strip chart recorders, magnetic tape recordings, and hand-held calculators have been replaced by analogue-to-digital converters and computer recording and analysis programs, the basic instrumentation required for data acquisition has not changed dramatically over the decades. Other readings to assist in understanding electromyographic concepts both for research and the clinic can include the fifth edition by Basmajian and DeLuca,5 as well as books by Loeb and Gans,7 Gnatz,8 and Kimura.9 Lastly, in consideration of EMG of the head and neck, the work of Faaborg-Anderson10 is a most worthy read. Although most of these citations are from older sources, this author considers them to be some of the best resources available for acquainting oneself with proper methodology.


Methods of Electromyography Recording

Whether referring to the older use of analogue systems, or the present use of digital systems in electromyographic signal acquisition and conditioning, the required equipment continues to consist of electrodes (various types of surface and intramuscular), as well as a preamplifier, amplifier, filter, display, and data storage unit. Electromyography units can be purchased in one of two ways: first, as an integrated system, which may come with unexpected limitations imposed by the system algorithms; and second, as a set of components, which will allow for greater flexibility but require development responsibility from the user. Systems used for diagnostic purposes are almost always integrated units, whereas those used in the laboratory are more frequently composed of individual components.

When an integrated system is to be used, it is important to consider a variety of factors. These factors include, but are not limited to, the type of electrode cable input, location of the preamplifier and amplifier, upper limits for amplification, limitations in sampling rate, site and type of filter, 3-dB down point of filter, and the site and extent of display smoothing. It is generally advisable to use a system that will record the full bandwidth of the amplified EMG signal and to store that signal onto the hard drive before the signal is filtered; this is to prevent the loss of information that was obtained during the acquisition process. However, antialiasing should be performed before the signal is sampled. Antialiasing is based on the Nyquist theorem that states that a signal must be sampled at no less than twice its frequency. If the sampling rate is too low, the signal will be incorrectly reconstructed and findings will be invalid. All systems do not meet the specifications for an intended project, and so it is up to the clinician/investigator to make a careful decision regarding appropriate application of an available unit.

Naturally, electrical safety aspects of EMG systems must be considered. Only equipment that is properly grounded and that has appropriate isolation built into the system is permitted for use in hospitals. Nonetheless, both component and integrated systems must be routinely examined in order to check for current leakage. Extension cords should never be used with EMG equipment, and patients should not be connected to other electrical devices during EMG testing. The safety of intramuscular EMG recording of the pharyngeal and laryngeal musculature was reinforced by a report from Mu and Yang.11 These investigators recorded intramuscular needle electrode EMG activity from the posterior cricoarytenoid muscle of 1200 patients, with no reported complications.

A primary decision to be made by clinicians and researchers is that relating to the type of electrodes to be used in a study. Investigations relating to various aspects of deglutition have been performed on humans as well as animals and on structures ranging from the lips through to the esophagus. These have been performed with surface, surface-suction, needle, hooked wire, monopolar, and bipolar electrodes. A description of the various types of electrodes and appropriate application of a particular type of electrode can be found in many sources within the literature.2, 3, 12, 13, 14, 15 At times, the type of electrode used for investigation has been a function of the level of scientific advancement at the time in which the study was performed as well as a function of the questions that were proposed. Therefore, even the best science from a particular period is influenced by the instrumentation of the time. Unfortunately, at times, it has also been a function of what the clinician had available. And so, a reader must evaluate the appropriateness of reported methodology before accepting the accuracy of reported results. For example, an EMG study using surface electrodes and in which the muscles of interest are deep to the surface is a study that is seriously open to question.

When EMG is performed for diagnostic purposes, concentric needle electrodes are, and generally should be, used. However, the use of needle electrodes and the use of monopolar electrodes are not the best selections for the study of the physiologic aspects of human deglutition. Because of the movements associated with deglutition, it is often quite difficult to execute a comfortable and normal swallow when a needle is placed within a muscle. An intramuscular EMG study with simultaneous recording from bipolar hooked wire electrodes and from a concentric bipolar needle electrode placed in the thyroarytenoid muscles of 10 adult canines compared the amplitude of activity during vagal stimulation. Signals from both electrode types showed similar complex action potentials. Furthermore, there was no reported difference in terms of electrode stability or vocal fold injury.16 Such reports reinforce the appropriate selection of hooked wire electrodes in the examination of a dynamic task such as deglutition. Also to be considered is the use of monopolar vs. bipolar recordings. A monopolar electrode records from a wide field, whereas a differential recording between two closely spaced (bipolar) electrodes records better from a more restricted area. Figures 1,2 to 3 are differential recordings obtained with bipolar, hooked wire electrodes.

Surface EMG is often used by investigators who are less concerned if the EMG activity is recorded from a group of muscles within a wide field, such as the submental region. Submental surface EMG is acceptable when the investigators are not interested in differentiating geniohyoid activity from that of the mylohyoid or digastric muscles. A commonly referenced textbook relating to the applications of surface EMG is that by Cram and Kasman.17 This book emphasizes that an appropriate use for surface EMG is for biofeedback, not as a diagnostic tool. The authors state that EMG biofeedback techniques fall roughly into three clinical categories: (1) systemic relaxation, (2) muscle strengthening or disinhibition, and (3) coordination. Coordination techniques are described as an advanced level of biofeedback and usually follow successful relaxation or strengthening therapy and is intended to help the patient learn how to obtain the correct balance of agonists/antagonists. Other sources for information relating to the use of surface EMG are DeLuca,18 Kasman et al.,3 and Merletti.19

It is important to stress that surface EMG is easy to use and equally as easy to abuse. Experienced electromyographers are quite aware that ease of use does not equate with acquisition of accurate information.20


Historical Background

According to Basmajian and DeLuca,5 the birth of the field of neurophysiology can be attributed to a 1791 experiment by Luiggi Galvani. Galvani developed his concept of "animal electricity" following a series of experiments in which he depolarized frog leg muscles by using metal rods to make contact with the leg muscles. A publication that is cited as reporting the first study of electromyographic signals was that by Piper.21 Other early and important work reported by Basmajian and DeLuca includes, but is not limited to, that of Gasser and Erlanger,22 Proebster,23 Adrian and Bronk,24 Kugelberg and Petersen,25 Denny-Brown,26 and Buchthal et al.27, 28

Electromyography has been used to study aspects of motor activity during swallowing for many decades. In 1956, Doty and Bosma29 described findings from an investigation performed on 28 animals from three species (cat, dog, monkey). In that report, the animals were anesthetized and a series of electrode placements began at the mylohyoid muscle and progressed caudally as far as the inferior pharyngeal constrictor; diaphragmatic activity was also recorded. The superior laryngeal nerve was stimulated to produce a swallow. The authors presented a detailed qualitative description of the reflexive stage of deglutition and the interaction of the muscles associated with the oral and pharyngeal stages of swallowing, and also provided a discussion of the similarities and differences across species.

The descriptive work by Doty and Bosma is still respected for its early contribution toward the understanding of, and interest in, the process of deglutition. Follow-up studies in other laboratories reported that the findings of Doty and Bosma were similar to their work with other mammals.30, 31, 32 A consistent pattern of contraction following muscle ablation was reported in a study of anesthetized dogs by Maeyama.33 Additionally, Maeyama concluded that the geniohyoid and thyrohyoid muscles were primarily responsible for laryngeal elevation; however, at that time, the author did not recognize the importance of these suprahyoid and infrahyoid muscles to safe oral intake. The early EMG studies of deglutition with both animal and human models are cited in the book by Basmajian and DeLuca.5

Other early and equally interesting EMG investigations examined activity in the striated and smooth muscles of the animal esophagus.34, 35, 36 As with the work of Doty and Bosma, these early esophageal studies were primarily descriptive; but they provided methodologic information that was important to the advancement of this field of study.

There are considerable esophageal EMG data reported from animal models; however, this review is directed toward work that has been reported on the adult human, and much of the swallowing research using the human model has been performed on muscles associated with the oral or pharyngeal stages of deglutition. This is reasonable because of the difficulty of inserting electrodes into the esophagus or lower esophageal sphincter of a human volunteer. However, Basmajian,37 cites a 1960 report by Petit, Milic-Emili, and Delhez in which diaphragmatic EMG was performed on a conscious man via the esophagus.

In an editorial article addressing the application of intraluminal suction electrodes to the esophagus, Zelter et al.38 stated five reasons why they believed that electromyographic recordings were difficult to perform for evaluation of esophageal function: (1) movement of the esophagus produced an artifact in the recorded signal, (2) occlusive mucous plugs could make it difficult to secure a strong fixation of suction electrodes to the esophageal wall, (3) electrical activity from the heart could interfere with the EMG signal, (4) reduced signal amplitude occurred because of extravasation of blood with subsequent reduction in the suction force, and (5) displacement of the electrode could easily occur due to insufficient suction. Given that these criticisms were published in 1990, it appears that investigators have resolved at least some of those issues. Intraluminal esophageal catheters have been reported in human subject research39, 40, 41, 42; additionally, intramuscular EMG research has been performed during surgical procedures.43

Because the lower esophageal sphincter (LES) is exposed to pressures changes within that region, gastroesophageal reflux can, and does, occur. In an effort to better understand the effects of intra-abdominal pressures on LES function, simultaneous pressure and intramuscular EMG recordings from the LES were obtained from 17 normal subjects who were scheduled for surgical repair unrelated to LES function. At the completion of the surgical procedures, the anesthetists were instructed to move the endotracheal tube so as to induce coughing and straining in the anesthetized patients. Electromyography activity in the LES during both coughing and straining showed a significant increase over resting activity; this was interpreted to suggest the LES had contracted in order to assist in the prevention of gastroesophageal reflux. The authors concluded that the straining-esophageal reflex is "deranged" in patients who experience gastroesophageal reflux disease (GERD).43

Much of the early EMG research with human subjects and relating to the muscles of the head and neck were originally performed by investigators from the disciplines of speech science and from otolaryngology.10, 44, 45, 46, 47 Few of these publications considered the actions of those muscles as they related to chewing or swallowing; nonetheless, the methodologic aspects of the research were strongly related to future studies of the oral and pharyngeal stages of deglutition.

Similarly, much of the early work relating to oral function was also performed by those in the area of speech science. Before long, dental researchers began to include EMG techniques into their work as well.48, 49, 50, 51, 52 A PubMed ( search suggested that even though EMG is used for human and animal studies of the esophagus, it is used less often for study of the esophageal stage of deglutition than for oral and pharyngeal stages of swallowing.

Electromyographic recordings from the submental region are often performed with surface electrodes. There are many publications in which surface electrodes were used, both properly and improperly. Therefore, the reader must use care in accepting the validity of reported data.


Anatomic Sites of Interest for Electromyographic Studies of Deglutition

Clinically, it is important to be able to interpret physiologic information on the anatomic sites that are important to the three stages of deglutition.53 The sites54 that are located above the upper esophageal sphincter are of relevance to the safe passage of a bolus during the oral and pharyngeal stages of deglutition. The locations of particular interest to effective and safe passage include the lips, muscles of the submental region, tongue, soft palate, hyoid bone, pharyngeal constrictors, vallecular spaces, epiglottis, thyroid cartilage, false and true vocal folds, arytenoid cartilages, pyriform sinuses, and upper esophageal sphincter.

Simultaneous EMG recordings from five sites, including submental, pharyngeal, and laryngeal musculature, has indicated that the sequence of a normal swallow is preceded by contraction of the submental musculature followed by contraction of the superior pharyngeal constrictor, and then relaxation of the cricopharyngeus (CP) muscle. During the period of CP relaxation, both the thyroarytenoid muscles and the interarytenoid muscles contract, but EMG activity in the CP redevelops before the interarytenoid muscles relax. With a bolus as small as 10 mL, the thyroarytenoid muscle also continues to contract until after the CP has begun to close.55 Thus as one of several safety features, the CP segment will have separated the esophagus from the pharynx before the airway opens fully. Raw EMG data from these five muscles are displayed in Figures 4 and 5. Figure 4 shows an EMG recording from the superior pharyngeal constrictor, thyroarytenoid, interarytenoid, and submental muscles. Figure 5 shows EMG recording of two swallows from the superior pharyngeal constrictor, thyroarytenoid, cricopharyngeus, and submental muscles.

An interesting review of brainstem mechanisms underlying the generation of sequential and rhythmic swallowing movements has been published.56 It analyzes the neuronal circuitry, the cellular properties of neurons, and the neurotransmitters possibly involved, as well as the peripheral and central inputs that shape the output of the network appropriately so that the swallowing movements correspond to the bolus to be swallowed. The mechanisms possibly involved in pattern generation and the possible flexibility of the swallowing central pattern generator are discussed. This review complements the anatomic and neurophysiologic information provided elsewhere.53, 57

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