DISCUSIÓN

Electromyography evaluations of the masticator muscles during the maximum bite force

Mercedes González Hidalgo

Sección de Neuromuscular. Servicio de Neurofisiología Clínica. Instituto de Neurociencias. Hospital Clínico Universitario San Carlos. Madrid. España

Electromyography (EMG) is based on recording the muscular action potential or the potential of the motor unit, which is the sum of the electrical changes produced by the contraction of all the muscular fibers that compose the motor unit. It provides information on the comprehensive function of the entire motor system: upper and lower motor neuron, neuromuscular junction, and muscle. The action potential of the motor unit is obtained with a mild voluntary contraction of the muscle recorded with a concentric needle electrode inserted in the muscle under study.

This type of EMG produces recordings of high quality and specificity that allow the study of the characteristics of the motor unit potentials to identify different neurologic diseases that can affect any component of the motor unit (alpha motor neuron of the spinal anterior horn, its axon, and all the muscle fibers innervated by this axon that are activated almost simultaneously), neuromuscular junction, and muscle.

Surface EMG is performed with surface electrodes, generally self-adhesive, that are positioned on the skin covering the muscles. They explore a large area of muscle and are nonspecific, but they serve to assess the changes in the global electrical muscular activity at rest and maximum muscular contraction, being the result of voltage variations that occur in muscular fibers as the expression of cell membrane depolarization during contraction. Basically they evaluate the pattern of contraction and changes in amplitude.¹

The changes in amplitude most often studied, which are automatically analyzed in the EMG recording during maximum effort, are: peak-to-peak (amplitude P-P or P-P amplitude), which is the relation between the maximum amplitude and the minimum amplitude throughout the entire recording; the mean square root of voltage (RCM Voltage, RMS Voltage, or RMS), calculated on the EMG tracing; mean rectified voltage (MRV), which is the mean amplitude after wave rectification and is calculated for the entire EMG tracing, and the number of turns per second (turns/s), which represents the number of "peaks" (maximum positive-negative amplitude) in the waves of the tracing above 100 microvolts during one second of recording. It usually is a standardized numerical value.

This type of recording also allows the evaluation of the movements resulting from muscular action per se. The EMG can be used to observe and record functional disturbances not recognizable by clinical observation.²

The normal muscle at rest is electrically silent and is activated progressively in relation to the voluntary muscular contraction.

Electromyographic studies have been used by different authors in recent years, not only to obtain knowledge of mandibular mechanics:^3-6 studies centered on activity at rest, activity during maximum effort, muscular balance and muscular symmetry. EMG has also been used to obtain knowledge of the alterations produced by that numerous craniomandibular syndromes of muscular origin:^7,8 muscular hyperactivity or hypoactivity, reflex muscular activity, or parafunctional activity. Standardization of the examination protocols and values of normality would be useful for purposes of comparing the results obtained by different authors.

Numerous muscles participate in mastication:^3,4 temporal, masseter, pterygoid, digastric, myohyoid, and geniohyoid. However, the muscles that have been studied in these EMG studies have been mainly the temporal and masseter,⁹ because they are more accessible, larger, and easily located. ¹⁰

The temporal muscle is located by placing the electrode two centimeters above the zygomatic arc and two centimeters posterior to the eye commissure. For the masseter muscle, the electrode is placed one centimeter below the jaw angle, although the masseter muscle is delimited easily by palpation of the muscular contraction produced when gritting the teeth. The two muscles are innervated by branches of the anterior trunk of the mandibular nerve issued by the third division of the trigeminal nerve (motor branch).

The authors made a prospective study in 17 healthy adult subjects of mandibular biomechanics by electromyographic analysis of the masseter muscles and the anterior region of the temporal muscle on both sides, left and right, during maximum bite effort.

Simultaneous polygraphic studies were made of the masseter and temporal muscles by surface EMG. Amplitude variations and the pattern of muscular activation were evaluated during the maximum bite effort, determined by means of a pressure transducer inserted in a rubber tube, with the sensor connected to the EMG recording system. The rubber tube is placed between the upper and lower molars on both sides. This effort is repeated three times consecutively, after a warm-up period, for a duration of 5 seconds and at 1-minute intervals.

The placement of the pressure transducer is an interesting and novel aspect due to the reproducibility of the maximum effort and the homogenization of force between different subjects.

The present study analyzes the variability of the muscular activity, demonstrating greater participation of the masseter muscles during the maximum bite and more muscular activation than in the temporal muscles. These results coincide with those obtained by other authors^3-5 in previous studies. This asymmetry is smaller at rest than during maximum opening.

As the authors of the study indicate, there is a certain asymmetry in the muscular activation of the masseter muscles, with predominance in the activation of the right muscle. This probably is related with hemispheric dominance and should be considered as normal, as has been pointed out by other authors.¹¹

This study evidenced more force in the masseter muscles of men, which the authors attributed to muscle size, although this gender-related asymmetry has not been observed in previous studies.⁶

Therefore, in confirmation of the conclusions of the authors and others,^5,7-9 it seems that the muscular electrical activity recorded by surface EMG provides a useful and reproducible functional guide to mandibular mechanics in healthy subjects. EMG can assist in understanding variations in muscular behavior⁸ in patients with prostheses, malocclusion, craniomandibular syndromes or muscular diseases.

References

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