USUAL CHEW AND ELETROMYOGRAPHIC ACTIVITY OF THE MASSETER AND TEMPORALIS MUSCLES IN SCHOOL CHILDREN FROM 7 TO 12 YEARS Mastigação habitual e atividade eletromiográfica dos músculos masseter e temporal em escolares de 7 a 12 anos

(1) Universidade Federal da Bahia UFBA, Salvador, BA, Brasil. (2) Instituto de Ciências da Saúde da Universidade Federal da Bahia UFBA, Salvador, BA, Brasil. (3) União Metropolitana de Educação e Cultura, Salvador, BA, Brasil. Conflict of interest: non-existent stimulation of supporting structures, such as bones and teeth and for favoring that the harmonious craniofacial growth occurs3 . The chewing pattern can be influenced by several factors, such as occlusal interferences, dental and/ or periodontal problems, tooth loss, muscle and/or temporomandibular joints problems2. It is known that in a unilateral mastication, the chewing muscles are mainly characterized by increased muscle strength on the working side, in other words, where chewing is occurring. The muscles of the balancing side, opposite side to the working side, is generally more elongated and with  NTRODUCTION

DMF11 and RDC protocol -Research Diagnostic Criteria -Criteria for Diagnosis in research 12 .
The acquisition of the electrical activity of the masseter and temporalis masticatory muscles was recorded during 5 seconds of maximum voluntary isometric contraction in maximal intercuspal position through MIOTEC equipment with software MIOTOOL 400 4-channel filter of 500 Hz Low Pass and High Pass of 20 Hz.
For the exam, was made a prior to cleansing the skin of the students with 70% alcohol, in order to reduce the impedance.The procedure also facilitates the adhesion and uptake of electric potentials from muscle contraction.
The electrodes were placed in the most massive region of the anterior temporalis and masseter muscles by palpation in these regions during voluntary isometric contraction in maximal intercuspal position.The individual was kept sitting with open eyes, in a vertical backrest chair, with the head lined as the Frankfurt Horizontal Plane, keeping upper body relaxed on the lower limbs and feet flat on the floor, on a rug rubber.
The children were instructed to clench tightly the teeth for 5 seconds, contracting the masticatory muscles bilaterally and simultaneously.This procedure was performed 3 times with intervals of 30 seconds.
Then underwent occlusal evaluation, conducted by an orthodontist, based on the DAI -Dental Aesthetic Index 13 , and also of the mastication record procedures, based on the adapted protocol MBGR 14 through a Sony digital camera 12x mp, with french bread used as food.
Before the start of mastication, the children were instructed to remain seated and looking ahead.They were handed with a piece of bread that could enable them biting and taking a piece.The request was: "You will chew and swallow the bread as you normally eat at home".
For analysis of electromyographic data, we adopted the average of the second and third records of the amplitude of the signal provided by the software in Root Mean Square (RMS).The first record was disregarded for analysis, once used as an understanding of the guidance provided before the exam test.The EMG data were normalized, taking as parameter to calculate the proportions of the maximum amplitude average (peak) obtained during the records.
Mastication was classified as unilateral preferential when observed a frequency of more than 60% of the cycles performed in a single side; bilateral when the cycles were run of 50 to 60 % of a side 4 .In relation of this latter mastication type, was considered alternating bilateral chewing when there reduced functional tonus, which can be a visually discernible muscle or even bone asymmetry 2,3 .
The assessment of functional asymmetry of the craniofacial complex usually involves the jaw movement pattern and the activity of masticatory muscles².The pattern of muscle pairs contraction can be investigated using electromyography of surface (EMG) -objective examination, capable to record the action potentials that occur by means of voluntary muscle activation or response to stimulation 5 .
In the Speech Therapy area, the use of electromyography of surface are recent and aim to aid in the diagnosis of oral motor disorders, changes in swallowing, chewing and speech 6 .
The masticatory pattern, as well as the electrical activity of masseter and temporalis masticatory muscles has been approached in the literature [7][8][9][10] .However, studies with children aged between 7 and 12 years are rare, especially with regard to the association between usual chewing and muscular electrical activity.
Thus, the analysis of the correlation between electromyographic activity of the masseter and temporalis muscles (during voluntary isometric contraction) and the preferred chewing side in children contributes to a better understanding of neuromuscular functioning of the craniofacial complex, for the functional differential diagnosis, and mainly to the therapeutic referring related to the balance of the stomatognathic system.
The aim of this study was to investigate the relationship of the rather side of masticatory with the electrical activity of the masseter and temporalis muscles.

METHODS
The study was approved by the Ethics Committee in Research of the College of Odontology of Federal University of Bahia (UFBa) by Opinion Number 17/10.This is a cross-sectional descriptive and analytical study.The study was developed with 115 children, of both genders, aged 7-12 years, enrolled in regular public schools in São Francisco do Conde, Bahia.The guardians signed, agreeing to the "Terms of Free and Clear Consent".
Children with histories of facial trauma, head and neck surgery, clinical diagnosis of neurological disorders, craniofacial malformations, signs and symptoms of temporomandibular disorders (TMD) were excluded from the study.
To identify the signs and symptoms of TMD, was used as instrument the Anamnesic Index Simplified

RESULTS
After examination of the masticatory preference side, we observed that of the total of 115 children, 76 (65.5%) had a preferred side during mastication, whereas 44 (38.3%) children showed preference masticatory to the right side and 32 (27.8%) preferred left chewing.Of the individuals without a preferential side, there was 23.5% (n = 27) alternating with bilateral chewing and 10.4% (n = 12) with simultaneous bilateral chewing.
According to the masticatory preference observed, the children were divided into four groups: Left Preferred Chewing Group (LPCG) Right Preferred Chewing Group (RPCG), Bilateral Simultaneous Chewing Group (BSCG) and Bilateral Alternating Chewing Group (BACG).The demographic data on gender, age and occlusion are described in Table 1.
was presence of lateral and vertical movement of the mandible and simultaneous bilateral chewing when a predominance of vertical movements 15 .
The collected data were properly analyzed using SPSS statistical software 17.0.The paired T' Student's test was adopted to verify the difference of normalized EMG averages ( % ) between the masseter and temporalid muscles pairs of left and right sides of the face.To verify the relationship between the type of occlusion and electrical activity of the muscles studied, as well as the electrical activity and masticatory pattern, the ANOVA test was used and the association between the variables "type of Occlusion" and "masticatory pattern" was analyzed using the Chi-square test.In all tests was considered p < 0.05 for statistical significance.levels of electrical activity of the muscles of the left side showed no statistically significant differences when compared with the average of its peers on the right side in all groups, p < 0.05 (Table 2).
According to each chewing group, comparisons between the electrical activity of the right and left masseter and right and left temporal muscles were performed.From this analysis it was found that the No statistically significant relationship (p < 0.05) between the electrical activity of the muscles studied and the type of occlusion was found in the sample studied.Records the electrical activity of masseter and temporal muscles, according to the occlusion and gender, were shown as Figure 1.When analyzing the association between the results of occlusion with electrical activity, also not statistically significant relationship was found.This result is similar to the study of Bevilaqua -Grossi et al. 20 , which aimed to compare the electrical activity of the masticatory muscles during mastication in children with crossbite and children with normal occlusion.On the other hand, the research of Rodrigues, Bérzin and Siqueira 21 found no significant difference in electromyographic activity in different types of occlusion with children aged between 7 and 9 years with posterior crossbite.
The characterization of the results regarding masticatory preference, showed that of 115 children, 76 (65.5%) had a preferred side during mastication, whereas 44 (38.3%) children showed preference to right masticatory and 32 (27,8%) to left preferred

DISCUSSION
Occlusion variable was not statistically different with the myoelectric activity of the masseter and temporal, bilaterally, as well as the chewing pattern.Thus, subjects were divided into four groups according to the preferred chewing side.
In this sample, the type of occlusion seems not to interfere as the chewing pattern.Findings agree with studies of Cavalcanti and Bianchini 16 and Duarte 17 , who also found no significant correlation between occlusion and the preferred chewing side.However, is quoted in the literature the influence of occlusal factors in masticatory pattern mainly involving the unilateral occlusal pattern with changes and temporomandibular dysfunction 18,19 .When analyzed the chewing pattern and occlusion type, for the studied sample, there was no relationship between the variables (Table 3).
The analysis of the relationship between the electrical activity of muscles and chewing pattern surveyed also showed no statistical significance, p < 0.05 (Table 4).
to the study of Ferla, Silva and Corrêa 28 , in which the pattern of electrical activity (during usual and maximal intercuspal chewing) of the anterior temporal and masseter muscles in children aged between 8 and 12 years old, with mouth breathing, proved to be lower than in nose breathers children, but statistically significant only in the left temporal muscle.
As previously shown, many studies 1,20,21,26,28 use electromyography as a tool for analysis of electrical activity of the masticatory muscles with different objectives.However, in the consulted literature no studies that assessed the relationship between the electrical activity of the masticatory muscles in isometric contraction and chewing pattern were found.
The results achieved in this study are consistent with the concept that electromyography effort evaluates the ability of the muscles to recruit as many motor units during maximal voluntary contraction.However, it is not possible to make a direct link between electromyographic findings and muscle strength, since muscle forces can be calculated directly through invasive procedures 23, 29 .The impracticality of this relationship due to possible variations in muscle strength due to changes in motor drive and speed of contraction without changing the amplitude of the eletromyographic signal 30,31 .This amplitude in turn, can be modulated by changes in the shape and speed of propagation of action potentials, without a concomitant change in the muscular strength 32,33 .
This neuromuscular fatigue, for example, is described in the literature as a factor that induces a decrease in the speed of conduction of the potential action, an increase in loss of strength and synchronization can be compensated by recruitment of additional motor units 23,31,34,35 .These phenomena may induce an increase in the amplitude of the signal while the strength remains constant 36 .
Although studies 29,37,38 try to find solutions featuring new methodological developments for this limitation, it is not yet possible to make a direct relationship between electromyographic findings and muscle strength.

CONCLUSION
From the results obtained, there was no relationship between habitual chewing pattern and the electrical activity of muscles, masseter and temporalis, according to the methodology adopted.
chewing.These results support the findings of previous studies, which report a high occurrence of mastication performed preferably on one of the sides 17,22 .
Skeletal muscle is composed of several fibers together with the neuron comprises the terminal motor unit 23 .The value provided by the EMG is the sum of the action potentials of all the muscle fibers of the motor unit, which come together in activity, because they are innervated by the same motor neuron 23 .
The motor unit is the functional unit investigated by EMG in this study and the electrical signal provided by electromyography, faced the type of habitual mastication.
It is believed that making prolonged use of chewing preferably by one of the right and left sides, the musculature of the side of work becomes stronger, whereas the balancing side, the muscles are more elongated and with reduced tonus, showing often visible muscle asymmetry 3 .Thus, also in a state of isometric contraction through maximal intercuspal, as in this study, it would be possible to observe such features.
However, when analyzing the results, the likely physiological changes of masticatory muscles influenced by the pattern of chewing function were not observed on electromyographic signal amplitude.That means that the average of the electrical activity of the muscles of the left side showed no statistically significant differences when compared with the average of its peers on the right side in all groups studied, showing no relationship with the chewing pattern.
These results corroborate studies of Trawitzki et al. 24 and Alarcon et al. 25 that when analyzing the EMG activity of the temporal and masseter muscles in children with posterior crossbite, did not find significant differences in EMG activity of the masseter and temporalis muscles between crossed and uncrossed sides during mastication.
The research of Miranda, Vieira and Bommarito 26 and Vianna-Lara et al. 27 in order to examine the electromyographic activity of the masseter muscle in different facial types during rest and chewing, also found no statistically significant difference in the relationship.
Differing from the present study, the analysis of Faria et al. 10

Table 2 -Eletromyographic activity normalized (%) of the right and left masseter and temporalis muscles. (N=115)
The eletromyographic activity is shown in average (standard deviation).The determination of the statistical difference between the electric activity of the right and left sides of temporalis and masseter muscles was performed by T Student Test Paired.It was considerate significant p<0,05.RM -Right Masseter LM -Left Masseter RT -Right Temporalis LT -Left Temporalis LPCG -Left Preferred Chewing Group RPCG -Right Preferred Chewing Group BSCG -Bilateral Simultaneously Chewing Group BACG -Bilateral Alternating Chewing Group

Table 3 -Association between the masticatory pattern and type of occlusion (N=115)
The association between the variables Masticatory Pattern and Type of Occlusion was checked through Chi-Square test, considering p<0,05 to statistical significance.