temporomandibular disorder ( tmd ) is related to clinical conditions that involve the 
 mastication muscles , temporomandibular joint and associated structures1 .
studies indicate a high prevalence rate of this disorder 
 among children and adolescents2,3,4,5,6,7 . therefore , multidisciplinary analysis directed toward adequate 
 treatment for such cases is needed .
the occurrence of parafunctional habits , such as bruxism ( teeth clenching / grinding ) , and 
 changes in occlusal contacts are important factors in the development of this disorder2 , 5 , 8 , 9 . 

moreover , emotional factors also play an important role in the occurrence of tmd , especially 
 in adolescence10 .
surface electromyography ( emg ) is a widely used in the evaluation and follow up of 
 individuals with tmd , and it allows the observation of the electrophysiological behavior of 
 muscles under different physiological conditions12,13,14,15,16 .
the frequency of the emg signal represents the excitation rate of 
 the muscle cells .
this frequency is affected by the composition of the muscle and the 
 characteristics of the action potential of active nerve fibers17 .
the evaluation of the mean and median frequencies ( mnf and mdf , respectively ) of the emg 
 signal enables analysis of the temporal spectrum of the muscle , the content of the muscle 
 fibers and valuable information for the diagnosis and treatment of individuals with tmd16 , 18 .
previous reports have demonstrated that individuals with tmd exhibit 
 alterations in the electrical activity of the mastication muscles either as a result of the 
 disorder itself or from a compensatory mechanism associated with the symptoms14 , 15 , 19 . considering the joint occurrence of tmd and stressors
commonly found in adolescence , it is 
 reasonable to hypothesize that teenagers develop important alterations in the mastication 
 muscles .
the investigation of electromyographic characteristics of adolescents with tmd is 
 essential for the early determination of problems that predispose such individuals to pain 
 and muscle / joint dysfunction in adulthood .
thus , the aim of the present study was to assess 
 the behavior of the mean and median frequencies of the emg signal of the mastication muscles 
 of adolescents with different degrees of tmd severity with the mandible at rest and during 
 maximum clenching effort .
eighty - one male and female adolescents between 14 and 18  years of age enrolled at the jos 
 de paiva neto education institute , sao paulo , brazil , were evaluated .
the inclusion criteria 
 were having all four permanent first molars erupted and a statement of informed consent 
 signed by the participant or his / her guardian .
the exclusion criteria were current medical 
 or psychological treatment , dento - facial anomalies , history of surgery or trauma in the 
 region of the temporomandibular joint and a history of orthopedic or orthodontic treatment 
 of the jaws .
this study was conducted in compliance with the principles of the declaration of helsinki 
 and received approval from the human research ethics committee of nove de julho university , 
 protocol number 332780 .
sample size was calculated based on a pilot study using the diman 1.0 sample calculation 
 program and the root mean square ( rms ) of the electromyographic signals of the masseter 
 muscles and anterior bundles of the temporal muscles , with =0.05 , =0.20 and 80% test 
 power .
a minimum of 11 participants in each group was determined . to provide a margin of 
 safety , 14 individuals in each group ( control , mild tmd and moderate / severe tmd )
while the research diagnostic 
 criteria for temporomandibular disorders ( rdc / tmd ) is better for the classification of tmd , 
 the helkimo index was employed due to the fact that it is more easily understood by 
 individuals of the age group studied in the present investigation . moreover , the helkimo 
 index has been frequently employed for the diagnosis of tmd20,21,22,23 .
this index classifies 
 the individual in severity categories based on clinical signs of tmd , and is divided into 
 five subscales addressing limitations of mandibular range of motion , limitations of 
 temporomandibular joint function , pain in the mastication muscles , pain in the 
 temporomandibular joint and pain during mandibular movements24 . on a specific chart ,
data were recorded from a clinical examination involving extra - oral 
 and intra - oral inspection of the teeth , type of occlusion and occlusal alterations as well 
 as palpation of the mastication muscles and temporomandibular joint .
the analysis of 
 mandibular movements involved observation of the joint noises , headaches , facial pain , 
 difficulty with chewing , breathing pattern and parafunctional habits . based on the criteria 
 of the helkimo index , each item received a score of 0 , 1 or 5 .
the sum of the item scores 
 allows classification of the individual into one of four categories : no symptoms of tmd ( 0 
 points ) , mild symptoms of tmd ( 1 to 4 points ) , moderate symptoms of tmd ( 5 to 9 points ) and 
 severe symptoms of tmd ( 10 to 25 points)24 . in the present study ,
the participants were classified into the 
 following three groups : control group ( cg  without tmd ) ; mild tmd group ( mg ) ; and moderate 
 to severe tmd group ( msg ) .
emg signals were obtained using an eight - channel module ( emg system do brasil 
 ltda ) , consisting of a conditioner with a band pass filter with cut - off 
 frequencies of 20 and1,000  hz , an amplifier gain of 1,000 and a common mode rejection ratio 
 > 120 db . all data were acquired and processed using a 16-bit analog to digital converter 
 ( emg system do brasil ltda ) , with a sampling frequency of 2 khz .
the system was 
 composed of active bipolar electrodes with a pre - amplification gain of 20  .
the participants were instructed to remain seated in a chair , feet apart , shoulders relaxed 
 and hands resting on their thighs , with their heads on the frankfurt parallel to the ground 
 and no visual feedback of the signals registered on the computer .
the attachment sites for 
 the electrodes were cleaned with a cotton ball soaked in alcohol to diminish the impedance 
 between the skin and electrodes .
disposable ag / agcl surface electrodes measuring 10  mm in 
 diameter were attached to the belly of the muscle in the region with the greatest tonus 
 after the volunteer performed moderate intercuspation .
bandage tape was used to secure the 
 electrodes further , with care taken to avoid micro movements .
the inter - electrode distance 
 was 20  mm from center to center , as suggested by seniam ( european recommendations for 
 surface electromyography)25 . a 
 rectangular metallic electrode measuring 3  cm by 2  cm coated with lectron ii conductive gel 
 ( pharmaceutical innovations ) to increase the conduction capacity and impede 
 interference from external noise was attached to the left wrist of the volunteers for 
 reference .
the muscles recorded with surface emg were the right and left masseter and right and left 
 anterior temporal bundle .
the evaluations were carried out at rest and the maximum clenching 
 effort ( mce ) . under each condition , the signals were recorded three times for 10 seconds 
 each . to avoid direct occlusal contact between the dental cusps , a strip of parafilm 
 m ( american national can tm , chicago , usa ) was folded five times to create a 
 texture and dimensions similar to commercial chewing gum and arranged bilaterally in the 
 molar region26 .
initially , three readings were performed in the rest position with a two - minute interval 
 between readings . after 3 minutes
, three readings were performed in mce , with a ten - minute 
 interval between readings . in all tests ,
a five - second period was selected ( the two initial and three final seconds of 
 the emg signal were discarded ) . in the frequency domain analysis ,
2,048-point running fast 
 fourier transforms with a resolution of 10  hz were computed with a 50% overlap ( hamming 
 window processing ) .
the mean and median frequencies ( mnf and mdf , respectively ) of the power 
 spectrum were calculated for a one - second window .
all emg signal processing was performed 
 using specific routines carried out in the matlab program , version 7.1 ( the mathworks inc . , 
 natick , massachusetts , usa ) .
the shapiro - wilk test revealed that the mnf and mdf were not normally distributed 
 ( p<0.05 ) .
consequently , medians and inter - quartile ( 25% and 75% ) ranges were calculated 
 and the non - parametric kruskal - wallis test and dunn 's post hoc test were used to compare 
 differences in the masseter and temporal muscles among the cg , mg and msg during maximum 
 clenching effort and at rest .
spearman 's correlation coefficients ( r ) were 
 calculated for the determination of correlations between mnf and mdf values .
all data were analyzed using the 
 statistical package for the social sciences ( spss ) version 17 .
the sample was made up of 14 individuals without tmd ( mean age : 14.92  0.95  years ) , 14 
 with mild tmd ( mean age : 15.28  0.82  years ) , 12 with moderate tmd ( mean age : 15.25  
 0.62  years ) and two with severe tmd ( both aged 15  years ) .
asterisks indicates significant differences between groups ( p < 0.05 ) table 1 displays the mnf and mdf of the 
 temporal and masseter muscles at rest and during maximum clenching in the cg , mg and msg . 

significant differences ( p<0.05 ) were found in all comparisons of the mnf and mdf among 
 the cg , mg and msg during maximum clenching effort .
however , the post hoc test revealed no 
 significant differences in mnf and mdf among groups for the right masseter muscle .
greater differences among groups were found under the condition of mandibular rest 
 ( p<0.05 ) .
the highest mnf and mdf values were found in the group with moderate to 
 severe tmd .
asterisks indicates significant differences between groups ( p < 0.05 ) table 1 displays the mnf and mdf of the 
 temporal and masseter muscles at rest and during maximum clenching in the cg , mg and msg . 

significant differences ( p<0.05 ) were found in all comparisons of the mnf and mdf among 
 the cg , mg and msg during maximum clenching effort .
however , the post hoc test revealed no 
 significant differences in mnf and mdf among groups for the right masseter muscle .
greater differences among groups were found under the condition of mandibular rest 
 ( p<0.05 ) .
the highest mnf and mdf values were found in the group with moderate to 
 severe tmd .
individuals with tmd exhibit muscle dysfunctions that can be progressive and have a variety 
 of etiological factors2 , 5 , 8 , 9 . in adolescence
, occlusal interferences can cause a functional 
 deviation in the mandible , which can lead to serious craniocervical disorders27 , 28 .
frequent harmful oral habits can lead to alterations in the 
 biomechanics of the temporomandibular joint and an increase in tension in the mastication 
 muscles2 , 5 , 
 8 , 9 , with the consequent retention of fluids in the muscle tissue , 
 reduced blood flow and the accumulation of metabolic products29 .
thus , the evaluation of electromyographic activity in the 
 mastication muscles and related occlusal factors is of fundamental importance in comparisons 
 among individuals and the evaluation of signs and symptoms of tmd .
surface emg is one of the main tools employed in the diagnosis of tmd , the analysis of 
 muscle performance , and the detection of physiopathological conditions .
previous studies of 
 adult individuals have shown that individuals with tmd exhibit greater asymmetry between the 
 masseter and temporal muscles , as well as hyperfunction of the temporal muscle12 , 16 , 18 .
knowledge of the electromyographic pattern of the mastication muscles of adolescents with 
 tmd helps in the formulation of treatment strategies aimed at normalizing muscle activity 
 and improving the function of the stomatognathic system .
previous studies have demonstrated 
 the benefits of the use of a bite plate and massage therapy30,31,32,33 .
other treatment options , 
 such as low - level laser therapy , have also demonstrated satisfactory results34 , 35 . in the present study , significant differences were found in the electromyographic 
 characteristics ( mdf and mnf ) among the gc , mg and msg .
these findings suggest muscle 
 hyperactivity in adolescents with tmd even when the mandible is at rest . this behavior may 
 be explained by the need for greater muscle recruitment in individuals with tmd and 
 orofacial pain when the mandible is at rest36 , 37 .
greater differences among groups were found under the condition of mandibular rest 
 ( p<0.001 ) .
minimal electromyographic activity of the mastication muscles in the resting 
 position is found in individuals without tmd , indicating a balance between elevator and 
 depressor muscles of the mandible38 , 39 . in the present study ,
significant 
 differences in mnf and mdf were found between adolescents with signs and symptoms of tmd and 
 the control group .
this finding may be explained by the increase in muscle tension resulting 
 from the frequency of parafunctional habits .
previous studies have indicated that oclusal 
 alterations and different facial pattern influence the electrical activity of the 
 musculature40 , 41 .
the literature also reports that hyperactivity of the mastication 
 muscles may be related to teeth clenching ( bruxism ) , which is a common occurrence in 
 adolescents with tmd1 , 4 .
future studies addressing fatigue in these muscles among 
 adolescents with tmd , may offer physiological data to explain these findings . in conclusion , the mastication muscles of adolescents with tmd , especially those with a 
 greater severity of signs and symptoms , exhibit higher mnf and mdf of the electromyographic 
 signal , indicating hyperactivity of these muscles while at rest and during maximum 
 intercuspation .
   [ purpose ] the aim of the present study was to assess the behavior of the mean and median 
 frequencies of the electromyography signal of the mastication muscles of adolescents with 
 different degrees of tmd severity . 
 [ subjects ] forty - two adolescents aged 14 to 18  years . 
 [ methods ] 
 the adolescents were classified according to severity using the helkimo index . 
 
 the control group consisted of 14 subjects with no signs or symptoms of tmd . 
 three 
 readings were taken in during maximum intercuspation and mandibular rest , with each 
 reading lasting 10 seconds . 
 [ results ] significant differences ( p=0.0001 ) were found in the 
 mean frequency ( hz ) between the control group ( cg ) , mild tmd group ( mg ) and 
 moderate / severe tmd group ( msg ) , especially during mandibular rest , for all muscles 
 evaluated : right temporal : cg ( 137.5 ) , mg ( 194.2 ) , msg ( 291.7 ) ; left temporal : cg ( 106.9 ) , 
 mg ( 200.6 ) , msg ( 294.2 ) ; right masseter : cg ( 155.7 ) , mg ( 242.8 ) , msg ( 278.3 ) ; left 
 masseter : cg ( 125.0 ) , mg ( 214.6 ) , msg ( 316.7 ) . 
 greater differences among groups were found 
 under the condition of mandibular rest . 
 conclusions : adolescents with tmd especially those 
 with more severe symptoms exhibit hyperactivity of the mastication muscles . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
EMG signal
DISCUSSION