Keywords
masticatory muscles - muscle fatigue - electromyography - mouth breathing - stomatognathic
system
Introduction
Little is known so far about the contraction patterns of the facial and masticatory
muscles, either in normal or unbalanced conditions.[1] In the case of mouth-breathers (MBs), the discussions mainly approach the clinical
characteristics of the craniofacial, stomatognathic, and body adaptations, which even
affect the quality of life of individuals.[2]
[3]
[4]
Changes in the breathing mode are known to lead to various changes in the harmony
and growth of the craniofacial structures, which result from compensations to make
airflow easier. These changes include mandible posture and occlusal changes and influences
on the facial growth pattern.[5]
[6]
[7]
[8]
[9]
Some studies have shown that MBs typically have an elongated face,[5] while others have found mesofacial[10]
[11] and brachyfacial growth patterns[12] as a characteristic of this population. All these imbalances may potentialize speech,[13] swallowing,[2]
[9] and mastication changes.[9]
[14]
[15]
Regarding mastication, specifically, mouth breathing may impair its efficiency. Because
of the need to breathe through the mouth, mastication is interrupted, and the person
takes longer to finish the masticatory movements. Changes in lip posture and masticatory
muscle action (which is often underused) also appear when eating.[9]
[14]
[15] However, the literature disagrees whether the masseter and temporal muscles are
hypofunctional in MBs,[14]
[16] and little is known about the extent to which this musculature can be required while
maintaining an efficient motor performance.[17] Hence, researching this muscle fatigue can help understand this issue.
Muscle fatigue, which is a natural muscle mechanism, occurs when it is incapable of
maintaining high force levels over time.[17]
[18]
[19]
[20]
[21] The fatigue depends on the type, duration, and intensity of the exercise; the muscle
fiber type; the training level of the individual; and the environmental conditions
where the exercise is performed.[5] This variable can be analyzed through electrical activity patterns, mainly obtained
with surface electromyography (EMG) by means of isometric contractions, which causes
this phenomenon more easily[1]
[22]
[23]–although usual isotonic situations are also investigated.[1]
[22]
[23] Isotonicity nears the usual training of the masticatory muscles,[24]
[25] which is why the present study seeks to investigate this situation more in depth.
One of the ways to measure muscle fatigue is the analysis of the EMG frequency spectrum,
more specifically the median frequency (MF), which is an objective measure of the
muscle fatigue process.[26] Much research approaching different pathologies has studied the muscle fatigue threshold.
Using MF enables the verification of muscle susceptibility to induced physiological
fatigue – that is, the desired force production momentum can no longer be maintained,
and contractile fatigue is observed.[21]
[26]
Surface EMG is an important tool in the objective analysis of facial and masticatory
muscle activity. Various methodologies are used to understand the signs and symptoms
of muscle fatigue, making the analysis between results more difficult and, consequently,
hindering the definition of parameters to choose orofacial muscle training exercises.[27]
Thus, the objective of the present research is to study the fatigue, during mastication,
of the masseter and anterior temporal muscles of nasal-breathing (NB) and MB children,
also considering their facial growth patterns.
Methods
Sample
The present cross-sectional study encompassed 70 children. Of these, 36 were NBs (21
girls and 15 boys) and 34 were MBs (13 girls and 21 boys), aged 6 years and 0 months
to 12 years and 11 months old (mean NB = 9.6 years ± 22 months old; mean MB = 8.9
years ± 21 months old). The inclusion criteria were as follows: presenting agreement
between the speech-language-hearing and otorhinolaryngological diagnoses for MB and
NB; having the permanent upper first molars already erupted; and having body mass
index (BMI) within the normal range for the age.[28] The exclusion criteria were as follows: having a history of speech-language-hearing
and/or orthodontic treatment; missing more than three teeth; presenting with signs
suggestive of pathological bruxism; having craniofacial syndromes or malformations;
and having a neuromuscular impairment. Sex and age homogeneity between the groups
was tested with the chi-squared test; no statistical differences between them were
found (respectively, p = 0.05 and p = 0.17). They were divided into three age ranges to group children with structural
similarities.
After obtaining this sample, the sample calculation was made, based on Callegari-Jacques,[29] considering the highest standard deviation found (36.18), 5% significance level,
and 15-Hz sample error. The resulting minimum sample size was 23 subjects in each
group, which had already been obtained.
The age range of the study participants was defined considering that it is potentially
difficult to submit children < 6 years old to EMG assessment and that the first molars
erupt at 6 or 7 years old. The BMI of the children was also delimited between 5 and
85 percent, considering that larger fat layers under the skin may interfere with the
EMG signal pickup.[18]
[28]
[30]
Mouth breathing was diagnosed with the agreement between the speech-language-hearing
and otorhinolaryngologic assessments; if they did not agree, the subject was excluded.
The speech-language-hearing assessment was based on the MBGR Protocol,[31] with information on breathing, occlusion, other treatments, and signs suggestive
of craniofacial syndromes or neuromuscular impairment. The otorhinolaryngologic assessment
investigated breathing changes and, as performed by Berwig et al.[32] and Ritzel et al.,[33] encompassed oroscopy, anterior rhinoscopy, and otoscopy, followed by fiberoptic
nasopharyngoscopy, when necessary. When cephalometry was enough to determine the degree
of pharyngeal tonsil hypertrophy, the fiberoptic nasopharyngoscopy was not performed.
After this assessment, the children were divided into NB (nasal breathing mode, without
signs and symptoms of daytime and/or nighttime mouth breathing) and MB (oronasal or
mouth breathing mode, with at least three signs and symptoms of daytime and/or nighttime
mouth breathing, such as open mouth/open lips, dry lips, infraorbital dark circles,
sagging/drooping face, among others). Of the 34 MB children, 15 were diagnosed from
symptoms related to mouth breathing and 17 underwent cephalometry or fiberoptic nasopharyngoscopy,
being classified as grades I, II or III of obstruction.
The children were also grouped according to their facial growth patterns. This diagnosis
was based on Ricketts cephalometry analysis, performed with lateral teleradiography,
using 18 × 24 cm Kodak film and a cephalostat to standardize the head position in
ray emission, at a distance of 1.5 meters. The VERT index[34] was calculated, determining the following facial types: brachyfacial (index value
> + 0.5), mesofacial (index value between - 0.5 and + 0.5), and dolichofacial (index
value < - 0.5).
Hence, the groups were initially formed with NB and MB children; they were afterward
subdivided into brachyfacial (Br), mesofacial (Me), and dolichofacial (Do), totaling
six groups.
All the children and their parents/guardians agreed to their participation in the
study and signed the informed consent form – which had been previously approved by
the Research Ethics Committee of the institution under approval protocol number 08105512.0.0000.5346.
Electromyography
The EMG signals were picked up with equipment available in the market – Miotool (Miotec
– Brazil), with 8 input channels, 14-bit A/D converter – and saved in a portable computer
not plugged into the electrical outlet. Active sensors with differential input (manufactured
by Miotec) were connected to Ag/AgCl double electrodes (Hal Indústria e Comércio ltda.),
placed on the belly of the right (RM) and left masseter muscles (LM) and of the right
anterior (RT) and left anterior temporal muscles (LT). To better locate the muscle
bellies, a function test was conducted with the isometric contraction of the mandibular
elevator muscles. These disc-shaped electrodes have a fixed 20-mm distance from each
other, 20x gain, 10 GΩ input impedance, and common-mode rejection rate > 100 dB.[35] To decrease skin impedance,[19] the sites where the electrodes would be positioned were cleaned with 70% ethyl alcohol
and cotton; if necessary, the hair in the region was removed.
The collection room was also treated, having its floor covered with paviflex rubber
flooring.[18] As a precaution, equipment that might interfere electromagnetically with the examination
was put aside and turned off. The reference electrode (connected to the ground wire)
was positioned on the glabela of the patient. The signal was picked up with 20- to
500-Hz filter and a maximum acquisition capacity of 2,000 samples/second/channel.
This assessment was performed always by the same researcher to avoid deviations and
differences in the collection procedure.
Fatigue Assessment Protocol
The children sat comfortably, hip, knees, and ankles flexed 90°, following the Frankfurt
plane. They were instructed on the examination procedures, collection room setting,
and equipment and were trained on the procedures before the collection.[12] Isotonicity (mastication) was used to test muscle fatigue, as this function is often
trained in clinical practice to strengthen the masticatory muscles.[22]
[24]
[25]
[36]
[37]
The mastication test was performed three times in sequence, with 2-minute intervals
in between them.[18] A digital 80-bpm metronome was used (Mendonça et al., 2005), as well as chewing
gum (Plic Ploc - Brazil) because it best resembles food without deteriorating or producing
residues that might interfere with the assessment. Two portions of chewing gum were
placed on the molars, one on the right and one on the left side of the arch. Initially,
the children were asked to chew the gum freely for 40 seconds, without removing it
from the sides, to diminish and standardize its resistance. After some rest, they
should chew rhythmically until they felt fatigued – that is, the first sign of discomfort
in this musculature.[22]
[36]
Electromyography Signal Analysis
This analysis was performed with the Miograph 2.0 software, for 60 seconds of activity
divided into four 15-second intervals (T1, T2, T3, and T4), encompassing ∼ 18 mastication
cycles. In these intervals, the signal MF was analyzed, considering the moments of
activation and inactivation of the cycle together. Of the three mastication collections,
the one with the best signal quality was selected (analyzed with the fast Fourier
transform [FFT]). Then, the initial 0.5 seconds were excluded to make the assessment
period homogeneous. The time reported by the children when they began to feel muscle
fatigue was analyzed by recording the moment when it occurred and then comparing them
later. The researcher was unaware of the group identification for record analysis.
Statistical Analysis
After testing the normality of the variables with the Shapiro-Wilk test, the repeated
measure analysis of variance (ANOVA) was conducted, with the Tukey post hoc test.
For the reported fatigue time, without normal distribution, the Mann-Whitney U test
and Kruskal-Wallis test were applied, according to the category of each group. The
analyses were made in Statistica 9.0 software, with the significance level set at
5% (p < 0.05).
Results
Electromyography Signal
The evolution of the MF of the masticatory muscles throughout the mastication, regardless
of the groups, is shown in [Table 1]. A statistical significance was found for RT (p < 0.01), LM (p = 0.018), and LT (p = 0.02). The post hoc analysis showed that this occurred mainly between 15 and 60 seconds
of activity. However, there was no defined MF decrease pattern.
Table 1
Distribution of means and standard deviations of the median frequency and statistical
analysis found in the tests of the masticatory muscles, regardless of the groups,
throughout the different collection moments (T1, T2, T3, and T4)
|
T1
|
T2
|
T3
|
T4
|
p-value
|
|
Mastication
|
RM
|
191.8
(19.5)
|
194.1
(19.6)
|
194.0
(18.1)
|
194.0
(18.9)
|
0.18
|
|
RT
|
176.7
(25.0)
|
181.6
(23.8)
|
180.8
(20.8)
|
182.3
(20.3)
|
< 0.01*
|
|
** T1 ≠ T2, T3, T4
|
|
LM
|
185.6
(22.4)
|
189.0
(20.6)
|
186.5
(22.9)
|
189.8
(20.4)
|
0.018*
|
|
** T1 ≠T4
|
|
LT
|
184.3
(25.9)
|
187.4
(26.4)
|
187.9
(23.4)
|
188.9
(22.4)
|
0.02*
|
|
** T1 ≠T4
|
Abbreviations: LM, left masseter; LT, left temporal; RM, right masseter; RT, right
temporal; T1, 15 seconds of activity; T2, 30 seconds of activity; T3, 45 seconds of
activity; T4, 60 seconds of activity.
*Significance with the repeated measure ANOVA test.
**Significance with Tukey post hoc, ≠ difference.
The MF of the masticatory muscles in interaction with the breathing mode ([Table 2]) revealed no MF continuous decrease. Only the LM and MB had statistical significance
(p < 0.05), although it referred to an MF increase, instead of a decrease.
Table 2
Distribution of means and standard deviations of the median frequency found for the
masticatory muscles during mastication and statistical analysis of the interaction
with the breathing mode, throughout the different collection moments (T1, T2, T3,
and T4)
|
Nasal breathers
|
Mouth breathers
|
p-value
|
|
T1
|
T2
|
T3
|
T4
|
T1
|
T2
|
T3
|
T4
|
|
Mastication
|
RM
|
194.1
(20.3)
|
195.8
(20.4)
|
195.3
(19.9)
|
195.3
(20.4)
|
189.0
(18.5)
|
191.9
(18.9)
|
192.4
(15.9)
|
192.3
(17.1)
|
0.78
|
|
RT
|
178.5
(20.1)
|
182.4
(20.2)
|
181.0
(17.4)
|
182.5
(16.3)
|
174.4
(30.4)
|
180.7
(28.0)
|
180.5
(24.8)
|
182.1
(24.8)
|
0.57
|
|
LM
|
189.1
(21.0)
|
189.9
(21.4)
|
189.5
(21.2)
|
190.1
(20.3)
|
181.3
(24.8)
|
188.0
(19.9)
|
182.8
(24.7)
|
189.5
(20.8)
|
0.04*
|
|
|
|
|
|
** T1 ≠ T2, T4 and T3 ≠ T4
|
|
LT
|
186.4
(26.2)
|
189.0
(28.1)
|
189.2
(22.7)
|
191.3
(22.1)
|
181.8
(25.9)
|
185.4
(24.7)
|
186.2
(24.6)
|
186.0
(23.0)
|
0.89
|
Abbreviations: LM, left masseter muscle; LT, left temporal muscle; RM, right masseter
muscle; RT, right temporal muscle; T1, 15 seconds of activity; T2, 30 seconds of activity;
T3, 45 seconds of activity; T4, 60 seconds of activity.
*Statistical significance with the repeated measure ANOVA test.
**Analysis with Tukey post hoc, ≠ difference.
Considering the interaction with the facial growth pattern ([Table 3]) and with its association with the breathing mode ([Table 4]), there was likewise no decreasing MF pattern or statistical significance.
Table 3
Distribution of means and standard deviation of the median frequency found for the
masticatory muscles during mastication and statistical analysis of the interaction
with the facial growth pattern, throughout the different collection moments (T1, T2,
T3, and T4)
|
Dolichofacial
|
Mesofacial
|
Brachyfacial
|
p-value
|
|
T1
|
T2
|
T3
|
T4
|
T1
|
T2
|
T3
|
T4
|
T1
|
T2
|
T3
|
T4
|
|
Mastication
|
RM
|
190.8
(16.5)
|
196.2
(13.5)
|
194.5
(13.0)
|
194.1
(13.9)
|
194.3
(22.9)
|
190.5
(26.9)
|
193.7
(21.9)
|
194.0
(23.0)
|
191.1
(19.3)
|
195.0
(17.9)
|
194.1
(18.0)
|
193.9
(18.7)
|
0.24
|
|
RT
|
178.8
(20.0)
|
188.1
(30.9)
|
185.8
(23.1)
|
186.6
(24.3)
|
169.9
(28.3)
|
172.4
(23.1)
|
173.4
(21.9)
|
177.0
(22.2)
|
178.8
(24.9)
|
183.7
(21.8)
|
182.4
(19.7)
|
183.4
(18.7)
|
0.77
|
|
LM
|
189.6
(16.6)
|
194.1
(19.0)
|
192.3
(20.6)
|
197.8
(17.2)
|
180.0
(31.1)
|
183.8
(19.1)
|
176.1
(30.2)
|
183.5
(24.4)
|
186.9
(20.6)
|
189.9
(21.6)
|
189.2
(19.3)
|
190.3
(19.1)
|
0.49
|
|
LT
|
173.2
(24.2)
|
178.1
(35.8)
|
183.1
(30.3)
|
184.5
(21.8)
|
183.9
(27.1)
|
185.4
(24.9)
|
184.2
(25.1)
|
185.2
(24.1)
|
187.2
(25.8)
|
190.5
(24.8)
|
190.4
(21.3)
|
191.5
(22.2)
|
0.49
|
Abbreviations: LM, left masseter muscle; LT, left temporal muscle; RM, right masseter
muscle; RT, right temporal muscle; T1, 15 seconds of activity; T2, 30 seconds of activity;
T3, 45 seconds of activity; T4, 60 seconds of activity.
Table 4
Distribution of means and standard deviations of the median frequency found for the
masticatory muscles during mastication and statistical analysis of the interaction
with the breathing mode in association with the facial growth pattern, throughout
the different collection moments (T1, T2, T3, and T4)
|
NBDo
|
NBMe
|
NBBr
|
|
|
T1
|
T2
|
T3
|
T4
|
T1
|
T2
|
T3
|
T4
|
T1
|
T2
|
T3
|
T4
|
|
|
Mastication
|
RM
|
193.8
(16.8)
|
194.4
(14.5)
|
192.7
(13.8)
|
192.0
(16.9)
|
203.1
(18.4)
|
198.3
(25.3)
|
204.5
(21.3)
|
202.3
(18.9)
|
191.5
(21.5)
|
195.4
(20.7)
|
193.1
(20.5)
|
193.9
(21.8)
|
|
|
RT
|
171.0
(7.3)
|
173.6
(20.9)
|
172.9
(13.8)
|
178.3
(13.8)
|
173.1
(19.8)
|
177.6
(15.2)
|
176.8
(12.5)
|
181.7
(11.8)
|
181.6
(21.7)
|
185.6
(21.5)
|
183.9
(19.1)
|
183.6
(18.2)
|
|
|
LM
|
188.6
(22.2)
|
191.5
(28.2)
|
192.8
(29.1)
|
196.3
(22.3)
|
192.5
(24.1)
|
186.7
(13.0)
|
187.1
(23.2)
|
186.3
(21.6)
|
188.1
(20.9)
|
190.5
(22.9)
|
189.6
(20.2)
|
190.0
(20.4)
|
|
|
LT
|
155.6
(15.6)
|
161.1
(40.9)
|
169.6
(29.3)
|
176.2
(26.9)
|
184.2
(25.7)
|
185.8
(14.6)
|
182.9
(21.0)
|
184.1
(17.6)
|
193.1
(24.1)
|
195.6
(25.9)
|
195.0
(20.2)
|
196.5
(21.3)
|
|
|
MBDo
|
MBMe
|
MBBr
|
|
T1
|
T2
|
T3
|
T4
|
T1
|
T2
|
T3
|
T4
|
T1
|
T2
|
T3
|
T4
|
p
|
|
Mastication
|
RM
|
187.8
(18.1)
|
197.9
(14.3)
|
196.3
(13.9)
|
196.3
(12.3)
|
186.7
(24.9)
|
183.9
(28.4)
|
184.4
(19.0)
|
186.8
(25.1)
|
190.5
(16.1)
|
194.3
(13.1)
|
195.6
(14.1)
|
194.0
(13.4)
|
0.49
|
|
RT
|
186.5
(26.9)
|
202.7
(35.0)
|
198.8
(24.6)
|
194.9
(31.8)
|
167.1
(35.4)
|
168.0
(28.8)
|
170.5
(28.3)
|
173.0
(28.7)
|
174.5
(29.6)
|
180.7
(23.0)
|
180.3
(21.3)
|
183.1
(20.3)
|
0.75
|
|
LM
|
190.5
(12.0)
|
196.7
(5.6)
|
191.9
(11.9)
|
199.3
(13.7)
|
169.2
(34.1)
|
181.3
(23.9)
|
166.7
(33.9)
|
181.2
(28.2)
|
184.9
(20.8)
|
188.9
(20.4)
|
188.7
(18.6)
|
190.9
(17.8)
|
0.08
|
|
LT
|
190.9
(17.1)
|
195.1
(23.3)
|
196.7
(28.1)
|
192.8
(14.0)
|
183.5
(30.3)
|
185.1
(32.6)
|
185.4
(29.8)
|
186.2
(30.1)
|
178.1
(26.6)
|
182.6
(21.4)
|
183.5
(21.8)
|
183.9
(22.2)
|
0.59
|
Abbreviations, LM, left masseter muscle; LT, left temporal muscle; MB, mouth breathing;
MBBr, mouth breathing with a brachyfacial pattern; MBDo, mouth breathing with a dolichofacial
pattern; MBMe, mouth breathing with a mesofacial pattern; NB, nasal breathing; NBBr,
nasal breathing with a brachyfacial pattern; NBDo, nasal breathing with a dolichofacial
pattern; NBMe, nasal breathing with a mesofacial pattern; RM, right masseter muscle;
RT, right temporal muscle; T1, 15 seconds of activity; T2, 30 seconds of activity;
T3, 45 seconds of activity; T4, 60 seconds of activity.
Perception of Fatigue
The analysis of the reported time of fatigue of the masticatory muscles, in the three
group interactions, is shown in [Table 5]. There was no significant difference in either of the cases regarding the time when
these groups perceived muscle fatigue. In the breathing mode, the MB perceived the
fatigue sooner (mean of 95 seconds). In the facial growth pattern, dolichofacial and
mesofacial children perceived it sooner (mean of 93 seconds). And in the association
of the groups, the MB children with mesofacial patterns were the first ones to perceive
it (78 seconds). In all cases, the standard deviations (SDs) were high.
Table 5
Distribution of means and standard deviations of the reported fatigue time (in seconds)
found for the masticatory muscles during mastication and statistical analysis of the
interaction with the groups
|
Groups
|
Mastication fatigue time
|
|
Mean and SD
|
p-value
|
|
Breathing Mode
|
NB
|
102.7 (63.5)
|
0.56
|
|
MB
|
95.5 (65.2)
|
|
Facial Growth Pattern
|
Dolichofacial
|
93.6 (36.8)
|
0.68
|
|
Mesofacial
|
93.2 (63.5)
|
|
Brachyfacial
|
103.2 (68.6)
|
|
Breathing Mode and
Facial Growth Pattern
|
NBDo
|
95.6 (32.9)
|
0.75
|
|
NBMe
|
118.3 (77.8)
|
|
NBBr
|
99.5 (63.0)
|
|
MBDo
|
92.0 (43.5)
|
|
MBMe
|
78.5 (51.5)
|
|
MBBr
|
108.6 (77.9)
|
MB, mouth breathing; MBBr, mouth breathing with a brachyfacial pattern; MBDo, mouth
breathing with a dolichofacial pattern; MBMe, mouth breathing with a mesofacial pattern;
NB, nasal breathing; NBBr, nasal breathing with a brachyfacial pattern; NBDo, nasal
breathing with a dolichofacial pattern; NBMe, nasal breathing with a mesofacial pattern;
SD, standard deviation.
Discussion
The analysis of the masticatory muscles, regardless of the interaction with the groups,
showed that RT, LM, and LT had a significant change in MF, particularly between 15
and 60 seconds of mastication. However, it was not a decreasing change and therefore
was not suggestive of muscle fatigue. These findings corroborate two aspects reported
in the literature: the fiber arrangement of this musculature and the dynamic test
condition. In the masticatory muscles, more specifically the masseter, there is a
predominance of type I[38] and hybrid fibers,[39] which are both more resistant to muscle fatigue because they have an aerobic pattern
of energy production.
Regarding the use of chewing gum in the mastication test, the literature reports that
the EMG activity of the masseter and temporal muscles is associated with the mechanical
properties of the selected foods. In this case, chewing gum enables stable mandible
movements and EMG activity, which are considered adequate to analyze the masticatory
pattern. Moreover, as the muscle blood flow is not interrupted, isotonicity would
not easily cause fatigue.[22]
[23]
[37]
The interaction of the masticatory muscles with the breathing mode revealed that only
the LM had a difference in MF throughout the mastication in the MB group. However,
since it referred to its increase, rather than decrease, it was likewise not suggestive
of muscle fatigue.[18] Studies with EMG have shown that MBs tends to have less masseter and temporal muscle
activity than NBs.[15]
[40] In another study, children without changes had a diminished masseter and temporal
muscle behavior, which may be associated with immature motor coordination, resulting
in incapacity to maintain high force levels.[21]
The diminished muscle behavior in MB may be caused by the hypofunctioning of the masticatory
muscles of these subjects, especially the mandibular elevators, as the mouth remains
open for breathing.[5]
[8] The masticatory preference pattern and changed head posture of MBs can also interfere
with the asymmetry of the musculature, though with no defined pattern.[16]
[40] Thus, it was hypothesized that the possible asymmetry in the masticatory muscles
of this population can also interfere with muscle fatigue – which, however, was not
observed.
The perception time of masticatory muscle fatigue reported by the sample children
was not statistically different between NBs and MBs, with an approximate mean of 102
and 95 seconds, respectively, and a SD of 63 and 65, respectively. Other authors[22] researched reports of the masseter and temporal fatigue feeling and likewise observed
large SDs of the means, suggesting great subjectivity in this variable.
Each facial pattern determines specific characteristics regarding the soft and hard
structures, which influence the development of the stomatognathic functions.[32]
[41] As for the clinical aspects, people with a brachyfacial pattern have thicker and
more powerful mandibular elevator muscles. The opposite occurs with the dolichofacial
pattern, in whom these muscles are feebler and less strong.[32]
Concerning the myoelectrical manifestations in the facial types, the literature only
researched EMG approaching the amplitude and, even so, with disagreements. Some authors
did not find differences in the masticatory muscles between the various facial patterns
during mastication,[42] whereas others observed that the masseter muscles of the dolichofacial pattern were
more active in this function.[43] Since there is a muscle imbalance, especially in the brachyfacial and dolichofacial
patterns,[6]
[32]
[42] it was hypothesized that the masticatory muscles could also have fatigue differences
– which, however, was not proved.
There was no difference in the reported time of masticatory muscle fatigue of the
children between the facial growth patterns. Another study[44] investigated reports of pain and fatigue in the masseter muscle of people with different
types of craniofacial morphology and observed that people with normal and elongated
facial patterns have greater resistance and take longer to start feeling pain. The
authors pointed out, as a plausible explanation for the observed differences, the
theory of mechanical advantage, in which subjects with an elongated face have a smaller
mechanical advantage in the mandibular elevator muscles. Hence, those with a short
face have more occlusal force and consequently greater intramuscular pressure. This
may limit the muscle blood flow, which is necessary to maintain force. This disagreement
may be explained by the difference between samples, as the one in the present study
encompassed children, whereas the one in the literature encompassed adults. Furthermore,
the subjectivity of this variable may indicate greater perception difficulty in children.
The breathing mode was also considered along with the facial growth pattern because
it was believed that changes in these two aspects may potentialize each other. Nevertheless,
although the sample distribution into these new groups showed their stomatognathic
characteristics in further detail, it did not occur in myoelectric terms for the masticatory
muscles. The analysis of the MF and the fatigue perception time for this musculature
throughout the collection time revealed no difference.
Thus, it seems clear to say that the masticatory muscles are rather resistant to fatigue,
especially in dynamic situations, which elongate the muscle belly and consequently
increase the blood flow. This increases the muscle temperature and metabolism, further
removing the substrates that cause fatigue.[45] However, future studies should address other aspects neither investigated nor controlled
in the present study – such as the amplitude of the EMG signal –, aiming at a more
precise interpretation of these findings.
It is important to highlight that the present study has some limitations, such as
the small sample size when the six groups were analyzed separately and the muscle
fatigue analysis based exclusively on the EMG signal frequency, without its amplitude
– which is already being addressed in other studies of this research group.
Conclusion
The EMG analysis of the masticatory muscles during mastication showed that they were
not suggestive of fatigue. However, the fatigue time was reported despite the absence
of physiological fatigue. The breathing mode, the facial growth pattern, and the association
between them seemed not to influence the behavior of the MF of the EMG signal and
the fatigue time perception.
