Introduction

The modern-day diet is based on the consumption of soft, industrialized foods, which contain an immense amount of sugar, high sodium concentration, and very few nutrients [1, 2]. These factors can affect the stomatognathic system, mainly due to the ingestion of soft foods, which provides less vigorous chewing, causing possible functional changes in the biomechanics of the masticatory muscles [3].

Factors such as eating habits, dentition status, malocclusion, teeth morphology, and symptoms of temporomandibular disorders play a role in the masticatory system that can lead to functional changes [4]. Indigenous people have general habits appropriate to human health and ideal for the proper functioning of the masticatory muscular system [5]. The satisfactory nutritional status and daily routines of these Natives are part of their cultural identity and allow the preservation of healthy lifestyle habits [6].

The food of the Brazilian indigenous population living in the Xingu National Park is poorly processed, without spices, basically composed of roots and dried fruits, red meat from wild animals, and fish [7]. However, the Brazilian white population has a diet based mainly on soft and processed foods that can lead to hypofunction of the complex interrelationship between the morphology and biomechanics of facial and masseter and temporalis muscles [8, 9].

Thus, the study of indigenous populations that still preserve part of their hunter-gatherer and natural lifestyle is a useful comparison when analyzing the effects of lifestyles adopted by the white urban society on the masticatory performance. This understanding can help identify if the lifestyle of a population has made a real contribution to dysfunctions of the stomatognathic system observed in white urban populations and can be helpful for the design of treatments for this problem.

Our null hypothesis was that differences exist between patterns of electromyographic activity of the masseter and temporal muscles at indigenous individuals from Brazilian Xingu villages with Brazilian urban white people.

Material and methods

Results

Normalized electromyographic values for each muscle studied at each clinical condition for the two groups are shown in Table 1. There was significant difference between groups with a decrease in the normalized electromyographic data during mandibular rest [right masseter (p = .002) and left masseter (p = .004) muscles] for the indigenous individuals group. There was increase in the normalized electromyographic activity during protrusion [left temporal (p = .03) muscle] for the indigenous individuals group.

Table 1

Mean, standard error (SE), lower (L), upper (U), and statistical significance of normalized electromyographic data during the mandibular taks in the groups: Xingu indigenous Brazilians group (G1), and white urban Brazilian group.

	Groups						p value
	GI			GII
Mandibular Task	mean (SE)			mean (SE)
Muscles		L	U		L	U
Rest
Right masseter	0.03(0.004)	0.01	0.09	0.07(0.01)	0.02	0.25	.002*
Left masseter	0.03(0.005)	0.01	0.14	0.05(0.004)	0.01	0.13	.004*
Right temporal	0.08(0.01)	0.02	0.49	0.08(0.01)	0.02	0.24	NS
Left temporal	0.09(0.01)	0.02	0.40	0.09(0.01)	0.02	0.32	NS
Protrusion
Right masseter	0.16(0.02)	0.02	0.47	0.21(0.03)	0.02	0.91	NS
Left masseter	0.15(0.02)	0.02	0.51	0.19(0.02)	0.05	0.64	NS
Right temporal	0.13(0.02)	0.03	0.44	0.09(0.01)	0.02	0.39	NS
Left temporal	0.15(0.02)	0.03	0.71	0.08(0.01)	0.02	0.40	.03*
Right Laterality
Right masseter	0.12(0.02)	0.01	0.78	0.10(0.02)	0.01	0.68	NS
Left masseter	0.10(0.01)	0.01	0.37	0.14(0.01)	0.03	0.37	NS
Right temporal	0.13(0.02)	0.02	0.81	0.12(0.01)	0.04	0.35	NS
Left temporal	0.12(0.02)	0.02	0.52	0.08(0.01)	0.02	0.43	NS
Left Laterality
Right masseter	0.14(0.02)	0.01	0.51	0.13(0.01)	0.02	0.59	NS
Left masseter	0.08(0.01)	0.01	0.41	0.07(0.007)	0.01	0.17	NS
Right temporal	0.11(0.02)	0.01	0.81	0.09(0.01)	0.02	0.28	NS
Left temporal	0.15(0.02)	0.03	0.61	0.11(0.01)	0.03	0.53	NS

*significant difference, NS: not significant, one-way analysis (i.e., p < .05)

The normalized electromyographic data for habitual (peanuts and raisins) and non-habitual (Parafilm M) chewing for the groups are shown in Table 2. Differences were found between groups in the habitual chewing (peanuts) in the right (p = .001) and left (p = .001) masseter muscles and right temporal muscle (p = .01); habitual chewing (raisins) in the right (p = .001) and left (p = .002) masseter muscles and right (p = .008) and left (p = .01) temporalis muscles; and non-habitual chewing (Parafilm M) in the left masseter muscle (p = .05) with increase in the electromyographic activity at indigenous individuals as compared with white Brazilian individuals.

Table 2

Mean, standard error (SE), lower (L), upper (U), and statistical significance of normalized electromyographic data during the habitual and non-habitual chewing in the groups: Xingu indigenous Brazilians group (G1), and white urban Brazilian group.

	Groups						p value
	GI			GII
Chewing	mean (SE)			mean (SE)
Muscles		L	U		L	U
Peanuts
Right masseter	0.47(0.06)	0.16	1.76	0.76(0.05)	0.19	1.47	.001*
Left masseter	0.43(0.03)	0.14	0.78	0.70(0.05)	0.23	1.47	.001*
Right temporal	0.48(0.05)	0.17	1.53	0.64(0.03)	0.38	1.34	.01*
Left temporal	0.57(0.05)	0.22	1.33	0.69(0.05)	0.37	1.74	NS
Raisins
Right masseter	0.33(0.03)	0.06	0.81	0.31(0.04)	0.11	1.15	.001*
Left masseter	0.31(0.04)	0.05	1.43	0.51(0.03)	0.14	1.17	.002*
Right temporal	0.36(0.04)	0.06	1.51	0.51(0.02)	0.13	0.99	008*
Left temporal	0.41(0.03)	0.19	1.06	0.55(0.03)	0.14	1.10	01*
Parafilm M
Right masseter	0.53(0.06)	0.13	1.97	0.70(0.05)	0.33	1.64	NS
Left masseter	0.46(0.04)	0.10	1.03	0.58(0.03)	0.21	1.13	.05*
Right temporal	0.55(0.07)	0.12	1.84	0.63(0.05)	0.15	1.95	NS
Left temporal	0.59(0.07)	0.13	1.69	0.63(0.04)	0.16	1.47	NS

*significant difference, NS: not significant, one-way analysis (i.e., p < .05)

Discussion

The present study investigated the presence of differences in the electromyographic patterns of two distinct populations. The differences found in this study were attributable probably due to different lifestyles between the groups, that is, Xingu indigenous group maintained a traditional lifestyle connected with its culture, whereas the world modernization process influenced the white Brazilian group.

Several studies have analyzed the electromyographic activity of masticatory muscles in different situations of urban populations such as age, sex, temporomandibular dysfunction, chronic degenerative diseases, dentition status, and oral parafunctional habits [15–18], but there is a gap in the literature that does not show the masticatory muscle behavior of specific populations with eating habits of native villagers, such as indigenous populations.

In the present investigation, electromyographic activity was recorded during mandibular rest conditions in the masseter and temporalis muscles in two groups, indicating that lower activity of the masseter muscles in the Xingu indigenous.

The indigenous population in Xingu has carefully preserved life habits transmitted from one generation to the next that might somehow contribute to the development of healthy individuals [19, 20].

During the assessment of the maintenance of rest, it can be argued that Brazilian whites have interferences in the functioning of the stomatognathic system that promote greater electromyographic activity when compared to the indigenous individuals. Studies state that the increased electrical activity of the muscle during rest is considered a sign of disharmony in the stomatognathic system [21, 22].

The change in the electromyographic activity of the masticatory muscles may be associated with occlusal interferences such as premature contacts that cause imbalances in the stomatognathic system [23], justified by the biofeedback of the information captured by the fibers of the periodontal ligament and the nervous system [24]. In this study, it was not assessed whether the white Brazilian group had premature dental contacts.

Our results showed that during the protrusion condition, both groups exhibited a contraction pattern to maintain postural position, with greater activation of the masseter muscles when compared to the temporalis muscles. Higher electromyographic recordings in the temporalis muscles were found in the Xingu indigenous group, with a significant difference for the left temporal muscle.

In the right and left laterality conditions, greater electromyographic activity is expected to be recorded in the temporalis muscles on the same side. The jaw extends to (work side), whereas for the masseter muscle greater contralateral activity is to be expected [25].

It was evident that our result did not follow the normality pattern of neuroanatomical muscle activation in the right laterality condition in which larger electromyographic records were expected in the masseter muscle on the contralateral side of the mandible for the Xingu indigenous group.

The factor that may have influenced the incorrect neuroanatomical movement in this clinical condition would be the difficulty of communication with the indigenous village, sometimes making it impossible to understand the mandibular movement [26, 27]. In the clinical condition of the left laterality, the neuroanatomic pattern was correct in both groups.

Fibrous and rigid feeding throughout life promotes important changes in the masticatory system, which is a characteristic of indigenous tribes, providing occlusal wear, which is a mechanism of physiological adjustment in the masticatory system, which influences the neuroanatomical pattern [28–30]. Several authors defend the opinion that this type of wear is useful for strengthening the periodontium of the teeth with the increased electromyographic activity of the masticatory muscles in mandibular postural conditions [31]. In this study, we observed greater electromyographic activity of the masseter and temporalis muscles in almost all clinical mandibular conditions.

The function of the lateral pterygoid muscles that present activation of their fibers arranged horizontally during protrusion and mandibular excursion movements [32], can be compromised by occlusal factors, such as interferences and dental wear, progressively overloading the other masticatory muscles, modifying the neuroanatomical pattern. This factor can also modify the neuroanatomical pattern in the clinical conditions of protrusion and laterality of the mandible.

The analysis of the electromyographic data recorded during habitual and non-habitual chewing provided evidence of muscle performance. The comparison of the data obtained from the two groups during masticatory activities revealed lower electromyographic activity for all muscles in the group of indigenous individuals.

The diet of the indigenous population of the Xingu has a diet rich in proteins with essential amino acids for the formation of nervous and muscular structures, and the abundance of fiber and hard food consumption that favors efficient training for the stomatognathic system [33, 34]. In this way, synchrony, and power of all the components that make up this system are achieved, which allows the best performance during its function.

The absence of daily stress may also have provided some training throughout the life of indigenous individuals, which led the fibers of the masseter and temporalis muscles of these individuals to have the best possible performance. This optimal function enables them to grind hard foods all day long without wearing out their muscles, at the same time that they recruit a minimum of motor units and muscle fibers to perform this task [14, 35].

The habitual (peanuts and raisins) and non-habitual (Parafilm M) chewing revealed the capacity of the indigenous participants’ stomatognathic system to contract the muscles involved as little as possible. In the chewing process of the Xingu indigenous group, more motor units were spared the effort to grind the food, thus favoring muscle health with greater masticatory efficiency concerning white Brazilian individuals. However, the stomatognathic system of white Brazilian individuals may have stopped receiving natural stimuli to develop, not adapting to dietary changes, due to the global industrialization that produced soft and super caloric foods [36, 37].

Considering that electromyographic activity of the masseter and temporalis muscles on the mandibular tasks and chewing (habitual and non-habitual) was significantly different in the comparison between groups, our null hypothesis was not rejected.

This study was carried out in a totally isolated environment and with a population of principles and customs different from the white population. These facts made that with each contact with the indigenous population, there was an awareness process, being necessary to wait for the moment that the indigenous population accepted the data collection procedure.

There are limitations to the electromyographic analysis of the masseter and temporalis muscles. The difficulty of understanding the Portuguese language, by the indigenous population, of what would be the movement of a mandibular clinical condition assessed in electromyographic activity may have interfered in the data collection, even having a local translator in the indigenous village. Among the thirty indigenous assessed in this study, one third did not speak Portuguese and did not even understand the language.

Conclusions

The indigenous people from Brazilian Xingu villagers showed a reduction in electromyographic activity in mandibular tasks and an increase in masticatory efficiency. These results are strong indications that dietary and culture influences, structure and activity of masticatory muscles, oral habits, proprioceptive input from periodontium /muscles during chewing, wear of occlusal surfaces (due to diet) and lifestyle interfere with the functioning of the stomatognathic system and that the ancient wisdom and tradition of the indigenous population enable the maintenance of healthy masticatory system functioning.