The effect of age on the function of the masticatory system--an electromyographical analysis

Background: Old age is the last stage of human life and, unfortunately, the ageing rhythm of the oral cavity and masticatory system seems to be rather accelerated. Hence, there is a reduction in the amount of food ingested, establishing the imbalance of nutritional phenomena. Objective: To assure a better quality of life for the elderly, it is necessary to understand their masticatory system. Materials and methods: An electromyographical analysis of the masticatory system in 10 individuals aged between 60–75 years (group 1) and 10 between 23–30 years old (group 2) was carried out. The analysis was performed using a MyoSystem‐Br1 electromyographer with differential active electrodes. The test was registered during positioning and functional conditions, such as chewing, and the muscles assessed were the temporalis and masseter. Data were normalised by maximum voluntary contraction (MVC), and the results were analysed statistically using independent t‐test during the comparison between groups. Results: Statistical significance (p ≤ 0.05) was found in the analysis of various activities, in which the elderly showed greater electromyographical activity values relative to their MVC during the different mandibular positions, such as left laterality. While the elderly subjects revealed values of 30% relative to their MVC, the young revealed 10%. However, during masticatory activities, the elderly revealed values of up to 79%, and the young of 108%, relative to their MVC, such as when they are chewing peanuts. Conclusion: It could be concluded that the elderly show hyperactivity of masticatory musculature during posture maintenance and a slight hypoactivity of this musculature during chewing when analysed side by side with young individuals.     

Temporalis and masseter muscle function during incision in macaques and humans

Most previously published electromyographic (EMG) studies have indicated that the temporalis muscles in humans become almost electrically quiet during incisai biting. These data have led various workers to conclude that these muscles may contribute little to the incisai bite force. The feeding behavior and comparative anatomy of the incisors and temporalis muscles of certain catarrhine primates, however, suggest that the temporalis muscle is an important and powerful contributor to the bite force during incision. One purpose of this study is to analyze the EMG activity of the masseter and temporalis muscles in both humans and macaques with the intention of focusing on the conflict between published EMG data on humans and inferences of muscle function based on the comparative anatomy and behavior of catarrhine primates. The EMG data collected from humans in the present study indicate that, in five of seven subjects, the masseter,anterior temporalis, and posterior temporalis muscles are very active during apple incision (i.e., relative to EMG activity levels during apple and almond mastication). In the other two human subjects the EMG levels of these muscles are lower during incision than during mastication, but in no instance are these muscles ever close to becoming electrically quiet. The EMG data on macaques indicate that, in all six subjects, the masseter, anterior temporalis, and posterior temporalis muscles are very active during incision. These data are in general agreement with inferences on muscle function that have been drawn from the comparative anatomy and behavior of primates, but they do not agree with previous experimental data. The reason for this disagreement is probably due to differences in the experimental procedure. In previous studies subjects simply bit isometrically on their incisors and the resulting EMG pattern was compared to the pattern associated with powerful clenching in centric occlusion. In the present study the subjects incised into actual food objects, and the resulting EMG pattern was compared to the pattern associated with mastication of various foods. It is not surprising that these two procedures result in markedly different EMG patterns, which in turn result in markedly different interpretations of jaw-muscle function. In an attempt to explain the evolution of the postorbital septum in anthropoids, it has been suggested that the anterior temporalis is more active than the masseter during incision (Cachel, 1979). The human and macaque EMG data do not support this hypothesis; during incision, the two muscles show no consistent differences in humans and the masseter appears to be in fact more active than the anterior temporalis in macaques.     

Morphology and variability of masticatory structures in the orangutan

The present study examines and describes variability and ontogenetic change in the masticatory structures of the orangutan. Mm. temporalis, masseter, pterygoideus lateralis, and pterygoideus medialis as well as m. digastricus are dissected, described, and removed unilaterally for drying from a sample of 10 orangutans. The temporomandibular joint and related structures are also described. Results indicate substantial interindividual variability in muscle morphology as well as possible age-related changes. Age-related changes in m. temporalis include an expansion of attachment area, adjustments in fiber direction, and changes in related bony structures. An increased attachment area is likewise seen in m. masseter in older individuals. Age-related variation is also seen in the position and extent of the origin of m. digastricus as well as in its fiber direction. A comparison of muscle weights indicates that m. pterygoideus lateralis is relatively larger in the neonate, whereas in the older individuals, it is relatively smaller compared to other masticatory muscles. The general morphology of the masticatory structures in the orangutan resembles that of other extant great apes and humans with the exception of m. digastricus. The orangutan differs in possessing only a posterior belly of this muscle.     

Masticatory muscular activation in elderly individuals during chewing

Background: Old‐age is the last stage of human evolution and, unfortunately, the ageing of the oral cavity and masticatory system seems accelerated. As a consequence, there is a reduction in the amount of food ingested, which can lead to an imbalance in nutrition. Objective: The purpose of this study was to investigate the levels of muscular activation of elderly individuals, during chewing, and to compare with young individuals. Materials and Methods: An electromyographical analysis of the masticatory system in 10 individuals aged between 60 and 75 years (elderly group) and a similar number between 23–30 years old (young group ‐ control) was carried out. The analysis was performed using a MyoSystem‐Br1 electromyographer with differential active electrodes. The test was recorded during functional conditions, and the muscles assessed were the temporalis and masseter. Data were normalised by maximum voluntary contraction (MVC), and the results were analysed using an independent t‐test for comparison between the groups. Results: The normalised electromyographic data obtained showed significant differences in both groups. Comparing the normalised values obtained for MVC, the mean values for the masseter and temporalis muscles of elderly group were statistically lower (p ≤ 0.05) than control group for harder foods, but there were no significant differences for food with the lowest consistency. Conclusion: It can be concluded that elderly individuals show slight hypoactivity of their masticatory musculature during chewing when compared to young individuals.     

Association between the lack of teeth and the expression of myosins in masticatory muscles of microphthalmic mouse

The purposes of the present study were to elucidate the influences of the deficiency of teeth on masticatory muscles, such as the masseter, temporalis and digastric muscles and compare the influence among masticatory muscles. We analysed the expressions of myosin heavy chain (MyHC) isoform messenger RNA (mRNA) and protein in these muscles in the microphthalmic (mi/mi) mouse, whose teeth cannot erupt because of a mutation in the mitf gene locus. The expression levels of MyHC mRNA and protein in the masseter, temporalis, digastric, tibialis anterior and gastrocnemius muscles of +/+ and mi/mi mice were analysed with real‐time polymerase chain reaction and sodium dodecyl sulfate‐polyacrylamide gel electrophoresis, respectively. The mi/mi masseter muscle at 8 weeks of age expressed 4·1‐fold (p < 0·05) and 3.3‐fold (p < 0·01) more MyHC neonatal mRNA and protein than that in the +/+, respectively; the expression level of MyHC neonatal protein was 19% of the total MyHC protein in the masseter muscle of mi/mi mice. In the digastric muscle, the expression levels of MyHC I mRNA and protein in the mi/mi mice were 4·7‐fold (p < 0·05) and 5‐fold (p < 0·01) higher than those in the +/+ mice. In the temporalis, tibialis anterior and gastrocnemius muscles, there was no significant difference in the expression levels of any MyHC isoform mRNA and protein between +/+ and mi/mi mice. These results indicate associations between the lack of teeth and the expression of MyHC in the masseter and digastric muscles but not such associations in the temporalis muscle, suggesting that the influence of tooth deficiency varies among the masticatory muscles. Copyright © 2011 John Wiley & Sons, Ltd.     

Interspecific analysis of covariance structure in the masticatory apparatus of galagos

The primate masticatory apparatus (MA) is a functionally integrated set of features, each of which performs important functions in biting, ingestive, and chewing behaviors. A comparison of morphological covariance structure among species for these MA features will help us to further understand the evolutionary history of this region. In this exploratory analysis, the covariance structure of the MA is compared across seven galago species to investigate 1) whether there are differences in covariance structure in this region, and 2) if so, how has this covariation changed with respect to size, MA form, diet, and/or phylogeny? Ten measurements of the MA functionally related to bite force production and load resistance were obtained from 218 adults of seven galago species. Correlation matrices were generated for these 10 dimensions and compared among species via matrix correlations and Mantel tests. Subsequently, pairwise covariance disparity in the MA was estimated as a measure of difference in covariance structure between species. Covariance disparity estimates were correlated with pairwise distances related to differences in body size, MA size and shape, genetic distance (based on cytochrome‐b sequences) and percentage of dietary foods to determine whether one or more of these factors is linked to differences in covariance structure. Galagos differ in MA covariance structure. Body size appears to be a major factor correlated with differences in covariance structure among galagos. The largest galago species, Otolemur crassicaudatus, exhibits large differences in body mass and covariance structure relative to other galagos, and thus plays a primary role in creating this association. MA size and shape do not correlate with covariance structure when body mass is held constant. Diet also shows no association. Genetic distance is significantly negatively correlated with covariance disparity when body mass is held constant, but this correlation appears to be a function of the small body size and large genetic distance for Galagoides demidoff. These exploratory results indicate that changing body size may have been a key factor in the evolution of the galago MA. Am. J. Primatol. 69:46–58, 2007. © 2006 Wiley‐Liss, Inc.     

Normalizing surface electromyographic measures of the masticatory muscles: Comparison of two different methods for clinical purpose

PurposeTo compare a new normalization technique (wax pad, WAX) with the currently utilized cotton roll (COT) method in surface electromyography (sEMG) of the masticatory muscles.MethodssEMG of the masseter and anterior temporalis muscles of 23 subjects was recorded while performing two repetitions of 5 s maximum voluntary clenches (MVC) on COT and WAX. For each task, the mean value of sEMG amplitude and its coefficient of variation were calculated, and the differences between the two repetitions computed. The standard error of measurement (SEM) was calculated. For each subject and muscle, the COT-to-WAX maximum activity increment was computed. Participant preference between tasks was also recorded.ResultsWAX MVC tasks had larger maximum EMG amplitude than COT MVC tasks (P < 0.001), with COT-to-WAX maximum amplitude increments of 61% (temporalis) and 94% (masseter) (P = 0.006). WAX MVC had better test-retest repeatability than COT. For both MVC modalities, the mean amplitude (P > 0.391) and its coefficient of variation were unchanged (P > 0.180). The WAX task was the more comfortable for 18/23 subjects (P = 0.007).ConclusionWAX normalization ensures the same stability level of maximum EMG amplitude as COT normalization, but it is more repeatable, elicits larger maximum muscular contraction, and is felt to be more comfortable by subjects.     

Embryonic and postnatal development of masticatory and tongue muscles

This review summarizes findings concerning the unique developmental characteristics of mouse head muscles (mainly the masticatory and tongue muscles) and compares their characteristics with those of other muscles. The developmental origin of the masticatory muscles is the somitomeres, whereas the tongue and other muscles, such as the trunk (deep muscles of the back, body wall muscles) and limb muscles, originate from the somites. The program controlling the early stages of masticatory myogenesis, such as the specification and migration of muscle progenitor cells, is distinctly different from those in trunk and limb myogenesis. Tongue myogenesis follows a similar regulatory program to that for limb myogenesis. Myogenesis and synaptogenesis in the masticatory muscles are delayed in comparison with other muscles and are not complete even at birth, whereas the development of tongue muscles proceeds faster than those of other muscles and ends at around birth. The regulatory programs for masticatory and tongue myogenesis seem to depend on the developmental origins of the muscles, i.e., the origin being either a somite or somitomere, whereas myogenesis and synaptogenesis seem to progress to serve the functional requirements of the masticatory and tongue muscles. This study was supported by grants-in-aid for funding scientific research (nos. 13671955 and 16591871), the Bio-ventures and High-Technology Research Center of the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and the Science Research Promotion Fund from the Promotion and Mutual Aid Corporation for Private Schools of Japan.     

Standardised surface electromyography allows effective submental muscles assessment

The aims of this pilot study were to evaluate: (i) the reproducibility and variability of an electromyographical protocol developed for the assessment of submental muscles (SM) (ii) to apply the new protocol to maximal teeth clenching, a simple and largely studied static task in order to quantify the relative contribution of submental muscles. In 20 healthy subjects, aged 19–35 years, surface electromyography of SM, masseter (MM) and anterior temporalis (TA) muscles was performed during maximal voluntary clenching (MVC) with and without cotton rolls and the pushing of the tongue against the palate. Clenching on cotton rolls and pushing the tongue against the palate were used to standardise respectively MM and TA, and SM muscular potentials. The exercises were repeated in two appointments (T1–T2); submental muscles standardisation was also repeated twice (A-B) in each session to assess repeatability. Symmetry and activity were calculated for each couple of muscles. A two-way analysis of variance was computed for SM: no Factor 1 (T1 vs T2) or Factor 2 (A vs B) or F1 × F2 significant effects were found. SM recruitment was 31% of the maximal activity, with symmetry values larger than 80%. In conclusion, standardised electromyography allows a reliable assessment of Submental muscles activity.     

Non-specific esterases and esterproteases in masticatory muscles from the muscular dystrophic mouse

With the aid of histochemical and electrophoretic techniques activities for esterase and esterprotease were investigated in the digastric and masseter muscles from normal and dystrophic mice. The substrates used were -naphthyl acetate and N-acetyl-l-alanine -naphthyl ester. According to the microscopic observations of the dystrophic muscles the histopathological changes in the masseter muscle were much more pronounced than in the digastric muscle. The connective tissue surrounding the myofibers of the dystrophic masseter contained a large number of cells with pronounced enzyme activity. Among them were mast cells that were strongly stained for esterprotease. The connective tissue of the dystrophic digastricus was much less infiltrated with cellular elements reacting for esterprotease. In zymograms the normal digastricus, the dystrophic masseter and the dystrophic digastricus showed a strong activity for certain isoenzymes that were absent or weakly expressed in the normal masseter.This study was supported by grand No. 12-6516 from the Danish Medical Research Council     

Electromyography of brachial muscles in Pan troglodytes and Pongo pygmaeus

Electromyographic recordings were taken from all heads of the triceps brachii and biceps brachii muscles and from the anconeus, brachialis, and brachioradialis muscles in a chimpanzee and an orangutan as they stood still and walked quadrupedally on horizontal and inclined surfaces, engaged in suspensory behavior, reached overhead, and manipulated a variety of foods and artifacts. Like the gorilla (Tuttle and Basmajian, 1974a), the chimpanzee and orangutan possess special close-packed positioning mechanisms that allow the bulky muscles that cross their elbow joints to remain silent during quiet pendant suspension. We found no major myological features that would dramatically separate the arms of knuckle-walking African apes from those of the orangutan. With a few exceptions, which could as well be attributed to individual variation as to interspecific differences, the brachial muscles acted similarly during quadrupedal positional behaviors, irrespective of whether the hands of the subjects were knuckled (African apes), fisted (chimpanzee and orangutan), or placed in modified palmigrade postures (orangutan). Evolutionary transformations, from brachial and elbow complexes like those of Pongo to ones like Pan, or vice versa, would probably be achieved quite readily as the species changed its substrate preferences and positional habits.     

Balancing function of the masticatory muscles during incisal biting in two murid rodents,Apodemus speciosus and Clethrionomys rufocanus

The functional significance of masticatory muscle direction was estimated using a mechanical model in two murid rodents: the Japanese field mouse (Apodemus speciosus) and the gray red-backed vole (Clethrionomys rufocanus). Theoretical analyses of the data suggest that a balancing mechanism among the muscle forces occurs during incisal power stroke. The activation of the large deep masseter in both murids results in marked tensile separation of two hemimandibles at the flexible mandibular symphysis. Activation of the internal pterygoid decreases this large tensile force at the symphysis more efficiently than other muscles. The lines of action of the deep masseter and internal pterygoid are aligned to produce such a balancing function in both species studied here. The resultant force generated by the deep masseter on both sides is opposite in direction to the reaction force at the lower incisor tip. Therefore, the large deep masseter forms an effective mandibular support mechanism when the reaction forces during biting push the mandible downward. Because of the area of insertion and the line of action, the posterior temporalis appears to have an important role in stabilizing the position of the mandibular condyle in the glenoid fossa during incisal biting. J. Morphol. 236:49–56, 1998. © 1998 Wiley-Liss, Inc.     

The influence of muscles activation on the dynamical behaviour of the tympano-ossicular system of the middle ear

The human ear is a complex biomechanical system and is divided into three parts: outer, middle and inner ear. The middle ear is formed by ossicles (malleus, incus and stapes), ligaments, muscles and tendons, which transfers sound vibrations from the eardrum to the inner ear, linking with mastoid and Eustachian tube. In this work, a finite element modelling of the tympano-ossicular system of the middle ear was developed. A dynamic study based on a structural response to harmonic vibrations, for a sound pressure level (SPL) of 110, 120 and 130 dB SPL applied in the eardrum, is presented. The connection between the ossicles is made using a contact formulation. The model includes the different ligaments considering its hyperelastic behaviour. The activation of the muscles is based on the constitutive model proposed by previous work. The harmonic responses of displacement and pressure obtained on the stapes footplate, for a frequency range between 100 Hz and 10 kHz, are obtained simulating the muscle activation. The results are compared considering the passive and active states. The results are discussed and they are in accordance with audiological data published with reference to the effects of the middle ear muscles contraction.     

Cortical control for mastication in cats: Changes in masticatory movements following lesions in the masticatory cortex

In a previous paper (Hiraba and Sato ) we reported that an accurate mastication might be executed by the cortical processing in bilateral masticatory area (MA)and motor cortices. The aim of this study was to determine if cats with lesion of either unilateral or bilateral MA showed changes in mastication. After exploring mechanoreceptive fields and motor effects of mastication-related neurons (MRNs) in MA using the single unit recording and intracortical microstimulation methods, we made various lesions in MAs with injections of kainic acid (0.1%, 2.0?µl). Since the MA was divided into facial (F) and intraoral (I) projection areas as reported in the previous paper, cats with the unilateral lesion in F or I, and with the bilateral lesion in F & F, I & I or F & I (F on one side and I on other side) were prepared. Cats with unilateral lesion in F or I and with bilateral lesion in F & I showed no changes in mastication except for prolongation of the food intake and masticatory periods. Cats with bilateral lesion into F & F, or I & I showed wider jaw-opening during mastication. Particularly, the latter group showed enormous jaw-opening, delay in the start of mastication and difficulty in manipulating food on the tongue. In all cats with lesions of each type, masticatory and swallowing rhythms remained normal. These findings suggest that accurate mastication is executed by the close integration between F & F and I & I of the bilateral MA.     

Effect of bone mineral density on masticatory performance and efficiency

doi: 10.1111/j.1741‐2358.2010.00414.x Effect of bone mineral density on masticatory performance and efficiency Objective: To evaluate the effect of bone mineral density (BMD) on masticatory performance and efficiency in dentate subjects. Background data: Osteoporosis is the most common disorder of the bone. It causes reduction in BMD of the all the skeletal tissue including jaw bones. It also promotes bone loss in jaw bones. In osteoporosis, a reduction of maximal bite force and greater electromyography activity of masticatory muscles is documented. This may lead to the development of masticatory dysfunction which can be assessed by a chewing test in the form of change in masticatory performance and efficiency. Materials and methods: Sixty subjects with equal numbers of men and women were selected for the study, in which BMD screening (T‐score) was carried out to identify the normal, osteopenic and osteoporotic subjects. Their masticatory performance and efficiency was evaluated by a chewing test (fractional sieving method). Results: A high ‘T’ score was associated with low masticatory efficiency and a low ‘T’ score with high masticatory efficiency. Masticatory performance and efficiency was significantly higher among males as compared to females with similar range of BMD. Conclusion: In both genders, high BMD groups (low ‘T’ score) had a significantly high percentage of masticatory efficiency compared to the low BMD (high ‘T’ score) group.     

sEMG activity of masticatory, neck, and trunk muscles during the treatment of scoliosis with functional braces. A longitudinal controlled study

Background

Studies on the relationship between occlusal problems and the spine are of increasing interest. In this study, we monitored the sEMG activity of masticatory, neck, and trunk muscles during the treatment of scoliosis in young patients, and compared the data with a control of untreated group.

Subjects and methods

Twelve white Caucasian patients (nine males and three females; mean age of 8.0 ± 1.5 years) with scoliosis and Class I occlusion (without crowding) were included in this study (study group). Fifteen healthy subjects (nine males and six females; mean age of 9.5 ± 0.8 years) were recruited as control group. The subjects were visited before they underwent the treatment of scoliosis, as well as after 3 (T1) and 6 months (T2) of their treatment for scoliosis. The patients were instructed to wear the device during sleep and during the day, according to the protocol given by their orthopedic.

Results

The treated group showed statistically significant changes in the sEMG activity of masticatory, neck, and trunk muscles, both at rest and during MVC of the mandible with respect to T0. The masseter and the anterior temporalis showed a significant improvement in the asymmetry index from T0 to T2. On the other hand, subjects in the control group did not register much change.

Conclusion

Our findings suggest that the use of a functional device for the treatment of scoliosis induces a significant reduction in the asymmetry index of the trunk muscles, as well as a significant increase in the contractility of masticatory muscles.     

Activation of shoulder muscles in healthy men and women under isometric conditions

Purpose: Due to the low osseous lead of the shoulder joint a large portion of the shoulder muscles, in addition to executing movements, deals with stabilising tasks. This requires a permanent readjustment of the intermuscular co-ordination of all involved muscles. The aim of the study was to verify the existence of gender dependent differences in intramuscular co-ordination patterns of shoulder muscles.

Method: Fifteen healthy men and nine healthy women, who executed 24 isometric exercises in sagittal, frontal and horizontal planes with a loading of 50% of their individual isometric maximum force, were investigated. In every plane, four angular positions were chosen and both opposite force directions were measured, respectively. SEMG was taken from 13 muscles of the shoulder and the upper arm. Due to inter-individual differences SEMG amplitudes were normalised. Results: Gender specific differences of functional intermuscular co-ordination patterns could be proven systematically. Women showed less activation of muscles acting in the main force direction. In addition, those muscles less necessary for the actual force production were more activated in women than in men.

Conclusions: Functionally comparable shoulder function showed a gender dependency in terms of functional intermuscular co-ordination.