Differences in fibre type composition between human masseter and biceps muscles in young and adults reveal unique masseter fibre type growth pattern

A Tutorial on Skeletal Muscle Metabolism and the Role of Blood Lactate: Implications for Speech Production

PURPOSE

The purpose of this tutorial is threefold: (a) present relevant exercise science literature on skeletal muscle metabolism and synthesize the limited available research on metabolism of the adult human speech musculature in an effort to elucidate the role of metabolism in speech production; (b) introduce a well-studied metabolic serum biomarker in exercise science, lactate, and the potential usefulness of investigating this metabolite, through a well-established exercise science methodology, to better understand metabolism of the musculature involved in voice production; and (c) discuss exercise physiology considerations for future voice science research that seeks to investigate blood lactate and metabolism in voice physiology in an ecologically valid manner.

METHOD

This tutorial begins with relevant exercise science literature on the basic cellular processes of muscle contraction that require energy and the metabolic mechanisms that regenerate the energy required for task execution. The tutorial next synthesizes the available research investigating metabolism of the adult human speech musculature. This is followed by the authors proposing a hypothesis of speech metabolism based on the voice science literature and the application of well-studied exercise science principles of muscle physiology. The tutorial concludes with a discussion and the potential usefulness of lactate in investigations to better understand the metabolism of the musculature involved in vocal demand tasks.

CONCLUSION

The role of metabolism during speech (respiratory, laryngeal, and articulatory) is an understudied yet critical aspect of speech physiology that warrants further study to better understand the metabolic systems that are used to meet vocal demands.

Skeletal muscle fibre morphology in childhood-insights into myopenia in pediatric liver disease

TAKE-HOME MESSAGE

Skeletal muscle morphology in healthy children changes with age. Liver disease may preferentially affect type II fibres in adults with end-stage liver disease (ESLD). More research is needed on the effects of ESLD on muscle morphology in children.

Hemodynamic Changes in the Masseter and Superior Orbicularis Oris Muscles before and after Exercise Load: A Comparison between Young Adult Women and Middle-Aged to Old Adult Women

Background

The vascularity index (VI) is useful for measuring the hemodynamics on ultrasound imaging. However, there are no reports concerning the application of the VI to facial muscles.

Objective

The aim of this study was (1) to establish a method of measuring the hemodynamics in facial muscles in a constant way and (2) to evaluate the hemodynamic changes in the masseter and superior orbicularis oris muscles (SOOMs) before and after exercise load in two subject groups of females of different ages.

Methods

(1) The VI in the SOOM was calculated, and the test-retest reliability was assessed in seven healthy adults. (2) The VIs in the left-side masseter and SOOM were calculated in 3 sessions: before exercise loading (T0), immediately after loading (T1), and 5 minutes after T1 (T2) for the young adult group (YAG, n = 20; age range, 20–35 years) and the middle-aged to old group (MOG, n = 20; age range, 50–70 years). Tasks were gum chewing for the masseter muscle and lip sealing for the SOOM. The differences in the mean peak flows between two sessions were examined.

Results

(1) Significant differences were not noted for the repeatedly measured average volumes of blood flow with good test-retest agreement (intraclass correlation coefficient = 0.81). (2) In both muscles of the YAG, there were a significant increase in T1 compared with T0 and a significant decrease in T2 compared with T1 (all p < 0.05). In both muscles of the MOG, no significant differences were noted in either comparison.

Conclusions

A method of measuring the hemodynamics in facial muscles was developed and showed good reliability. Changes in the blood flow after exercise load in these muscles may vary with age in women.

Differential control of blood flow in masseter and biceps brachii muscles during stress

OBJECTIVE

The present study aimed to compare sympathetic hemodynamic effects in masticatory and limb muscles in response to different stressors.

DESIGN

Twelve healthy participants were subjected to a randomized series of stressors, including cold pressor test (CPT), mental arithmetic test, apnea, isometric handgrip (IHG) and post-handgrip muscle ischemia (PHGMI), while in the supine position. Spatially-resolved near-infrared spectroscopy was used to measure relative changes in blood volume and oxygenation (TOI) of the resting masseter and biceps muscles. Cardiac output, heart rate, and arterial blood pressure (ABP) were also monitored.

RESULTS

Except apnea, all tests increased ABP. Different response patterns were observed in the 2 muscles: TOI significantly increased during contralateral IHG (1.24 ± 1.17%) but markedly decreased during CPT (-4.84 ± 4.09%) and PHGMI (-6.65 ± 5.31%) in the biceps muscle, while exhibiting consistent increases in the masseter (1.88 ± 1.85%; 1.60 ± 1.75%; 1.06 ± 3.29%, respectively) (p < 0.05).

CONCLUSIONS

The results allow us to infer differential control of blood flow in head and limb muscles. In general, the masseter appears more prone to dilatation than the biceps, exhibiting opposite changes in response to painful stimuli (CPT and PHGMI). Several mechanisms may mediate this effect, including reduced sympathetic outflow to the extracranial vasculature of the head, generally exposed to lower hydrostatic loads than the rest of the body.

Decline of oral functions in old-old adults and their relationship with age and sex: The SONIC study

BACKGROUND

Many physical functions decline with aging, but it is not known whether oral functions vary according to sex or decline with aging, as it occurs with physical functions. The present study aimed to examine the association of sex, age, and elapsed years with occlusal force and tongue pressure using a generalized linear mixed-effect model (GLMM) over a 3-year period among old-old Japanese adults.

METHODS

Participants were community-dwelling older adults who participated in a survey (June 2014-March 2017) and a follow-up survey (July 2017-December 2019) after 3 years (n = 951: 70-year group, n = 466; 80-year group, n = 391; 90-year group, n = 94). Dental examinations including the number of teeth, occlusal force, and tongue pressure were conducted, and a GLMM was used to estimate the association of sex, age, and elapsed years with occlusal force and tongue pressure, adjusting for the number of teeth.

RESULTS

The GLMM showed that occlusal force was significantly associated with sex (reference; male, non-standardized coefficient: B = -66.9 [female], p < 0.001), age (reference; 70-year group, B = -81.7 [80-year group], p < 0.001, B = -87.2 [90-year group], p < 0.001), and the number of teeth (B = 13.8, p < 0.001), but did not significantly decrease with elapsed years. Tongue pressure was significantly associated with sex (reference; male, B = -0.94 [female], p = 0.034) and age (reference; 70-year group, B = -1.78 [80-year group], p < 0.001, B = -5.47 [90-year group], p < 0.001). Tongue pressure decreased significantly with elapsed years (B = -0.82, p < 0.001).

CONCLUSIONS

These findings suggest that tongue pressure significantly decreased over time, but occlusal force did not. Tongue-related muscles may be more susceptible to aging than masticatory muscles.

Age-related changes in oral motor-control strategies during unpredictable load demands in humans

To investigate age-related changes in oral motor strategies in response to unpredictable load demands. Sixty-five healthy children (aged 3-17 yr) were divided into five age-groups based on their dental eruption stages and compared with a group of healthy adults (aged 18-35 yr). Each participant was asked to perform a standardized motor control task involving 'pulling' and 'holding' a force transducer with the anterior teeth. Different loads were attached to the force transducer in an unpredictable manner. The temporal force profile was divided into two time-segments (an initial segment and a later segment). The peak force and peak force rate during the initial time-segment, and the holding force and intra-trial variability (coefficient of variation) during the later time-segment, were measured. The results showed no differences in the peak force, peak force rate, holding force, and force variability in children compared with adults. However, the trends in the data evaluated using a segmented regression analysis showed that a breakpoint (abrupt change) consistently occurred in the late-mixed dentition group (age 9-11 yr) for most of the outcome variables. The results indicate that while the motor control strategies in children appear to be similar to those in adults, there is a shift in the oral motor developmental trend during the late-mixed dentition stage.

Differences in muscle mechanics underlie divergent optimality criteria between feeding and locomotor systems

Tetrapod musculoskeletal diversity is usually studied separately in feeding and locomotor systems. However, direct comparisons between these systems promise important insight into how natural selection deploys the same basic musculoskeletal toolkit-connective tissues, bones, nerves, and skeletal muscle-to meet the differing performance criteria of feeding and locomotion. Recent studies using this approach have proposed that the feeding system is optimized for precise application of high forces and the locomotor system is optimized for wide and rapid joint excursions for minimal energetic expenditure. If this hypothesis is correct, then it stands to reason that other anatomical and biomechanical variables within the feeding and locomotor systems should reflect these diverging functions. To test this hypothesis, we compared muscle moment arm lengths, mechanical advantages, and force vector orientations of two jaw elevator muscles (m. temporalis and m. superficial masseter), an elbow flexor (m. brachialis) and extensor (m. triceps- lateral head), and a knee flexor (m. biceps femoris-short head) and extensor (m. vastus lateralis) across 18 species of primates. Our results show that muscles of the feeding system are more orthogonally oriented relative to the resistance arm (mandible) and operate at relatively large moment arms and mechanical advantages. Moreover, these variables show relatively little change across the range of jaw excursion. In contrast, the representative muscles of the locomotor system have much smaller mechanical advantages and, depending on joint position, smaller muscle moment arm lengths and almost parallel orientations relative to the resistance arm. These patterns are consistent regardless of phylogeny, body mass, locomotor mode, and feeding specialization. We argue that these findings reflect fundamental functional dichotomies between tetrapod locomotor and feeding systems. By organizing muscles in a manner such that moment arms and mechanical advantage are relatively small, the locomotor system can produce broad joint excursions and high angular velocities with only small muscular contraction. As such, the anatomical organization of muscles within the limbs allows striding animals to move relatively rapidly and with minimal energetic expenditure. In contrast, the anatomical configuration of muscles in the feeding system, at least m. superficial masseter and m. temporalis, favors their force-producing capacity at the expense of excursion and velocity.

Effect of food hardness on chewing behavior in children

Objective

To investigate the effects of food hardness on chewing behavior in children compared with adults.

Materials and methods

Healthy children (3–17 years) were equally divided into five groups based on their dental eruption stages. Each participant ate soft and hard viscoelastic test food models (3 each), while the three-dimensional jaw movements and electromyographic (EMG) activity of the bilateral masseter muscles were recorded. The data from the children were compared with a control group of healthy adults (18–35 years). The data were analyzed with nonparametric tests.

Results

There was no significant difference in the number of chewing cycles and the duration of the chewing sequence between children groups and adults. Children with primary dentition (3–5 years) showed shorter lateral jaw movement and higher muscle activity at the end of the chewing sequence, compared with adults. Further, children’s age-groups (3–14 years) failed to adapt their jaw muscle activity to food hardness. However, at the late-permanent dentition stage (15–17 years), children were capable of performing adult-like chewing behavior.

Conclusions

Overall, it seems that children as young as 3-year-old are quite competent in performing basic chewing function similar to adults. Yet, there are differences in the anticipation or adaption of jaw muscle activity and jaw kinematics to food hardness.

Clinical relevance

The study may have clinical implication in the diagnosis and management of children with chewing impairment associated with dental malocclusions and other orofacial dysfunctions.

Fine-wire recordings of flexor hallucis brevis motor units up to maximal voluntary contraction reveal a flexible, nonrigid mechanism for force control

Our current knowledge on the neurophysiological properties of intrinsic foot muscles is limited, especially at high forces. This study therefore aimed to investigate the discharge characteristics of single motor units in an intrinsic foot muscle, namely flexor hallucis brevis, during voluntary contractions up to 100% of maximal voluntary contraction. We measured the recruitment threshold and discharge rate of flexor hallucis brevis motor units using indwelling fine-wire electrodes. Ten participants followed a target ramp up to maximal voluntary contraction by applying a metatarso-phalangeal flexion torque. We observed motor unit recruitment thresholds across a wide range of isometric forces (ranging from 10 to 98% of maximal voluntary contraction) as well as across a wide range of discharge rates (ranging from 4.8 to 23.3 Hz for initial discharge rate and 9.5 to 34.2 Hz for peak discharge rate). We further observed patterns of high variability in recruitment threshold and discharge rate as well as crossover in discharge rate between motor units within the same participant. These findings suggest that the force output of a muscle is generated through a mechanism with substantial variability rather than relying on a rigid organization, which is in contrast to the proposed onion-skin theory. The demands placed on the plantar intrinsic foot muscles during high- and low-force tasks may explain these observed neurophysiological properties.NEW & NOTEWORTHY We recorded for the first time single motor unit action potential trains in the flexor hallucis brevis, a short toe muscle, over the full range of maximum voluntary contraction. Its motor units are recruited up to very high (98%) recruitment thresholds with a substantial range of discharge rates. We further show high variability with crossover of discharge rates as a function of recruitment threshold both between participants and between motor units within participants.

A Comparison of Ovine Facial and Limb Muscle as a Primary Cell Source for Engineered Skeletal Muscle

Volumetric muscle loss (VML) contributes to the number of soft tissue injuries that necessitate reconstructive surgery, but treatment options are often limited by tissue availability and donor site morbidity. To combat these issues, our laboratory has developed scaffold-free tissue-engineered skeletal muscle units (SMUs) as a novel treatment for VML injuries. Recently, we have begun experiments addressing VML in facial muscle, and the optimal starting cell population for engineered skeletal muscle tissue for this application may not be cells derived from hindlimb muscles due to reported heterogeneity of cell populations. Thus, the purpose of this study was to compare SMUs fabricated from both craniofacial and hindlimb sources to determine which cell source is best suited for the engineering of skeletal muscle. Herein, we assessed the development, structure, and function of SMUs derived from four muscle sources, including two hindlimb muscles (i.e., soleus and semimembranosus [SM]) and two craniofacial muscles (i.e., zygomaticus major and masseter). Overall, the zygomaticus major exhibited the least efficient digestion, and SMUs fabricated from this muscle exhibited the least aligned myosin heavy chain staining and consequently, the lowest average force production. Conversely, the SM muscle exhibited the most efficient digestion and the highest number of myotubes/mm²; however, the SM, masseter, and soleus groups were roughly equivalent in terms of force production and histological structure. Impact Statement An empirical comparison of the development, structure, and function of engineered skeletal muscle tissue fabricated from different muscles, including both craniofacial and hindlimb sources, will not only provide insight into innate regenerative mechanisms of skeletal muscle but also will give our team and other researchers the information necessary to determine which cell sources are best suited for the skeletal muscle tissue engineering.

Functional and molecular outcomes of the human masticatory muscles

The masticatory muscles achieve a broad range of different activities such as chewing, sucking, swallowing, and speech. In order to accomplish these duties, masticatory muscles have a unique and heterogeneous structure and fiber composition, enabling them to produce their strength and contraction speed largely dependent on their motor units and myosin proteins that can change in response to genetic and environmental factors. Human masticatory muscles express unique myosin isoforms, including a combination of thick fibers, expressing myosin light chains (MyLC) and myosin class I and II heavy chains (MyHC) -IIA, -IIX, α-cardiac, embryonic and neonatal and thin fibers, respectively. In this review, we discuss the current knowledge regarding the importance of fiber-type diversity in masticatory muscles versus supra- and infrahyoid muscles, and versus limb and trunk muscles. We also highlight new information regarding the adaptive response and specific genetic variations of muscle fibers on the functional significance of the masticatory muscles, which influences craniofacial characteristics, malocclusions, or asymmetry. These findings may offer future possibilities for the prevention of craniofacial growth disturbances.

Spectral components in electromyograms from four regions of the human masseter, in natural dentate and edentulous subjects with removable prostheses and implants

OBJECTIVE

To compare the frequency or spectral components between different regions of the superficial masseter in young natural dentate and total edentulous older adults rehabilitated with removable prostheses and fixed-implant support. A secondary objective was to compare these components between the three groups.

DESIGN

21 young natural dentate and 28 edentulous (14 with removable prostheses and 14 with fixed-implant support) were assessed. High-density surface electromyography (sEMG) was recorded in four portions of the superficial masseter during submaximal isometric bites. Spectral components were obtained through a spectral analysis of the sEMG signals. An analysis of mixed models was used to compare the spectral components.

RESULTS

In all groups, the spectral components of the anterior portion were lower than in the posterior region (p < 0.05). Both edentulous groups showed lower spectral components and median frequency slope than the natural dentate group (p < 0.05). The removable prostheses group showed the greatest differences with natural dentate group.

CONCLUSIONS

There were significant differences in the spectral components recorded in the different regions of the superficial masseter. The lower spectral components and fatigability of older adults rehabilitated with prostheses could be a cause of a greater loss of type II fibers, especially in the removable prostheses group.

Relationship of aging, skeletal muscle mass, and tooth loss with masseter muscle thickness

Background

Previous studies have reported a relationship between masseter muscle thickness and tooth loss or limb muscle thickness. However, it is not yet known whether masseter muscle thickness is related to appendicular skeletal muscle mass, and grip strength. The purpose of this study was to determine which of the two variables—tooth loss or appendicular skeletal muscle mass index—is more strongly related to masseter muscle thickness, and to identify a suitable indicator of decreasing masseter muscle thickness in healthy elderly individuals.

Methods

Grip strength, walking speed, body weight, skeletal muscle mass index, tooth loss, and masseter muscle thickness at rest and during contraction were determined in 97 community-dwelling elderly individuals aged ≥65 years (men: 44, women: 53). Masseter muscle thickness was chosen as the dependent variable, while age, skeletal muscle mass index, body weight, grip strength, and tooth loss were chosen as the independent variables. Multiple regression analysis was conducted using the stepwise regression method.

Results

In men, grip strength was the only independent predictor of masseter muscle thickness at rest. Tooth loss and grip strength were independent predictor of masseter muscle thickness during contraction. In women, tooth loss was the independent predictor of masseter muscle thickness both at rest and during contraction, while grip strength and body weight were the independent predictor of masseter muscle thickness at rest only.

Conclusions

We confirmed that in healthy elderly individuals, tooth loss has a stronger relationship with masseter muscle thickness than aging and skeletal muscle mass index do. Masseter muscle thickness in both elderly men and women is also associated with grip strength, suggesting that grip strength can be used as an indicator of masseter muscle thickness in this population.

Functional compartmentalization of the human superficial masseter muscle

Some muscles have demonstrated a differential recruitment of their motor units in relation to their location and the nature of the motor task performed; this involves functional compartmentalization. There is little evidence that demonstrates the presence of a compartmentalization of the superficial masseter muscle during biting. The aim of this study was to describe the topographic distribution of the activity of the superficial masseter (SM) muscle's motor units using high-density surface electromyography (EMGs) at different bite force levels. Twenty healthy natural dentate participants (men: 4; women: 16; age 20±2 years; mass: 60±12 kg, height: 163±7 cm) were selected from 316 volunteers and included in this study. Using a gnathodynamometer, bites from 20 to 100% maximum voluntary bite force (MVBF) were randomly requested. Using a two-dimensional grid (four columns, six electrodes) located on the dominant SM, EMGs in the anterior, middle-anterior, middle-posterior and posterior portions were simultaneously recorded. In bite ranges from 20 to 60% MVBF, the EMG activity was higher in the anterior than in the posterior portion (p-value = 0.001).The center of mass of the EMG activity was displaced towards the posterior part when bite force increased (p-value = 0.001). The topographic distribution of EMGs was more homogeneous at high levels of MVBF (p-value = 0.001). The results of this study show that the superficial masseter is organized into three functional compartments: an anterior, a middle and a posterior compartment. However, this compartmentalization is only seen at low levels of bite force (20-60% MVBF).

Myosinopathies: pathology and mechanisms

The myosin heavy chain (MyHC) is the molecular motor of muscle and forms the backbone of the sarcomere thick filaments. Different MyHC isoforms are of importance for the physiological properties of different muscle fiber types. Hereditary myosin myopathies have emerged as an important group of diseases with variable clinical and morphological expression depending on the mutated isoform and type and location of the mutation. Dominant mutations in developmental MyHC isoform genes (MYH3 and MYH8) are associated with distal arthrogryposis syndromes. Dominant or recessive mutations affecting the type IIa MyHC (MYH2) are associated with early-onset myopathies with variable muscle weakness and ophthalmoplegia as a consistent finding. Myopathies with scapuloperoneal, distal or limb-girdle muscle weakness including entities, such as myosin storage myopathy and Laing distal myopathy are the result of usually dominant mutations in the gene for slow/β cardiac MyHC (MYH7). Protein aggregation is part of the features in some of these myopathies. In myosin storage myopathy protein aggregates are formed by accumulation of myosin beneath the sarcolemma and between myofibrils. In vitro studies on the effects of different mutations associated with myosin storage myopathy and Laing distal myopathy indicate altered biochemical and biophysical properties of the light meromyosin, which is essential for thick filament assembly. Protein aggregates in the form of tubulofilamentous inclusions in association with vacuolated muscle fibers are present at late stage of dominant myosin IIa myopathy and sometimes in Laing distal myopathy. These protein aggregates exhibit features indicating defective degradation of misfolded proteins. In addition to protein aggregation and muscle fiber degeneration some of the myosin mutations cause functional impairment of the molecular motor adding to the pathogenesis of myosinopathies.

Remarkable heterogeneity in myosin heavy-chain composition of the human young masseter compared with young biceps brachii

Adult human jaw muscles differ from limb and trunk muscles in enzyme-histochemical fibre type composition. Recently, we showed that the human masseter and biceps differ in fibre type pattern already at childhood. The present study explored the myosin heavy-chain (MyHC) expression in the young masseter and biceps muscles by means of gel electrophoresis (GE) and immuno-histochemical (IHC) techniques. Plasticity in MyHC expression during life was evaluated by comparing the results with the previously reported data for adult muscles. In young masseter, GE identified MyHC-I, MyHC-IIa MyHC-IIx and small proportions of MyHC-fetal and MyHC-α cardiac. Western blots confirmed the presence of MyHC-I, MyHC-IIa and MyHC-IIx. IHC revealed in the masseter six isomyosins, MyHC-I, MyHC-IIa, MyHC-IIx, MyHC-fetal, MyHC α-cardiac and a previously not reported isoform, termed MyHC-IIx'. The majority of the masseter fibres co-expressed two to four isoforms. In the young biceps, both GE and IHC identified MyHC-I, MyHC-IIa and MyHC-IIx. MyHC-I predominated in both muscles. Young masseter showed more slow and less-fast and fetal MyHC than the adult and elderly masseter. These results provide evidence that the young masseter muscle is unique in MyHC composition, expressing MyHC-α cardiac and MyHC-fetal isoforms as well as hitherto unrecognized potential spliced isoforms of MyHC-fetal and MyHC-IIx. Differences in masseter MyHC expression between young adult and elderly suggest a shift from childhood to adulthood towards more fast contractile properties. Differences between masseter and biceps are proposed to reflect diverse evolutionary and developmental origins and confirm that the masseter and biceps present separate allotypes of muscle.