Twitch Fibers Research Articles

Exercise in Chronic Kidney Disease Patients Does Not Significantly Alter Fibrosis in Quadriceps Muscle

Background: Chronic kidney disease (CKD) is commonly associated with decreased physical activity and muscle dysfunction. Muscles in CKD become fibrotic, containing excess extracellular matrix (ECM), particularly collagen. Collagen is cross-linked in the ECM, and increased cross-linking is often associated with fibrosis, stiffness, and reduced ECM remodeling. However, it is not known how exercise in patients with CKD may impact the extent of collagen cross-linking or fibrosis. Additionally, muscle fibrosis may impair muscle adaptations to exercise including, fiber size, fiber type, or vasculature. We predicted that collagen content and cross-linking would decrease, and there would be more Type I fibers and larger fiber size in the patients that were in the exercise group. Methods: Participants were randomized to exercise (n=15) or no exercise (n=5) groups in the Exercise Study Testing Enhanced Energetics of Mitochondria (ESTEEM) clinical trial of non-dialysis patients with CKD. The exercise program consisted of 35 min sessions, 3 times a week for 12 weeks consisting of high-intensity interval training, strength training, and power walking. Vastus lateralis biopsies were obtained before and after the 12 weeks of exercise. A bundle of muscle tissue was powdered and separated into a pepsin soluble and insoluble fraction before applying a hydroxyproline assay for collagen content, with the insoluble fraction representing more cross-linked collagen. For histological analysis 10 μm thick cryosections were stained with Picrosirius Red, or immunofluorescent stains to investigate satellite cells, vasculature, fibro-adipogenic progenitors (FAPs), and muscle fiber types. Picrosirius red sections were imaged under polarized light to look at birefringence as a measure of collagen density. Sections that were stained for fiber type and vasculature were analyzed using semi-automatic muscle analysis using segmentation of histology (SMASH). Results: The overall amount of collagen and collagen cross-linking were not altered in either the exercise or no exercise groups. Picrosirius red stained sections did not reveal a change in ECM area or collagen packing density with exercise. Surprisingly minimum ferret diameter and percentage of type I (slow twitch) fibers decreased in most patients that underwent the exercise program. Type I and type IIa (fast oxidative) fiber size also significantly decreased, while there was no change in type IIx (fast glycolytic) fiber size. Conclusion: This study shows that exercise does not dramatically reduce ECM content or muscle fibrosis in patients with CKD. Surprisingly, the response to exercise was opposite of that anticipated in healthy subject with exercise in CKD producing smaller muscle fibers and a shift away from more oxidative type I fibers. This shows that exercise may have limited capacity to enhance function in CKD muscle, potentially due to irreversible fibrosis. Work supported by mitochondrial bioenergetics in persons with chronic kidney disease (NIH R03DK114502). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Finite element model of ocular adduction with unconstrained globe translation

Details of the anatomy and behavior of the structures responsible for human eye movements have been extensively elaborated since the first modern biomechanical models were introduced. Based on these findings, a finite element model of human ocular adduction is developed based on connective anatomy and measured optic nerve (ON) properties, as well as active contractility of bilaminar extraocular muscles (EOMs), but incorporating the novel feature that globe translation is not otherwise constrained so that realistic kinematics can be simulated. Anatomy of the hemisymmetric model is defined by magnetic resonance imaging. The globe is modeled as suspended by anatomically realistic connective tissues, orbital fat, and contiguous ON. The model incorporates a material subroutine that implements active EOM contraction based on fiber twitch characteristics. Starting from the initial condition of 26° adduction, the medial rectus (MR) muscle was commanded to contract as the lateral rectus (LR) relaxed. We alternatively modeled absence or presence of orbital fat. During pursuit-like adduction from 26 to 32°, the globe translated 0.52 mm posteriorly and 0.1 mm medially with orbital fat present, but 1.2 mm posteriorly and 0.1 mm medially without fat. Maximum principal strains in the optic disk and peripapillary reached 0.05–0.06, and von-Mises stress 96 kPa. Tension in the MR orbital layer was ~ 24 g-force after 6° adduction, but only ~ 3 gm-f in the whole LR. This physiologically plausible simulation of EOM activation in an anatomically realistic globe suspensory system demonstrates that orbital connective tissues and fat are integral to the biomechanics of adduction, including loading by the ON.

A comparative study of lower limb strength during vertical jump of male college students in table tennis, badminton and tennis

The objectives of this paper are to explore the correlation of kinetic indices of SJ (squat jump) and CMJ (counter-movement jump) and the differences of lower limb strength mechanics indexes of table tennis, badminton and tennis players under SJ and CMJ. This study included 60 male collegiate athletes, 20 each from table tennis, badminton, and tennis. Subjects performed SJ and CMJ tests on a force platform and the vertical jump data were analyzed. Table tennis, badminton and tennis players demonstrated significant differences in the SJ and CMJ assessments. Badminton players outperformed table tennis players in terms of peak power (PP) (p = 0.005) and peak velocity (PV) (p = 0.017). Badminton players beat table tennis players in PV (p = 0.028), PP (p = 0.022), fast twitch fibers (FTF) (p = 0.033) and pre-stretch effect (EP) (p = 0.0004). Tennis players exhibited lower peak force (PF) (p = 0.006) indicators than badminton players. For athletes in all three sports, the SJ test markers (vertical jump displacement, PF, PP and PV) demonstrated a strong positive correlation. There was a highly significant positive correlation between VJD and PF, PP, PV and FTF among badminton players. Significant positive connections were discovered between PF, PP, PV, FTF and EP, as well as between PP and PV and FTF and EP. PV and FTF had a very strong positive correlation, as did EP, PV and FTF. College badminton players had higher vertical jumps than table tennis and tennis players. In addition to vertical jump height, PP (power production), PV (power velocity) and FTF (force-time factors) are important markers for assessing the success of vertical jumps in athletes’ daily training. These findings can assist coaches and athletes develop better vertical jump training programs.

The effects of crank power and cadence on muscle fascicle shortening velocities, muscle activations and joint-powers during cycling.

Whilst people typically chose to locomote in most economical fashion, during bicycling they will, unusually, chose cadences that are higher than metabolically optimal. Empirical measurements of the intrinsic contractile properties of the vastus lateralis (VL) muscle during submaximal cycling suggest that the cadences that people self-selected (SSC) might allow for optimal muscle fascicle shortening velocity for the production of knee extensor muscle power. It remains unclear, however, whether this is consistent across different power outputs where the SSC varies. We examined the effect of cadence and external power requirements on muscle neuromechanics and joint powers during cycling. VL fascicle shortening velocities, muscle activations and joint-specific powers were measured during cycling between 60 and 120rpm (including SSC), while participants produced 10%, 30%, and 50% of peak maximal power. VL shortening velocity increased as cadence increased but was similar across the different power outputs. Although no differences were found in the distribution of joint powers across cadence conditions, the absolute knee joint power increased with increasing crank power output. Muscle fascicle shortening velocities increased in VL at the SSC as pedal power demands increased from submaximal towards maximal cycling. A secondary analysis of muscle activation patterns showed minimized activation of VL and other muscles near the SSC at the 10% and 30% power conditions. Minimization of activation with progressively increasing fascicle shortening velocities at the SSC may be consistent with the theory that the optimum shortening velocity for maximizing power increases with intensity of exercise and recruitment of fast twitch fibers.

Exercise in chronic kidney disease patients reduces percentage of slow twitch fibers in quadriceps muscle

Study Objective: Determine the responsiveness of skeletal muscle to exercise in patients with chronic kidney disease. Background: Chronic kidney disease (CKD) is commonly associated with decreased physical activity and muscle dysfunction. Muscles in CKD become fibrotic, containing excess extracellular matrix (ECM), particularly collagen. Collagen is crosslinked in the ECM, and increased crosslinking is often associated with fibrosis, stiffness, and reduced ECM remodeling. Muscle fibers can adapt to resistance exercise by having increased Type II (fast twitch) fibers. However, it is not known how exercise in patients with CKD may impact the extent of collagen crosslinking or fibrosis, fiber type, or vasculature within the muscle. We predicted that collagen content and crosslinking would decrease, and there would be more Type II fibers and less Type I fibers in the patients that were in the exercise group. Methods: Participants were randomized to exercise (n=9) or no exercise (n=3) groups, unless specified below, in the ESTEEM clinical trial of non-dialysis CKD. Exercise consisted of 35 min sessions, 3 times a week for 12 weeks consisting of high-intensity interval training, strength training, and power walking. Vastus lateralis biopsies were obtained pre and post the 12 weeks of exercise. Biochemical analysis used tissue powdered and separated into a pepsin and acidic acid soluble and insoluble fraction before applying a hydroxyproline assay (exercise n=12, no exercise n=5, healthy control n=1). The insoluble fraction represents the more crosslinked collagen. For histology tissue frozen in liquid nitrogen cooled isopentane was cut in 10 μm thick sections and stained with Picrosirius Red and for fiber types. Sections that were stained for fiber type were analyzed using semi-automatic muscle analysis using segmentation of histology (SMASH). Results: The overall amount of collagen and collagen crosslinking were not altered in either the exercise or no exercise groups. Picrosirius red stained sections did not reveal a change in ECM area with exercise. Minimum ferret diameter decreases in most patients that underwent the exercise program. The exercise group had less Type I (slow twitch) fibers and most patients had increased Type IIa (fast twitch) fibers after exercise, while the no exercise group increased Type I fibers. Conclusion: This study shows that CKD patient’s muscle fibers adapt to exercise with decrease in slow fibers, and smaller fiber size in most patients. Overall collagen content and crosslinked collagen were not influenced by exercise. ECM area also did not significantly change in either group. Mitochondrial bioenergetics in persons with chronic kidney disease (NIH R03DK114502), NIDDK R01DK129793, NIDDK R01DK125794, Dialysis Clinics Incorporated C-4112 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

多チャンネル表面筋電図伝播波解析による速筋線維活動評価の検討

It is significant to evaluate the recruitment of fast twitch fibers quantitively in means of preventing sarcopenia. In a previous study, m-ch method was proposed to observe propagation wave which originates from action potential of each muscle fiber. In this study, relation between strength of muscle contraction and propagation wave had been examined in purpose of evaluating recruitment of fast twitch fibers during training. The result suggests that most propagation waves acquired band's velocity has increased by increase of muscle contraction strength. There is a hope for evaluating recruitment of fast twitch fibers during training.

Impacts of thermal acclimatization on fish skeletal muscle

Thermal acclimation allows ectotherms to maintain physiological homeostasis while occupying habitats with constantly changing temperatures. This process is especially important in skeletal muscle which powers most movements necessary for life. We aimed to understand how fish skeletal muscle is impacted by acclimatization in the laboratory. To accomplish this, we compared muscle contraction kinetics of four-week lab acclimatized fish (at 20 °C) to fish taken directly from the field when sea surface temperatures were similar to lab treatment temperature (ocean temperature ranged from 17.7 to 19.9 °C in the four weeks prior to collection at 20 °C). To examine these effects, we chose to study tautog (Tautoga onitis) and cunner (Tautogolabrus adspersus) from Long Island Sound. We found that timing of contraction kinetics in cunner and tautog did not differ from the lab acclimatized and field acclimatized groups. However, lab acclimatized cunner produced greater contraction force than fish taken directly from the field. This increased force production allowed lab acclimatized cunner to produce greater power when compared to cunner from the field treatment. Furthermore, laboratory acclimatized cunner did not express any slow myosin heavy chain, suggesting that their muscle had transitioned to mostly fast twitch fibers after being held at a constant temperature in the lab. None of these effects were seen in tautog. In this work we highlight the importance of considering the impacts laboratory conditions have on experimental conditions.

Muscle architecture a new dimension of talent identification: A mini review

Talent Identification in the field of exercise science is growing exponentially, where muscle is considered as most reliable method for determining muscle fiber type and selecting sports activity accordingly. Muscle architecture a new fiber type determination technique is now in trend, which refers to the macroscopic arrangement of muscle fibres that influences a muscle's mechanical function. Previous research has shown a relationship between muscle architecture (such as fascicle length, muscle thickness, and pennation angle) and muscle fibre types (e.g. slow twitch fibres and fast twitch fibers). This small review attempts to briefly discuss how muscle architecture has been investigated in the realm of sport/exercise performance, as well as how various muscle fibre types have varied muscle architecture characteristics.

A comparative study of vertical jump parameters in men's college basketball and volleyball players

This study aimed to compare lower limb strength disparities between male college volleyball and basketball players in four distinct vertical jump types. We recruited 100 second-level (above the national standard) male college athletes (50 basketball and 50 volleyball players) from Capital University of Physical Education and Sports who volunteered to participate. We assessed the performance of each athlete with regards to the squat jump (SJ), countermovement jump (CMJ), continuous jump with straight legs (CJs), and continuous jump with bent legs (CJb). Data analysis was performed by independent samples t-tests in SPSS version 25.0. software. In the CMJ test, volleyball players had a significantly higher vertical jump displacement (VJD; p = 0.047, t = −2.018), squat displacement (SD; p = 0.005, t = −2.883), peak force (p = 0.023, t = −1.964), and peak velocity (p = 0.011, t = −1.687) than basketball players. In the SJ test, volleyball players had a significantly higher VJD (p = 0.041, t = −1.21) and SD (p = 0.008, t = −1.46) than basketball players. In the CJs test, volleyball players had a significantly higher average vertical jump displacement (AVJD; p = 0.042, t = −2.067) than basketball players. In the CJb test, volleyball players had a significantly higher AVJD (p = 0.001, t = −3.448), average squat displacement (p = 0.017, t = −2.44), and average contact time (p = 0.045, t = −2.038) than basketball players. The overall vertical jump ability of college volleyball players was better than that of college basketball players. CMJ, SJ, CJs and CJb training should be included in daily training sessions and considered in the selection of basketball and volleyball players. In addition, coaches can use fast twitch Fibers, the effect of pre-stretch and k as the main evaluation indicators to assess daily training progress.

K+-induced Potentiation And Post-tetanic Potentiation Act Via Independent Mechanisms In Mouse Fast-twitch Muscle

A potentiation of force in skeletal muscle has been described as a consequence of moderate increases in extracellular [K+]. This K+-induced potentiation exhibits similarities to the well-described phenomenon of post-tetanic potentiation (PTP), in that both affect force at low frequency of stimulation and that the effect is largest in muscles predominated by fast twitch fibers. Mechanistically PTP is caused largely by skeletal myosin light chain kinase (skMLCK) catalyzed phosphorylation of the myosin regulatory light chains (RLC), while the mechanism of K+-induced potentiation is still not fully elucidated. PURPOSE: To investigate the interactive effects of K+-induced potentiation and PTP and their possible mechanistic overlap between the two potentiation phenomena. METHODS: We assessed the influence of elevated [K+] and PTP on the isometric twitch force response in extensor digitorum longus muscles isolated from wild type and skeletal myosin light chain kinase absent mice (skMLCK-/-). Muscles were incubated in Tyrode solution with either 5 or 10 mM K+ at 25 °C. PTP was elicited by 4 tetanic contractions elicited by brief stimulation trains (0.2-0.4 s at 50-200 Hz) all delivered within 10 s. RESULTS: Increasing [K+] of the incubation medium from 5 to 10 mM increased isometric twitch force by a similar amount in wildtype and skMLCK-/- muscles (~ 12-14 in % both genotypes) (all data n = 7-8, P < 0.05). At 5 mM K+, the relative twitch potentiation caused by a tetanic stimulus series (PTP) was much greater for wildtype than for skMLCK-/- muscles (i.e. 24 and 8%, respectively). In addition, for both genotypes, in muscles already potentiated by 10 mM K+, PTP elicited the same degree of twitch potentiation as that seen at 5 mM K+ (25 and 10% increase in wildtype and skMLCK-/-, respectively). CONCLUSION: We conclude that [K+] and PTP are additive and that [K+]-induced potentiation is independent of RLC phosphorylation.

Extended Match Time Exacerbates Fatigue and Impacts Physiological Responses in Male Soccer Players.

This study evaluated how extended match time (90 + 30 min) affected physiological responses and fatigue in male soccer players. Twenty competitive players (mean ± SD: age, 20 ± 1 yr; maximal oxygen uptake, 59 ± 4 mL·min -1 ·kg -1 ) completed an experimental match with their activity pattern and heart rate assessed throughout the game, whereas countermovement jump performance and repeated sprint ability were tested and quadriceps muscle biopsies and venous blood samples were taken at baseline and after 90 and 120 min of match play. Less high-intensity running (12%) was performed in extra time in association with fewer intense accelerations and decelerations per minute compared with normal time. Peak sprint speed was 11% lower in extra time compared with normal time, and fatigue also manifested in impaired postmatch repeated sprint ability and countermovement jump performance (all P < 0.05). Muscle glycogen declined from 373 ± 59 mmol·kg -1 dry weight (dw) at baseline to 266 ± 64 mmol·kg -1 dw after 90 min, with a further decline to 186 ± 56 mmol·kg -1 dw after extra time ( P < 0.05) and with single-fiber analyses revealing depleted or very low glycogen levels in ~75% of both slow and fast twitch fibers. Blood glucose did not change during the first 90-min but declined ( P < 0.05) to 81 ± 8 mg·dL -1 after extra time. Plasma glycerol and ammonia peaked at 236 ± 33 mg·dL -1 and 75 ± 21 μmol·L -1 after the extra period. These findings demonstrate exacerbated fatigue after extra time compared with normal time, which seems to be associated with muscle glycogen depletion, reductions in blood glucose levels, and hyperammonemia. Together, this points to metabolic disturbances being a major part of the integrated and multifaceted fatigue response during extended soccer match play.

Leptin mediates the regulation of muscle mass and strength by adipose tissue.

Adipose tissue secretes numerous cytokines (termed ‘adipokines’) that have known or hypothesized actions on skeletal muscle. The majority of adipokines have been implicated in the pathological link between excess adipose and muscle insulin resistance, but approximately half also have documented in vitro effects on myogenesis and/or hypertrophy. This complexity suggests a potential dual role for adipokines in the regulation of muscle mass in homeostasis and the development of pathology. In this study, we used lipodystrophic ‘fat‐free’ mice to demonstrate that adipose tissue is indeed necessary for the development of normal muscle mass and strength. Fat‐free mice had significantly reduced mass (∼15%) and peak contractile tension (∼20%) of fast‐twitch muscles, a slowing of contractile dynamics and decreased cross‐sectional area of fast twitch fibres compared to wild‐type littermates. These deficits in mass and contractile tension were fully rescued by reconstitution of ∼10% of normal adipose mass, indicating that this phenotype is the direct consequence of absent adipose. We then showed that the rescue is solely mediated by the adipokine leptin, as similar reconstitution of adipose from leptin‐knockout mice fails to rescue mass or strength. Together, these data indicate that the development of muscle mass and strength in wild‐type mice is dependent on adipose‐secreted leptin. This finding extends our current understanding of the multiple roles of adipokines in physiology as well as disease pathophysiology to include a critical role for the adipokine leptin in muscle homeostasis. Key points Adipose‐derived cytokines (adipokines) have long been implicated in the pathogenesis of insulin resistance in obesity but likely have other under‐appreciated roles in muscle physiology.Here we use a fat‐free mouse to show that adipose tissue is necessary for the normal development of muscle mass and strength.Through add‐back of genetically modified adipose tissue we show that leptin is the key adipokine mediating this regulation.This expands our understanding of leptin's role in adipose–muscle signalling to include development and homeostasis and adds the surprising finding that leptin is the sole mediator of the maintenance of muscle mass and strength by adipose tissue.

Resistance exercise training and the motor unit.

Resistance exercise training (RET) is a key modality to enhance sports performance, injury prevention and rehabilitation, and improving overall health via increases in muscular strength. Yet, the contribution of neural mechanisms to increases in muscular strength are highly debated. This is particularly true for the involvement of the motor unit, which is the link between neural (activation) and mechanical (muscle fiber twitch forces) mechanisms. A plethora of literature that examines the effects of RET on skeletal muscle speculate the role of motor units, such as increases in firing rates partially explains muscular strength gains. Results, however, are mixed regarding changes in firing rates in studies that utilize single motor unit recordings. The lack of clarity could be related to vast or subtle differences in RET programs, methods to record motor units, muscles tested, types of contractions and intensities used to record motor units, etc. Yet to be discussed, mixed findings could be the result of non-uniform MU behavior that is not typically accounted for in RET research. The purpose of this narration is to discuss the effects of acute resistance exercise training studies on MU behavior and to provide guidance for future research.

Correlation analysis of vertical jump variables in male track and field athletes

Background: The purpose of this study is threefold: (1) compare differences in countermovement jump (CMJ) variables and squat jump (SJ) variables in male track and field athletes; (2) explore the correlation of Fast Twitch Fibers (FT), Effect of Pre-stretch (EP) and other variables during the CMJ in male track and field athletes; (3) explore the correlation of SJ variables in male track and field athletes. Methods: 96 male university athletes (21.25 ± 1.04 years; 71.96 ± 8.58 kg; 180.05 ± 5.66 cm) in track and field volunteered to participate in this study. They are from Capital University of Physical Education and Sports, and all athletes are above the national standard at the second level. Subjects sequentially completed 3 CMJs and 3 SJs on the force plate. Throughout the entire range of motion, the CMJ and SJ were performed with both hands on the hips. In a laboratory, all of the individuals were assessed at the same time. SPSS 25.0 (Chicago, IL, USA) was used to run independent samples t-test and Pearson correlation analysis. Results: The vertical jump displacement (VJD) (p &lt; 0.01), squat displacement (SD) (p &lt; 0.01), peak velocity (PV) (p &lt; 0.01), peak power (PP) (p &lt; 0.05), average power (AP) (p &lt; 0.01) were significantly higher during the CMJ than during the SJ. The peak force (PF) (p &lt; 0.01) was significantly smaller during the CMJ than during the SJ. The FT and EP during the CMJ were associated with low test-retest reliability (coefficient of variation (CV): 9.73–8.86%). VJD, SD, PF, PP, and AP produced high test-retest reliability (CV: 2.29–4.48%) during both the CMJ and SJ movements. The correlation results were as follows, the VJD during the CMJ was significantly related to SD, PF, PP, AP (r = 0.21, r = 0.42, r = 0.8, r = 0.69, respectively). The PF during the CMJ was significantly related to PP and AP (r = 0.87, r = 0.72, respectively). The PV during the CMJ was significantly related to PP and AP (r = 0.63, r = 0.79, respectively). During the CMJ, there were significant connections between PP and AP (r = 0.94). Except for SD showed no significant relationships and the results for the correlation of other variables were the same as CMJ during the SJ. Furthermore, the Fast Twitch Fibers (FT) during the CMJ was significantly related to PP and AP (r = 0.49, r = 0.46, respectively). The Effect of Pre-stretch (EP) during the CMJ was significantly related to PV, PP, AP and FT (r = 0.36, r = 0.24, r = 0.27, r = 0.22, respectively). Conclusions: Our results indicate that both FT and EP were highly significantly correlated with PP in CMJ, and both FT and EP were significantly correlated with AP in CMJ. In addition, FT and EP data have good reliability. It means that FT and EP may be important indicators of lower limb strength in male track and field athletes under certain conditions. This will inform the training of male track and field athletes.

Analysis of Masticatory Muscle Tendon-aponeurosis Hyperplasia by Using Next-generation Sequencing.

Masticatory muscle tendon-aponeurosis hyperplasia (MMTAH) is a disease associated with a mouth opening limitation. Here, we conducted a bioinformatics analysis to examine gene expression patterns in patients with MMTAH in comparison to those with facial deformity (FD). Seven MMTAH patients and three FD patients were recruited. We conducted RNA sequencing analysis, quantitative reverse transcription polymerase chain reaction and immunoblot analysis. Of the identified 19,767 mapped read tags that showed clear differential expression, 2,471 genes were significantly up-regulated and 2,849 genes were significantly down-regulated in patients with MMTAH compared to those in patients with FD. Among the up-regulated genes, ten genes were significantly increased. The distribution of up-regulated and down-regulated genes at different ages tended to be similar. Moreover, the protein levels of Ankyrin Repeat Domain 2, Troponin T1 and myosin heavy chain 7, which are associated with slow twitch fibers and mechanical loading, were strongly expressed in patients with MMTAH compared to those in patients with FD. The gene expression pattern in MMTAH patients was similar regardless of age. As the transition of fast-to-slow twitch in the skeletal muscle is induced by mechanical loading, and up-regulation of slow twitch molecules was observed in MMTAH patients, mechanical loading is suggested to be implicated in MMTAH.

Immunohistochemistry of kangaroo rat hindlimb muscles.

Kangaroo rats (Dipodomys spp.) use specialized bipedal hopping like that of kangaroos. In contrast to kangaroos that have elastic tendons capable of storing energy, kangaroo rats have inelastic tendons that are unable to store large amounts of energy. Thus, the musculature of the ankle joint provides the greatest power contribution to kangaroo rat hopping. Skeletal muscle can be characterized by several fiber types, including slow twitch (Type I) and fast twitch (Type II) fibers. Fast fibers are found in higher concentration in muscles that perform quick, dynamic movements, whereas slow fibers are found in higher proportion in muscles that perform slow, endurant movements. Using fiber type specific antibodies, we identified four pure (Types I, IIA, IIB, and IIX) and two hybrid (Types I/IIA and IIA/IIX) fiber types in six hindlimb muscles from three kangaroo rats (Dipodomys merriami) to investigate the relationship between fiber composition and hindlimb muscle function. Hindlimb muscles (except soleus) were dominated by Type IIB fibers, which were largest in cross-sectional area, and are known to be best suited for rapid and explosive movements. Oxidative Type IIA and Type IIX fibers were found at moderate concentrations and likely function in maintaining continual saltatory locomotion. Thus, kangaroo rats can use these two fiber type populations as "gears" for both endurant and explosive behaviors.

The missing 'link'? β-Catenin's role in skeletal muscle glucose uptake during exercise and contraction.

The missing 'link'? β-Catenin's role in skeletal muscle glucose uptake during exercise and contraction.

Troponin Variants as Markers of Skeletal Muscle Health and Diseases.

Ca2+-regulated contractility is a key determinant of the quality of muscles. The sarcomeric myofilament proteins are essential players in the contraction of striated muscles. The troponin complex in the actin thin filaments plays a central role in the Ca2+-regulation of muscle contraction and relaxation. Among the three subunits of troponin, the Ca2+-binding subunit troponin C (TnC) is a member of the calmodulin super family whereas troponin I (TnI, the inhibitory subunit) and troponin T (TnT, the tropomyosin-binding and thin filament anchoring subunit) are striated muscle-specific regulatory proteins. Muscle type-specific isoforms of troponin subunits are expressed in fast and slow twitch fibers and are regulated during development and aging, and in adaptation to exercise or disuse. TnT also evolved with various alternative splice forms as an added capacity of muscle functional diversity. Mutations of troponin subunits cause myopathies. Owing to their physiological and pathological importance, troponin variants can be used as specific markers to define muscle quality. In this focused review, we will explore the use of troponin variants as markers for the fiber contents, developmental and differentiation states, contractile functions, and physiological or pathophysiological adaptations of skeletal muscle. As protein structure defines function, profile of troponin variants illustrates how changes at the myofilament level confer functional qualities at the fiber level. Moreover, understanding of the role of troponin modifications and mutants in determining muscle contractility in age-related decline of muscle function and in myopathies informs an approach to improve human health.

Characterization of laryngeal motor neuron properties in the American bullfrog, Lithobates catesbieanus

Motor neurons represent the final output from the central respiratory network. American bullfrogs, Lithobates catesbieanus, have provided insight into development and plasticity of the breathing control system, yet cellular aspects of bullfrog motor neurons are not well-described. In this study, we characterized properties of laryngeal motor neurons that produce motor outflow to the glottal dilator, a muscle that gates airflow to the lungs of anurans. To this end, we measured several intrinsic membrane properties of labeled laryngeal motor neurons in brain slices. Using unsupervised clustering analyses, we identified two broad classes of motor neurons: those with high firing rates and strong adaptation (∼70 %), and those with lower firing rates and less adaptation (∼30 %). These results suggest that two neuronal cell types innervate the glottal dilator, roughly aligning with the composition of fast and slower twitch fibers of this muscle. In sum, these data reinforce the need to consider cell-type when assessing motor neuron function in the respiratory network.

Effect of maternal overnutrition on predisposition to insulin resistance in the foal: Foal skeletal muscle development and insulin signaling

Skeletal muscle plays an integral role in the ability of a horse to perform at high levels. Shifts in skeletal muscle development in response to maternal plane of nutrition may have substantial and lasting impacts on athletic performance and whole-body metabolism. Therefore, sixteen Quarter Horse mares were used in a completely randomized design and maintained at a body condition score (BCS) 6 until start of third trimester. On d 235 of gestation, mares were randomly assigned to receive one of two dietary treatments with a diet formulated to meet requirements during late gestation (CON; n = 8), and an overfed diet (HIGH; n = 8) where mares received an additional 40% above CON. Five h after parturition, foals were euthanized, and gluteus medius, triceps brachii, and semitendinosus were harvested for analyses. Gene expression was determined by qPCR and western immunoblotting was used to quantify total and phosphorylated forms of proteins involved in skeletal muscle metabolism with tubulin as the loading control. All data were analyzed using PROC MIXED of SAS. Foals from HIGH mares exhibited larger skeletal muscle fibers by area (P <0.05), and a shift in muscle fiber development towards type I slow twitch muscle fibers (P <0.05). Relative expression of glucose transporter 4 (GLUT4) was lower in HIGH foals compared to CON in gluteus medius (P = 0.05). Insulin receptor isoform B (INSR-B) and insulin-like growth factor 1 receptor (IGF1R) were greater in triceps brachii of HIGH foals compared to CON (P ≤ 0.03). Insulin receptor isoform A (INSR-A), however, tended to be lower in triceps brachii of HIGH compared to CON (P = 0.10). Ratios of phosphorylated to total extracellular signal-regulated protein kinase 1/2 (ERK1/2) and c-June N-terminal kinase (JNK) were higher in HIGH foals compared to CON (P ≤0.04) in gluteus medius. There were no differences observed for phosphorylated to total protein ratios in semitendinosus and triceps brachii muscles; however, total ERK1/2 tended to be elevated (P <0.10) in semitendinosus from CON foals compared to HIGH. There was no difference in phosphorylated or total protein kinase B (AKT) (P >0.14). These data indicate hypertrophy of skeletal muscle fibers and a shift towards type I slow twitch fibers in HIGH foals. Furthermore, this study identifies muscle specific changes in gene expression and downstream insulin receptor signaling, which may contribute to future metabolic abnormalities in response to maternal overnutrition.