Muscle Fiber Type Distribution Research Articles

Neuromuscular Impairment Following Chronic Moderate-Severe Contusion in Spinal Cord Injured Rats

Muscle atrophy and neuromuscular impairment are consequences of spinal cord injury (SCI) that impede quality of life and functional recovery. PURPOSE: To examine time course changes in muscle fiber type distribution, fiber cross-sectional area (fCSA), and other histologic characteristics of muscle pathology occurring in rats in response to moderate-severe contusion SCI. METHODS: Twenty-four 4-month old male Sprague-Dawley rats received SHAM surgery or T9 laminectomy plus moderate-severe (250 kilodyne) contusion SCI by a computer-guided impactor. Body weight and Basso-Beattie-Bresnahan (BBB) hindlimb locomotor rating scores were measured weekly. Animals were euthanized and soleus were harvested at 2-weeks, 1-month, 2-months, or 3-months post-surgery. Soleus fiber-type distribution, fCSA, and muscle-nerve bundle morphology were assessed by immunofluorescent staining, imaged using an epifluorescent microscope, and quantified with semi-automatic muscle analysis using segmentation of histology (SMASH). RESULTS: At 1-week post-surgery, SCI animals exhibited near-complete hindlimb paralysis (indicated by BBB scores <3), with minimal improvement in voluntary hindlimb locomotor function thereafter. Body weight, soleus mass, and median fCSA were significantly lower in SCI vs SHAM animals (p<0.01 at all timepoints). A slow-to-fast fiber-type shift was observed in SCI animals, with a progressive ~20% decrease in the number of type I fibers, ~8% increase in type IIa fibers, and ~5% increase in hybrid type I/IIa fibers at each consecutive timepoint, along with the emergence of unstained type IIx/b muscle fibers (~30% of total) at 3-months. In addition, muscle fiber splitting was present in SCI animals at 2-months, as well as reduced neurofilament staining in SCI muscle-nerve bundles. Furthermore, evidence of growth related remodeling occurred in SHAM muscles from increased centrally nucleated fibers across timepoints but not in SCI. CONCLUSION: Deterioration in motor ability accompanying SCI produced muscle atrophy and progressive impairments in muscle oxidative capacity that may have resulted from repetitive denervation-reinnervation cycles. These factors may have contributed to muscle pathology resulting in limited capacity for muscle growth and remodeling.

Cellular Aspects of Muscle Specialization Demonstrate Genotype - Phenotype Interaction Effects in Athletes.

IntroductionGene polymorphisms are associated with athletic phenotypes relying on maximal or continued power production and affect the specialization of skeletal muscle composition with endurance or strength training of untrained subjects. We tested whether prominent polymorphisms in genes for angiotensin converting enzyme (ACE), tenascin-C (TNC), and actinin-3 (ACTN3) are associated with the differentiation of cellular hallmarks of muscle metabolism and contraction in high level athletes.MethodsMuscle biopsies were collected from m. vastus lateralis of three distinct phenotypes; endurance athletes (n = 29), power athletes (n = 17), and untrained non-athletes (n = 63). Metabolism-, and contraction-related cellular parameters (such as capillary-to-fiber ratio, capillary length density, volume densities of mitochondria and intramyocellular lipid, fiber mean cross sectional area (MCSA) and volume densities of myofibrils) and the volume densities of sarcoplasma were analyzed by quantitative electron microscopy of the biopsies. Gene polymorphisms of ACE (I/D (insertion/deletion), rs1799752), TNC (A/T, rs2104772), and ACTN3 (C/T, rs1815739) were determined using high-resolution melting polymerase chain reaction (HRM-PCR). Genotype distribution was assessed using Chi2 tests. Genotype and phenotype effects were analyzed by univariate or multivariate analysis of variance and post hoc test of Fisher. P-values below 0.05 were considered statistically significant.ResultsThe athletes demonstrated the specialization of metabolism- and contraction-related cellular parameters. Differences in cellular parameters could be identified for genotypes rs1799752 and rs2104772, and localized post hoc when taking the interaction with the phenotype into account. Between endurance and power athletes these concerned effects on capillary length density for rs1799752 and rs2104772, fiber type distribution and volume densities of myofibrils (rs1799752), and MSCA (rs2104772). Endurance athletes carrying the I-allele of rs1799752 demonstrated 50%-higher volume densities of mitochondria and sarcoplasma, when power athletes that carried only the D-allele showed the highest fiber MCSAs and a lower percentage of slow type muscle fibers.DiscussionACE and tenascin-C gene polymorphisms are associated with differences in cellular aspects of muscle metabolism and contraction in specifically-trained high level athletes. Quantitative differences in muscle fiber type distribution and composition, and capillarization in knee extensor muscle explain, in part, identified associations of the insertion/deletion genotypes of ACE (rs1799752) with endurance- and power-type Sports.

AMPK Mediates Muscle Mass Change But Not the Transition of Myosin Heavy Chain Isoforms during Unloading and Reloading of Skeletal Muscles in Mice.

5′AMP-activated protein kinase (AMPK) plays an important role in the regulation of skeletal muscle mass and fiber-type distribution. However, it is unclear whether AMPK is involved in muscle mass change or transition of myosin heavy chain (MyHC) isoforms in response to unloading or increased loading. Here, we checked whether AMPK controls muscle mass change and transition of MyHC isoforms during unloading and reloading using mice expressing a skeletal-muscle-specific dominant-negative AMPKα1 (AMPK-DN). Fourteen days of hindlimb unloading reduced the soleus muscle weight in wild-type and AMPK-DN mice, but reduction in the muscle mass was partly attenuated in AMPK-DN mice. There was no difference in the regrown muscle weight between the mice after 7 days of reloading, and there was concomitantly reduced AMPKα2 activity, however it was higher in AMPK-DN mice after 14 days reloading. No difference was observed between the mice in relation to the levels of slow-type MyHC I, fast-type MyHC IIa/x, and MyHC IIb isoforms following unloading and reloading. The levels of 72-kDa heat-shock protein, which preserves muscle mass, increased in AMPK-DN-mice. Our results indicate that AMPK mediates the progress of atrophy during unloading and regrowth of atrophied muscles following reloading, but it does not influence the transition of MyHC isoforms.

Altered Lipid Metabolism Impairs Skeletal Muscle Force in Young Rats Submitted to a Short-Term High-Fat Diet.

Obesity and ensuing disorders are increasingly prevalent in young populations. Prolonged exposure to high-fat diets (HFD) and excessive lipid accumulation were recently suggested to impair skeletal muscle functions in rodents. We aimed to determine the effects of a short-term HFD on skeletal muscle function in young rats. Young male Wistar rats (100–125 g) were fed HFD or a regular chow diet (RCD) for 14 days. Specific force, resistance to fatigue and recovery were tested in extensor digitorum longus (EDL; glycolytic) and soleus (SOL; oxidative) muscles using an ex vivo muscle contractility system. Muscle fiber typing and insulin signaling were analyzed while intramyocellular lipid droplets (LD) were characterized. Expression of key markers of lipid metabolism was also measured. Weight gain was similar for both groups. Specific force was decreased in SOL, but not in EDL of HFD rats. Muscle resistance to fatigue and force recovery were not altered in response to the diets. Similarly, muscle fiber type distribution and insulin signaling were not influenced by HFD. On the other hand, percent area and average size of intramyocellular LDs were significantly increased in the SOL of HFD rats. These effects were consistent with the increased expression of several mediators of lipid metabolism in the SOL muscle. A short-term HFD impairs specific force and alters lipid metabolism in SOL, but not EDL muscles of young rats. This indicates the importance of clarifying the early mechanisms through which lipid metabolism affects skeletal muscle functions in response to obesogenic diets in young populations.

Correction: Mice Hemizygous for a Pathogenic Mitofusin-2 Allele Exhibit Hind Limb/Foot Gait Deficits and Phenotypic Perturbations in Nerve and Muscle.

[This corrects the article DOI: 10.1371/journal.pone.0167573.].

Skeletal Muscle Gene Expression Study of Monozygous Twins with 35 Years of Divergent Exercise History

Variations in physical ability and capacities among individuals depend on both genetic inheritance and lifestyle. Previous research has yet to utilize a human model of monozygous (MZ) twins with substantial and long-term differences in exercise backgrounds to identify molecular and cellular mechanisms underlying exercise-mediated adaptations in muscle and whole-body phenotype. PURPOSE: In particular, this study examined skeletal muscle expression of genes related to muscle growth, inflammation, metabolism, and fiber type distribution in MZ twins with 35 years of substantially differing exercise history. METHODS: Muscle biopsies were collected from the vastus lateralis of male, 52-year-old MZ twins. Reverse transcriptase-polymerase chain reaction was used for the quantification of mRNAs associated with the following gene markers of various adaptive responses: a) skeletal muscle fiber type shift: MyHC-1, MyHC-2a, and MyHC-2x; b) adaptations in oxidative capacity: transcription factor A of the mitochondria (TFAM), and citrate synthase; c) angiogenesis: endothelial nitric oxide synthase, and vascular endothelial growth factor; d) muscular growth and satellite cell activation: myostatin, Pax7 (PAX7), mechano-growth factor (IGF1Ec), insulin-like growth factor a (IGF1Ea), and MyoD; and e) the inflammatory response: TWEAK (TNFSF12), the FN14 TWEAK receptor (TNFRSF12A) and tumor necrosis factor-α (TNF). RESULTS: MyHC-2x was expressed at a lower level in the trained subject relative to the untrained subject. No differences were observed for other markers of fiber type or metabolic or angiogenesis gene products. Some differences were observed in the expression of genes related to muscle growth including elevations in PAX7, MGF and IGF1Ea, and a reduction in MyoD. Finally, the only difference in expression of markers for the inflammatory response was an elevation in TNFRSF12A in the trained twin. CONCLUSIONS: At the mRNA-level, differences in expression of some key markers related to muscle fiber type, muscle growth, and the inflammatory response were observed in the trained vs the untrained twin. These data highlight the adaptability of skeletal muscle at the molecular level with decades of divergent physical activity patterns.

Diversity in Myosin Heavy Chain Composition of the Papionin Masseter Muscle Indicates the Importance of Hybrid Phenotypes for Feeding

The contractile performance of a muscle fiber is directly related to fiber type. Variation in fiber type within and among muscles leads to a diversity of phenotypes, providing the flexibility needed to perform an assortment of behaviors with varying functional demands. Previous analyses have demonstrated variation in fiber phenotype of the chewing muscles in a number of primates and nonprimate mammals. Some of this variation has been linked to species‐specific feeding behaviors. To further evaluate the relationship between fiber phenotype and feeding behavior, we compare the distribution of masseter muscle fiber types in four papionin species: the hard‐object feeding sooty mangabey (Cercocebus atys) and three papionin species that do not specialize on hard objects.Immunohistological staining was used to determine the presence of slow (MyHC‐1 and MyHC α –cardiac) and fast (MyHC‐2 and MyHC‐M) isoforms. Samples were taken from the anterior superficial masseter of C. atys (n=8), M. mulatta (n=2), M. fascicularis (n=2), and P. anubis (n=2). Histological sections were subsequently digitized to determine the relative proportions of MyHC types and fiber phenotype in each specimen.The presence of MyHC α‐cardiac was ubiquitous throughout the sample (C. atys=99%; M. mulatta=76%; M. fascicularis=77%; P. anubis=79%). MyHC α‐cardiac maintains fatigue resistance while producing twitch tension more rapidly than MyHC‐1, thus providing an advantage for animals that require a fiber phenotype combining fatigue resistance and intermediate speed of contraction. Previously, we reported high proportions of MyHC α‐cardiac in C. atys, which we functionally linked to the need for sustained and repetitive high bite forces required for crushing hard seeds. Our finding that C. atys consistently express a higher percentage of MyHC α‐cardiac in comparison to the other papionins further supports this interpretation. The presence of hybrid fibers in all four papionin species was notable. Slow types MyHC‐1 and MyHC α‐cardiac were co‐expressed in nearly 100% of fibers sampled in each of the species. Hybrid fibers co‐expressing slow and fast‐type phenotypes were also prevalent. The abundance of hybrid fibers across papionins suggests a shared fiber phenotype that provides for versatility in the contractile properties of the anterior superficial masseter, a muscular region that must meet diverse performance requirements. This initial work is part of a larger ongoing study to assess jaw‐muscle fiber phenotype in non‐human primates. Future analyses will include larger samples and a greater diversity of species to enable us to test key functional hypotheses about fiber type diversity in primates.Support or Funding InformationFunded in part by National Science Foundation Grants (BCS 1719743, BCS 0962677, BCS 0095522) and Yerkes National Primate Research Center Base Grant ORIP/OD P51OD011132 and the Comparative AIDS CoreThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Revisiting the peculiar regional distribution of muscle fiber types in rat Sternomastoid Muscle.

The sternomastoid (SM) muscle in rodents is known to have a peculiar distribution of fiber types with a steep gradient from surface to deep region. We here further characterize this peculiar regional distribution by quantitative histochemical morphometrys. In Hematoxylin-Eosin (H-E) stained transverse cryosections harvested in the medial portion of the muscle we counted around 10.000 myofibers with a mean diameter of 51.3±12.6 (μm). Cryisections of the SM stained by SDH reaction clearly show two distinct regions, toward the deep surface of the muscle a 40% area that contains packed SDH-positive myofibers, while the remaining area of the SM toward the external surface presents a more checker-board appearance. On the other hand, in the deep region of SM type 1 (slow contracting) muscle fibers, caracterized by positive acidic ATPase pH 4.35 reaction, are only the 24.5% of the fibers in the deep area of SM muscles, being restricted to the deepest region. The 75.5% of the myofibers in the deep region are of the fast contracting types (either 48.4% 2A, SDH –positive fibers or 27.1% 2B, SDH-negative fibers, respectively). As expected the 2B muscle fibers, acidic ATPase pH 4.3-negative and SDH-negative, present the largest size, while Type 1 fibers, acidic ATPase pH 4.3-positive and SDH-positive, present the smallest size in rat SM muscle. Based on present and previous observations, comparison of change in absolute number and/or percentage of the fiber types in any experimental model of muscle atrophy/hypertrophy/plasticity/pathology /recovery in the rat SM, and possibly of all mammals, will ask for morphometry of the whole muscle cross-sections, muscle sampling by bioptic approches will provide only comparable data on the size of the different types of muscle fibers.

Dietary nitrate-induced increases in human muscle power: high versus low responders.

Maximal neuromuscular power is an important determinant of athletic performance and also quality of life, independence, and perhaps even mortality in patient populations. We have shown that dietary nitrate (NO 3 −), a source of nitric oxide (NO), improves muscle power in some, but not all, subjects. The present investigation was designed to identify factors contributing to this interindividual variability. Healthy men (n = 13) and women (n = 7) 22–79 year of age and weighing 52.1–114.9 kg were studied using a randomized, double‐blind, placebo‐controlled, crossover design. Subjects were tested 2 h after ingesting beetroot juice (BRJ) either containing or devoid of 12.3 ± 0.8 mmol of NO 3 −. Plasma NO 3 − and nitrite (NO 2 −) were measured as indicators of NO bioavailability and maximal knee extensor speed (V max), power (P max), and fatigability were determined via isokinetic dynamometry. On average, dietary NO 3 − increased (P < 0.05) P max by 4.4 ± 8.1%. Individual changes, however, ranged from −9.6 to +26.8%. This interindividual variability was not significantly correlated with age, body mass (inverse of NO 3 − dose per kg), body mass index (surrogate for body composition) or placebo trial V max or fatigue index (in vivo indicators of muscle fiber type distribution). In contrast, the relative increase in Pmax was significantly correlated (r = 0.60; P < 0.01) with the relative increase in plasma NO 2 − concentration. In multivariable analysis female sex also tended (P = 0.08) to be associated with a greater increase in Pmax. We conclude that the magnitude of the dietary NO 3 −‐induced increase in muscle power is dependent upon the magnitude of the resulting increase in plasma NO 2 − and possibly female sex.

Effects of endurance training on skeletal muscle mitochondrial function in Huntington disease patients

BackgroundMitochondrial dysfunction may represent a pathogenic factor in Huntington disease (HD). Physical exercise leads to enhanced mitochondrial function in healthy participants. However, data on effects of physical exercise on HD skeletal muscle remains scarce. We aimed at investigating adaptations of the skeletal muscle mitochondria to endurance training in HD patients.MethodsThirteen HD patients and 11 healthy controls completed 26 weeks of endurance training. Before and after the training phase muscle biopsies were obtained from M. vastus lateralis. Mitochondrial respiratory chain complex activities, mitochondrial respiratory capacity, capillarization, and muscle fiber type distribution were determined from muscle samples.ResultsCitrate synthase activity increased during the training intervention in the whole cohort (P = 0.006). There was no group x time interaction for citrate synthase activity during the training intervention (P = 0.522). Complex III (P = 0.008), Complex V (P = 0.043), and succinate cytochrome c reductase (P = 0.008) activities increased in HD patients and controls by endurance training. An increase in mass-specific mitochondrial respiratory capacity was present in HD patients during the endurance training intervention. Overall capillary-to-fiber ratio increased in HD patients by 8.4% and in healthy controls by 6.4% during the endurance training intervention.ConclusionsSkeletal muscle mitochondria of HD patients are equally responsive to an endurance-training stimulus as in healthy controls. Endurance training is a safe and feasible option to enhance indices of energy metabolism in skeletal muscle of HD patients and may represent a potential therapeutic approach to delay the onset and/or progression of muscular dysfunction.Trial registrationClinicalTrials.gov NCT01879267. Registered May 24, 2012.

Reduced dietary intake of micronutrients with antioxidant properties negatively impacts muscle health in aged mice

BackgroundInadequate intake of micronutrients with antioxidant properties is common among older adults and has been associated with higher risk of frailty, adverse functional outcome, and impaired muscle health. However, a causal relationship is less well known. The aim was to determine in old mice the impact of reduced dietary intake of vitamins A/E/B6/B12/folate, selenium, and zinc on muscle mass, oxidative capacity, strength, and physical activity (PA) over time.MethodsTwenty‐one‐month‐old male mice were fed either AIN‐93‐M (control) or a diet low in micronutrients with antioxidant properties (=LOWOX‐B: 50% of mouse recommended daily intake of vitamins A, E, B6, and B12, folate, selenium, and zinc) for 4 months. Muscle mass, grip strength, physical activity (PA), and general oxidative status were assessed. Moreover, muscle fatigue was measured of m. extensor digitorum longus (EDL) during an ex vivo moderate exercise protocol. Effects on oxidative capacity [succinate dehydrogenase (SDH) activity], muscle fibre type, number, and fibre cross‐sectional area (fCSA) were assessed on m. plantaris (PL) using histochemistry.ResultsAfter 2 months on the diet, bodyweight of LOWOX‐B mice was lower compared with control (P < 0.0001), mainly due to lower fat mass (P < 0.0001), without significant differences in food intake. After 4 months, oxidative status of LOWOX‐B mice was lower, demonstrated by decreased vitamin E plasma levels (P < 0.05) and increased liver malondialdehyde levels (P = 0.018). PA was lower in LOWOX‐B mice (P < 0.001 vs. control). Muscle mass was not affected, although PL‐fCSA was decreased (~16%; P = 0.028 vs. control). SDH activity and muscle fibre type distribution remained unaffected. In LOWOX‐B mice, EDL force production was decreased by 49.7% at lower stimulation frequencies (P = 0.038), and fatigue resistance was diminished (P = 0.023) compared with control.ConclusionsReduced dietary intake of vitamins A, E, B6, and B12, folate, selenium, and zinc resulted in a lower oxidative capacity and has major impact on muscle health as shown by decreased force production and PA, without effects on muscle mass. The reduced fCSA in combination with similar SDH activity per fibre might explain the reduced oxidative capacity resulting in the increased fatigue after exercise in LOWOX‐B mice.

Intramuscular variations of proteome and muscle fiber type distribution in semimembranosus and semitendinosus muscles associated with pork quality

Proteome analysis was performed to understand intramuscular variations in muscle fiber distribution in semimembranosus (SM) and semitendinosus (ST) muscles associated with pork quality. Fifteen SM and ST muscles were separated into dark and light portions. The relative area of oxidative fiber was higher (P < .0001) in dark portion than that in light portion, while glycolytic fiber types were distributed primarily (P < .01) in light portions regardless of muscle types. Myosin-1, myosin-4, troponin complex (fast), myosin light chains, and metabolic enzymes responsible for fast-twitch glycolytic types were overexpressed in light portions (P < .05). However, myosin-2, myosin-7, myoglobin, and mitochondrial oxidative metabolic enzymes were closely related to slow-twitch oxidative fibers. These resulted in high pH, redness, and tenderness but low lightness and drip loss of pork quality. In conclusion, differentially expressed muscle proteins are associated with fiber type (oxidative vs. glycolytic) distribution, resulting in intramuscular variations of pork quality.

Phonation Analysis Combined with 3D Reconstruction of the Thyroarytenoid Muscle in Aged Ovine Ex Vivo Larynx Models

The aim of the study was to establish a basic data set of combined functional and anatomical measures of aged sheep larynges using ex vivo models. Combining these two approaches in one and the same larynx is an unmet goal so far yet is important as newer treatment strategies aim to preserve the organ structure and new assessment tools are required. Ovine larynges were used as their dimensions, and muscle fiber type distribution highly resemble the human larynx. Ex vivo animal study. Larynges of six sheep (~9 years of age) were subjected to ex vivo functional phonatory experiments. Phonatory characteristics were analyzed as a function of longitudinal vocal fold (VF) prestress. Anatomical measurements of the same larynges comprised micro-computed tomography scans followed by three-dimensional (3D) reconstructions. Using specially adapted radiological scan protocols with subsequent 3D reconstruction, muscle volumes, surface areas, and anatomical measurements were computed. Increasing longitudinal prestress yielded higher subglottal pressure (PS) for the same airflow. Quantitative differences to previous studies-such as the increased PS and increased phonation threshold pressure-were detected. We achieved excellent visualization of the laryngeal muscles and framework, resulting in accurate 3D reconstructions for quantitative analysis. We found no significant intraindividual volume differences of the thyroarytenoid muscles. The established protocol allows precise functional and anatomical measures. The data created provide a reference data set for upcoming therapeutic strategies (eg, growth factor therapy, functional electrical stimulation) that target essential structures of the VFs such as the laryngeal muscles and/or the VF mucosa.

Vestigial-like 2 contributes to normal muscle fiber type distribution in mice

Skeletal muscle is composed of heterogeneous populations of myofibers that are classified as slow- and fast-twitch fibers. The muscle fiber-type is regulated in a coordinated fashion by multiple genes, including transcriptional factors and microRNAs (miRNAs). However, players involved in this regulation are not fully elucidated. One of the members of the Vestigial-like factors, Vgll2, is thought to play a pivotal role in TEA domain (TEAD) transcription factor-mediated muscle-specific gene expression because of its restricted expression in skeletal muscles of adult mice. Here, we generated Vgll2 null mice and investigated Vgll2 function in adult skeletal muscles. These mice presented an increased number of fast-twitch type IIb fibers and exhibited a down-regulation of slow type I myosin heavy chain (MyHC) gene, Myh7, which resulted in exercise intolerance. In accordance with the decrease in Myh7, down-regulation of miR-208b, encoded within Myh7 gene and up-regulation of targets of miR-208b, Sox6, Sp3, and Purβ, were observed in Vgll2 deficient mice. Moreover, we detected the physical interaction between Vgll2 and TEAD1/4 in neonatal skeletal muscles. These results suggest that Vgll2 may be both directly and indirectly involved in the programing of slow muscle fibers through the formation of the Vgll2-TEAD complex.

Aging leads to inferior Achilles tendon mechanics and altered ankle function in rodents

Spontaneous rupture of the Achilles tendon is increasingly common in the middle aged population. However, the cause for the particularly high incidence of injury in this age group is not well understood. Therefore, the objective of this study was to identify age-specific differences in the Achilles tendon-muscle complex using an animal model. Functional measures were performed in vivo and tissues were harvested following euthanasia for mechanical, structural, and histological analysis from young, middle aged, and old rats. Numerous alterations in tendon properties were detected across age groups, including inferior material properties (maximum stress, modulus) with increasing age. Differences in function were also observed, as older animals exhibited increased ankle joint passive stiffness and decreased propulsion force during locomotion. Macroscale differences in tendon organization were not observed, although cell density and nuclear shape did vary between age groups. Muscle fiber size and type distribution were not notably affected by age, indicating that other factors may be more responsible for age-specific Achilles tendon rupture rates. This study improves our understanding of the role of aging in Achilles tendon biomechanics and ankle function, and helps provide a potential explanation for the disparate incidence of Achilles tendon ruptures in varying age groups.

Determinants of skeletal muscle work efficiency in patients with COPD

Skeletal muscle work inefficiency plays a major role in limiting exercise capacity in patients with chronic Obstructive Pulmonary Disease (COPD). However, the physiological mechanisms responsible for this excessive energy demand from the exercising muscle of patients with COPD remains poorly understood. To address this gap in our knowledge, we measured the cost of muscle contraction (ATP cost) in the quadriceps of patients with moderate to severe COPD (n=11) and age‐matched sedentary controls (n=12) with phosphorus magnetic resonance spectroscopy (31P‐MRS) during 1) maximal intermittent contractions to elicit a metabolic demand from both cross‐bridge cycling and ion pumping and 2) a continuous maximal contraction to predominantly tax cross‐bridge cycling. In addition, muscle fiber type distribution was determined in the vastus lateralis using immunofluorescence analysis. The ATP cost of contraction for both the continuous (Control: 0.11 ± 0.07 mM.min−1.N m−1; COPD: 0.12 ± 0.07 mM.min−1.N m−1) and intermittent (Control: 0.24 ± 0.17 mM.min−1.N m−1; COPD: 0.24 ± 0.18 mM.min−1.N m−1) protocols were not significantly different between groups with or without normalization for muscle mass (P&gt;0.05). Somewhat surprisingly, fiber type distribution was also not different between groups (Type I; Control: 41 ± 12 %; COPD: 40 ± 18 %; Type II; Control: 59 ± 12 %; COPD: 60 ± 18 % respectively P&gt;0.05). Of note, this finding of similar fiber type composition in controls and COPD was partly the consequence of a relatively high proportion of type II fibers in the elderly controls. These, somewhat serendipitous, data reveal that excessive energy demand from ion pumping or myosin ATPase per se is not an obligatory component of the skeletal muscle work inefficiency in COPD and suggest that the previously documented elevated ATP cost of contraction in the locomotor muscles of these patients was likely mediated by a fiber type shift. In practical terms, these findings highlight the potential importance of developing effective therapeutic strategies to limit the likely deleterious consequences of a skeletal muscle fiber type shift in the lower limbs of patients with COPD.Support or Funding InformationThis work was funded in part by grants from the Flight Attendant Medical Research Institute (FAMRI), NIH National Heart, Lung, and Blood Institute (PO1 HL 091830, K99HL125756) and VA Merit grant E6910R.

Improved skeletal muscle mass and strength after heavy strength training in very old individuals

Age-related loss of muscle mass and function represents personal and socioeconomic challenges. The purpose of this study was to determine the adaptation of skeletal musculature in very old individuals (83+ years) performing 12weeks of heavy resistance training (3×/week) (HRT) compared to a non-training control group (CON). Both groups received similar protein supplementations. We studied 26 participants (86.9±3.2 (SD) (83–94, range) years old) per-protocol. Quadriceps cross-sectional area (CSA) differed between groups at post-test (P<0.05) and increased 1.5±0.7cm2 (3.4%) (P<0.05) in HRT only. The increase in CSA is correlated inversely with the baseline level of CSA (R2=0.43, P<0.02). Thigh muscle isometric strength, isokinetic peak torque and power increased significantly only in HRT by 10–15%, whereas knee extension one-repetition maximum (1 RM) improved by 91%. Physical functional tests, muscle fiber type distribution and size did not differ significantly between groups. We conclude that in protein supplemented very old individuals, heavy resistance training can increase muscle mass and strength, and that the relative improvement in mass is more pronounced when initial muscle mass is low.

Effect of Intramuscular Fiber Type Variation on Beef Semitendinosus Steak Metmyoglobin Accumulation during Retail Display

ObjectivesThe objective of this study was to examine effects of steak location (LOC) on muscle fiber type distribution and metmyoglobin accumulation of Semitendinosus (ST) steaks.Materials and MethodsTwenty ST muscles (IMPS 171C) from a commercial abattoir were wet aged for 22 d. Progressing from the proximal to distal end, each ST was fabricated into twelve 2.54-cm thick steaks. Steaks 1 through 4 were designated proximal (PROX), 5 through 8 were designated middle (MID), and 9 through 12 were designated distal (DIST), with steaks 1, 6, and 12 utilized for fiber type analysis. Remaining steaks within each location were randomly assigned to 0, 4, or 9 d of simulated retail display under fluorescent light. Day-0 and -4 steaks were utilized for metmyoglobin reducing ability (MRA) analysis. Day-9 steaks were subjected to daily objective and subjective steak surface metmyoglobin analyses, and day-9 MRA analysis.ResultsThere were Location × Day interactions (P < 0.01) for surface metmyoglobin percentage and visual panel percent discoloration scores. On d 0 of display, PROX steaks had less surface metmyoglobin than the other locations (P < 0.01), which were not different (P = 0.51). On d 1, MID steaks had more metmyoglobin than the other locations (P < 0.04), and DIST steaks had more (P < 0.01) metmyoglobin than PROX steaks. From d 2 to 6, MID steaks had more metmyoglobin than steaks from other locations (P < 0.01), which did not differ (P > 0.17). On d 7 of display, MID steaks tended to have more metmyoglobin than steaks from other locations (P < 0.09), which did not differ (P = 0.65). On d 8 and 9, MID steaks had more metmyoglobin than PROX steaks (P < 0.02), and DIST steaks did not differ from the two locations (P > 0.15). No differences in panel percent discoloration scores were found between muscle locations on d 0 (P = 1.00); however from d 1 to 5, MID steaks had more discoloration than PROX and DIST steaks (P < 0.04), which did not differ (P > 0.12). From d 6 to 8, MID steaks had more discoloration than PROX steaks (P < 0.05), and steaks from both locations did not differ from DIST steaks (P > 0.16). On d 9, PROX steaks had less discoloration than MID and DIST steaks (P < 0.03), which did not differ (P = 0.72). At d 0 and 4 of display, PROX and DIST steaks had greater reducing ability than MID steaks (P < 0.01), but were not different (P = 0.33) from one another. At d 9 of display, all locations possessed the same MRA (P > 0.51).Location affected percentage of all 3 fiber types (P < 0.01). There were less type I fibers in PROX steaks than the other 2 locations (P < 0.01), and MID steaks tended to have more (P = 0.10) than DIST steaks. Proximal steaks had more (P < 0.01) type IIA fibers than the MID location, and tended to have more (P = 0.07) than DIST steaks. The DIST steaks tended to have more (P = 0.08) type IIA fibers than MID steaks. Steaks from PROX and MID locations did not differ (P = 0.72) in type IIX fiber percentage, but did possess more type IIX fibers than the DIST steaks (P < 0.01).ConclusionThroughout most of display, ST MID steaks accumulated more surface metmyoglobin than PROX and DIST steaks, which was also detected by a visual panel. Steaks from the MID location possessed less MRA compared to the other 2 locations on d 0 and 4 of display. Differences in MRA and discoloration may be due to the MID location possessing less type IIA fibers.

Mice Hemizygous for a Pathogenic Mitofusin-2 Allele Exhibit Hind Limb/Foot Gait Deficits and Phenotypic Perturbations in Nerve and Muscle.

Charcot-Marie-Tooth disease type 2A (CMT2A), the most common axonal form of hereditary sensory motor neuropathy, is caused by mutations of mitofusin-2 (MFN2). Mitofusin-2 is a GTPase required for fusion of mitochondrial outer membranes, repair of damaged mitochondria, efficient mitochondrial energetics, regulation of mitochondrial-endoplasmic reticulum calcium coupling and axonal transport of mitochondria. We knocked T105M MFN2 preceded by a loxP-flanked STOP sequence into the mouse Rosa26 locus to permit cell type-specific expression of this pathogenic allele. Crossing these mice with nestin-Cre transgenic mice elicited T105M MFN2 expression in neuroectoderm, and resulted in diminished numbers of mitochondria in peripheral nerve axons, an alteration in skeletal muscle fiber type distribution, and a gait abnormality.

Modulating effects of exercise training regimen on skeletal muscle properties in female polo ponies.

BackgroundThe match play patterns in equestrian polo are unique and require specific training programs to ensure sport performance. The effect of commonly used exercise training regimens on the adaptation of skeletal muscle is unclear. The present study investigated the modulating effects of the classic training regimen, comprised of aerobic exercise training with increasing exercise intensities and varying duration combined with match play, on the properties of muscle in polo ponies. Nine healthy adult female polo ponies were subjected to four consecutive subsets of 1 year classic training regimen including basal activity (B), low intensity (L), low to moderate intensity (LM), and low to moderate intensity training plus match play during polo tournament (LMP), respectively. At the end of each training period, gluteus medius muscle samples were taken for determination of muscle fiber type distribution, muscle metabolic capacity, capillary density, and lipid and glycogen content. The expression profile of metabolic genes including succinate dehydrogenase (SDH), phosphofructokinase (PFK), glycogen phosphorylase (PYG), and glycogen synthase (GYS) were also measured.ResultsAmong all exercise training subsets, only LMP exercise period caused an increase in the number of oxidative fibers (type IIa), along with increases in properties related to oxidative metabolism including high capillary density, intramuscular lipid content, and expression of SDH and PYG genes, with a corresponding decrease in the number of type IIx muscle fibers.ConclusionThe combination of low to moderate and high intensity training in LMP are only sufficient to induce changes in oxidative characteristics. As the first scientific evidence providing such insight about the classic polo training regimen, the data forms a basis for further consideration in training program design.Electronic supplementary materialThe online version of this article (doi:10.1186/s12917-016-0874-6) contains supplementary material, which is available to authorized users.