Myosin Heavy Chain IIa Fibers Research Articles

Title: Influence of the NASA SPRINT exercise protocol on myonuclear and satellite cell content of the vastus lateralis and soleus during 70 days of bedrest

Objectives: The purpose of this study was to assess the effcacy of the NASA SPRINT exercise protocol in preserving myonuclear and satellite cell number of the quadriceps (vastus lateralis) and triceps surae (soleus) muscles following 70 days of bedrest (i.e., simulated microgravity). These two muscles were the focus of this study because they both atrophy significantly with microgravity exposure in the absence of countermeasures. Hypotheses: The SPRINT exercise protocol will preserve myonuclear number (MNN), myonuclear domain (MND), and satellite cell number (SCN) during 70 days of bedrest. Methods: Subjects were randomized into three groups: bedrest only (BR; n=8, 37±3 y), bedrest with endurance and resistance exercise (BRE; n=9, 34±2 y) and bedrest with endurance and resistance exercise and low dose testosterone injections (BRE+T; n=8, 33±3 y). Muscle biopsies were taken before and at the end of bedrest from the vastus lateralis and soleus, and were sectioned, immunofluorescently stained, and analyzed for MNN (myonuclei/fiber), MND (μm2/myonuclei), and SCN (satellite cells/fiber) in slow (myosin heavy chain (MHC) I) and fast (MHC IIa) muscle fibers. SCN was only determined in MHC I fibers for the soleus. Results: MNN, MND, and SCN in MHC I and IIa fibers of the vastus lateralis and soleus were unchanged (p<0.05) from pre- to post-bedrest within BR, BRE, and BRE+T. However, there was a main effect for time (p<0.05) for a decrease in MND for vastus lateralis MHC I fibers (-8%), as well as soleus MHC I (-18%) and IIa (-18%) fibers. Also, there was a trend for a main effect for time (P=0.059) for an increase (+37%) in SCN for vastus lateralis MHC IIa fibers, which was driven primarily by the exercise groups (BR: -4%, BRE: +36%, BRE+T: +67%). Conclusions: Myonuclear number appears to be unaltered during 70 days of simulated microgravity with or without countermeasures exercise. As a result, any changes in myonuclear domain correspond with alterations in muscle mass. Satellite cell number also appears to be relatively refractory to change, but may be influenced in fast muscle fibers under the combined dynamic cellular control mechanisms of exercise and atrophy encountered during relatively long-duration simulated microgravity. Funding Source: This research was supported by the National Aeronautics and Space Administration, NNJ06HF59G; the Institute for Translational Sciences at the University of Texas Medical Branch, supported in part by a Clinical and Translational Science Award (UR1TR000071) from the National Center for Advancing Translational Sciences, National Institutes of Health; the NASA Flight Analogs Project; and the Human Bioenergetic Program at Ball State University. 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.

Exercise-induced changes to the fiber type-specific redox state in human skeletal muscle are associated with aerobic capacity.

The benefits of exercise involve skeletal muscle redox state alterations of nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). We determined the fiber-specific effects of acute exercise on the skeletal muscle redox state in healthy adults. Muscle biopsies were obtained from 19 participants (11 M, 8 F; 26 ± 4 yr) at baseline (fasted) and 30 min and 3 h after treadmill exercise at 80% maximal oxygen consumption (V̇o2max). Muscle samples were probed for autofluorescence of NADH (excitation at 340-360 nm) and oxidized flavoproteins (Fp; excitation at 440-470 nm) and subsequently, fiber typed to quantify the redox signatures of individual muscle fibers. Redox state was calculated as the oxidation-to-reduction redox ratio: Fp/(Fp + NADH). At baseline, pair-wise comparisons revealed that the redox ratio of myosin heavy chain (MHC) I fibers was 7.2% higher than MHC IIa (P = 0.023, 95% CI: 5.2, 9.2%) and the redox ratio of MHC IIa was 8.0% higher than MHC IIx (P = 0.035, 95% CI: 6.8, 9.2%). MHC I fibers also displayed greater NADH intensity than MHC IIx (P = 0.007) and greater Fp intensity than both MHC IIa (P = 0.019) and MHC IIx (P < 0.0001). Fp intensities increased in all fiber types (main effect, P = 0.039) but redox ratios did not change (main effect, P = 0.483) 30 min after exercise. The change in redox ratio was positively correlated with capillary density in MHC I (rho = 0.762, P = 0.037), MHC IIa fibers (rho = 0.881, P = 0.007), and modestly in MHC IIx fibers (rho = 0. 771, P = 0.103). These findings support the use of redox autofluorescence to interrogate skeletal muscle metabolism.NEW & NOTEWORTHY This study is the first to use autofluorescent imaging to describe differential redox states within human skeletal muscle fiber types with exercise. Our findings highlight an easy and efficacious technique for assessing skeletal muscle redox in humans.

Skeletal muscle single fiber force production declines early in juvenile male mice with chronic kidney disease.

Children with chronic kidney disease (CKD) frequently exhibit delayed physical development and reduced physical performance, presumably due to skeletal muscle dysfunction. However, the cellular and molecular basis of skeletal muscle impairment in juvenile CKD remains poorly understood. Cellular (single fiber) and molecular (myosin-actin interactions and myofilament properties) function was examined ex vivo in slow (soleus) and fast (extensor digitorum longus) contracting muscles of juvenile male (6 weeks old) CKD and control mice. CKD was induced by 0.2% adenine diet for 3 weeks starting at 3 weeks of age. Specific tension (maximal isometric force divided by cross-sectional area) was reduced in larger myosin heavy chain (MHC) I and IIA fibers and in all IIB fibers in juvenile male mice with CKD due to fewer strongly bound myosin-actin cross-bridges. Fiber cross-sectional area in juvenile CKD mice was unchanged in MHC I and IIB fibers and increased in MHC IIA fibers, compared to controls. CKD slowed cross-bridge kinetics (slower rate of myosin force production and longer myosin attachment time, ton ) in MHC IIA fibers, and accelerated kinetics (shorter ton ) in MHC IIB fibers, which may indicate fiber type dependent shifts in contractile velocity in juvenile CKD. Overall, our findings show that single fiber myopathy is an early event during juvenile CKD, manifesting prior to the development of cellular atrophy as reduced force generation due to fewer strongly bound myosin heads. These results warrant clinical translation and call for early interventions to preserve physical function in children with CKD.

Iron supplementation improves skeletal muscle contractile properties in mice with chronic kidney disease.

Adult chronic kidney disease (CKD) patients commonly display decreased physical function, leading to increased mortality, which is exacerbated with iron deficiency and anemia. Interventions to mitigate CKD myopathy are scarce, potentially because the underlying contractile deficits have not been fully investigated. We examined single fiber force production, myosin-actin interactions and myofilament properties in soleus and extensor digitorum longus muscles of 16-week-old CKD mice without (CKD, N=12) and with iron supplementation (CKDFE, N=6, weekly injections of 0.2 mg/kg iron dextran) and controls (N=10). Late-stage CKD was induced by 0.2% adenine diet for 8 weeks. CKD with iron supplementation (CKDFE) improved anemia status compared to CKD, but not back to control levels. Maximally activated (pCa 4.5, 25°C) specific tension (maximal isometric force divided by cross-sectional area) was reduced 36-51% with CKD due, in a large part, to a reduction in strongly-bound myosin-actin cross-bridges in all myosin heavy chain (MHC) I, IIA, and IIB fibers. Iron supplementation improved specific tension and number of strongly-bound cross-bridges in MHC IIA and IIB fibers. Myosin-actin cross-bridge kinetics were slower with CKD in MHC I and IIA fibers, with iron supplementation restoring kinetics closer to control values in MHC IIA fibers. Higher hemoglobin levels were correlated to improvements in specific tension and strongly-bound cross-bridges in MHC IIA and IIB fibers, and with faster cross-bridge kinetics in MHC IIA fibers. However, CKD induced decreases in myofilament lattice stiffness for MHC IIA and IIB fibers were not improved with iron supplementation. Our findings show that iron supplementation improved various aspects of the cellular and molecular CKD-induced myopathy in fast-contracting fiber types. Thus, correction of iron imbalance and/or anemia may be an effective countermeasure to reduce contractile deficits and improve quality of life in late-stage CKD patients.

Skeletal muscle fiber force declines with chronic kidney disease in juvenile and adult mice.

Juvenile and adult chronic kidney disease (CKD) patients frequently have compromised physical performance, which worsens with disease severity. Examining the underlying mechanisms behind this contractile dysfunction can be difficult to impossible in humans, especially in juveniles, but can be accomplished using animal models. We characterized CKD-induced changes in cellular (single fiber force) and molecular (myosin-actin interactions, myofilament properties) function in soleus and extensor digitorum longus muscles of 6-week-old (juvenile, control = 5, CKD = 6) and 16-week-old (adults, control = 10, CKD = 12) mice. CKD was induced by 0.2% adenine diet for 3 weeks (early-stage CKD) for juveniles and 8 weeks (late-stage CKD) for adults. Maximally activated (pCa 4.5, 25°C) specific tension (maximal isometric force divided by cross-sectional area) was reduced in all myosin heavy chain (MHC) I, IIA, and IIB fibers in juvenile (23-30%) and adult (36-51%) mice with CKD due, in a large part, to a reduction in strongly-bound myosin-actin cross-bridges. Additionally, myofilament stiffness was reduced in MHC IIA fibers from juveniles and adults as well as MHC IIB fibers from adults, potentially reducing force transmission. For adults, myosin-actin interactions were slower in MHC I and IIA fibers. In juveniles, myosin-actin interactions were slower only in MHC IIA fibers but faster in MHC IIB fibers. Fiber cross-sectional area with CKD was increased or unchanged in all fiber types, except for a decrease in MHC IIB fibers from adults. Overall, juvenile early-stage and adult late-stage CKD showed similar molecular mechanisms leading to reduced force production and cross-bridge kinetics, although alterations were generally larger in adult late-stage CKD. Thus, physical function in juvenile early-stage and adult late-stage CKD is being impaired on the molecular level due to intrinsic contractile deficits independent of skeletal muscle atrophy.

Stretch activated force production increases during fatigue in fast-contracting skeletal muscle fibers.

Stretch activation (FSA) is the delayed increase in fiber tension (force per cross-sectional area) following a rapid stretch and can improve muscle performance during repetitive cyclical contractions. Historically considered minimal in skeletal muscle, our recent work showed the ratio of stretch activation to calcium-activated tension (FSA/F0) increased from 10 to 40% with greater inorganic phosphate (Pi) levels in soleus muscle fibers (Straight et al., 2019). Given Pi increases with muscle fatigue, we hypothesize that FSA helps maintain force generation during fatigue. To test this, FSA, induced by a stretch of 0.5% fiber length, was examined during control (pCa 4.5, pH 7.0, Pi 5 mM), high calcium fatigue (pCa 4.5, pH 6.2, Pi 30 mM) and low calcium fatigue (pCa 5.1, pH 6.2, Pi 30 mM) in fibers expressing myosin heavy chain (MHC) I, IIA, IIX and IIB isoforms from soleus and extensor digitorum longus muscles of C57BL/6NJ mice. F0 of all MHC isoforms decreased from control to high calcium fatigue to low calcium fatigue, as expected. In MHC IIX and IIB fibers, FSA occurred under all conditions and FSA/F0 increased from control (17-20%) to high calcium fatigue (32-35%) to low calcium fatigue (42-44%). In MHC IIA fibers, FSA/F0 was similar to MHC IIX and IIB fibers in control and high calcium fatigue, however MHC I fiber values were much less (5% and 18%). For MHC I and IIA fibers, no discernable FSA was apparent in low calcium fatigue. These results show that FSA is a significant modulator of force production under fatiguing conditions in fast-contracting muscle fibers. This mechanism could play an important physiological role during cyclical contractions, when the antagonistic muscle rapidly stretches the agonist muscle, by reducing the effect of fatigue on force production.

Iron Supplementation Improves Skeletal Muscle Contractile Properties in Mice with CKD.

Patients with chronic kidney disease (CKD) frequently have compromised physical performance, which increases their mortality; however, their skeletal muscle dysfunction has not been characterized at the single-fiber and molecular levels. Notably, interventions to mitigate CKD myopathy are scarce. The effect of CKD in the absence and presence of iron supplementation on the contractile function of individual skeletal muscle fibers from the soleus and extensor digitorum longus muscles was evaluated in 16-week-old mice. CKD was induced by the adenine diet, and iron supplementation was by weekly iron dextran injections. Maximally activated and fatigued fiber force production was decreased 24%-52% in untreated CKD, independent of size, by reducing strongly bound myosin/actin cross-bridges and/or decreasing myofilament stiffness in myosin heavy chain (MHC) I, IIA, and IIB fibers. Additionally, myosin/actin interactions in untreated CKD were slower for MHC I and IIA fibers and unchanged or faster in MHC IIB fibers. Iron supplementation improved anemia and did not change overall muscle mass in CKD mice. Iron supplementation ameliorated CKD-induced myopathy by increasing strongly bound cross-bridges, leading to improved specific tension, and/or returning the rate of myosin/actin interactions toward or equivalent to control values in MHC IIA and IIB fibers. Skeletal muscle force production was significantly reduced in untreated CKD, independent of fiber size, indicating that compromised physical function in patients is not solely due to muscle mass loss. Iron supplementation improved multiple aspects of CKD-induced myopathy, suggesting that timely correction of iron imbalance may aid in ameliorating contractile deficits in CKD patients.

Skeletal muscle cellular contractile dysfunction after anterior cruciate ligament reconstruction contributes to quadriceps weakness at 6-month follow-up.

Muscle dysfunction following anterior cruciate ligament reconstruction (ACLR) may evolve from alterations in muscle contractility at the myofilament protein level. Using a prospective, within-subject case-control design, we evaluated cellular-level contractility, cross-sectional area (CSA), and myosin heavy chain (MHC) isoform expression on single muscle fibers 3 weeks post ACLR, and evaluated their relationship to whole muscle strength and patient-oriented outcomes 6 months post operation. Biopsies of the vastus lateralis were performed 3 weeks post ACLR in 11 subjects (5 females, mean age ± SD = 24.7 ± 6.5 years, height = 172.7 ± 8.2 cm, mass = 75.7 ± 12.5 kg) following first-time ACL rupture and whole muscle strength and self-reported pain, function, and quality of life assessed 6 months post ACLR. At 3 weeks post ACLR, force production was reduced (p < 0.01) in MHC I (-36%) and IIA (-48%) fibers compared with the non-injured leg. When force production was expressed relative to CSA to account for fiber atrophy, reductions remained in MHC IIA fibers (-40%; p < 0.001), but MHC I fibers showed only a trend toward being lower (-13%; p = 0.09). Finally, skeletal muscle fiber functional deficits at 3 weeks post ACLR were associated with whole muscle weakness and less favorable patient-reported outcomes at 6-month follow-up. Thus, ACLR promotes early cellular contractile dysfunction that may contribute to decreased whole muscle strength and patient function, and increased patient-reported symptoms, at 6-month follow-up.

Super-relaxed state of myosin in human skeletal muscle is fiber-type dependent.

The myosin super-relaxed state (SRX) in skeletal muscle is hypothesized to play an important role in regulating muscle contractility and thermogenesis in humans but has only been examined in model organisms. Here we report the first human skeletal muscle SRX measurements, using quantitative epifluorescence microscopy of fluorescent 2'/3'-O-(N-methylanthraniloyl) ATP (mantATP) single-nucleotide turnover. Myosin heavy chain (MHC) isoform expression was determined using gel electrophoresis for each permeabilized vastus lateralis fiber, to allow for novel comparisons of SRX between fiber types. We find that the fraction of myosin in SRX is less in MHC IIA fibers than in MHC I and IIAX fibers (P = 0.008). ATP turnover of SRX is faster in MHC IIAX fibers compared with MHC I and IIA fibers (P = 0.001). We conclude that SRX biochemistry is measurable in human skeletal muscle, and our data indicate that SRX depends on fiber type as classified by MHC isoform. Extension from this preliminary work would provide further understanding regarding the role of SRX in human muscle physiology.

Influenza Infection has Fiber Type-Specific Effects on Cellular and Molecular Skeletal Muscle Function in Aged Mice.

Skeletal muscle myopathies represent a common non-pulmonary manifestation of influenza infection, leading to reduced physical function and hospitalization in older adults. However, underlying mechanisms remain poorly understood. Our study examined the effects of influenza virus A pulmonary infection on contractile function at the cellular (single fiber) and molecular (myosin-actin interactions and myofilament properties) levels in soleus and extensor digitorum longus muscles of aged (20 months) C57BL/6 male mice that were healthy or flu-infected for 7 (7-days post-infection; 7-DPI) or 12 days (12-DPI). Cross-sectional area (CSA) of myosin heavy chain (MHC) IIA and IIB fibers was reduced at 12-DPI relative to 7-DPI and healthy. Maximal isometric force in MHC IIA fibers was also reduced at 12-DPI relative to 7-DPI and healthy, resulting in no change in specific force (maximal isometric force divided by CSA). In contrast, MHC IIB fibers produced greater isometric force and specific force at 7-DPI compared to 12-DPI or healthy. The increased specific force in MHC IIB fibers was likely due to greater myofilament lattice stiffness and/or an increased number or stiffness of strongly bound myosin-actin cross-bridges. At the molecular level, cross-bridge kinetics were slower in MHC IIA fibers with infection, while changes in MHC IIB fibers were largely absent. In both fiber types, greater myofilament lattice stiffness was positively related to specific force. This study provides novel evidence that cellular and molecular contractile function is impacted by influenza infection in a fiber type-specific manner, suggesting potential molecular mechanisms to help explain the impact of flu-induced myopathies.

Sex and fiber type independently influence AMPK, TBC1D1, and TBC1D4 at rest and during recovery from high-intensity exercise in humans.

Women and men present different metabolic responses to exercise, yet whether this phenomenon results from differences in fiber type (FT) composition or other sex-specific factors remains unclear. Therefore, our aim was to examine the effects of sex and FT independently on AMP-activated protein kinase (AMPK), acetyl-CoA carboxylase (ACC), Tre-2/BUB2/CDC1 domain family (TBC1D)1, and TBC1D4 in response to acute exercise. Segregated pools of myosin heavy chain (MHC) I and MHC IIa fibers were prepared from vastus lateralis biopsies of young trained men and women at rest and during recovery (0 min, 45 min, 90 min, or 180 min) from high-intensity interval exercise (6 × 1.5 min at 95% maximum oxygen uptake). In resting MHC I vs. IIa fibers, AMPKα2, AMPKγ3, and TBC1D1 were higher and TBC1D4 expression was lower in both sexes, along with higher phospho (p)-TBC1D1Ser660 and lower p-TBC1D4Thr642. Women expressed higher ACC than men in MHC IIa fibers and higher AMPKβ1, AMPKβ2, TBC1D1, and TBC1D4 in both FTs. Immediately after exercise, p-AMPKαThr172 increased only in MHC IIa fibers, whereas p-ACCSer221 increased in both FTs, with no change in p-TBC1D1Ser660 or p-TBC1D4Thr642. During recovery, delayed responses were observed for p-AMPKαThr172 in MHC I (45 min), p-TBC1D4Thr642 in both FTs (45 min), and p-TBC1D1Ser660 (180 min). FT-specific phosphorylation responses to exercise were similar between men and women. Data indicate that sex and FT independently influence expression of AMPK and its substrates. Thus failing to account for sex or FT may reduce accuracy and precision of metabolic protein measurements and conceal key findings.NEW & NOTEWORTHY This investigation is the first to compare muscle fiber type (FT)-specific analysis of proteins between the sexes, providing comprehensive data on AMP-activated protein kinase (AMPK), acetyl-CoA carboxylase (ACC), Tre-2/BUB2/CDC1 domain family (TBC1D)1, and TBC1D4 before and in the hours following high-intensity interval exercise (HIIT). Expression and phosphorylation of specific AMPK isoforms, ACC, TBC1D1, and TBC1D4 were shown to be FT dependent, sex dependent, or both, and TBC1D1 showed an unexpected delay in FT-dependent phosphorylation in the time period following HIIT.

Skeletal Muscle Fiber Type and Morphology in a Middle-Aged Elite Male Powerlifter Using Anabolic Steroids

Powerlifting regularly exposes athletes to extreme stimuli such as chronic heavy resistance training (HRT), and many powerlifters choose to augment their performance with anabolic–androgenic steroids (AAS). However, little is known about the myocellular adaptations that occur from long-term HRT and AAS use, especially into middle age. We were presented with the unique opportunity to study muscle cells from an elite-level powerlifter (EPL; age 40 years) with ≥ 30 years of HRT experience and ≥ 15 years of AAS use. The purpose of this case study was to identify myocellular characteristics [myosin heavy chain (MHC) fiber type, fiber size, and myonuclear content] in EPL, as well as compare these data to existing literature. The participant underwent a resting vastus lateralis muscle biopsy and single fibers were analyzed for MHC content via SDS-PAGE. A subset of fibers underwent MHC-specific imaging analysis via confocal microscopy to identify cell size (cross-sectional area, CSA) and myonuclear domain (MND) size. MHC fiber type distribution was 9% I, 12% I/IIa, 79% IIa, and 0% other isoforms. This pure MHC IIa (fast-twitch) fiber content was amongst the highest reported in the literature. Imaging analysis of MHC IIa fibers revealed a mean CSA of 4218 ± 933 μm2 and MND of 12,548 ± 3181 μm3. While the fast-twitch fiber CSA was comparable to values in previous literature, mean MND was smaller than has been reported in untrained men, implying greater capacity for growth and repair. These findings showcase the unique muscle cell structure of an elite powerlifter, extending the known physiological limits of human muscle size and strength.

Quadriceps Lipid Content Has Sex-Specific Associations With Whole-Muscle, Cellular, and Molecular Contractile Function in Older Adults.

Increased adiposity is associated with reduced skeletal muscle function in older adults, but the mechanisms underlying this relationship remain unclear. To explore whether skeletal muscle properties track with adiposity, whole-muscle, cellular, and molecular function were examined in relation to adiposity measured at various anatomical levels in healthy older (60-80 years) men and women. Although women had greater absolute and relative body and thigh fat than men, quadriceps muscle attenuation, an index of intramuscular lipid content, was similar between sexes. At the whole-muscle level, greater quadriceps attenuation was associated with reduced knee extensor function in women, but not men. In women, decreased myosin heavy chain I and IIA fiber-specific force was associated with higher intramuscular lipid content, which may be explained, in part, by the reduced myofilament lattice stiffness found in myosin heavy chain IIA fibers. Longer myosin attachment times in myosin heavy chain I fibers from men and women were associated with greater amounts of adipose tissue, suggesting that fat deposits lead to slower myosin-actin cross-bridge kinetics. Our results indicate greater quantities of adipose tissue alter myofilament properties and cross-bridge kinetics, which may partially explain the adiposity-induced decrements in single-fiber and whole-muscle function of older adults, especially women.

Age- and sex-related differences in myosin heavy chain isoforms and muscle strength, function, and quality: a cross sectional study.

[Purpose]Declining muscle strength and function are hallmarks of the aging process. This study aimed to determine sex-related differences in myosin heavy chain (MHC) isoforms and muscle mass, strength, and quality with aging.[Methods]This cross-sectional study recruited 53 healthy participants (32 men, 21 women) aged 20–85 years who were divided into four groups: young men (n=17, YM, 29.23±4.51), older men (n=15, OM, 71.87±3.42), young women (n=11, YW, 29.64±4.88), and older women (n=10, OW, 68.1±1.91). Body composition and muscle strength and quality were analyzed. Muscle specimens were obtained from the vastus lateralis in all participants to analyze the type of MHC isoforms.[Results]Men showed a greater age-related decline in skeletal muscle mass (18.6%, p<0.01), lean body mass (10.1%, p<0.05), grip strength (35.3%, p<0.001), isometric strength (29.6%, p<0.001), isotonic power (42.5%, p<0.001), isokinetic strength (up to 44.3%, p<0.001), and muscle quality (up to 24.8%, p<0.01). In contrast, women had significantly lower isometric strength (24.2%, p<0.05), isotonic power (36.5%, p<0.01), and upper-body muscle quality (24.7%, p<0.001) with aging. In addition, the proportion of MHC IIa was significantly lower in OM (p<0.05) and OW (p<0.05) than in YM and YW, respectively. However, the proportion of MHC I was significantly higher in OM (p<0.01) than in YM but was high in both YW and OW. MHC I and MHC IIa negatively and positively correlated, respectively, with muscle strength and function.[Conclusion]These results indicate the existence of sex-related differences in muscle mass, strength, and quality and MHC isoform composition with increasing age. The effects on muscle strength and function with aging were significant in men, but not in women. Higher and lower proportions of MHC I and MHC IIa fibers, respectively, were inversely associated with muscle strength and quality. In particular, Korean YW showed lower muscle strength and quality, and the proportion of MHC isoforms was similar to that in the muscles of OW.

Myocellular Responses to Concurrent Flywheel Training during 70 Days of Bed Rest.

This investigation evaluated myocellular responses to an integrated resistance and aerobic training program during 70 d of bed rest. Training was 6 d·wk on a small-footprint gravity-independent flywheel resistance and aerobic device; 3 d of maximal flywheel supine quadriceps and calf exercises with continuous rowing separated by 4 to 6 h, and 3 d of interval rowing. Vastus lateralis (VL) and soleus (SOL) muscle biopsies were obtained from eight healthy males (age, 28 ± 4 yr; BMI, 25 ± 3 kg·m; V˙O2max, 42 ± 6 mL·kg·min) before and after 6° head-down tilt bed rest. Vastus lateralis and SOL myosin heavy chain (MHC) I and IIa single muscle fiber size and functional characteristics, as well as overall fiber type distribution, capillarization, and metabolic enzyme activities were evaluated. In the VL, MHC I size and power (absolute and normalized) were preserved. The MHC IIa fibers hypertrophied (+6%, P < 0.05) without a change in absolute power, so normalized power declined (-7%, P < 0.05). In the SOL, MHC I fibers atrophied (-9%) and absolute power declined (-17%) (P < 0.05), whereas normalized power was maintained. Size, absolute power, and normalized power were protected in the less-abundant MHC IIa fibers. Reduced MHC coexpressing hybrid fibers, generally indicative of an exercise training effect, was apparent in the VL, whereas fiber type was maintained in the SOL. Capillarization and metabolic enzymes were generally preserved or increased in VL and SOL. The integrated resistance and aerobic training protocol on a device maintains several key myocellular characteristics during prolonged unloading, but further refinement of the exercise approach to fully protect the SOL is warranted.

Differential impact of tongue exercise on intrinsic lingual muscles.

Aging is associated with muscle fiber hypotrophy and decreased percentages of rapidly contracting myosin heavy chain (MyHC) type IIb muscle fibers. Tongue exercise programs used to treat dysphagia target age-related decline in tongue muscle function, but the impact of exercise on the intrinsic tongue muscles is unknown. We hypothesized that exercise would induce muscle fiber hypertrophy and increase the percentage of MyHC IIa fibers in the intrinsic tongue. Animal model. Eight old and eight young-adult rats underwent 8 weeks of tongue exercise training, and 8 animals of each age group served as controls. Longitudinal, transverse, and verticalis muscle samples from the anterior, middle, and posterior regions of the tongue were sectioned and stained to determine muscle fiber diameter and MyHC composition. MyHC fiber type distribution was altered by exercise, and the effects differed by muscle and region of the tongue. In the exercise groups, as compared to the control groups, the anterior transverse and middle superior longitudinal muscles had significantly reduced percentages of MyHC IIx positive fibers and higher percentages of rapidly contracting fatigable MyHC IIb positive muscle fibers, whereas the middle transverse and posterior longitudinal muscles had increased percentages of the less rapidly contracting and more fatigue-resistant MyHC IIa fibers. The impact of exercise did not differ with age, as there was no significant interaction between age and exercise. Tongue exercise had no significant effect on muscle fiber diameter. The impact of exercise varied among the tongue muscles, which may indicate different functional contributions to the tongue exercise task. NA Laryngoscope, 128:2245-2251, 2018.

Age-related reduction in single muscle fiber calcium sensitivity is associated with decreased muscle power in men and women

Age-related declines in human skeletal muscle performance may be caused, in part, by decreased responsivity of muscle fibers to calcium (Ca2+). This study examined the contractile properties of single vastus lateralis muscle fibers with various myosin heavy chain (MHC) isoforms (I, I/IIA, IIA and IIAX) across a range of Ca2+ concentrations in 11 young (24.1±1.1years) and 10 older (68.8±0.8years) men and women. The normalized pCa-force curve shifted rightward with age, leading to decreased activation threshold (pCa10) and/or Ca2+ sensitivity (pCa50) for all MHC isoforms examined. In older adults, the slope of the pCa-force curve was unchanged in MHC I-containing fibers (I, I/IIA), but was steeper in MHC II-containing fibers (IIA, IIAX), indicating greater cooperativity compared to young adults. At sub-maximal [Ca2+], specific force was reduced in MHC I-containing fibers, but was minimally decreased in MHC IIA fibers as older adults produced greater specific forces at high [Ca2+] in these fibers. Lessor pCa50 in MHC I fibers independently predicted reduced isokinetic knee extensor power across a range of contractile velocities, suggesting that the Ca2+ response of slow-twitch fibers contributes to whole muscle dysfunction. Our findings show that aging attenuates Ca2+ responsiveness across fiber types and that these cellular alterations may lead to age-related reductions in whole muscle power output.

Improved single muscle fiber quality in the oldest-old.

We examined single muscle fiber contractile function of the oldest-old (3F/2M, 89 ± 1 yr old) enrolled in The Health, Aging, and Body Composition Study (The Health ABC Study). Vastus lateralis muscle biopsies were obtained and single muscle fiber function was determined (n = 105) prior to myosin heavy chain (MHC) isoform identification with sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Cross-sectional area of MHC I muscle fibers (5,576 ± 333 μm2; n = 58) was 21% larger (P < 0.05) than MHC IIa fibers (4,518 ± 386 μm2; n = 47). Normalized power (an indicator of muscle fiber quality incorporating size, strength, and speed) of MHC I and IIa muscle fibers was 2.3 ± 0.1 and 17.4 ± 0.8 W/l, respectively. Compared with previous research from our lab using identical procedures, MHC I normalized power was 28% higher than healthy 20 yr olds and similar to younger octogenarians (∼80 yr old). Normalized power of MHC IIa fibers was 63% greater than 20 yr olds and 39% greater than younger octogenarians. These comparative data suggest that power output per unit size (i.e., muscle quality) of remaining muscle fibers improves with age, a phenomenon more pronounced in MHC IIa fibers. Age-related single muscle fiber quality improvements may be a compensatory mechanism to help offset decrements in whole muscle function.

Regional adaptation of collagen in skeletal muscle to repeated bouts of strenuous eccentric exercise.

This study investigated the injured region-specific alterations of factors related to the "repeated bout effect" (RBE), i.e., when the first bout of eccentric exercise generates resistance to injuries from the second bout of the same exercise. Wistar rats were divided into single injury (SI) and repeated injury (RI) groups. The right gastrocnemius muscle was subjected to a bout of eccentric contractions (ECs) at the age of 14weeks in the SI group and 10 and 14weeks in the RI group. The number of injured fibers after the last bout of ECs was lower in RI than in SI. In the SI group, injured fibers after ECs were mainly located in the superficial region of muscle and expressed myosin heavy chain (MHC) IIx and IIb. Prior to the second bout of ECs, the fiber-type composition in the RI group showed decreased MHC IIx and IIb fibers and increased MHC IIa fibers compared with those in the SI group. However, most regenerating fibers showed either MHC IIx or IIb expression. Heat shock protein 72 and total collagen contents in whole muscle were higher in the RI group than in the SI group; however, only the collagen expression in the RI group was more intense than that in the SI group in the superficial region of muscle. These findings suggest that increased collagen may play a more important role in the injured region of muscle than the other factors in RBE.

Skeletal muscle fiber size and fiber type distribution in human cancer: Effects of weight loss and relationship to physical function

Cancer patients frequently experience weight loss, with negative consequences for functionality and prognosis. The extent to which muscle atrophy contributes to weight loss, however, is not clear, as few studies have directly measured muscle fiber morphology in cancer patients. Whole body and regional tissue composition were measured, along with the cross-sectional area (CSA) and fiber type of mechanically-isolated, single muscle fibers, in 19 cancer patients (8 with a history of weight loss, 11 weight-stable) and 15 non-diseased controls. Whole body fat mass was reduced in cancer patients with a history of weight loss, but no differences in whole body or leg fat-free mass were apparent. In contrast, reductions (∼20%) in single muscle fiber CSA were found in both slow-twitch, myosin heavy chain (MHC) I and fast-twitch, MHC IIA fibers in both weight-stable patients and those with a history of weight loss. Fiber type distribution showed a shift towards a fast-twitch phenotype compared to controls, which may preserve muscle function in cancer patients despite atrophy, as positive relationships were found between the fractions of hybrid MHC IIAX and I/IIA fibers and 6-min walk performance. Our results suggest that, although not apparent from whole body or regional measurements, cancer is associated with reduced skeletal muscle fiber size independent of weight loss history and a shift towards fast-twitch fibers, phenotypes that resemble adaptations to muscle disuse.