Skeletal Muscle Myofibrillar Protein Abundance Is Higher in Resistance-Trained Men, and Aging in the Absence of Training May Have an Opposite Effect

Skeletal muscle myosin heavy chain fragmentation as a potential marker of protein degradation in response to resistance training and disuse atrophy

We examined how resistance exercise (RE), cycling exercise and disuse atrophy affect myosin heavy chain (MyHC) protein fragmentation. The 1boutRE study involved younger men (n = 8; 5 ± 2 years of RE experience) performing a lower body RE bout with vastus lateralis (VL) biopsies being obtained prior to and acutely following exercise. With the 10weekRT study, VL biopsies were obtained in 36 younger adults before and 24 h after their first/naïve RE bout. Participants also engaged in 10 weeks of resistance training and donated VL biopsies before and 24 h after their last RE bout. VL biopsies were also examined in an acute cycling study (n = 7) and a study involving 2 weeks of leg immobilization (n = 20). In the 1boutRE study, fragmentation of all MyHC isoforms (MyHCTotal) increased 3 h post-RE (∼200%, P = 0.018) and returned to pre-exercise levels by 6 h post-RE. Interestingly, a greater magnitude increase in MyHC type IIa versus I isoform fragmentation occurred 3 h post-RE (8.6 ± 6.3-fold vs. 2.1 ± 0.7-fold, P = 0.018). In 10weekRT participants, the first/naïve and last RE bouts increased MyHCTotal fragmentation 24 h post-RE (+65% and +36%, P < 0.001); however, the last RE bout response was attenuated compared to the first bout (P = 0.045). Although cycling exercise did not alter MyHCTotal fragmentation, ∼8% VL atrophy with 2 weeks of leg immobilization increased MyHCTotal fragmentation (∼108%, P < 0.001). Mechanistic C2C12 myotube experiments indicated that MyHCTotal fragmentation is likely due to calpain proteases. In summary, RE and disuse atrophy increase MyHC protein fragmentation. Research into how ageing and disease-associated muscle atrophy affect these outcomes is needed. HIGHLIGHTS: What is the central question of this study? How different exercise stressors and disuse affect skeletal muscle myosin heavy chain fragmentation. What is the main finding and its importance? This investigation is the first to demonstrate that resistance exercise and disuse atrophy lead to skeletal muscle myosin heavy chain protein fragmentation in humans. Mechanistic in vitro experiments provide additional evidence that MyHC fragmentation occurs through calpain proteases.

Skeletal muscle myosin heavy chain protein fragmentation as a potential marker of protein degradation in response to resistance training and disuse atrophy

We sought to examine how resistance exercise (RE), cycling exercise, and disuse atrophy affect myosin heavy chain (MyHC) protein fragmentation in humans. In the first study (1boutRE), younger adult men (n=8; 5±2 years of RE experience) performed a lower body RE bout with vastus lateralis (VL) biopsies obtained immediately before, 3-, and 6-hours post-exercise. In the second study (10weekRT), VL biopsies were obtained in untrained younger adults (n=36, 18 men and 18 women) before and 24 hours (24h) after their first/naïve RE bout. These participants also engaged in 10 weeks (24 sessions) of resistance training and donated VL biopsies before and 24h after their last RE bout. VL biopsies were also examined from a third acute cycling study (n=7) and a fourth study involving two weeks of leg immobilization (n=20, 15 men and 5 women) to determine how MyHC fragmentation was affected. In the 1boutRE study, the fragmentation of all MyHC isoforms (MyHCTotal) increased 3 hours post-RE (~ +200%, p=0.018) and returned to pre-exercise levels by 6 hours post-RE. Immunoprecipitation of MyHCTotal revealed ubiquitination levels remained unaffected at the 3- and 6-hour post-RE time points. Interestingly, a greater increase in magnitude for MyHC type IIa versus I isoform fragmentation occurred 3-hours post-RE (8.6±6.3-fold versus 2.1±0.7-fold, p=0.018). In all 10weekRT participants, the first/naïve and last RE bouts increased MyHCTotal fragmentation 24h post-RE (+65% and +36%, respectively; p<0.001); however, the last RE bout response was attenuated compared to the first bout (p=0.045). The first/naïve bout response was significantly elevated in females only (p<0.001), albeit females also demonstrated a last bout attenuation response (p=0.002). Although an acute cycling bout did not alter MyHCTotal fragmentation, ~8% VL atrophy with two weeks of leg immobilization led to robust MyHCTotal fragmentation (+108%, p<0.001), and no sex-based differences were observed. In summary, RE and disuse atrophy increase MyHC protein fragmentation. A dampened response with 10 weeks of resistance training, and more refined responses in well-trained men, suggest this is an adaptive process. Given the null polyubiquitination IP findings, more research is needed to determine how MyHC fragments are processed. Moreover, further research is needed to determine how aging and disease-associated muscle atrophy affect these outcomes, and whether MyHC fragmentation is a viable surrogate for muscle protein turnover rates.

A novel deep proteomic approach in human skeletal muscle unveils distinct molecular signatures affected by aging and resistance training

The skeletal muscle proteome alterations to aging and resistance training have been reported in prior studies. However, conventional proteomics in skeletal muscle typically yields wide protein abundance ranges that mask the detection of lowly expressed proteins. Thus, we adopted a novel deep proteomics approach whereby myofibril (MyoF) and non-MyoF fractions were separately subjected to protein corona nanoparticle complex formation prior to digestion and Liquid Chromatography Mass Spectrometry (LC-MS). Specifically, we investigated MyoF and non-MyoF proteomic profiles of the vastus lateralis muscle of younger (Y, 22±2 years old; n=5) and middle-aged participants (MA, 56±8 years old; n=6). Additionally, MA muscle was analyzed following eight weeks of resistance training (RT, 2d/week). Across all participants, the number of non-MyoF proteins detected averaged to be 5,645±266 (range: 4,888-5,987) and the number of MyoF proteins detected averaged to be 2,611±326 (range: 1,944-3,101). Differences in the non-MyoF (8.4%) and MyoF (2.5%) proteomes were evident between age cohorts, and most differentially expressed non-MyoF proteins (447/543) were more enriched in MA versus Y. Biological processes in the non-MyoF fraction were predicted to be operative in MA versus Y including increased cellular stress, mRNA splicing, translation elongation, and ubiquitin-mediated proteolysis. RT in MA participants only altered ~0.3% of MyoF and ~1.0% of non-MyoF proteomes. In summary, aging and RT predominantly affect non-contractile proteins in skeletal muscle. Additionally, marginal proteome adaptations with RT suggest more rigorous training may stimulate more robust effects or that RT, regardless of age, subtly alters basal state skeletal muscle protein abundances.

Mechanisms of mechanical overload-induced skeletal muscle hypertrophy: current understanding and future directions

Mechanisms underlying mechanical overload-induced skeletal muscle hypertrophy have been extensively researched since the landmark report by Morpurgo (1897) of "work-induced hypertrophy" in dogs that were treadmill trained. Much of the preclinical rodent and human resistance training research to date supports that involved mechanisms include enhanced mammalian/mechanistic target of rapamycin complex 1 (mTORC1) signaling, an expansion in translational capacity through ribosome biogenesis, increased satellite cell abundance and myonuclear accretion, and postexercise elevations in muscle protein synthesis rates. However, several lines of past and emerging evidence suggest that additional mechanisms that feed into or are independent of these processes are also involved. This review first provides a historical account of how mechanistic research into skeletal muscle hypertrophy has progressed. A comprehensive list of mechanisms associated with skeletal muscle hypertrophy is then outlined, and areas of disagreement involving these mechanisms are presented. Finally, future research directions involving many of the discussed mechanisms are proposed.

A novel deep proteomic approach in human skeletal muscle unveils distinct molecular signatures affected by aging and resistance training

We examined the myofibrillar (MyoF) and non-myofibrillar (non-MyoF) proteomic profiles of the vastus lateralis (VL) muscle of younger (Y, 22±2 years old; n=5) and middle-aged participants (MA, 56±8 years old; n=6), and MA following eight weeks of knee extensor resistance training (RT, 2d/week). Shotgun/bottom-up proteomics in skeletal muscle typically yields wide protein abundance ranges that mask lowly expressed proteins. Thus, we adopted a novel approach whereby the MyoF and non-MyoF fractions were separately subjected to protein corona nanoparticle complex formation prior to digestion and Liquid Chromatography Mass Spectrometry (LC-MS) analysis. A total of 10,866 proteins (4,421 MyoF and 6,445 non-MyoF) were identified. Across all participants, the number of non-MyoF proteins detected averaged to be 5,645±266 (range: 4,888-5,987) and the number of MyoF proteins detected averaged to be 2,611±326 (range: 1,944-3,101). Differences in the non-MyoF (8.4%) and MyoF (2.5%) proteome were evident between age cohorts. Further, most of these age-related non-MyoF proteins (447/543) were more enriched in MA versus Y. Several biological processes in the non-MyoF fraction were predicted to be operative in MA versus Y including (but not limited to) increased cellular stress, mRNA splicing, translation elongation, and ubiquitin-mediated proteolysis. Non-MyoF proteins associated with splicing and proteostasis were further interrogated, and in agreement with bioinformatics, alternative protein variants, spliceosome-associated proteins (snRNPs), and proteolysis-related targets were more abundant in MA versus Y. RT in MA non-significantly increased VL muscle cross-sectional area (+6.5%, p=0.066) and significantly increased knee extensor strength (+8.7%, p=0.048). However, RT modestly altered the MyoF (~0.3%, 11 upregulated and two downregulated proteins) and non-MyoF proteomes (~1.0%, 56 upregulated and eight downregulated proteins, p<0.01). Further, RT did not affect predicted biological processes in either fraction. Although participant numbers were limited, these preliminary results using a novel deep proteomic approach in skeletal muscle suggest that aging and RT predominantly affects protein abundances in the non-contractile protein pool. However, the marginal proteome adaptations occurring with RT suggest either: a) this may be an aging-associated phenomenon, b) more rigorous RT may stimulate more robust effects, or c) RT, regardless of age, subtly affects skeletal muscle protein abundances in the basal state.

Extracellular matrix content and remodeling markers do not differ in college-aged men classified as higher and lower responders to resistance training

We determined if skeletal muscle extracellular matrix (ECM) content and remodeling markers adapted with resistance training or were associated with hypertrophic outcomes. Thirty-eight untrained males (21 ± 3 yr) participated in whole body resistance training (10 wk, 2 × weekly). Participants completed testing [ultrasound, peripheral quantitative computed tomography (pQCT)] and donated a vastus lateralis (VL) biopsy 1 wk before training and 72 h following the last training bout. Higher responders (HR, n = 10) and lower responders (LR, n = 10) were stratified based on a composite score considering changes in pQCT-derived mid-thigh cross-sectional area (mCSA), ultrasound-derived VL thickness, and mean fiber cross-sectional area (fCSA). In all participants, training reduced matrix metalloprotease (MMP)-14 protein (P < 0.001) and increased satellite cell abundance (P < 0.001); however, VL fascial thickness, ECM protein content per myofiber, MMP-2/-9 protein content, tissue inhibitor of metalloproteinase (TIMP)-1/-2 protein content, collagen-1/-4 protein content, macrophage abundance, or fibroadipogenic progenitor cell abundance were not altered. Regarding responder analysis, MMP-14 exhibited an interaction (P = 0.007), and post hoc analysis revealed higher protein content in HR versus LR before training (P = 0.026) and a significant decrease from pre to posttraining in HR only (P = 0.002). In summary, basal skeletal muscle ECM markers are minimally affected with 10 wk of resistance training, and these findings could be related to not capturing more dynamic alterations in the assayed markers earlier in training. However, the downregulation in MMP-14 in college-aged men classified as HR is a novel finding and warrants continued investigation, and further research is needed to delineate muscle connective tissue strength attributes between HR and LR.NEW & NOTEWORTHY Although past studies have examined aspects of extracellular matrix remodeling in relation to mechanical overload or resistance training, this study serves to expand our knowledge on a multitude of extracellular matrix markers and whether these markers adapt to resistance training or are associated with differential hypertrophic responses.

Fiber-Type Shifting in Sarcopenia of Old Age: Proteomic Profiling of the Contractile Apparatus of Skeletal Muscles

The progressive loss of skeletal muscle mass and concomitant reduction in contractile strength plays a central role in frailty syndrome. Age-related neuronal impairments are closely associated with sarcopenia in the elderly, which is characterized by severe muscular atrophy that can considerably lessen the overall quality of life at old age. Mass-spectrometry-based proteomic surveys of senescent human skeletal muscles, as well as animal models of sarcopenia, have decisively improved our understanding of the molecular and cellular consequences of muscular atrophy and associated fiber-type shifting during aging. This review outlines the mass spectrometric identification of proteome-wide changes in atrophying skeletal muscles, with a focus on contractile proteins as potential markers of changes in fiber-type distribution patterns. The observed trend of fast-to-slow transitions in individual human skeletal muscles during the aging process is most likely linked to a preferential susceptibility of fast-twitching muscle fibers to muscular atrophy. Studies with senescent animal models, including mostly aged rodent skeletal muscles, have confirmed fiber-type shifting. The proteomic analysis of fast versus slow isoforms of key contractile proteins, such as myosin heavy chains, myosin light chains, actins, troponins and tropomyosins, suggests them as suitable bioanalytical tools of fiber-type transitions during aging.

Proteomic features of skeletal muscle adaptation to resistance exercise training as a function of age

Resistance exercise training (RET) can counteract negative features of muscle ageing but older age associates with reduced adaptive capacity to RET. Altered muscle protein networks likely contribute to ageing RET adaptation; therefore, associated proteome-wide responses warrant exploration. We employed quantitative sarcoplasmic proteomics to compare age-related proteome and phosphoproteome responses to RET. Thigh muscle biopsies were collected from eight young (25 ± 1.1 years) and eight older (67.5 ± 2.6 years) adults before and after 20 weeks supervised RET. Muscle sarcoplasmic fractions were pooled for each condition and analysed using Isobaric Tags for Relative and Absolute Quantification (iTRAQ) labelling, tandem mass spectrometry and network-based hub protein identification. Older adults displayed impaired RET-induced adaptations in whole-body lean mass, body fat percentage and thigh lean mass (P > 0.05). iTRAQ identified 73 differentially expressed proteins with age and/or RET. Despite possible proteomic stochasticity, RET improved ageing profiles for mitochondrial function and glucose metabolism (top hub; PYK (pyruvate kinase)) but failed to correct altered ageing expression of cytoskeletal proteins (top hub; YWHAZ (14-3-3 protein zeta/delta)). These ageing RET proteomic profiles were generally unchanged or oppositely regulated post-RET in younger muscle. Similarly, RET corrected expression of 10 phosphoproteins altered in ageing, but these responses were again different vs. younger adults. Older muscle is characterised by RET-induced metabolic protein profiles that, whilst not present in younger muscle, improve untrained age-related proteomic deficits. Combined with impaired cytoskeletal adhesion responses, these results provide a proteomic framework for understanding and optimising ageing muscle RET adaptation.

Daily protein-polyphenol ingestion increases daily myofibrillar protein synthesis rates and promotes early muscle functional gains during resistance training

Factors underpinning the time-course of resistance-type exercise training (RET) adaptations are not fully understood. This study hypothesized that consuming a twice-daily protein-polyphenol beverage (PPB; n = 15; age, 24 ± 1 yr; BMI, 22.3 ± 0.7 kg·m-2) previously shown to accelerate recovery from muscle damage and increase daily myofibrillar protein synthesis (MyoPS) rates would accelerate early (10 sessions) improvements in muscle function and potentiate quadriceps volume and muscle fiber cross-sectional area (fCSA) following 30 unilateral RET sessions in healthy, recreationally active, adults. Versus isocaloric placebo (PLA; n = 14; age, 25 ± 2 yr; BMI, 23.9 ± 1.0 kg·m-2), PPB increased 48 h MyoPS rates after the first RET session measured using deuterated water (2.01 ± 0.15 vs. 1.51 ± 0.16%·day-1, respectively; P < 0.05). In addition, PPB increased isokinetic muscle function over 10 sessions of training relative to the untrained control leg (%U) from 99.9 ± 1.8 pretraining to 107.2 ± 2.4%U at session 10 (vs. 102.6 ± 3.9 to 100.8 ± 2.4%U at session 10 in PLA; interaction P < 0.05). Pre to posttraining, PPB increased type II fCSA (PLA: 120.8 ± 8.2 to 109.5 ± 8.6%U; PPB: 92.8 ± 6.2 to 108.4 ± 9.7%U; interaction P < 0.05), but the gain in quadriceps muscle volume was similar between groups. Similarly, PPB did not further increase peak isometric torque, muscle function, or MyoPS measured posttraining. This suggests that although PPB increases MyoPS and early adaptation, it may not influence longer term adaptations to unilateral RET.NEW & NOTEWORTHY Using a unilateral model of resistance training, we show for the first time that a protein-polyphenol beverage increases initial rates of myofibrillar protein synthesis and promotes early functional improvements. Following a prolonged period of training, this strategy also increases type II fiber hypertrophy and causes large individual variation in gains in quadricep muscle cross-sectional area.

Frequent Manipulation of Resistance Training Variables Promotes Myofibrillar Spacing Changes in Resistance-Trained Individuals

We sought to determine if manipulating resistance training (RT) variables differentially altered the expression of select sarcoplasmic and myofibril proteins as well as myofibrillar spacing in myofibers. Resistance-trained men (n = 20; 26 ± 3 years old) trained for 8 weeks where a randomized leg performed either a standard (CON) or variable RT protocol (VAR: manipulation of load, volume, muscle action, and rest intervals at each RT session). A pre-training (PRE) vastus lateralis biopsy was obtained from a randomized single leg, and biopsies were obtained from both legs 96 h following the last training bout. The sarcoplasmic protein pool was assayed for proteins involved in energy metabolism, and the myofibril protein pool was assayed for relative myosin heavy chain (MHC) and actin protein abundances. Sections were also histologically analyzed to obtain myofibril spacing characteristics. VAR resulted in ~12% greater volume load (VL) compared to CON (p < 0.001). The mean fiber cross-sectional area increased following both RT protocols [CON: 14.6% (775.5 μm²), p = 0.006; VAR: 13.9% (743.2 μm²), p = 0.01 vs. PRE for both], but without significant differences between protocols (p = 0.79). Neither RT protocol affected a majority of assayed proteins related to energy metabolism, but both training protocols increased hexokinase 2 protein levels and decreased a mitochondrial beta-oxidation marker (VLCAD protein; p < 0.05). Citrate synthase activity levels increased with CON RT (p < 0.05), but not VAR RT. The relative abundance of MHC (summed isoforms) decreased with both training protocols (p < 0.05). However, the relative abundance of actin protein (summed isoforms) decreased with VAR only (13.5 and 9.0%, respectively; p < 0.05). A decrease in percent area occupied by myofibrils was observed from PRE to VAR (−4.87%; p = 0.048), but not for the CON (4.53%; p = 0.979). In contrast, there was an increase in percent area occupied by non-contractile space from PRE to VAR (10.14%; p = 0.048), but not PRE to CON (0.72%; p = 0.979). In conclusion, while both RT protocols increased muscle fiber hypertrophy, a higher volume-load where RT variables were frequently manipulated increased non-contractile spacing in resistance-trained individuals.

Myofibril and Mitochondrial Area Changes in Type I and II Fibers Following 10 Weeks of Resistance Training in Previously Untrained Men

Resistance training increases muscle fiber hypertrophy, but the morphological adaptations that occur within muscle fibers remain largely unresolved. Fifteen males with minimal training experience (24±4years, 23.9±3.1kg/m² body mass index) performed 10weeks of conventional, full-body resistance training (2× weekly). Body composition, the radiological density of the vastus lateralis muscle using peripheral quantitative computed tomography (pQCT), and vastus lateralis muscle biopsies were obtained 1week prior to and 72h following the last training bout. Quantification of myofibril and mitochondrial areas in type I (positive for MyHC I) and II (positive for MyHC IIa/IIx) fibers was performed using immunohistochemistry (IHC) techniques. Relative myosin heavy chain and actin protein abundances per wet muscle weight as well as citrate synthase (CS) activity assays were also obtained on tissue lysates. Training increased whole-body lean mass, mid-thigh muscle cross-sectional area, mean and type II fiber cross-sectional areas (fCSA), and maximal strength values for leg press, bench press, and deadlift (p<0.05). The intracellular area occupied by myofibrils in type I or II fibers was not altered with training, suggesting a proportional expansion of myofibrils with fCSA increases. However, our histological analysis was unable to differentiate whether increases in myofibril number or girth occurred. Relative myosin heavy chain and actin protein abundances also did not change with training. IHC indicated training increased mitochondrial areas in both fiber types (p=0.018), albeit CS activity levels remained unaltered with training suggesting a discordance between these assays. Interestingly, although pQCT-derived muscle density increased with training (p=0.036), suggestive of myofibril packing, a positive association existed between training-induced changes in this metric and changes in mean fiber myofibril area (r=0.600, p=0.018). To summarize, our data imply that shorter-term resistance training promotes a proportional expansion of the area occupied by myofibrils and a disproportional expansion of the area occupied by mitochondria in type I and II fibers. Additionally, IHC and biochemical techniques should be viewed independently from one another given the lack of agreement between the variables assessed herein. Finally, the pQCT may be a viable tool to non-invasively track morphological changes (specifically myofibril density) in muscle tissue.

UBE2L3, a Partner of MuRF1/TRIM63, Is Involved in the Degradation of Myofibrillar Actin and Myosin

The ubiquitin proteasome system (UPS) is the main player of skeletal muscle wasting, a common characteristic of many diseases (cancer, etc.) that negatively impacts treatment and life prognosis. Within the UPS, the E3 ligase MuRF1/TRIM63 targets for degradation several myofibrillar proteins, including the main contractile proteins alpha-actin and myosin heavy chain (MHC). We previously identified five E2 ubiquitin-conjugating enzymes interacting with MuRF1, including UBE2L3/UbcH7, that exhibited a high affinity for MuRF1 (KD = 50 nM). Here, we report a main effect of UBE2L3 on alpha-actin and MHC degradation in catabolic C2C12 myotubes. Consistently UBE2L3 knockdown in Tibialis anterior induced hypertrophy in dexamethasone (Dex)-treated mice, whereas overexpression worsened the muscle atrophy of Dex-treated mice. Using combined interactomic approaches, we also characterized the interactions between MuRF1 and its substrates alpha-actin and MHC and found that MuRF1 preferentially binds to filamentous F-actin (KD = 46.7 nM) over monomeric G-actin (KD = 450 nM). By contrast with actin that did not alter MuRF1-UBE2L3 affinity, binding of MHC to MuRF1 (KD = 8 nM) impeded UBE2L3 binding, suggesting that differential interactions prevail with MuRF1 depending on both the substrate and the E2. Our data suggest that UBE2L3 regulates contractile proteins levels and skeletal muscle atrophy.

Effect of a 12-week endurance training program on force transfer and membrane integrity proteins in lean, obese, and type 2 diabetic subjects

The mechanisms accounting for the loss of muscle function with obesity and type 2 diabetes are likely the result of a combination of neural and muscular factors. One muscular factor that is important, yet has received little attention, is the protein machinery involved in longitudinal and lateral force transmission. The purpose of this study was to compare the levels of force transfer and membrane integrity proteins before and after a 12‐week endurance training program in lean, obese, and obese type 2 diabetic adults. Nineteen sedentary subjects (male = 8 and female = 11) were divided into three groups: Lean (n = 7; 50.3 ± 4.1 y; 69.1 ± 7.2 kg); Obese (n = 6; 49.8 ± 4.1 y; 92.9 ± 19.5 kg); and Obese with type 2 diabetes (n = 6; 51.5 ± 7.9 years; 88.9 ± 15.1 kg). Participants trained 150 min/week between 55% and 75% of VO_2max for 12 weeks. Skeletal muscle biopsies were taken before and after the training intervention. Baseline dystrophin and muscle LIM protein levels were higher (~50% p < .01) in lean compared to obese and type 2 diabetic adults, while the protein levels of the remaining force transfer and membrane integrity proteins were similar between groups. After training, obese individuals decreased (−53%; p < .01) the levels of the muscle ankyrin repeat protein and lean individuals decreased dystrophin levels (−45%; p = .01), while the levels of the remaining force transfer and membrane integrity proteins were not affected by training. These results suggest that there are modest changes to force transfer and membrane integrity proteins in middle‐aged individuals as a result of 12 weeks of lifestyle and training interventions.

Are muscle fibres of body builders intrinsically weaker? A comparison with single fibres of aged-matched controls

AIM

Skeletal muscles of Body Builders (BB) represent an interesting model to study muscle mass gains in response to high volume resistance training. It is debated whether muscle contractile performance improves in proportion to mass. Here, we aim to assess whether muscle hypertrophy does not occur at the expense of performance.

METHODS

Six BB and Six untrained controls (CTRL) were recruited. Cross-sectional area (CSA) and maximum voluntary contraction (MVC) of quadriceps femoris muscle (QF) and CSA and architecture of vastus lateralis (VL) were determined. Moreover, a biopsy was taken from VL mid-portion and single fibres were analysed.

RESULTS

QF CSA and MVC were 32% (n.s., P = .052) and 58% (P = .009) higher in BB than in CTRL, respectively. VL CSA was 37% higher in BB (P = .030). Fast 2A fibres CSA was 24% (P = .048) greater in BB than in CTRL, when determined in immunostained sections of biopsy samples. Single permeabilized fast fibres CSA was 37% (n.s., P = .052) higher in BB than in CTRL, and their force was slightly higher in BB (n.s.), while specific tension (P₀ ) was 19% (P = .024) lower. The lower P₀ was not explained either by lower myosin content or by impaired calcium diffusion. Conversely, the swelling caused by skinning-induced permeabilization was different and, when used to correct P₀ , differences between populations disappeared.

CONCLUSIONS

The results show that high degree of muscle hypertrophy is not detrimental for force generation capacity, as increases in fibre size and force are strictly proportional once the differential swelling response is accounted for.

Emphasizing Task-Specific Hypertrophy to Enhance Sequential Strength and Power Performance

While strength is indeed a skill, most discussions have primarily considered structural adaptations rather than ultrastructural augmentation to improve performance. Altering the structural component of the muscle is often the aim of hypertrophic training, yet not all hypertrophy is equal; such alterations are dependent upon how the muscle adapts to the training stimuli and overall training stress. When comparing bodybuilders to strength and power athletes such as powerlifters, weightlifters, and throwers, while muscle size may be similar, the ability to produce force and power is often inequivalent. Thus, performance differences go beyond structural changes and may be due to the muscle's ultrastructural constituents and training induced adaptations. Relative to potentiating strength and power performances, eliciting specific ultrastructural changes should be a variable of interest during hypertrophic training phases. By focusing on task-specific hypertrophy, it may be possible to achieve an optimal amount of hypertrophy while deemphasizing metabolic and aerobic components that are often associated with high-volume training. Therefore, the purpose of this article is to briefly address different types of hypertrophy and provide directions for practitioners who are aiming to achieve optimal rather than maximal hypertrophy, as it relates to altering ultrastructural muscular components, to potentiate strength and power performance.

Sarcoplasmic Hypertrophy in Skeletal Muscle: A Scientific "Unicorn" or Resistance Training Adaptation?

Skeletal muscle fibers are multinucleated cells that contain mostly myofibrils suspended in an aqueous media termed the sarcoplasm. Select evidence suggests sarcoplasmic hypertrophy, or a disproportionate expansion of the sarcoplasm relative to myofibril protein accretion, coincides with muscle fiber or tissue growth during resistance training. There is also evidence to support other modes of hypertrophy occur during periods of resistance training including a proportional accretion of myofibril protein with fiber or tissue growth (i.e., conventional hypertrophy), or myofibril protein accretion preceding fiber or tissue growth (i.e., myofibril packing). In this review, we discuss methods that have been used to investigate these modes of hypertrophy. Particular attention is given to sarcoplasmic hypertrophy throughout. Thus, descriptions depicting this process as well as the broader implications of this phenomenon will be posited. Finally, we propose future human and rodent research that can further our understanding in this area of muscle physiology.

Identifying the Structural Adaptations that Drive the Mechanical Load-Induced Growth of Skeletal Muscle: A Scoping Review

The maintenance of skeletal muscle mass plays a critical role in health and quality of life. One of the most potent regulators of skeletal muscle mass is mechanical loading, and numerous studies have led to a reasonably clear understanding of the macroscopic and microscopic changes that occur when the mechanical environment is altered. For instance, an increase in mechanical loading induces a growth response that is mediated, at least in part, by an increase in the cross-sectional area of the myofibers (i.e., myofiber hypertrophy). However, very little is known about the ultrastructural adaptations that drive this response. Even the most basic questions, such as whether mechanical load-induced myofiber hypertrophy is mediated by an increase in the size of the pre-existing myofibrils and/or an increase in the number myofibrils, have not been resolved. In this review, we thoroughly summarize what is currently known about the macroscopic, microscopic and ultrastructural changes that drive mechanical load-induced growth and highlight the critical gaps in knowledge that need to be filled.

Skeletal Muscle Protein Composition Adaptations to 10 Weeks of High-Load Resistance Training in Previously-Trained Males

While high-load resistance training increases muscle hypertrophy, the intramuscular protein responses to this form of training remains largely unknown. In the current study, recreationally resistance-trained college-aged males (N = 15; mean ± SD: 23 ± 3 years old, 6 ± 5 years training) performed full-body, low-volume, high-load [68–90% of one repetition maximum (1RM)] resistance training over 10 weeks. Back squat strength testing, body composition testing, and a vastus lateralis biopsy were performed before (PRE) and 72 h after the 10-week training program (POST). Fiber type-specific cross-sectional area (fCSA), myofibrillar protein concentrations, sarcoplasmic protein concentrations, myosin heavy chain and actin protein abundances, and muscle tissue percent fluid were analyzed. The abundances of individual sarcoplasmic proteins in 10 of the 15 participants were also assessed using proteomics. Significant increases (p < 0.05) in type II fCSA and back squat strength occurred with training, although whole-body fat-free mass paradoxically decreased (p = 0.026). No changes in sarcoplasmic protein concentrations or muscle tissue percent fluid were observed. Myosin heavy chain protein abundance trended downward (−2.9 ± 5.8%, p = 0.069) and actin protein abundance decreased (−3.2 ± 5.3%, p = 0.034) with training. Proteomics indicated only 13 sarcoplasmic proteins were altered with training (12 up-regulated, 1 down-regulated, p < 0.05). Bioinformatics indicated no signaling pathways were affected, and proteins involved with metabolism (e.g., ATP-PCr, glycolysis, TCA cycle, or beta-oxidation) were not affected. These data comprehensively describe intramuscular protein adaptations that occur following 10 weeks of high-load resistance training. Although previous data from our laboratory suggests high-volume resistance training enhances the ATP-PCr and glycolytic pathways, we observed different changes in metabolism-related proteins in the current study with high-load training.