The molecular basis for load-induced skeletal muscle hypertrophy

Twenty-five years of blood flow restriction training: What we know, what we don't, and where to next?

Blood flow restriction is a technique that involves inflating a cuff at the proximal portion of the limb with the goal of reducing arterial inflow into the muscle and venous outflow from the muscle. Low-load or low-intensity exercise in combination with blood flow restriction has been consistently shown to augment adaptations over the same/similar exercise without restriction, with changes in muscle size and strength being two of the most commonly measured adaptations. The purpose of this manuscript is to provide an updated narrative review on blood flow restriction. Blood flow restriction's history, methodology, safety, and efficacy are highlighted. We discuss the effects of blood flow restriction on changes in muscle size and strength, and also review work completed on other variables (e.g. bone, resting blood flow, tendon, pain sensitivity, cognition, orthostatic intolerance). We finish by highlighting six possible areas for future research: 1) identifying mechanisms for growth and strength; 2) sex differences in the effects of blood flow restriction; 3) individual responses to blood flow restriction; 4) influence of pressure versus amount of blood flow restricted; 5) application of blood flow restriction with higher-loads; and 6) what considerations should be made to test the effects of blood flow restriction.

The effects of branched-chain amino acids on muscle protein synthesis, muscle protein breakdown and associated molecular signalling responses in humans: an update

Branched-chain amino acids (BCAA: leucine, isoleucine and valine) are three of the nine indispensable amino acids, and are frequently consumed as a dietary supplement by athletes and recreationally active individuals alike. The popularity of BCAA supplements is largely predicated on the notion that they can stimulate rates of muscle protein synthesis (MPS) and suppress rates of muscle protein breakdown (MPB), the combination of which promotes a net anabolic response in skeletal muscle. To date, several studies have shown that BCAA (particularly leucine) increase the phosphorylation status of key proteins within the mechanistic target of rapamycin (mTOR) signalling pathway involved in the regulation of translation initiation in human muscle. Early research in humans demonstrated that BCAA provision reduced indices of whole-body protein breakdown and MPB; however, there was no stimulatory effect of BCAA on MPS. In contrast, recent work has demonstrated that BCAA intake can stimulate postprandial MPS rates at rest and can further increase MPS rates during recovery after a bout of resistance exercise. The purpose of this evidence-based narrative review is to critically appraise the available research pertaining to studies examining the effects of BCAA on MPS, MPB and associated molecular signalling responses in humans. Overall, BCAA can activate molecular pathways that regulate translation initiation, reduce indices of whole-body and MPB, and transiently stimulate MPS rates. However, the stimulatory effect of BCAA on MPS rates is less than the response observed following ingestion of a complete protein source providing the full complement of indispensable amino acids.

The biphasic activity of autophagy and heat shock protein response in peripheral blood mononuclear cells following acute resistance exercise in resistance-trained males

PURPOSE

Autophagy and heat shock protein (HSP) response are proteostatic systems involved in the acute and adaptive responses to exercise. These systems may upregulate sequentially following cellular stress including acute exercise, however, currently few data exist in humans. This study investigated the autophagic and HSP responses to acute intense lower body resistance exercise in peripheral blood mononuclear cells (PBMCs) with and without branched-chain amino acids (BCAA) supplementation.

METHODS

Twenty resistance-trained males (22.3 ± 1.5 yr; 175.4 ± .7 cm; 86.4 ± 15.6 kg) performed a bout of intense lower body resistance exercise and markers of autophagy and HSP70 were measured immediately post- (IPE) and 2, 4, 24, 48, and 72 h post-exercise. Prior to resistance exercise, 10 subjects were randomly assigned to BCAA supplementation of 0.22 g/kg/d for 5 days pre-exercise and up to 72 h following exercise while the other 10 subjects consumed a placebo (PLCB).

RESULTS

There were no difference in autophagy markers or HSP70 expression between BCAA and PLCB groups. LC3II protein expression was significantly lower 2 and 4 h post-exercise compared to pre-exercise. LC3II: I ratio was not different at any time point compared to pre-exercise. Protein expression of p62 was lower IPE, 2, and 4 h post-exercise and elevated 24 h post-exercise. HSP70 expression was elevated 48 and 72 h post-exercise.

CONCLUSIONS

Autophagy and HSP70 are upregulated in PBMCs following intense resistance exercise with autophagy increasing initially post-exercise and HSP response in the latter period. Moreover, BCAA supplementation did not affect this response.

Changes in BDNF Concentration in Men after Foam Roller Massage

Massage is one of the oldest forms of therapy practiced since ancient times. Nowadays, it is used in sports practice, recovery from injury, or supportive therapy for various conditions. The practice of massage uses a variety of instruments that facilitate massaging while relieving the stress on the masseur. One of them is a foam roller. Although roller massage is widely used, there are still no scientific studies describing the biological mechanisms of its effects on the body. The purpose of our study was to analyze the effect of roller massage on BDNF levels in men undergoing self-massage 4x/week/7 weeks. The control group consisted of men who did not perform self-massage. Before the test and after the first, third, fifth, and seventh weeks of self-massage, the study participants' blood was drawn, the serum BDNF was determined, and the results were subjected to analysis of variance by ANOVA test. After the first week of self-massage, an increase in BDNF concentration was observed in the self-massage group compared to the control group (p = 0.023). Similarly, changes were observed in week five (p = 0.044) and week seven (p = 0.046). In the massaged group, BDNF concentrations were significantly higher after the first week of self-massage compared to baseline. In the third week of the study, BDNF decreased to a value comparable to the baseline study, then increased significantly in the fifth and seventh weeks compared to the value recorded in the third week (p = 0.049 and p = 0.029). It was significantly higher in week seven compared to week five (p = 0.03). Higher concentrations of BDNF in subjects undergoing roller self-massage may be one of the biological mechanisms justifying the therapeutic effects of massage in both sports and clinical practice. Studies analyzing the stimulation of BDNF synthesis through various massage techniques should be performed on a larger group of healthy individuals, patients after trauma of multiple origins, and sick people with indications for therapeutic massage.

Progression of total training volume in resistance training studies and its application to skeletal muscle growth

Progressive overload describes the gradual increase of stress placed on the body during exercise training, and is often quantified (i.e. in resistance training studies) through increases in total training volume (i.e. sets × repetitions × load) from the first to final week of the exercise training intervention. Within the literature, it has become increasingly common for authors to discuss skeletal muscle growth adaptations in the context of increases in total training volume (i.e. the magnitude progression in total training volume). The present manuscript discusses a physiological rationale for progressive overload and then explains why, in our opinion, quantifying the progression of total training volume within research investigations tells very little about muscle growth adaptations to resistance training. Our opinion is based on the following research findings: (1) a noncausal connection between increases in total training volume (i.e. progressively overloading the resistance exercise stimulus) and increases in skeletal muscle size; (2) similar changes in total training volume may not always produce similar increases in muscle size; and (3) the ability to exercise more and consequently amass larger increases in total training volume may not inherently produce more skeletal muscle growth. The methodology of quantifying changes in total training volume may therefore provide a means through which researchers can mathematically determine the total amount of external 'work' performed within a resistance training study. It may not, however, always explain muscle growth adaptations.

Comparison of Adaptations in the Gastrocnemius Muscle from an Exercise Program with and without Low-Intensity Blood Flow Restriction Banding in 18 Male Amateur Basketball Players Aged 16-45 Years

BACKGROUND Basketball is a sport with a global impact and recognized major leagues, and is one of the most studied and analyzed sports for improvement at the level of the high-performance athlete. Increasing the jump height of basketball players is an essential factor for high athletic performance. MATERIAL AND METHODS This study aimed to identify the effect of low-intensity training with flow restriction versus the eccentric exercise protocol on amateur athletes. Eighteen amateur basketball players aged 16-45 years were divided into 2 groups: Group A consisted of 9 participants with low-intensity training with flow restriction (40% intensity) with 200 mmHg occlusion applying flow restriction bands in the popliteal area, while Group B consisted of 9 participants who performed an eccentric exercises protocol on the gastrocnemius. An anthropometric evaluation was applied, which consisted of perception of effort, range of movement (ROM), muscle strength intensity, and the power of the jump measured with a jump platform. RESULTS Notable changes were observed in favor of Group A for the right dorsiflexion, with mean difference (MD)=-2.444 (P=0.018); left dorsiflexion with MD=-2.778 (P=0.027) and left foot perimeter variable with MD=-0.667 (P=0.026) at 95% confidence interval (CI); while the vertical jump was in favor of Group B, with MD=-2.899 (P=0.006). CONCLUSIONS Low-intensity training with flow restriction and eccentric exercise protocol were both effective in improving jumping performance. A significant improvement was shown in the jump height and ROM of the 2 study groups.

The effects of blood flow restriction training on PAP and lower limb muscle activation: a meta-analysis

Objective: This study aims to systematically evaluate the effects of blood flow restriction (BFR) training on lower limb muscle activation and post-activation potentiation (PAP) in athletes through a meta-analysis and discuss methods to improve instant muscle strength so as to provide a reference for training in this field. Methods: Randomized controlled trials (RCTs) that examined the impact of BFR training on muscle activation and PAP were gathered through database searches, such as CNKI, Wanfang, Web of Science, PubMed, and others. The Cochrane risk of bias tool was used to include and exclude literature. Quality evaluation and statistical analysis were conducted using ReviewManager 5.3 software, STATA 16.0, and other software programs. The sensitivity analysis and funnel plots were employed to assess result stability and publication bias. Results: In total, 18 literature studies were included with a total of 267 subjects. The meta-analysis showed that BFR could significantly improve the RMS value of lower limb muscles [SMD = 0.98, 95% CI (0.71, 1.24), and p < 0.00001]. BFR had a significant effect on the immediate explosive power of the lower limbs [SMD = 0.28, 95% CI (0.02, 0.53), and p = 0.03], but the heterogeneity was obvious (I 2 = 51%). The subgroup analysis showed that different training methods may be influencing factors that lead to the heterogeneity between studies. The measurement indexes were the counter movement jump (CMJ) [SMD = 0.45, 95% CI (0.20, 0.69), and p = 0.0004], training mode to overcome body weight [SMD = 0.57, 95% CI (0.33, 0.82), and p < 0.00001], and compressive strength of 40%-60% arterial occlusion pressure (AOP) [SMD = 0.57, 95% CI (0.31, 0.83), and p < 0.0001], which reached the maximum effect and was statistically significant. Conclusion: BFR training can induce lower extremity muscle activation and PAP. Combining self-weight training with BFR exercises set at 40%-60% AOP appears to be particularly effective in inducing PAP, especially for enhancing CMJ. Furthermore, combining body-weight training with BFR is considered an effective warm-up method to improve CMJ. Systematic Review Registration: http://inplasy.com, identifier INPLASY2023100087.

Interstitial Injection of Hydrogels with High-Mechanical Conductivity Relieves Muscle Atrophy Induced by Nerve Injury

Injectable hydrogels have been extensively used in tissue engineering where high mechanical properties are key for their functionality at sites of high physiological stress. In this study, an injectable, conductive hydrogel is developed exhibiting remarkable mechanical strength that can withstand a pressure of 500 kPa (85% deformation rate) and display good fatigue resistance, electrical conductivity, and tissue adhesion. A stable covalent cross-linked network with a slip-ring structure by threading amino β-cyclodextrin is formed onto the chain of a four-armed (polyethylene glycol) amino group, and then reacted with the four-armed (polyethylene glycol) maleimide under physiological conditions. The addition of silver nanowires enhances the hydrogel's electrical conductivity, enabling it to act as a good conductor in vivo. The hydrogel is injected into the fascial space, and the results show that the weight and muscle tone of the atrophied gastrocnemius muscle improve, subsequently alleviating muscle atrophy. Overall, this study provides a simple method for the preparation of a conductive hydrogel with high mechanical properties. In addition, the interstitial injection provides a strategy for the use of hydrogels in vivo.

Molecular clocks, satellite cells, and skeletal muscle regeneration

Skeletal muscle comprises approximately 50% of individual body mass and plays vital roles in locomotion, heat production, and whole body metabolic homeostasis. This tissue exhibits a robust diurnal rhythm that is under control of the suprachiasmatic nucleus (SCN) region of the hypothalamus. The SCN acts as a "central" coordinator of circadian rhythms, while cell-autonomous "peripheral" clocks are located within almost all other tissues/organs in the body. Synchronization of the peripheral clocks in muscles (and other tissues) together with the central clock is crucial to ensure temporally coordinated physiology across all organ systems. By virtue of its mass, human skeletal muscle contains the largest collection of peripheral clocks, but within muscle resides a local stem cell population, satellite cells (SCs), which have their own functional molecular clock, independent of the numerous muscle clocks. Skeletal muscle has a daily turnover rate of 1%-2%, so the regenerative capacity of this tissue is important for whole body homeostasis/repair and depends on successful SC myogenic progression (i.e., proliferation, differentiation, and fusion). Emerging evidence suggests that SC-mediated muscle regeneration may, in part, be regulated by molecular clocks involved in SC-specific diurnal transcription. Here we provide insights on molecular clock regulation of muscle regeneration/repair and provide a novel perspective on the interplay between SC-specific molecular clocks, myogenic programs, and cell cycle kinetics that underpin myogenic progression.

Harnessing conserved signaling and metabolic pathways to enhance the maturation of functional engineered tissues

The development of induced-pluripotent stem cell (iPSC)-derived cell types offers promise for basic science, drug testing, disease modeling, personalized medicine, and translatable cell therapies across many tissue types. However, in practice many iPSC-derived cells have presented as immature in physiological function, and despite efforts to recapitulate adult maturity, most have yet to meet the necessary benchmarks for the intended tissues. Here, we summarize the available state of knowledge surrounding the physiological mechanisms underlying cell maturation in several key tissues. Common signaling consolidators, as well as potential synergies between critical signaling pathways are explored. Finally, current practices in physiologically relevant tissue engineering and experimental design are critically examined, with the goal of integrating greater decision paradigms and frameworks towards achieving efficient maturation strategies, which in turn may produce higher-valued iPSC-derived tissues.

The Role of IL-6 Released During Exercise to Insulin Sensitivity and Muscle Hypertrophy

Interleukin-6 (IL-6) influences both inflammatory response and anti-inflammatory processes. This cytokine can be released by the exercising skeletal muscle, which characterizes it as a myokine. Unlike what is observed in inflammation, IL-6 produced by skeletal muscle is not preceded by the release of other pro-inflammatory cytokines, but is seems to be dependent on the lactate produced during exercise, thus causing different effects from those of seen in inflammatory state. After binding to its receptor, myokine IL-6 activates the PI3K-Akt pathway. One consequence of this upregulation is the potentiation of insulin signaling, which enhances insulin sensitivity. IL-6 increases GLUT-4 vesicle mobilization to muscle cell periphery, increasing the glucose transport into the cell, and also glycogen synthesis. Muscle glycogen provides energy for the ATP resynthesis, and regulates Ca2+ release by the sarcoplasmic reticulum, influencing muscle contraction, and, hence, muscle function by multiple pathways. Another implication for the upregulation of PI3K-Akt pathway is the activation of mTORC1, which regulates mRNA translational efficiency by regulating translation machinery, and translational capacity by inducing ribosomal biogenesis. Thus, IL-6 may contribute for skeletal muscle hypertrophy and function by increasing contractile protein synthesis.

Wearables-Only Analysis of Muscle and Joint Mechanics: An EMG-Driven Approach

Complex sensor arrays prohibit practical deployment of existing wearables-based algorithms for free-living analysis of muscle and joint mechanics. Machine learning techniques have been proposed as a potential solution, however, they are less interpretable and generalizable when compared to physics-based techniques. Herein, we propose a hybrid method utilizing inertial sensor- and electromyography (EMG)-driven simulation of muscle contraction to characterize knee joint and muscle mechanics during walking gait. Machine learning is used only to map a subset of measured muscle excitations to a full set thereby reducing the number of required sensors. We demonstrate the utility of the approach for estimating net knee flexion moment (KFM) as well as individual muscle moment and work during the stance phase of gait across nine unimpaired subjects. Across all subjects, KFM was estimated with 0.91%BW•H RMSE and strong correlations (r = 0.87) compared to ground truth inverse dynamics analysis. Estimates of individual muscle moments were strongly correlated (r = 0.81-0.99) with a reference EMG-driven technique using optical motion capture and a full set of electrodes as were estimates of muscle work (r = 0.88-0.99). Implementation of the proposed technique in the current work included instrumenting only three muscles with surface electrodes (lateral and medial gastrocnemius and vastus medialis) and both the thigh and shank segments with inertial sensors. These sensor locations permit instrumentation of a knee brace/sleeve facilitating a practically deployable mechanism for monitoring muscle and joint mechanics with performance comparable to the current state-of-the-art.

CCN2 participates in overload-induced skeletal muscle hypertrophy

The regulation of skeletal muscle growth following pro-hypertrophic stimuli requires a coordinated response by different cell types that leads to extracellular matrix (ECM) remodeling and increases in muscle cross-sectional area. Indeed, matricellular proteins serve a key role as communication vehicles that facilitate the propagation of signaling stimuli required for muscle adaptation to environmental challenges. We found that the matricellular protein cellular communication network factor 2 (CCN2), also known as connective tissue growth factor (CTGF), is induced during a time course of overload-driven skeletal muscle hypertrophy in mice. To elucidate the role of CCN2 in mediating the hypertrophic response, we utilized genetically engineered mouse models for myofiber-specific CCN2 gain- and loss-of-function and then examined their response to mechanical stimuli through muscle overload. Interestingly, myofiber-specific deletion of CCN2 blunted muscle's hypertrophic response to overload without interfering with ECM deposition. On the other hand, when in excess through transgenic CCN2 overexpression, CCN2 was efficient in promoting overload-induced aberrant ECM accumulation without affecting myofiber growth. Altogether, our genetic approaches highlighted independent ECM and myofiber stress adaptation responses, and positioned CCN2 as a central mediator of both. Mechanistically, CCN2 acts by regulating focal adhesion kinase (FAK) mediated transduction of overload-induced extracellular signals, including interleukin 6 (IL6), and their regulatory impact on global protein synthesis in skeletal muscle. Overall, our study highlights the contribution of muscle-derived extracellular matrix factor CCN2 for proper hypertrophic muscle growth.

Aerobic Adaptations to Resistance Training: The Role of Time under Tension

Generally, skeletal muscle adaptations to exercise are perceived through a dichotomous lens where the metabolic stress imposed by aerobic training leads to increased mitochondrial adaptations while the mechanical tension from resistance training leads to myofibrillar adaptations. However, there is emerging evidence for cross over between modalities where aerobic training stimulates traditional adaptations to resistance training (e.g., hypertrophy) and resistance training stimulates traditional adaptations to aerobic training (e.g., mitochondrial biogenesis). The latter is the focus of the current review in which we propose high-volume resistance training (i.e., high time under tension) leads to aerobic adaptations such as angiogenesis, mitochondrial biogenesis, and increased oxidative capacity. As time under tension increases, skeletal muscle energy turnover, metabolic stress, and ischemia also increase, which act as signals to activate the peroxisome proliferator-activated receptor gamma coactivator 1-alpha, which is the master regulator of mitochondrial biogenesis. For practical application, the acute stress and chronic adaptations to three specific forms of high-time under tension are also discussed: Slow-tempo, low-intensity resistance training, and drop-set resistance training. These modalities of high-time under tension lead to hallmark adaptations to resistance training such as muscle endurance, hypertrophy, and strength, but little is known about their effect on traditional aerobic training adaptations.

High-throughput, real-time monitoring of engineered skeletal muscle function using magnetic sensing

Engineered muscle tissues represent powerful tools for examining tissue level contractile properties of skeletal muscle. However, limitations in the throughput associated with standard analysis methods limit their utility for longitudinal study, high throughput drug screens, and disease modeling. Here we present a method for integrating 3D engineered skeletal muscles with a magnetic sensing system to facilitate non-invasive, longitudinal analysis of developing contraction kinetics. Using this platform, we show that engineered skeletal muscle tissues derived from both induced pluripotent stem cell and primary sources undergo improvements in contractile output over time in culture. We demonstrate how magnetic sensing of contractility can be employed for simultaneous assessment of multiple tissues subjected to different doses of known skeletal muscle inotropes as well as the stratification of healthy versus diseased functional profiles in normal and dystrophic muscle cells. Based on these data, this combined culture system and magnet-based contractility platform greatly broadens the potential for 3D engineered skeletal muscle tissues to impact the translation of novel therapies from the lab to the clinic.

The Effect of Low-Load Resistance Training on Skeletal Muscle Hypertrophy in Trained Men: A Critically Appraised Topic

Clinical Scenario: Resistance training (RT) programs promote skeletal muscle hypertrophy through the progressive physiological stress applied to an individual. Currently, the vast majority of studies regarding the hypertrophic response to RT have focused on either sedentary or untrained individuals. This critically appraised topic focuses on the hypertrophic response to high- and low-load RT in resistance-trained men. Clinical Question: In experienced male weightlifters, does high-load RT lead to greater increases in muscle mass than low-load RT? Summary of Key Findings: Six studies met the inclusion criteria, while 4 studies were included in the analysis. Each of the 4 studies showed that low-load RT elicited hypertrophic gains similar to high-load RT when sets were taken to failure. Three of the studies were not volume equated, indicating a dose-response relationship between training volume-load and skeletal muscle hypertrophy. One of the studies was volume equated, indicating that skeletal muscle hypertrophy could be achieved at levels comparable to those observed in high-load protocols as a result of high levels of metabolic stress and the concomitant recruitment of high-threshold motor units that can occur during fatiguing contractions. Clinical Bottom Line: Evidence suggests that low-load training produces hypertrophic gains similar to those observed in high-load RT protocols when sets are taken to failure in resistance-trained men. Strength of Recommendation: There is moderate to strong evidence to suggest that low-load RT elicits hypertrophic gains similar to those observed in high-load RT protocols when sets are taken to failure in resistance-trained men.

Skeletal muscle atrophy: From mechanisms to treatments

Skeletal muscle is a crucial tissue for movement, gestural assistance, metabolic homeostasis, and thermogenesis. It makes up approximately 40% of the total body weight and 50% of total protein. However, several pathological abnormalities (e.g., chronic diseases, cancer, long-term infection, aging) can induce an imbalance in skeletal muscle protein synthesis and degradation, which triggers muscle wasting and even leads to atrophy. Skeletal muscle atrophy is characterized by weakening, shrinking, and decreasing muscle mass and fiber cross-sectional area at the histological level. It manifests as a reduction in force production, easy fatigue and decreased exercise capability, along with a lower quality of life. Mechanistically, there are several pathophysiological processes involved in skeletal muscle atrophy, including oxidative stress and inflammation, which then activate signal transduction, such as the ubiquitin proteasome system, autophagy lysosome system, and mTOR. Considering the great economic and social burden that muscle atrophy can inflict, effective prevention and treatment strategies are essential but still limited. Exercise is widely acknowledged as the most effective therapy for skeletal muscle atrophy; unfortunately, it is not applicable for all patients. Several active substances for skeletal muscle atrophy have been discovered and evaluated in clinical trials, however, they have not been marketed to date. Knowledge is being gained on the underlying mechanisms, highlighting more promising treatment strategies in the future. In this paper, the mechanisms and treatment strategies for skeletal muscle atrophy are briefly reviewed.

LINCing Nuclear Mechanobiology With Skeletal Muscle Mass and Function

Skeletal muscle demonstrates a high degree of adaptability in response to changes in mechanical input. The phenotypic transformation in response to mechanical cues includes changes in muscle mass and force generating capabilities, yet the molecular pathways that govern skeletal muscle adaptation are still incompletely understood. While there is strong evidence that mechanotransduction pathways that stimulate protein synthesis play a key role in regulation of muscle mass, there are likely additional mechano-sensitive mechanisms important for controlling functional muscle adaptation. There is emerging evidence that the cell nucleus can directly respond to mechanical signals (i.e., nuclear mechanotransduction), providing a potential additional level of cellular regulation for controlling skeletal muscle mass. The importance of nuclear mechanotransduction in cellular function is evident by the various genetic diseases that arise from mutations in proteins crucial to the transmission of force between the cytoskeleton and the nucleus. Intriguingly, these diseases preferentially affect cardiac and skeletal muscle, suggesting that nuclear mechanotransduction is critically important for striated muscle homeostasis. Here we discuss our current understanding for how the nucleus acts as a mechanosensor, describe the main cytoskeletal and nuclear proteins involved in the process, and propose how similar mechanoresponsive mechanisms could occur in the unique cellular environment of a myofiber. In addition, we examine how nuclear mechanotransduction fits into our current framework for how mechanical stimuli regulates skeletal muscle mass.

Mapping of the contraction-induced phosphoproteome identifies TRIM28 as a significant regulator of skeletal muscle size and function

Mechanical signals, such as those evoked by maximal-intensity contractions (MICs), can induce an increase in muscle mass. Rapamycin-sensitive signaling events are widely implicated in the regulation of this process; however, recent studies indicate that rapamycin-insensitive signaling events are also involved. Thus, to identify these events, we generate a map of the MIC-regulated and rapamycin-sensitive phosphoproteome. In total, we quantify more than 10,000 unique phosphorylation sites and find that more than 2,000 of these sites are significantly affected by MICs, but remarkably, only 38 of the MIC-regulated events are mediated through a rapamycin-sensitive mechanism. Further interrogation of the rapamycin-insensitive phosphorylation events identifies the S473 residue on Tripartite Motif-Containing 28 (TRIM28) as one of the most robust MIC-regulated phosphorylation sites, and extensive follow-up studies suggest that TRIM28 significantly contributes to the homeostatic regulation of muscle size and function as well as the hypertrophy that occurs in response to increased mechanical loading.

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.

Strength Training: In Search of Optimal Strategies to Maximize Neuromuscular Performance

Training with low-load exercise performed under blood flow restriction can augment muscle hypertrophy and maximal strength to a similar extent as the classical high-load strength training method. However, the blood flow restriction method elicits only minor neural adaptations. In an attempt to maximize training-related gains, we propose using other protocols that combine high voluntary activation, mechanical tension, and metabolic stress.

ER-Mitochondria Contacts and Insulin Resistance Modulation through Exercise Intervention

The endoplasmic reticulum (ER) makes physical contacts with mitochondria at specific sites, and the hubs between the two organelles are called mitochondria-associated ER membranes (MAMs). MAMs are known to play key roles in biological processes, such as intracellular Ca²⁺ regulation, lipid trafficking, and metabolism, as well as cell death, etc. Studies demonstrated that dysregulation of MAMs significantly contributed to insulin resistance. Alterations of MAMs’ juxtaposition and integrity, impaired expressions of insulin signaling molecules, disruption of Ca²⁺ homeostasis, and compromised metabolic flexibility are all actively involved in the above processes. In addition, exercise training is considered as an effective stimulus to ameliorate insulin resistance. Although the underlying mechanisms for exercise-induced improvement in insulin resistance are not fully understood, MAMs may be critical for the beneficial effects of exercise.

The Failed Clinical Story of Myostatin Inhibitors against Duchenne Muscular Dystrophy: Exploring the Biology behind the Battle

Myostatin inhibition therapy has held much promise for the treatment of muscle wasting disorders. This is particularly true for the fatal myopathy, Duchenne Muscular Dystrophy (DMD). Following on from promising pre-clinical data in dystrophin-deficient mice and dogs, several clinical trials were initiated in DMD patients using different modality myostatin inhibition therapies. All failed to show modification of disease course as dictated by the primary and secondary outcome measures selected: the myostatin inhibition story, thus far, is a failed clinical story. These trials have recently been extensively reviewed and reasons why pre-clinical data collected in animal models have failed to translate into clinical benefit to patients have been purported. However, the biological mechanisms underlying translational failure need to be examined to ensure future myostatin inhibitor development endeavors do not meet with the same fate. Here, we explore the biology which could explain the failed translation of myostatin inhibitors in the treatment of DMD.

Strength Training Reduces Fat Accumulation and Improves Blood Lipid Profile Even in the Absence of Skeletal Muscle Hypertrophy in High-Fat Diet-Induced Obese Condition

The aim was to investigate the effect of strength training on skeletal muscle morphology and metabolic adaptations in obese rats fed with unsaturated high-fat diet (HFD). The hypothesis was that strength training induces positive metabolic adaptations in obese rats despite impaired muscle hypertrophy. Male Wistar rats (n = 58) were randomized into two groups and fed a standard diet or a high-fat diet (HFD) containing 49.2% of fat. After induction and maintenance to obesity, the rats were divided into four groups: animals distributed in sedentary control (CS), control submitted to strength training protocol (CT), obese sedentary (ObS), and obese submitted to strength training protocol (ObT). The exercise protocol consisted of 10 weeks of training on a vertical ladder (three times a week) with a load attached to the animal’s tail. At the end of 10 weeks, strength training promoted positive changes in the body composition and metabolic parameters in obese animals. Specifically, ObT animals presented a reduction of 22.6% and 14.3% in body fat and adiposity index when compared to ObS, respectively. Furthermore, these rats had lower levels of triglycerides (ObT = 23.1 ± 9.5 vs. ObS = 30.4 ± 6.9 mg/dL) and leptin (ObT = 13.2 ± 7.2 vs. ObS = 20.5 ± 4.3 ng/mL). Training (ObT and CT) induced a greater strength gain when compared with the respective control groups. In addition, the weight of the flexor hallucis longus (FHL) muscle was higher in the ObT group than in the CT group, representing an increase of 26.1%. However, training did not promote hypertrophy as observed by a similar cross-sectional area of the FHL and plantar muscles. Based on these results, high-intensity strength training promoted an improvement of body composition and metabolic profile in obese rats that were fed a high-fat diet without skeletal muscle adaptations, becoming a relevant complementary strategy for the treatment of obesity.

Recovery of Diabetic Rats After Physical Exhaustion: Kinetic Alterations in Muscle Inflammation and Muscle-Signaling Proteins to Atrophy and Hypertrophy

The complexity of the adaptive response of diabetics to intense exercise is still poorly understood. To optimize exercise interventions in diabetics, the chronology of inflammatory mediators in muscle and the signaling involved in muscle hypertrophy/atrophy must be understood. Herein, we studied the kinetic inflammatory profile and cellular signaling pathways modulated by physical exhaustion after the induction of type 1 diabetes by streptozotocin in rats. Soleus muscle samples were obtained from diabetic and control groups at the following moments: baseline (no exercise); immediately after exhaustive exercise; and at 2 h, 24 h, 48 h, and 72 h after a treadmill exhaustive exercise. Kinetic production of cytokines and kinetic activation of proteins related to muscle synthesis (p70S6K and Akt) and degradation (GSK3, MuRF1, and MAFbx) were measured in the soleus muscle. We observed that the muscle TNF-α (0.9-fold; p = 0.0007), IL-1β (0.8-fold; p = 0.01), IL-6 (0.8-fold; p = 0.0013), L-selectin (1.0-fold; p = 0.0019), and CINC-2α/β (0.9-fold; p = 0.04) levels were higher in almost all stages of the study in the diabetic animals compared with the control group. Our data showed that exhaustive exercise decreased MAFbx expression in diabetic animals compared to the control group in a time-dependent manner. The decreased activation ratios of MAFbx were followed by a decrease in TNF-α, IL-1β, and IL-6 levels. p70S6k phosphorylation was also decreased in the diabetic group compared to the control group after physical exhaustion. Regarding the activation of proteins related to muscle synthesis and degradation, we found that the alterations induced by exhaustive exercise in the diabetic rats might involve pathways related to synthesis and muscle breakdown. Moreover, after an exhaustive exercise session, the recovery of the inflammatory response in the diabetic animals was slower than that in the control rats while the return of inflammatory cytokines to baseline levels was more effective in the diabetic animals.

Effect of Silymarin Supplementation on Physical Performance, Muscle and Myocardium Histological Changes, Bodyweight, and Food Consumption in Rats Subjected to Regular Exercise Training

(1) Background: Regular exercise induces physiological and morphological changes in the organisms, but excessive training loads may induce damage and impair recovery or muscle growth. The purpose of the study was to evaluate the impact of Silymarin (SM) consumption on endurance capacity, muscle/cardiac histological changes, bodyweight, and food intake in rats subjected to 60 min of regular exercise training (RET) five days per week. (2) Methods: Male Wistar rats were subjected to an eight-week RET treadmill program and were previously administered SM and vitamin C. Bodyweight and food consumption were measured and registered. The maximal endurance capacity (MEC) test was performed at weeks one and eight. After the last training session, the animals were sacrificed, and samples of quadriceps/gastrocnemius and cardiac tissue were obtained and process for histological analyzes. (3) Results: SM consumption improved muscle recovery, inflammation, and damaged tissue, and promoted hypertrophy, vascularization, and muscle fiber shape/appearance. MEC increased after eight weeks of RET in all trained groups; moreover, the SM-treated group was enhanced more than the group with vitamin C. There were no significant changes in bodyweight and in food and nutrient consumption along the study. (5) Conclusion: SM supplementation may enhance physical performance, recovery, and muscle hypertrophy during the eight-week RET program.

Safety and Effectiveness of Progressive Moderate-to-Vigorous Intensity Elastic Resistance Training on Physical Function and Pain in People With Hemophilia

OBJECTIVE

Strength training is recommended for people with hemophilia; however, published data are anecdotal and have methodological limitations. The purpose of this study was to evaluate the safety and effectiveness of progressive moderate-to-vigorous intensity elastic resistance training on physical function and pain in this patient population.

METHODS

A randomized controlled trial was conducted in a university laboratory setting where 20 patients (17 with severe, 1 with moderate, and 2 with mild hemophilia) aged 21 to 53 years received evaluations at baseline and 8-week follow-up. Participants were allocated to intervention (progressive strength training) or control (usual daily activities) groups. The intervention group trained 2 days per week during 8 weeks with elastic resistance. Intensity during the first 2 weeks was a 20-repetition maximum and increased progressively toward 15, 12, and finally 10 repetition maximum. The primary outcome was muscle strength. Secondary outcomes were the Timed "Up and Go" Test score, sit-to-stand, range of motion, Haemophilia Joint Health Score, kinesiophobia score, global impression of pain change, general self-rated health status, and desire to exercise.

RESULTS

The intervention group showed greater strength improvements than the control group in almost all of the joints, with moderate to high effect sizes. The intervention group also showed better Timed "Up and Go" and sit-to-stand scores than the control group (moderate effect size), greater range of motion at the knee flexion with the right leg (trivial effect size), and better Haemophilia Joint Health Score at the left knee (small effect size). The intervention group showed greater overall pain reduction, self-rated overall status, and desire to exercise than the control group.

CONCLUSIONS

Progressive strength training with elastic resistance performed twice a week during 8 weeks is safe and effective in people with hemophilia to improve muscle strength and functional capacity, reduce general pain, and improve self-rated health status and desire to exercise.

IMPACT

This study provides evidence for the use of a specific strength training regimen for people with hemophilia.

LAY SUMMARY

People with hemophilia of differing levels of severity, with adequate coverage with clotting factor, can safely engage in progressive strength training and can improve their functioning.

Preserved Capacity for Adaptations in Strength and Muscle Regulatory Factors in Elderly in Response to Resistance Exercise Training and Deconditioning

Aging is related to an inevitable loss of muscle mass and strength. The mechanisms behind age-related loss of muscle tissue are not fully understood but may, among other things, be induced by age-related differences in myogenic regulatory factors. Resistance exercise training and deconditioning offers a model to investigate differences in myogenic regulatory factors that may be important for age-related loss of muscle mass and strength. Nine elderly (82 ± 7 years old) and nine young, healthy persons (22 ± 2 years old) participated in the study. Exercise consisted of six weeks of resistance training of the quadriceps muscle followed by eight weeks of deconditioning. Muscle biopsy samples before and after training and during the deconditioning period were analyzed for MyoD, myogenin, insulin-like growth-factor I receptor, activin receptor IIB, smad2, porin, and citrate synthase. Muscle strength improved with resistance training by 78% (95.0 ± 22.0 kg) in the elderly to a similar extent as in the young participants (83.5%; 178.2 ± 44.2 kg) and returned to baseline in both groups after eight weeks of deconditioning. No difference was seen in expression of muscle regulatory factors between elderly and young in response to exercise training and deconditioning. In conclusion, the capacity to gain muscle strength with resistance exercise training in elderly was not impaired, highlighting this as a potent tool to combat age-related loss of muscle function, possibly due to preserved regulation of myogenic factors in elderly compared with young muscle.

Resistance Training and Skeletal Muscle Protein Metabolism in Eumenorrheic Females: Implications for Researchers and Practitioners

Resistance training is essential for health and performance and confers many benefits such as increasing skeletal muscle mass, increasing strength and power output, and improving metabolic health. Resistance training is a major component of the physical activity guidelines, yet research in female populations is limited. Recent increases in the promotion of, and the participation by, females in sport and exercise, highlight the need for an increase in understanding of evidence-based best practice exercise prescription for females. The aim of this review is to provide an overview of the current research regarding resistance training performance and skeletal muscle adaptation in females, with a focus on the hormonal variables that may influence resistance training outcomes. Findings suggest that the menstrual cycle phase may impact strength, but not skeletal muscle protein metabolism. In comparison, oral contraception use in females may reduce skeletal muscle protein synthesis, but not strength outcomes, when compared to non-users. Future research should investigate the role of resistance training in the maintenance of skeletal muscle protein metabolism during pregnancy, menopause and in athletes experiencing relative energy deficiency in sport. The review concludes with recommendations for researchers to assist them in the inclusion of female participants in resistance training research specifically, with commentary on the most appropriate methods of controlling for, or understanding the implications of, hormonal fluctuations. For practitioners, the current evidence suggests possible resistance training practices that could optimise performance outcomes in females, although further research is warranted.

Effects of Blood Flow Restriction Training with Protein Supplementation on Muscle Mass And Strength in Older Men

Blood flow restriction (BFR) training has been shown to induce favorable changes in muscle mass and strength with a considerably low training load (20 - 30% 1RM). However, it has never been evaluated if an additional post-exercise protein supplementation enhances the effects of this training regimen. Thirty healthy older men (60.1 ± 7.6 years) were enrolled in the 8-week intervention and randomly allocated to one of the following groups: low-load BFR training with protein (collagen hydrolysate) supplementation (BFR-CH), low-load BFR training with placebo (BFR-PLA), or a control group without training, but with protein supplementation (CON). Muscle cross-sectional area (CSA), muscle strength, circulating reactive oxygen species and IGF-1 were measured before and after the intervention. Muscle CSA increased in both BFR-CH and BFR-PLA groups by 6.7 ± 3.2 % (p < 0.001) and 5.7 ± 2.7 % (p < 0.001) respectively. No significant changes were observed in the CON group (1.1 ± 1.7 %, p = 0.124). Evaluation of isometric strength (p = 0.247), insulin-like growth factor 1 (p = 0.705) and the production of reactive oxygen species (p^t1 = 0.229; p^t2 = 0.741) revealed no significant interaction effect but a significant long-term time effect (p < 0.001). Our results demonstrate that BFR training is an effective alternative for increasing muscle CSA in older men. Although there was a trend towards greater muscle mass adaptations in the BFR-CH group, these findings showed no statistical significance. Further research with larger sample sizes is needed to confirm these results.

Molecular regulation of human skeletal muscle protein synthesis in response to exercise and nutrients: a compass for overcoming age-related anabolic resistance

Skeletal muscle mass, a strong predictor of longevity and health in humans, is determined by the balance of two cellular processes, muscle protein synthesis (MPS) and muscle protein breakdown. MPS seems to be particularly sensitive to changes in mechanical load and/or nutritional status; therefore, much research has focused on understanding the molecular mechanisms that underpin this cellular process. Furthermore, older individuals display an attenuated MPS response to anabolic stimuli, termed anabolic resistance, which has a negative impact on muscle mass and function, as well as quality of life. Therefore, an understanding of which, if any, molecular mechanisms contribute to anabolic resistance of MPS is of vital importance in formulation of therapeutic interventions for such populations. This review summarizes the current knowledge of the mechanisms that underpin MPS, which are broadly divided into mechanistic target of rapamycin complex 1 (mTORC1)-dependent, mTORC1-independent, and ribosomal biogenesis-related, and describes the evidence that shows how they are regulated by anabolic stimuli (exercise and/or nutrition) in healthy human skeletal muscle. This review also summarizes evidence regarding which of these mechanisms may be implicated in age-related skeletal muscle anabolic resistance and provides recommendations for future avenues of research that can expand our knowledge of this area.

Blood Flow Restriction During Futsal Training Increases Muscle Activation and Strength

The aim of this study was to investigate the effect of leg blood flow restriction (BFR) applied during a 3-a-side futsal game on strength-related parameters. Twelve male futsal players were randomly assigned into two groups (n = 6 for each group) during 10 training sessions either with or without leg BFR. Prior to and post-training sessions, participants completed a series of tests to assess anabolic hormones and leg strength. Pneumatic cuffs were initially inflated to 110% of leg systolic blood pressure and further increased by 10% after every two completed sessions. In comparison with baseline, the resting post-training levels of myostatin (p = 0.002) and IGF-1/MSTN ratio (p = 0.006) in the BFR group changed, whereas no change in the acute level of IGF-1 and myostatin after exercise was observed. Peak torque of knee extension and flexion increased in both groups (p < 0.05). A trend of increased neural activation of all heads of the quadriceps was observed in both groups, however, it was statistically significant only for rectus femoris in BFR (p = 0.02). These findings indicated that the addition of BFR to normal futsal training might induce greater neuromuscular benefits by increasing muscle activation and augmenting the hormonal response.

Moderate-intensity aerobic exercise improves skeletal muscle quality in older adults

Sarcopenia, age-associated involuntary loss of muscle and strength, can progress to clinically relevant functional decline. Resistance exercise attenuates muscle and strength loss but may not be feasible for some older adults. Aerobic exercise training (AET) improves cardiopulmonary health; however, effects on protein turnover, muscle mass, and strength are less clear. We aimed to determine whether AET improves basal myofibrillar protein synthesis (MPS) and capillarization, promoting hypertrophy and strength. We hypothesized that AET improves strength with increased MPS and capillarization. Older adults were randomized to non-exercise (NON; n = 11, 71.4 ± 4.18 years) or exercise (EX; n = 12, 73.7 ± 4.05 years). EX completed 24 weeks of AET (walking 3×/week, 45 minutes, 70% heart rate reserve); NON remained sedentary. A stable isotope tracer was infused. MPS and capillarization were analyzed from vastus lateralis muscle biopsies. Strength was measured via isokinetic dynamometry. Lean mass was determined with dual-energy X-ray absorptiometry. Basal MPS increased in EX (+50.7%, P = 0.01) along with capillary density (+66.4%, P = 0.03), peak oxygen consumption (+15.8%, P = 0.01), quadriceps strength (+15.1%, P = 0.01), and muscle quality (peak torque divided by leg lean mass, +15.5%, P = 0.01). Lean mass did not change (P > 0.05). AET increases muscle protein turnover and capillarization in older adults, improving muscle quality.

Electrical stimulation increases hypertrophy and metabolic flux in tissue-engineered human skeletal muscle

In vitro models of contractile human skeletal muscle hold promise for use in disease modeling and drug development, but exhibit immature properties compared to native adult muscle. To address this limitation, 3D tissue-engineered human muscles (myobundles) were electrically stimulated using intermittent stimulation regimes at 1 Hz and 10 Hz. Dystrophin in myotubes exhibited mature membrane localization suggesting a relatively advanced starting developmental maturation. One-week stimulation significantly increased myobundle size, sarcomeric protein abundance, calcium transient amplitude (∼2-fold), and tetanic force (∼3-fold) resulting in the highest specific force generation (19.3mN/mm2) reported for engineered human muscles to date. Compared to 1 Hz electrical stimulation, the 10 Hz stimulation protocol resulted in greater myotube hypertrophy and upregulated mTORC1 and ERK1/2 activity. Electrically stimulated myobundles also showed a decrease in fatigue resistance compared to control myobundles without changes in glycolytic or mitochondrial protein levels. Greater glucose consumption and decreased abundance of acetylcarnitine in stimulated myobundles indicated increased glycolytic and fatty acid metabolic flux. Moreover, electrical stimulation of myobundles resulted in a metabolic shift towards longer-chain fatty acid oxidation as evident from increased abundances of medium- and long-chain acylcarnitines. Taken together, our study provides an advanced in vitro model of human skeletal muscle with improved structure, function, maturation, and metabolic flux.

The role of raptor in the mechanical load-induced regulation of mTOR signaling, protein synthesis, and skeletal muscle hypertrophy

It is well known that an increase in mechanical loading can induce skeletal muscle hypertrophy, and a long standing model in the field indicates that mechanical loads induce hypertrophy via a mechanism that requires signaling through the mechanistic target of rapamycin complex 1 (mTORC1). Specifically, it has been widely proposed that mechanical loads activate signaling through mTORC1 and that this, in turn, promotes an increase in the rate of protein synthesis and the subsequent hypertrophic response. However, this model is based on a number of important assumptions that have not been rigorously tested. In this study, we created skeletal muscle specific and inducible raptor knockout mice to eliminate signaling by mTORC1, and with these mice we were able to directly demonstrate that mechanical stimuli can activate signaling by mTORC1, and that mTORC1 is necessary for mechanical load-induced hypertrophy. Surprisingly, however, we also obtained multiple lines of evidence that indicate that mTORC1 is not required for a mechanical load-induced increase in the rate of protein synthesis. This observation highlights an important shortcoming in our understanding of how mechanical loads induce hypertrophy and illustrates that additional mTORC1-independent mechanisms play a critical role in this process.-You, J.-S., McNally, R. M., Jacobs, B. L., Privett, R. E., Gundermann, D. M., Lin, K.-H., Steinert, N. D., Goodman, C. A., Hornberger, T. A. The role of raptor in the mechanical load-induced regulation of mTOR signaling, protein synthesis, and skeletal muscle hypertrophy.

Translocation and protein complex co-localization of mTOR is associated with postprandial myofibrillar protein synthesis at rest and after endurance exercise

Translocation and colocalization of mechanistic target of rapamycin complex 1 (mTORC1) with regulatory proteins represents a critical step in translation initiation of protein synthesis in vitro. However, mechanistic insight into the control of postprandial skeletal muscle protein synthesis rates at rest and after an acute bout of endurance exercise in humans is lacking. In crossover trials, eight endurance‐trained men received primed‐continuous infusions of L‐[ring‐²H₅]phenylalanine and consumed a mixed‐macronutrient meal (18 g protein, 60 g carbohydrates, 17 g fat) at rest (REST) and after 60 min of treadmill running at 70% VO_2peak (EX). Skeletal muscle biopsies were collected to measure changes in phosphorylation and colocalization in the mTORC1‐pathway, in addition to rates of myofibrillar (MyoPS) and mitochondrial (MitoPS) protein synthesis. MyoPS increased (P < 0.05) above fasted in REST (~2.1‐fold) and EX (~twofold) during the 300 min postprandial period, with no corresponding changes in MitoPS (P > 0.05). TSC2/Rheb colocalization decreased below fasted at 60 and 300 min after feeding in REST and EX (P < 0.01). mTOR colocalization with Rheb increased above fasted at 60 and 300 min after feeding in REST and EX (P < 0.01), which was consistent with an increased phosphorylation 4E‐BP1^Thr37/46 and rpS6^ser240/244 at 60 min. Our data suggest that MyoPS, but not MitoPS, is primarily nutrient responsive in trained young men at rest and after endurance exercise. The postprandial increase in MyoPS is associated with an increase in mTOR/Rheb colocalization and a reciprocal decrease in TSC2/Rheb colocalization and thus likely represent important regulatory events for in vivo skeletal muscle myofibrillar mRNA translation in humans.

Stimuli and sensors that initiate skeletal muscle hypertrophy following resistance exercise

One of the most striking adaptations to exercise is the skeletal muscle hypertrophy that occurs in response to resistance exercise. A large body of work shows that a mammalian target of rapamycin complex 1 (mTORC1)-mediated increase of muscle protein synthesis is the key, but not sole, mechanism by which resistance exercise causes muscle hypertrophy. While much of the hypertrophy signaling cascade has been identified, the initiating, resistance exercise-induced and hypertrophy-stimulating stimuli have remained elusive. For the purpose of this review, we define an initiating, resistance exercise-induced and hypertrophy-stimulating signal as "hypertrophy stimulus," and the sensor of such a signal as "hypertrophy sensor." In this review we discuss our current knowledge of specific mechanical stimuli, damage/injury-associated and metabolic stress-associated triggers, as potential hypertrophy stimuli. Mechanical signals are the prime hypertrophy stimuli candidates, and a filamin-C-BAG3-dependent regulation of mTORC1, Hippo, and autophagy signaling is a plausible albeit still incompletely characterized hypertrophy sensor. Other candidate mechanosensing mechanisms are nuclear deformation-initiated signaling or several mechanisms related to costameres, which are the functional equivalents of focal adhesions in other cells. While exercise-induced muscle damage is probably not essential for hypertrophy, it is still unclear whether and how such muscle damage could augment a hypertrophic response. Interventions that combine blood flow restriction and especially low load resistance exercise suggest that resistance exercise-regulated metabolites could be hypertrophy stimuli, but this is based on indirect evidence and metabolite candidates are poorly characterized.

Increased autophagy signaling but not proteasome activity in human skeletal muscle after prolonged low-intensity exercise with negative energy balance

Little is known about the molecular regulation of skeletal muscle protein turnover during exercise in field conditions where energy is intake inadequate. Here, 17 male and 7 female soldiers performed an 8 days long field-based military operation. Vastus lateralis muscle biopsies, in which autophagy, the ubiquitin-proteasome system, and the mTORC1 signaling pathway were studied, were collected before and after the operation. The 187 h long operation resulted in a 15% and 29% negative energy balance as well as a 4.1% and 4.6% loss of body mass in women and men, respectively. After the operation protein levels of ULK1 as well as the phosphorylation of ULK1^Ser317 and ULK1^Ser555 had increased by 11%, 39%, and 13%, respectively, and this was supported by a 17% increased phosphorylation of AMPK^Thr172 (P < 0.05). The LC3b-I/II ratio was threefold higher after compared to before the operation (P < 0.05), whereas protein levels of p62/SQSTM1 were unchanged. The β1, β2, and β5 activity of the proteasome and protein levels of MAFbx did not change, whereas levels of MuRF-1 were slightly reduced (6%, P < 0.05). Protein levels and phosphorylation status of key components in the mTORC1 signaling pathway remained at basal levels after the operation. Muscle levels of glycogen decreased from 269 ± 12 to 181 ± 9 mmol·kg dry·muscle^-1 after the exercise period (P < 0.05). In conclusion, the 8 days of field-based exercise resulted in induction of autophagy without any increase in proteasome activity or protein ubiquitination. Simultaneously, the regulation of protein synthesis through the mTORC1 signaling pathway was maintained.

Genes Whose Gain or Loss-Of-Function Increases Skeletal Muscle Mass in Mice: A Systematic Literature Review

Skeletal muscle mass differs greatly in mice and humans and this is partially inherited. To identify muscle hypertrophy candidate genes we conducted a systematic review to identify genes whose experimental loss or gain-of-function results in significant skeletal muscle hypertrophy in mice. We found 47 genes that meet our search criteria and cause muscle hypertrophy after gene manipulation. They are from high to small effect size: Ski, Fst, Acvr2b, Akt1, Mstn, Klf10, Rheb, Igf1, Pappa, Ppard, Ikbkb, Fstl3, Atgr1a, Ucn3, Mcu, Junb, Ncor1, Gprasp1, Grb10, Mmp9, Dgkz, Ppargc1a (specifically the Ppargc1a4 isoform), Smad4, Ltbp4, Bmpr1a, Crtc2, Xiap, Dgat1, Thra, Adrb2, Asb15, Cast, Eif2b5, Bdkrb2, Tpt1, Nr3c1, Nr4a1, Gnas, Pld1, Crym, Camkk1, Yap1, Inhba, Tp53inp2, Inhbb, Nol3, Esr1. Knock out, knock down, overexpression or a higher activity of these genes causes overall muscle hypertrophy as measured by an increased muscle weight or cross sectional area. The mean effect sizes range from 5 to 345% depending on the manipulated gene as well as the muscle size variable and muscle investigated. Bioinformatical analyses reveal that Asb15, Klf10, Tpt1 are most highly expressed hypertrophy genes in human skeletal muscle when compared to other tissues. Many of the muscle hypertrophy-regulating genes are involved in transcription and ubiquitination. Especially genes belonging to three signaling pathways are able to induce hypertrophy: (a) Igf1-Akt-mTOR pathway, (b) myostatin-Smad signaling, and (c) the angiotensin-bradykinin signaling pathway. The expression of several muscle hypertrophy-inducing genes and the phosphorylation of their protein products changes after human resistance and high intensity exercise, in maximally stimulated mouse muscle or in overloaded mouse plantaris.

Select Skeletal Muscle mRNAs Related to Exercise Adaptation Are Minimally Affected by Different Pre-exercise Meals that Differ in Macronutrient Profile

Background: Substantial research has been done on the impact of carbohydrate and fat availability on endurance exercise adaptation, though its role in the acute adaptive response to resistance exercise has yet to be fully characterized. Purpose: We aimed to assess the effects of a pre-resistance exercise isocaloric mixed meal containing different amounts of carbohydrates and fat, on post-resistance exercise gene expression associated with muscle adaptation. Methods: Thirteen young (age 21.2 ± 1.6 year), recreationally trained (VO_2max 51.3 ± 4.8 ml/kg/min) men undertook an aerobic exercise session of 90-min continuous cycling (70% VO_2max) in the morning with pre- and post-exercise protein ingestion (10 and 15 g casein in a 500 ml beverage pre- and post-exercise, respectively). Subjects then rested for 2 h and were provided with a meal consisting of either 3207 kJ; 52 g protein; 51 g fat; and 23 g carbohydrate (FAT) or 3124 kJ; 53 g protein; 9 g fat; and 109 g carbohydrate (CHO). Two hours after the meal, subjects completed 5 × 8 repetitions (80% 1-RM) for both bilateral leg press and leg extension directly followed by 25 g of whey protein (500 ml beverage). Muscle biopsies were obtained from the vastus lateralis at baseline (morning) and 1 and 3 h post-resistance exercise (afternoon) to determine intramuscular mRNA response. Results: Muscle glycogen levels were significantly decreased post-resistance exercise, without any differences between conditions. Plasma free fatty acids increased significantly after the mixed meal in the FAT condition, while glucose and insulin were higher in the CHO condition. However, PDK4 mRNA quantity was significantly higher in the FAT condition at 3 h post-resistance exercise compared to CHO. HBEGF, INSIG1, MAFbx, MURF1, SIRT1, and myostatin responded solely as a result of exercise without any differences between the CHO and FAT group. FOXO3A, IGF-1, PGC-1α, and VCP expression levels remained unchanged over the course of the day. Conclusion: We conclude that mRNA quantity associated with muscle adaptation after resistance exercise is not affected by a difference in pre-exercise nutrient availability. PDK4 was differentially expressed between CHO and FAT groups, suggesting a potential shift toward fat oxidation and reduced glucose oxidation in the FAT group.

Interpreting Signal Amplitudes in Surface Electromyography Studies in Sport and Rehabilitation Sciences

Surface electromyography (sEMG) is a popular research tool in sport and rehabilitation sciences. Common study designs include the comparison of sEMG amplitudes collected from different muscles as participants perform various exercises and techniques under different loads. Based on such comparisons, researchers attempt to draw conclusions concerning the neuro- and electrophysiological underpinning of force production and hypothesize about possible longitudinal adaptations, such as strength and hypertrophy. However, such conclusions are frequently unsubstantiated and unwarranted. Hence, the goal of this review is to discuss what can and cannot be inferred from comparative research designs as it pertains to both the acute and longitudinal outcomes. General methodological recommendations are made, gaps in the literature are identified, and lines for future research to help improve the applicability of sEMG are suggested.

Enhanced skeletal muscle regrowth and remodelling in massaged and contralateral non-massaged hindlimb

KEY POINTS

Muscle fibre cross sectional area is enhanced with massage in the form of cyclic compressive loading during regrowth after atrophy. Massage enhances protein synthesis of the myofibrillar and cytosolic, but not the mitochondrial fraction, in muscle during regrowth. Focal adhesion kinase activation and satellite cell number are elevated in muscles undergoing massage during regrowth. Muscle fibre cross sectional area and protein synthesis of the myofibrillar fraction, but not DNA synthesis, are elevated in muscle of the contralateral non-massaged limb. Massage in the form of cyclic compressive loading is a potential anabolic intervention during muscle regrowth after atrophy.

ABSTRACT

Massage, in the form of cyclic compressive loading (CCL), is associated with multiple health benefits, but its potential anabolic effect on atrophied muscle has not been investigated. We hypothesized that the mechanical activity associated with CCL induces an anabolic effect in skeletal muscle undergoing regrowth after a period of atrophy. Fischer-Brown Norway rats at 10 months of age were hindlimb unloaded for a period of 2 weeks. The rats were then allowed reambulation with CCL applied at a 4.5 N load at 0.5 Hz frequency for 30 min every other day for four bouts during a regrowth period of 8 days. Muscle fibre cross sectional area was enhanced by 18% with massage during regrowth compared to reloading alone, and this was accompanied by elevated myofibrillar and cytosolic protein as well as DNA synthesis. Focal adhesion kinase phosphorylation indicated that CCL increased mechanical stimulation, while a higher number of Pax7+ cells likely explains the elevated DNA synthesis. Surprisingly, the contralateral non-massaged limb exhibited a comparable 17% higher muscle fibre size compared to reloading alone, and myofibrillar protein synthesis, but not DNA synthesis, was also elevated. We conclude that massage in the form of CCL induces an anabolic response in muscles regrowing after an atrophy-inducing event. We suggest that massage can be used as an intervention to aid in the regrowth of muscle lost during immobilization.

The Molecular Mechanisms and Prevention Principles of Muscle Atrophy in Aging

Muscle atrophy in aging is characterized by progressive loss of muscle mass and function. Muscle mass is determined by the balance of synthesis and degradation of protein, which are regulated by several signaling pathways such as ubiquitin-proteasome system, autophagy-lysosome systems, oxidative stress, proinflammatory cytokines, hormones, and so on. Sufficient nutrition can enhance protein synthesis, while exercise can improve the quality of life in the elderly. This chapter will discuss the epidemiology, pathogenesis, as well as the current treatment for aging-induced muscular atrophy.

Differential localization and anabolic responsiveness of mTOR complexes in human skeletal muscle in response to feeding and exercise

Mechanistic target of rapamycin (mTOR) resides as two complexes within skeletal muscle. mTOR complex 1 [mTORC1-regulatory associated protein of mTOR (Raptor) positive] regulates skeletal muscle growth, whereas mTORC2 [rapamycin-insensitive companion of mTOR (Rictor) positive] regulates insulin sensitivity. To examine the regulation of these complexes in human skeletal muscle, we utilized immunohistochemical analysis to study the localization of mTOR complexes before and following protein-carbohydrate feeding (FED) and resistance exercise plus protein-carbohydrate feeding (EXFED) in a unilateral exercise model. In basal samples, mTOR and the lysosomal marker lysosomal associated membrane protein 2 (LAMP2) were highly colocalized and remained so throughout. In the FED and EXFED states, mTOR/LAMP2 complexes were redistributed to the cell periphery [wheat germ agglutinin (WGA)-positive staining] (time effect; P = 0.025), with 39% (FED) and 26% (EXFED) increases in mTOR/WGA association observed 1 h post-feeding/exercise. mTOR/WGA colocalization continued to increase in EXFED at 3 h (48% above baseline) whereas colocalization decreased in FED (21% above baseline). A significant effect of condition (P = 0.05) was noted suggesting mTOR/WGA colocalization was greater during EXFED. This pattern was replicated in Raptor/WGA association, where a significant difference between EXFED and FED was noted at 3 h post-exercise/feeding (P = 0.014). Rictor/WGA colocalization remained unaltered throughout the trial. Alterations in mTORC1 cellular location coincided with elevated S6K1 kinase activity, which rose to a greater extent in EXFED compared with FED at 1 h post-exercise/feeding (P < 0.001), and only remained elevated in EXFED at the 3 h time point (P = 0.037). Collectively these data suggest that mTORC1 redistribution within the cell is a fundamental response to resistance exercise and feeding, whereas mTORC2 is predominantly situated at the sarcolemma and does not alter localization.

Do metabolites that are produced during resistance exercise enhance muscle hypertrophy?

Many reviews conclude that metabolites play an important role with respect to muscle hypertrophy during resistance exercise, but their actual physiologic contribution remains unknown. Some have suggested that metabolites may work independently of muscle contraction, while others have suggested that metabolites may play a secondary role in their ability to augment muscle activation via inducing fatigue. Interestingly, the studies used as support for an anabolic role of metabolites use protocols that are not actually designed to test the importance of metabolites independent of muscle contraction. While there is some evidence in vitro that metabolites may induce muscle hypertrophy, the only study attempting to answer this question in humans found no added benefit of pooling metabolites within the muscle post-exercise. As load-induced muscle hypertrophy is thought to work via mechanotransduction (as opposed to being metabolically driven), it seems likely that metabolites simply augment muscle activation and cause the mechanotransduction cascade in a larger proportion of muscle fibers, thereby producing greater muscle growth. A sufficient time under tension also appears necessary, as measurable muscle growth is not observed after repeated maximal testing. Based on current evidence, it is our opinion that metabolites produced during resistance exercise do not have anabolic properties per se, but may be anabolic in their ability to augment muscle activation. Future studies are needed to compare protocols which produce similar levels of muscle activation, but differ in the magnitude of metabolites produced, or duration in which the exercised muscles are exposed to metabolites.

Overload-mediated skeletal muscle hypertrophy is not impaired by loss of myofiber STAT3

Although the signal pathways mediating muscle protein synthesis and degradation are well characterized, the transcriptional processes modulating skeletal muscle mass and adaptive growth are poorly understood. Recently, studies in mouse models of muscle wasting or acutely exercised human muscle have suggested a potential role for the transcription factor signal transducer and activator of transcription 3 (STAT3), in adaptive growth. Hence, in the present study we sought to define the contribution of STAT3 to skeletal muscle adaptive growth. In contrast to previous work, two different resistance exercise protocols did not change STAT3 phosphorylation in human skeletal muscle. To directly address the role of STAT3 in load-induced (i.e., adaptive) growth, we studied the anabolic effects of 14 days of synergist ablation (SA) in skeletal muscle-specific STAT3 knockout (mKO) mice and their floxed, wild-type (WT) littermates. Plantaris muscle weight and fiber area in the nonoperated leg (control; CON) was comparable between genotypes. As expected, SA significantly increased plantaris weight, muscle fiber cross-sectional area, and anabolic signaling in WT mice, although interestingly, this induction was not impaired in STAT3 mKO mice. Collectively, these data demonstrate that STAT3 is not required for overload-mediated hypertrophy in mouse skeletal muscle.