Six Weeks of Low-Load Blood Flow Restricted and High-Load Resistance Exercise Training Produce Similar Increases in Cumulative Myofibrillar Protein Synthesis and Ribosomal Biogenesis in Healthy Males

Short-term repeated human biopsy sampling contributes to changes in muscle morphology and higher outcome variability

Changes in skeletal muscle are an important aspect of overall health. The collection of human muscle to study cellular and molecular processes for research requires a needle biopsy procedure which, in itself, can induce changes in the tissue. To investigate the effect of repeat tissue sampling, we collected skeletal muscle biopsy samples from vastus lateralis separated by 7 days. Cellular infiltrate, central nucleation, enlarged extracellular matrix, and rounding of muscle fibers were used as indices to define muscle damage, and we found that 16/26 samples (61.5%) revealed at least two of these symptoms in the secondary biopsy. The presence of damage influenced outcome measures usually obtained in human biopsies. Damaged muscle showed an increase in the number of small fibers even though average fiber and fiber type-specific cross-sectional area (CSA) were not different. This included higher numbers of embryonic myosin heavy chain-positive fibers (P = 0.001) as well as elevated satellite cell number (P = 0.02) in the damaged areas and higher variability in satellite cell count in the total area (P = 0.04). Collagen content was higher in damaged (P = 0.0003) as well as nondamaged areas (P = 0.05) of the muscle sections of the damaged compared with the nondamaged group. Myofibrillar protein and ribonucleic acid (RNA) fractional synthesis rates were not significantly different between the damaged compared with the nondamaged group. Results indicate that common outcomes as well as outcome variability in human muscle tissue are affected by previous biopsies. Therefore, the extent of potential damage should be assessed when performing repeated biopsies.NEW & NOTEWORTHY Indices of damage can be found in repeated biopsy samples of nonintervened control legs. Variables, directly and not directly related to muscle damage or regeneration, were compromised in second biopsy. There is a need to determine potential damage within muscle tissue when repeated muscle sampling is part of the study design. Muscle biopsy sampling may be a source of increased heterogeneity in human muscle data.

Low-intensity resistance exercise with blood flow restriction for patients with claudication: A randomized controlled feasibility trial

BACKGROUND

Claudication is a common and debilitating symptom of peripheral artery disease, resulting in poor exercise performance and quality of life (QoL). Supervised exercise programs are an effective rehabilitation for patients with claudication, but they are poorly adhered to, in part due to the high pain and effort associated with walking, aerobic, and resistance exercise. Low-intensity resistance exercise with blood flow restriction (BFR) represents an alternative exercise method for individuals who are intolerant to high-intensity protocols. The aim of this study was to evaluate the feasibility of a supervised BFR program in patients with claudication.

METHODS

Thirty patients with stable claudication completed an 8-week supervised exercise program and were randomized to either BFR (n = 15) or a control of matched exercise without BFR (control; n = 15). Feasibility, safety, and efficacy were assessed.

RESULTS

All success criteria of the feasibility trial were met. Exercise adherence was high (BFR = 78.3%, control = 83.8%), loss to follow up was 10%, and there were no adverse events. Clinical improvement in walking was achieved in 86% of patients in the BFR group but in only 46% of patients in the control group. Time to claudication pain during walking increased by 35% for BFR but was unchanged for the control. QoL for the BFR group showed improved mobility, ability to do usual activities, pain, depression, and overall health at follow up.

CONCLUSION

A supervised blood flow restriction program is feasible in patients with claudication and has the potential to increase exercise performance, reduce pain, and improve QoL. (Clinicaltrials.gov Identifier: NCT04890275).

Where Does Blood Flow Restriction Fit in the Toolbox of Athletic Development? A Narrative Review of the Proposed Mechanisms and Potential Applications

Blood flow-restricted exercise is currently used as a low-intensity time-efficient approach to reap many of the benefits of typical high-intensity training. Evidence continues to lend support to the notion that even highly trained individuals, such as athletes, still benefit from this mode of training. Both resistance and endurance exercise may be combined with blood flow restriction to provide a spectrum of adaptations in skeletal muscle, spanning from myofibrillar to mitochondrial adjustments. Such diverse adaptations would benefit both muscular strength and endurance qualities concurrently, which are demanded in athletic performance, most notably in team sports. Moreover, recent work indicates that when traditional high-load resistance training is supplemented with low-load, blood flow-restricted exercise, either in the same session or as a separate training block in a periodised programme, a synergistic and complementary effect on training adaptations may occur. Transient reductions in mechanical loading of tissues afforded by low-load, blood flow-restricted exercise may also serve a purpose during de-loading, tapering or rehabilitation of musculoskeletal injury. This narrative review aims to expand on the current scientific and practical understanding of how blood flow restriction methods may be applied by coaches and practitioners to enhance current athletic development models.

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.

Muscle strength adaptation between high-load resistance training versus low-load blood flow restriction training with different cuff pressure characteristics: a systematic review and meta-analysis

Purpose: In this systematic review and meta-analysis, blood flow restriction (BFR) with low-load resistance training (BFR-RT) was compared with high-load resistance training (HL-RT) on muscle strength in healthy adults. The characteristics of cuff pressure suitable for muscle strength gain were also investigated by analyzing the effects of applying different occlusion pressure prescriptions and cuff inflation patterns on muscle strength gain. Methods: Literature search was conducted using PubMed, Ovid Medline, ProQuest, Cochrane Library, Embase, and Scopus databases to identify literature published until May 2023. Studies reporting the effects of BFR-RT interventions on muscle strength gain were compared with those of HL-RT. The risk of bias in the included trials was assessed using the Cochrane tool, followed by a meta-analysis to calculate the combined effect. Subgroup analysis was performed to explore the beneficial variables. Results: Nineteen articles (42 outcomes), with a total of 458 healthy adults, were included in the meta-analysis. The combined effect showed higher muscle strength gain with HL-RT than with BFR-RT (p = 0.03, SMD = -0.16, 95% CI: -0.30 to -0.01). The results of the subgroup analysis showed that the BFR-RT applied with incremental and individualized pressure achieved muscle strength gain similar to the HL-RT (p = 0.8, SMD = -0.05, 95% CI: -0.44 to 0.34; p = 0.68, SMD = -0.04, 95% CI: -0.23 to 0.15), but muscle strength gain obtained via BFR-RT applied with absolute pressure was lower than that of HL-RT (p < 0.05, SMD = -0.45, 95% CI: -0.71 to -0.19). Furthermore, muscle strength gain obtained by BFR-RT applied with intermittent pressure was similar to that obtained by HL-RT (p = 0.88, SMD = -0.02, 95% CI: -0.27 to 0.23), but muscle strength gain for BFR-RT applied with continuous pressure showed a less prominent increase than that for HL-RT (p < 0.05, SMD = -0.3, 95% CI: -0.48 to -0.11). Conclusion: In general, HL-RT produces superior muscle strength gains than BFR-RT. However, the application of individualized, incremental, and intermittent pressure exercise protocols in BFR-RT elicits comparable muscle strength gains to HL-RT. Our findings indicate that cuff pressure characteristics play a significant role in establishing a BFR-RT intervention program for enhancing muscle strength in healthy adults. Clinical Trial Registration: https://www.crd.york.ac.uk/PROSPERO/#recordDetails; Identifier: PROSPERO (CRD42022364934).

Phytochemical compound PB125 attenuates skeletal muscle mitochondrial dysfunction and impaired proteostasis in a model of musculoskeletal decline

Impaired mitochondrial function and disrupted proteostasis contribute to musculoskeletal dysfunction. However, few interventions simultaneously target these two drivers to prevent musculoskeletal decline. Nuclear factor erythroid 2-related factor 2 (Nrf2) activates a transcriptional programme promoting cytoprotection, metabolism, and proteostasis. We hypothesized daily treatment with a purported Nrf2 activator, PB125, in Hartley guinea pigs, a model of musculoskeletal decline, would attenuate the progression of skeletal muscle mitochondrial dysfunction and impaired proteostasis and preserve musculoskeletal function. We treated 2- and 5-month-old male and female Hartley guinea pigs for 3 and 10 months, respectively, with the phytochemical compound PB125. Longitudinal assessments of voluntary mobility were measured using Any-MazeTM open-field enclosure monitoring. Cumulative skeletal muscle protein synthesis rates were measured using deuterium oxide over the final 30 days of treatment. Mitochondrial oxygen consumption in soleus muscles was measured using high resolution respirometry. In both sexes, PB125 (1) increased electron transfer system capacity; (2) attenuated the disease/age-related decline in coupled and uncoupled mitochondrial respiration; and (3) attenuated declines in protein synthesis in the myofibrillar, mitochondrial and cytosolic subfractions of the soleus. These effects were not associated with statistically significant prolonged maintenance of voluntary mobility in guinea pigs. Collectively, treatment with PB125 contributed to maintenance of skeletal muscle mitochondrial respiration and proteostasis in a pre-clinical model of musculoskeletal decline. Further investigation is necessary to determine if these documented effects of PB125 are also accompanied by slowed progression of other aspects of musculoskeletal dysfunction. KEY POINTS: Aside from exercise, there are no effective interventions for musculoskeletal decline, which begins in the fifth decade of life and contributes to disability and cardiometabolic diseases. Targeting both mitochondrial dysfunction and impaired protein homeostasis (proteostasis), which contribute to the age and disease process, may mitigate the progressive decline in overall musculoskeletal function (e.g. gait, strength). A potential intervention to target disease drivers is to stimulate nuclear factor erythroid 2-related factor 2 (Nrf2) activation, which leads to the transcription of genes responsible for redox homeostasis, proteome maintenance and mitochondrial energetics. Here, we tested a purported phytochemical Nrf2 activator, PB125, to improve mitochondrial function and proteostasis in male and female Hartley guinea pigs, which are a model for musculoskeletal ageing. PB125 improved mitochondrial respiration and attenuated disease- and age-related declines in skeletal muscle protein synthesis, a component of proteostasis, in both male and female Hartley guinea pigs.

Myofiber hypertrophy adaptations following 6 weeks of low-load resistance training with blood flow restriction in untrained males and females

The effects of low-load resistance training with blood flow restriction (BFR) on hypertrophy of type I/II myofibers remains unclear, especially in females. The purpose of the present study is to examine changes in type I/II myofiber cross-sectional area (fCSA) and muscle CSA (mCSA) of the vastus lateralis (VL) from before (Pre) to after (Post) 6 wk of high-load resistance training (HL; n = 15, 8 females) and low-load resistance training with BFR (n = 16, 8 females). Mixed-effects models were used to analyze fCSA with group (HL, BFR), sex (M, F), fiber type (I, II), and time (Pre, Post) included as factors. mCSA increased from pre- to posttraining (P < 0.001, d = 0.91) and was greater in males compared with females (P < 0.001, d = 2.26). Type II fCSA increased pre- to post-HL (P < 0.05, d = 0.46) and was greater in males compared with females (P < 0.05, d = 0.78). There were no significant increases in fCSA pre- to post-BFR for either fiber type or sex. Cohen's d, however, revealed moderate effect sizes in type I and II fCSA for males (d = 0.59 and 0.67), although this did not hold true for females (d = 0.29 and 0.34). Conversely, the increase in type II fCSA was greater for females than for males after HL. In conclusion, low-load resistance training with BFR may not promote myofiber hypertrophy to the level of HL resistance training, and similar responses were generally observed for males and females. In contrast, comparable effect sizes for mCSA and 1-repetition maximum (1RM) between groups suggest that BFR could play a role in a resistance training program.NEW & NOTEWORTHY This is the first study, to our knowledge, to examine myofiber hypertrophy from low-load resistance training with blood flow restriction (BFR) in females. Although this type of training did not result in myofiber hypertrophy, there were comparable increases in muscle cross-sectional area compared with high-load resistance training. These findings possibly highlight that males and females respond in a similar manner to high-load resistance training and low-load resistance training with BFR.

Fiber-Type-Specific Hypertrophy with the Use of Low-Load Blood Flow Restriction Resistance Training: A Systematic Review

Emerging evidence indicates that the use of low-load resistance training in combination with blood flow restriction (LL-BFR) can be an effective method to elicit increases in muscle size, with most research showing similar whole muscle development of the extremities compared to high-load (HL) training. It is conceivable that properties unique to LL-BFR such as greater ischemia, reperfusion, and metabolite accumulation may enhance the stress on type I fibers during training compared to the use of LLs without occlusion. Accordingly, the purpose of this paper was to systematically review the relevant literature on the fiber-type-specific response to LL-BFR and provide insights into future directions for research. A total of 11 studies met inclusion criteria. Results of the review suggest that the magnitude of type I fiber hypertrophy is at least as great, and sometimes greater, than type II hypertrophy when performing LL-BFR. This finding is in contrast to HL training, where the magnitude of type II fiber hypertrophy tends to be substantially greater than that of type I myofibers. However, limited data directly compare training with LL-BFR to nonoccluded LL or HL conditions, thus precluding the ability to draw strong inferences as to whether the absolute magnitude of type I hypertrophy is indeed greater in LL-BFR vs. traditional HL training. Moreover, it remains unclear as to whether combining LL-BFR with traditional HL training may enhance whole muscle hypertrophy via greater increases in type I myofiber cross-sectional area.

Six weeks of high-load resistance and low-load blood flow restricted training increase Na/K-ATPase sub-units α2 and β1 equally, but does not alter ClC-1 abundance in untrained human skeletal muscle

Contractile function of skeletal muscle relies on the ability of muscle fibers to trigger and propagate action potentials (APs). These electrical signals are created by transmembrane ion transport through ion channels and membrane transporter systems. In this regard, the Cl^- ion channel 1 (ClC-1) and the Na⁺/K^--ATPase (NKA) are central for maintaining ion homeostasis across the sarcolemma during intense contractile activity. Therefore, this randomized controlled trial aimed to investigate the changes in ClC-1 and specific NKA subunit isoform expression in response to six weeks (18 training sessions) of high-load resistance exercise (HLRE) and low-load blood flow restricted resistance exercise (BFRRE), respectively. HLRE was conducted as 4 sets of 12 repetitions of knee extensions performed at 70% of 1 repetition maximum (RM), while BFRRE was conducted as 4 sets of knee extensions at 30% of 1RM performed to volitional fatigue. Furthermore, the potential associations between protein expression and contractile performance were investigated. We show that muscle ClC-1 abundance was not affected by either exercise modality, whereas NKA subunit isoforms [Formula: see text]² and [Formula: see text]¹ increased equally by appx. 80-90% with BFRRE (p < 0.05) and 70-80% with HLRE (p < 0.05). No differential impact between exercise modalities was observed. At baseline, ClC-1 protein expression correlated inversely with dynamic knee extensor strength (r=-0.365, p = 0.04), whereas no correlation was observed between NKA subunit content and contractile performance at baseline. However, training-induced changes in NKA [Formula: see text]² subunit (r = 0.603, p < 0.01) and [Formula: see text]¹ subunit (r = 0.453, p < 0.05) correlated with exercise-induced changes in maximal voluntary contraction. These results suggest that the initial adaptation to resistance-based exercise does not involve changes in ClC-1 abundance in untrained skeletal muscle, and that increased content of NKA subunits may facilitate increases in maximal force production.

Torque and Discomfort During Electrically Evoked Muscle Contractions in Healthy Young Adults: Influence of Stimulation Current and Pulse Frequency

OBJECTIVE

To investigate (1) how current and pulse frequency of electrical stimulation (ES) as well as contraction mode (isometric, concentric, and eccentric) influence torque output and discomfort and (2) how familiarization by repeated ES sessions influences ratings of perceived discomfort.

DESIGN

An experimental study, 3 sessions.

SETTING

A university laboratory.

PARTICIPANTS

Eight healthy participants (5 men, 3 women; mean age 25.2 years; N=8).

INTERVENTIONS

Participants completed 3 trial days, each including 17 electrically evoked thigh muscle contractions. On each trial day, the first 6 contractions consisted of 2 isometric, 2 concentric, and 2 eccentric muscle contractions randomly ordered with a fixed stimulation current and pulse frequency (200 mA, 20 Hz), while the remaining 11 muscle contractions were all isometric with randomly ordered combinations of current (100-250 mA) and pulse frequency (20-100 Hz).

MAIN OUTCOME MEASURES

Torque and perceived discomfort were measured for each ES-evoked contraction.

RESULTS

Overall, the findings revealed that a higher stimulation frequency was associated with an increased torque without increased discomfort, while higher currents were associated with increases of both torque and discomfort. Contraction type did not influence level of discomfort, despite eccentric contractions eliciting higher torque compared with concentric and isometric contractions (P<.001). Finally, a significant familiarization to ES (P<.001) was observed after just 1 of 3 identical stimulation sessions.

CONCLUSIONS

The outlined data suggest that to elicit high torque levels while minimizing levels of discomfort in young subjects, eccentric muscle contractions evoked with a low stimulation current, and a high pulse frequency are preferable. Furthermore, a single familiarization session significantly lowers rating of perceived discomfort during ES.

Effects of transdermal estrogen therapy on satellite cell number and molecular markers for muscle hypertrophy in response to resistance training in early postmenopausal women

PURPOSE

To investigate the effects of resistance training with or without transdermal estrogen therapy (ET) on satellite cell (SC) number and molecular markers for muscle hypertrophy in early postmenopausal women.

METHODS

Using a double-blinded randomized controlled design, we allocated healthy, untrained postmenopausal women to perform 12 weeks of resistance training with placebo (PLC, n = 16) or ET (n = 15). Muscle biopsies obtained before and after the intervention, and two hours after the last training session were analyzed for fiber type, SC number and molecular markers for muscle hypertrophy and degradation (real-time PCR, western blotting).

RESULTS

The analysis of SCs per Type I fiber showed a time x treatment interaction caused by a 47% decrease in PLC, and a 26% increase after ET after the training period. Also, SCs per Type II fiber area was lower after the intervention driven by a 57% decrease in PLC. Most molecular markers changed similarly in the two groups.

CONCLUSION

A decline in SC per muscle fiber was observed after the 12-week training period in postmenopausal women, which was counteracted when combined with use of transdermal ET.

CLINICAL TRIAL REGISTRATION NUMBER

nct03020953.

Comparative Perceptual, Affective, and Cardiovascular Responses between Resistance Exercise with and without Blood Flow Restriction in Older Adults

Older adults and patients with chronic disease presenting with muscle weakness or musculoskeletal disorders may benefit from low-load resistance exercise (LLRE) with blood flow restriction (BFR). LLRE-BFR has been shown to increase muscle size, strength, and endurance comparable to traditional resistance exercise but without the use of heavy loads. However, potential negative effects from LLRE-BFR present as a barrier to participation and limit its wider use. This study examined the perceptual, affective, and cardiovascular responses to a bout of LLRE-BFR and compared the responses to LLRE and moderate-load resistance exercise (MLRE). Twenty older adults (64.3 ± 4.2 years) performed LLRE-BFR, LLRE and MLRE consisting of 4 sets of leg press and knee extension, in a randomised crossover design. LLRE-BFR was more demanding than LLRE and MLRE through increased pain (p ≤ 0.024, d = 0.8-1.4) and reduced affect (p ≤ 0.048, d = -0.5--0.9). Despite this, LLRE-BFR was enjoyed and promoted a positive affective response (p ≤ 0.035, d = 0.5-0.9) following exercise comparable to MLRE. This study supports the use of LLRE-BFR for older adults and encourages future research to examine the safety, acceptability, and efficacy of LLRE-BFR in patients with chronic disease.

Investigating effects of sodium beta-hydroxybutyrate on metabolism in placebo-controlled, bilaterally infused human leg with focus on skeletal muscle protein dynamics

Systemic administration of beta-hydroxybutyrate (BHB) decreases whole-body protein oxidation and muscle protein breakdown in humans. We aimed to determine any direct effect of BHB on skeletal muscle protein turnover when administered locally in the femoral artery. Paired design with each subject being investigated on one single occasion with one leg being infused with BHB and the opposing leg acting as a control. We studied 10 healthy male volunteers once with bilateral femoral vein and artery catheters. One artery was perfused with saline (Placebo) and one with sodium-BHB. Labelled phenylalanine and palmitate were used to assess local leg fluxes. Femoral vein concentrations of BHB were significantly higher in the intervention leg (3.4 (3.2, 3.6) mM) compared with the placebo-controlled leg (1.9 (1.8, 2.1) mM) with a peak difference of 1.4 (1.1, 1.7) mM, p < 0.0005. Net loss of phenylalanine for BHB vs Placebo -6.7(-10.8, -2.7) nmol/min vs -8.7(-13.8, -3.7) nmol/min, p = 0.52. Palmitate flux and arterio-venous difference of glucose did not differ between legs. Under these experimental conditions, we failed to observe the direct effects of BHB on skeletal muscle protein turnover. This may relate to a combination of high concentrations of BHB (close to 2 mM) imposed systemically by spillover leading to high BHB concentrations in the saline-infused leg and a lack of major differences in concentration gradients between the two sides-implying that observations were made on the upper part of the dose-response curve for BHB and the relatively small number of subjects studied.

The Role of Plasma Extracellular Vesicles in Remote Ischemic Conditioning and Exercise-Induced Ischemic Tolerance

Ischemic conditioning and exercise have been suggested for protecting against brain ischemia-reperfusion injury. However, the endogenous protective mechanisms stimulated by these interventions remain unclear. Here, in a comprehensive translational study, we investigated the protective role of extracellular vesicles (EVs) released after remote ischemic conditioning (RIC), blood flow restricted resistance exercise (BFRRE), or high-load resistance exercise (HLRE). Blood samples were collected from human participants before and at serial time points after intervention. RIC and BFRRE plasma EVs released early after stimulation improved viability of endothelial cells subjected to oxygen-glucose deprivation. Furthermore, post-RIC EVs accumulated in the ischemic area of a stroke mouse model, and a mean decrease in infarct volume was observed for post-RIC EVs, although not reaching statistical significance. Thus, circulating EVs induced by RIC and BFRRE can mediate protection, but the in vivo and translational effects of conditioned EVs require further experimental verification.

State of Knowledge on Molecular Adaptations to Exercise in Humans: Historical Perspectives and Future Directions

For centuries, regular exercise has been acknowledged as a potent stimulus to promote, maintain, and restore healthy functioning of nearly every physiological system of the human body. With advancing understanding of the complexity of human physiology, continually evolving methodological possibilities, and an increasingly dire public health situation, the study of exercise as a preventative or therapeutic treatment has never been more interdisciplinary, or more impactful. During the early stages of the NIH Common Fund Molecular Transducers of Physical Activity Consortium (MoTrPAC) Initiative, the field is well-positioned to build substantially upon the existing understanding of the mechanisms underlying benefits associated with exercise. Thus, we present a comprehensive body of the knowledge detailing the current literature basis surrounding the molecular adaptations to exercise in humans to provide a view of the state of the field at this critical juncture, as well as a resource for scientists bringing external expertise to the field of exercise physiology. In reviewing current literature related to molecular and cellular processes underlying exercise-induced benefits and adaptations, we also draw attention to existing knowledge gaps warranting continued research effort. © 2021 American Physiological Society. Compr Physiol 12:3193-3279, 2022.

The role of the neural stimulus in regulating skeletal muscle hypertrophy

Resistance training is frequently performed with the goal of stimulating muscle hypertrophy. Due to the key roles motor unit recruitment and mechanical tension play to induce muscle growth, when programming, the manipulation of the training variables is oriented to provoke the correct stimulus. Although it is known that the nervous system is responsible for the control of motor units and active muscle force, muscle hypertrophy researchers and trainers tend to only focus on the adaptations of the musculotendinous unit and not in the nervous system behaviour. To better guide resistance exercise prescription for muscle hypertrophy and aiming to delve into the mechanisms that maximize this goal, this review provides evidence-based considerations for possible effects of neural behaviour on muscle growth when programming resistance training, and future neurophysiological measurement that should be tested when training to increase muscle mass. Combined information from the neural and muscular structures will allow to understand the exact adaptations of the muscle in response to a given input (neural drive to the muscle). Changes at different levels of the nervous system will affect the control of motor units and mechanical forces during resistance training, thus impacting the potential hypertrophic adaptations. Additionally, this article addresses how neural adaptations and fatigue accumulation that occur when resistance training may influence the hypertrophic response and propose neurophysiological assessments that may improve our understanding of resistance training variables that impact on muscular adaptations.

Optimization of Exercise Countermeasures to Spaceflight Using Blood Flow Restriction

INTRODUCTION: During spaceflight missions, astronauts work in an extreme environment with several hazards to physical health and performance. Exposure to microgravity results in remarkable deconditioning of several physiological systems, leading to impaired physical condition and human performance, posing a major risk to overall mission success and crew safety. Physical exercise is the cornerstone of strategies to mitigate physical deconditioning during spaceflight. Decades of research have enabled development of more optimal exercise strategies and equipment onboard the International Space Station. However, the effects of microgravity cannot be completely ameliorated with current exercise countermeasures. Moreover, future spaceflight missions deeper into space require a new generation of spacecraft, which will place yet more constraints on the use of exercise by limiting the amount, size, and weight of exercise equipment and the time available for exercise. Space agencies are exploring ways to optimize exercise countermeasures for spaceflight, specifically exercise strategies that are more efficient, require less equipment, and are less time-consuming. Blood flow restriction exercise is a low intensity exercise strategy that requires minimal equipment and can elicit positive training benefits across multiple physiological systems. This method of exercise training has potential as a strategy to optimize exercise countermeasures during spaceflight and reconditioning in terrestrial and partial gravity environments. The possible applications of blood flow restriction exercise during spaceflight are discussed herein.Hughes L, Hackney KJ, Patterson SD. Optimization of exercise countermeasures to spaceflight using blood flow restriction. Aerosp Med Hum Perform. 2021; 93(1):32-45.

Skeletal Muscle Ribosome and Mitochondrial Biogenesis in Response to Different Exercise Training Modalities

Skeletal muscle adaptations to resistance and endurance training include increased ribosome and mitochondrial biogenesis, respectively. Such adaptations are believed to contribute to the notable increases in hypertrophy and aerobic capacity observed with each exercise mode. Data from multiple studies suggest the existence of a competition between ribosome and mitochondrial biogenesis, in which the first adaptation is prioritized with resistance training while the latter is prioritized with endurance training. In addition, reports have shown an interference effect when both exercise modes are performed concurrently. This prioritization/interference may be due to the interplay between the 5’ AMP-activated protein kinase (AMPK) and mechanistic target of rapamycin complex 1 (mTORC1) signaling cascades and/or the high skeletal muscle energy requirements for the synthesis and maintenance of cellular organelles. Negative associations between ribosomal DNA and mitochondrial DNA copy number in human blood cells also provide evidence of potential competition in skeletal muscle. However, several lines of evidence suggest that ribosome and mitochondrial biogenesis can occur simultaneously in response to different types of exercise and that the AMPK-mTORC1 interaction is more complex than initially thought. The purpose of this review is to provide in-depth discussions of these topics. We discuss whether a curious competition between mitochondrial and ribosome biogenesis exists and show the available evidence both in favor and against it. Finally, we provide future research avenues in this area of exercise physiology.

Acute cellular and molecular responses and chronic adaptations to low-load blood flow restriction and high-load resistance exercise in trained individuals

Blood flow restriction (BFR) with low-load resistance exercise (RE) is often used as a surrogate to traditional high-load RE to stimulate muscular adaptations, such as hypertrophy and strength. However, it is not clear whether such adaptations are achieved through similar cellular and molecular processes. We compared changes in muscle function, morphology and signaling pathways between these differing training protocols. Twenty-one males and females (mean ± SD: 24.3 ± 3.1 years) experienced with resistance training (4.9 ± 2.6 years) performed nine weeks of resistance training (three times per week) with either high-loads (75-80% 1RM; HL-RT), or low-loads with BFR (30-40% 1RM; LL-BFR). Before and after the training intervention, resting muscle biopsies were collected, and quadricep cross-sectional area (CSA), muscular strength and power were measured. Approximately 5 days following the intervention, the same individuals performed an additional 'acute' exercise session under the same conditions, and serial muscle biopsies were collected to assess hypertrophic- and ribosomal-based signaling stimuli. Quadricep CSA increased with both LL-BFR (7.4±4.3%) and HL-RT (4.6±2.9%), with no significant differences between training groups (p=0.37). Muscular strength also increased in both training groups, but with superior gains in squat 1RM occurring with HL-RT (p<0.01). Acute phosphorylation of several key proteins involved in hypertrophy signaling pathways, and expression of ribosomal RNA transcription factors occurred to a similar degree with LL-BFR and HL-RT (all p>0.05 for between-group comparisons). Together, these findings validate low-load resistance training with continuous BFR as an effective alternative to traditional high-load resistance training for increasing muscle hypertrophy in trained individuals.

Blood flow restriction training and the high-performance athlete: science to application

The manipulation of blood flow in conjunction with skeletal muscle contraction has greatly informed the physiological understanding of muscle fatigue, blood pressure reflexes, and metabolism in humans. Recent interest in using intentional blood flow restriction (BFR) has focused on elucidating how exercise during periods of reduced blood flow affects typical training adaptations. A large initial appeal for BFR training was driven by studies demonstrating rapid increases in muscle size, strength, and endurance capacity, even when notably low intensities and resistances, which would typically be incapable of stimulating change in healthy populations, were used. The incorporation of BFR exercise into the training of strength- and endurance-trained athletes has recently been shown to provide additive training effects that augment skeletal muscle and cardiovascular adaptations. Recent observations suggest BFR exercise alters acute physiological stressors such as local muscle oxygen availability and vascular shear stress, which may lead to adaptations that are not easily attained with conventional training. This review explores these concepts and summarizes both the evidence base and knowledge gaps regarding the application of BFR training for athletes.

Muscle from aged rats is resistant to mechanotherapy during atrophy and reloading

Massage is a viable mechanotherapy to improve protein turnover during disuse atrophy and improve muscle regrowth during recovery from disuse atrophy in adult muscle. Therefore, we investigated whether massage can cause beneficial adaptations in skeletal muscle from aged rats during normal weight-bearing (WB) conditions, hindlimb suspension (HS), or reloading (RE) following HS. Aged (30 months) male Fischer 344/Brown Norway rats were divided into two experiments: (1) WB for 7 days (WB, n = 8), WB with massage (WBM, n = 8), HS for 7 days (HS7, n = 8), or HS with massage (HSM, n = 8), and (2) WB for 14 days (WB14, n = 8), HS for 14 days (HS14, n = 8), reloading (RE, n = 10), or reloading with massage (REM, n = 10) for 7 days following HS. Deuterium oxide (D2O) labeling was used to assess dynamic protein and ribosome turnover in each group and anabolic signaling pathways were assessed. Massage did have an anabolic benefit during RE or WB. In contrast, massage during HS enhanced myofibrillar protein turnover in both the massaged limb and contralateral non-massaged limb compared with HS, but this did not prevent muscle loss. Overall, the data demonstrate that massage is not an effective mechanotherapy for prevention of atrophy during muscle disuse or recovery of muscle mass during reloading in aged rats.

Ribosome biogenesis and degradation regulate translational capacity during muscle disuse and reloading

BACKGROUND

Translational capacity (i.e. ribosomal mass) is a key determinant of protein synthesis and has been associated with skeletal muscle hypertrophy. The role of translational capacity in muscle atrophy and regrowth from disuse is largely unknown. Therefore, we investigated the effect of muscle disuse and reloading on translational capacity in middle-aged men (Study 1) and in rats (Study 2).

METHODS

In Study 1, 28 male participants (age 50.03 ± 3.54 years) underwent 2 weeks of knee immobilization followed by 2 weeks of ambulatory recovery and a further 2 weeks of resistance training. Muscle biopsies were obtained for measurement of total RNA and pre-ribosomal (r)RNA expression, and vastus lateralis cross-sectional area (CSA) was determined via peripheral quantitative computed tomography. In Study 2, male rats underwent hindlimb suspension (HS) for either 24 h (HS 24 h, n = 4) or 7 days (HS 7d, n = 5), HS for 7 days followed by 7 days of reloading (Rel, n = 5) or remained as ambulatory weight bearing (WB, n = 5) controls. Rats received deuterium oxide throughout the study to determine RNA synthesis and degradation, and mTORC1 signalling pathway was assessed.

RESULTS

Two weeks of immobilization reduced total RNA concentration (20%) and CSA (4%) in men (both P ≤ 0.05). Ambulatory recovery restored total RNA concentration to baseline levels and partially restored muscle CSA. Total RNA concentration and 47S pre-rRNA expression increased above basal levels after resistance training (P ≤ 0.05). In rats, RNA synthesis was 30% lower while degradation was ~400% higher in HS 7d in soleus and plantaris muscles compared with WB (P ≤ 0.05). mTORC1 signalling was lower in HS compared with WB as was 47S pre-rRNA (P ≤ 0.05). With reloading, the aforementioned parameters were restored to WB levels while RNA degradation was suppressed (P ≤ 0.05).

CONCLUSIONS

Changes in RNA concentration following muscle disuse and reloading were associated with changes in ribosome biogenesis and degradation, indicating that both processes are important determinants of translational capacity. The pre-clinical data help explain the reduced translational capacity after muscle immobilization in humans and demonstrate that ribosome biogenesis and degradation might be valuable therapeutic targets to maintain muscle mass during disuse.

Massage as a mechanotherapy promotes skeletal muscle protein and ribosomal turnover but does not mitigate muscle atrophy during disuse in adult rats

AIM

Interventions that decrease atrophy during disuse are desperately needed to maintain muscle mass. We recently found that massage as a mechanotherapy can improve muscle regrowth following disuse atrophy. Therefore, we aimed to determine if massage has similar anabolic effects when applied during normal weight bearing conditions (WB) or during atrophy induced by hindlimb suspension (HS) in adult rats.

METHODS

Adult (10 months) male Fischer344-Brown Norway rats underwent either hindlimb suspension (HS, n = 8) or normal WB (WB, n = 8) for 7 days. Massage was applied using cyclic compressive loading (CCL) in WB (WBM, n = 9) or HS rats (HSM, n = 9) and included four 30-minute bouts of CCL applied to gastrocnemius muscle every other day.

RESULTS

Massage had no effect on any anabolic parameter measured under WB conditions (WBM). In contrast, massage during HS (HSM) stimulated protein turnover, but did not mitigate muscle atrophy. Atrophy from HS was caused by both lowered protein synthesis and higher degradation. HS and HSM had lowered total RNA compared with WB and this was the result of significantly higher ribosome degradation in HS that was attenuated in HSM, without differences in ribosomal biogenesis. Also, massage increased protein turnover in the non-massaged contralateral limb during HS. Finally, we determined that total RNA degradation primarily dictates loss of muscle ribosomal content during disuse atrophy.

CONCLUSION

We conclude that massage is an effective mechanotherapy to impact protein turnover during muscle disuse in both the massaged and non-massaged contralateral muscle, but it does not attenuate the loss of muscle mass.

CORP: The use of deuterated water for the measurement of protein synthesis

The use of deuterium oxide (D₂O) has greatly expanded the scope of what is possible for the measurement of protein synthesis. The greatest asset of D₂O labeling is that it facilitates the measurement of synthesis rates over prolonged periods of time from single proteins through integrated tissue-based measurements. Because the ease of administration, the method is amenable for use in a variety of models and conditions. Although the method adheres to the same rules as other isotope methods, the flexibility can create conditions that are not the same as other approaches and thus requires careful execution to maintain validity and reliability. For this CORP article, we provide a history that gave rise to the method and discuss the advantages and disadvantages of the method, the critical assumptions, guidelines, and best practices based on instrumentation, models, and experimental design. The goal of this CORP article is to propagate additional use of D₂O in a manner that produces reliable and valid data.

Low-load resistance training to task failure with and without blood flow restriction: muscular functional and structural adaptations

The application of blood flow restriction (BFR) during resistance exercise is increasingly recognized for its ability to improve rehabilitation and for its effectiveness in increasing muscle hypertrophy and strength among healthy populations. However, direct comparison of the skeletal muscle adaptations to low-load resistance exercise (LL-RE) and low-load BFR resistance exercise (LL-BFR) performed to task failure is lacking. Using a within-subject design, we examined whole muscle group and skeletal muscle adaptations to 6 wk of LL-RE and LL-BFR training to repetition failure. Muscle strength and size outcomes were similar for both types of training, despite ~33% lower total exercise volume (load × repetition) with LL-BFR than LL-RE (28,544 ± 1,771 vs. 18,949 ± 1,541 kg, P = 0.004). After training, only LL-BFR improved the average power output throughout the midportion of a voluntary muscle endurance task. Specifically, LL-BFR training sustained an 18% greater power output from baseline and resulted in a greater change from baseline than LL-RE (19 ± 3 vs. 3 ± 4 W, P = 0.008). This improvement occurred despite histological analysis revealing similar increases in capillary content of type I muscle fibers following LL-RE and LL-BFR training, which was primarily driven by increased capillary contacts (4.53 ± 0.23 before training vs. 5.33 ± 0.27 and 5.17 ± 0.25 after LL-RE and LL-BFR, respectively, both P < 0.05). Moreover, maximally supported mitochondrial respiratory capacity increased only in the LL-RE leg by 30% from baseline (P = 0.006). Overall, low-load resistance training increased indexes of muscle oxidative capacity and strength, which were not further augmented with the application of BFR. However, performance on a muscle endurance test was improved following BFR training.

Sarcopenia Coexisting with High Adiposity Exacerbates Insulin Resistance and Dyslipidemia in Saudi Adult Men

OBJECTIVE

The associations between sarcopenia, adiposity indices and metabolic dysregulation still remain controversial. We aimed to assess and compare insulin resistance and metabolic profile in sarcopenic and non-sarcopenic obese Saudi adult men.

METHODS

This cross sectional study was conducted at the College of Sports Sciences, King Saud University, Riyadh. We recruited 312 Saudi adult male individuals and 288 were finally selected for the study. Body composition analysis and hand grip strength (HGS) were estimated by bioimpedance analysis (BIA) and dynamometer in all subjects, respectively. Fasting blood samples were collected for glucose (FBG), basal insulin (BI) and lipid profile. The subjects were divided into three groups based on the body composition parameters, appendicular lean mass (ALM) and body fat percentage (BF%), into non-obese (NonOb) [Normal ALM+<25 BF%], obese without sarcopenia (ObNonS) [Normal ALM+>25 BF%] and obese with sarcopenia (ObS) [Low ALM+>25 BF%].

RESULTS

Obese subjects had significantly higher BI, HOMA-IR and HOMA-β compared to non-obese. Moreover, comparison between two obese groups revealed that both BI and HOMA-IR levels were higher in ObS subjects compared to ObNonS individuals revealing that sarcopenia exacerbates the insulin resistance profile. There was a significant trend of higher resistance and lower sensitivity from non-obese to obesity with sarcopenia. Total cholesterol (TC) and triglycerides (TG) were significantly higher and high density lipoprotein cholesterol (HDL-C) was significantly lower in sarcopenic obese subjects compared to non-sarcopenic obese individuals. The worsening effects were more significant at cutoff point of 7.46 on insulin indices and lipid profile showing that sarcopenia associated with obesity exacerbates the dyslipidemia.

CONCLUSION

Our study shows that obesity associated with sarcopenia exhibits significantly greater insulin resistance and dyslipidemia than sarcopenia or obesity per se. Therefore, sarcopenic obesity might be an independent risk factor for metabolic disease progression.

Effect of Blood Flow Restricted Resistance Exercise and Remote Ischemic Conditioning on Functional Capacity and Myocellular Adaptations in Patients With Heart Failure

BACKGROUND

Patients with congestive heart failure (CHF) have impaired functional capacity and inferior quality of life. The clinical manifestations are associated with structural and functional impairments in skeletal muscle, emphasizing a need for feasible rehabilitation strategies beyond optimal anticongestive medical treatment. We investigated whether low-load blood flow restricted resistance exercise (BFRRE) or remote ischemic conditioning (RIC) could improve functional capacity and quality of life in patients with CHF and stimulate skeletal muscle myofibrillar and mitochondrial adaptations.

METHODS

We randomized 36 patients with CHF to BFRRE, RIC, or nontreatment control. BFRRE and RIC were performed 3× per week for 6 weeks. Before and after intervention, muscle biopsies, tests of functional capacity, and quality of life assessments were performed. Deuterium oxide was administered throughout the intervention to measure cumulative RNA and subfraction protein synthesis. Changes in muscle fiber morphology and mitochondrial respiratory function were also assessed.

RESULTS

BFRRE improved 6-minute walk test by 39.0 m (CI, 7.0-71.1, P=0.019) compared with control. BFRRE increased maximum isometric strength by 29.7 Nm (CI, 10.8-48.6, P=0.003) compared with control. BFRRE improved quality of life by 5.4 points (CI, -0.04 to 10.9; P=0.052) compared with control. BFRRE increased mitochondrial function by 19.1 pmol/s per milligram (CI, 7.3-30.8; P=0.002) compared with control. RIC did not produce similar changes.

CONCLUSIONS

Our results demonstrate that BFRRE, but not RIC, improves functional capacity, quality of life, and muscle mitochondrial function. Our findings have clinical implications for rehabilitation of patients with CHF and provide new insights on the myopathy accompanying CHF.

CLINICAL TRIAL REGISTRATION

URL: https://www.clinicaltrials.gov. Unique identifier: NCT03380663.

Revisiting the roles of protein synthesis during skeletal muscle hypertrophy induced by exercise

Protein synthesis is deemed the underpinning mechanism enhancing protein balance required for skeletal muscle hypertrophy in response to resistance exercise. The current model of skeletal muscle hypertrophy induced by resistance training states that the acute increase in the rates of protein synthesis after each bout of resistance exercise is the basis for muscle growth. Within this paradigm, each resistance exercise session would add a specific amount of muscle mass; therefore, muscle hypertrophy could be defined as the result of intermittent and short-lived increases in muscle protein synthesis rates following each resistance exercise session. Although a substantial amount of data has accumulated in the last decades regarding the acute changes in protein synthesis (or translational efficiency) following resistance exercise, considerable gaps on the mechanism of muscle growth still exist. Ribosome biogenesis and translational capacity have emerged as important mediators of skeletal muscle hypertrophy. Recent advances in the field have demonstrated that skeletal muscle hypertrophy is associated with markers of translational capacity and long-term changes in protein synthesis under resting conditions. This review will discuss the caveats of the current model of skeletal muscle hypertrophy induced by resistance training while proposing a working model that takes into consideration the novel data generated by independent laboratories utilizing different methodologies. It is argued, herein, that the role of protein synthesis in the current model of muscle hypertrophy warrants revisiting.