Fourteen days of bed rest induces a decline in satellite cell content and robust atrophy of skeletal muscle fibers in middle-aged adults

Daily blood flow restriction does not affect muscle fiber capillarization and satellite cell content during 2 wk of bed rest in healthy young men

The present study assessed whether single-leg daily blood flow restriction (BFR) treatment attenuates the decline in muscle fiber size, capillarization, and satellite cell (SC) content during 2 wk of bed rest in healthy, young men. Twelve healthy, young men (age: 24 ± 3 yr; BMI: 23.7 ± 3.1 kg/m2) were subjected to 2 wk of bed rest, during which one leg was exposed to three times daily 5 min of BFR, whereas the contralateral leg received sham treatment [control (CON)]. Muscle biopsies were obtained from the m. vastus lateralis from both the BFR and CON legs before and immediately after 2 wk of bed rest. Types I and II muscle fiber size, myonuclear content, capillarization, and SC content were assessed by immunohistochemistry. No significant decline in either type I or type II muscle fiber size was observed following bed rest, with no differences between the CON and BFR legs (P > 0.05). Type I muscle fiber capillary density increased in response to bed rest in both legs (P < 0.05), whereas other muscle fiber capillarization measures remained unaltered. SC content decreased in both type I (from 7.4 ± 3.2 to 5.9 ± 2.7 per 100 fibers) and type II (from 7.2 ± 3.4 to 6.5 ± 3.2 per 100 fibers) muscle fibers (main effect of time P = 0.018), with no significant differences between the BFR and CON legs (P > 0.05). In conclusion, 2 wk of bed rest has no effect on muscle capillarization and decreases the SC content, and daily BFR treatment does not affect skeletal muscle fiber size and SC content in healthy, young men.NEW & NOTEWORTHY We recently reported that the application of daily blood flow restriction (BFR) treatment does not preserve muscle mass or strength and does not modulate daily muscle protein synthesis rates during 2 wk of bed rest. Here, we show that 2 wk of bed rest resulted in a decrease in satellite cell (SC) content. In addition, the BFR treatment did not affect muscle fiber size, capillarization, and SC content during 2 wk of bed rest.

Myonuclear and satellite cell content of the vastus lateralis and soleus with 70 days of simulated microgravity and the NASA SPRINT exercise program

We previously observed a range of whole muscle and individual slow and fast myofiber size responses (mean: +4 to -24%) in quadriceps (vastus lateralis) and triceps surae (soleus) muscles of individuals undergoing 70 days of simulated microgravity with or without the NASA SPRINT exercise countermeasures program. The purpose of the current investigation was to further explore, in these same individuals, the content of myonuclei and satellite cells, both of which are key regulators of skeletal muscle mass. Individuals completed 6° head-down-tilt bedrest (BR, n = 9), bedrest with resistance and aerobic exercise (BRE, n = 9), or bedrest with resistance and aerobic exercise and low-dose testosterone (BRE + T, n = 8). The number of myonuclei and satellite cells associated with each slow [myosin heavy chain (MHC) I] and fast (MHC IIa) myofiber in the vastus lateralis was not changed (P > 0.05) pre- to postbedrest within the BR, BRE, or BRE + T groups. Similarly, in the soleus, the number of myonuclei associated with each slow and fast myofiber, and the number of satellite cells associated with each slow myofiber were not changed (P > 0.05) pre- to postbedrest within the BR, BRE, or BRE + T groups. It appears that even with relatively large perturbations in muscle mass over a few months of simulated microgravity, or with partially or completely effective exercise countermeasures, human skeletal muscle tightly regulates the abundance of myonuclei and satellite cells. Thus, exercise countermeasures efficacy for skeletal muscle atrophy appears to be independent of myonuclei and satellite cell abundance.NEW & NOTEWORTHY This study showed that after 70 days of simulated microgravity, human skeletal muscle does not alter the number of nuclei or satellite cells associated with slow or fast myofibers in the two muscle groups most negatively influenced by microgravity exposure [i.e., quadriceps (vastus lateralis) and triceps surae (soleus)]. Furthermore, the efficacy of exercise countermeasures for maintaining the mass of these muscles does not appear to be related to the myocellular content of nuclei or satellite cells.

No detectable loss of myonuclei from human muscle fibers after 6 wk of immobilization following an Achilles tendon rupture

Muscle disuse has rapid and debilitating effects on muscle mass and overall health, making it an important issue from both scientific and clinical perspectives. However, the myocellular adaptations to muscle disuse are not yet fully understood, particularly those related to the myonuclear permanence hypothesis. Therefore, in this study, we assessed fiber size, number of myonuclei, satellite cells, and capillaries in human gastrocnemius muscle after a period of immobilization following an Achilles tendon rupture. Six physically active patients (5 males/1 female, 43 ± 15 yr) were recruited to participate after sustaining an acute unilateral Achilles tendon rupture. Muscle biopsies were obtained from the lateral part of the gastrocnemius before and after 6 wk of immobilization using a plaster cast and orthosis. Muscle fiber characteristics were analyzed in tissue cross-sections and isolated single fibers using immunofluorescence and high-resolution microscopy. Immobilization did not change muscle fiber type composition nor cross-sectional area of type I or type II fibers, but muscle fiber volume tended to decline by 13% (P = 0.077). After immobilization, the volume per myonucleus was significantly reduced by 20% (P = 0.008). Myonuclei were not lost in response to immobilization but tended to increase in single fibers and type II fibers. No significant changes were observed for satellite cells or capillaries. Myonuclei were not lost in the gastrocnemius muscle after a prolonged period of immobilization, which may provide support to the myonuclear permanence hypothesis in human muscle. Capillaries remained stable throughout the immobilization period, whereas the response was variable for satellite cells, particularly in type II fibers.NEW & NOTEWORTHY The impact of prolonged immobilization on muscle fiber characteristics is difficult to study in humans and therefore remains poorly understood. We analyzed cross-sections and single fibers from gastrocnemius before and after 6 wk of immobilization due to an Achilles tendon rupture. Our data suggest that myonuclei are not lost in response to such stimuli, thus lending support to the hypothesis of myonuclear permanency in human muscle.

The expanding roles of myonuclei in adult skeletal muscle health and function

Skeletal muscle cells (myofibers) require multiple nuclei to support a cytoplasmic volume that is larger than other mononuclear cell types. It is dogmatic that mammalian resident myonuclei rely on stem cells (specifically satellite cells) for adding new DNA to muscle fibers to facilitate cytoplasmic expansion that occurs during muscle growth. In this review, we discuss the relationship between cell size and supporting genetic material. We present evidence that myonuclei may undergo DNA synthesis as a strategy to increase genetic material in myofibers independent from satellite cells. We then describe the details of our experiments that demonstrated that mammalian myonuclei can replicate DNA in vivo. Finally, we present our findings in the context of expanding knowledge about myonuclear heterogeneity, myonuclear mobility and shape. We also address why myonuclear replication is potentially important and provide future directions for remaining unknowns. Myonuclear DNA replication, coupled with new discoveries about myonuclear transcription, morphology, and behavior in response to stress, may provide opportunities to leverage previously unappreciated skeletal muscle biological processes for therapeutic targets that support muscle mass, function, and plasticity.

Motor unit adaptation to disuse: crossing the threshold from firing rate suppression to neuromuscular junction transmission

Neural conditioning to scenarios of muscle disuse is undoubtedly a cause of functional decrements that typically exceed losses of muscle size. Yet establishing the relative contribution of neural adaptation and the specific location in the motor pathway remains technically challenging. Several studies of healthy humans have targeted this system and have established that motor unit firing rate is suppressed following disuse, with a number of critical caveats. It is suppressed in the immobilized limb only, at relative and absolute force levels, and preferentially targets lower-threshold motor units. Concomitantly, electrophysiological investigation of neuromuscular junction transmission (NMJ) stability of lower-threshold motor units reveals minimal change following disuse. These findings contrast with numerous other methods, which show clear involvement of the NMJ but are unable to characterize the motor unit to which they belong. It is physiologically plausible that decrements observed following disuse are a result of suppressed firing rate of lower-threshold motor units and impairment of transmission of the NMJ of higher-threshold motor units. As such, motor units within the pool should be viewed in light of their varying susceptibility to disuse.

Acute Sarcopenia: Mechanisms and Management

BACKGROUND

Acute sarcopenia refers to the swift decline in muscle function and mass following acute events such as illness, surgery, trauma, or burns that presents significant challenges in hospitalized older adults.

METHODS

narrative review to describe the mechanisms and management of acute sarcopenia.

RESULTS

The prevalence of acute sarcopenia ranges from 28% to 69%, likely underdiagnosed due to the absence of muscle mass and function assessments in most clinical settings. Systemic inflammation, immune-endocrine dysregulation, and anabolic resistance are identified as key pathophysiological factors. Interventions include early mobilization, resistance exercise, neuromuscular electrical stimulation, and nutritional strategies such as protein supplementation, leucine, β-hydroxy-β-methyl-butyrate, omega-3 fatty acids, and creatine monohydrate. Pharmaceuticals show variable efficacy.

CONCLUSIONS

Future research should prioritize serial monitoring of muscle parameters, identification of predictive biomarkers, and the involvement of multidisciplinary teams from hospital admission to address sarcopenia. Early and targeted interventions are crucial to improve outcomes and prevent long-term disability associated with acute sarcopenia.

Exploring Radiomics Features Based on H&E Images as Potential Biomarkers for Evaluating Muscle Atrophy: A Preliminary Study

Radiomics features have been widely used as novel biomarkers in the diagnosis of various diseases, but whether radiomics features derived from hematoxylin and eosin (H&E) images can evaluate muscle atrophy has not been studied. Therefore, this study aims to establish a new biomarker based on H&E images using radiomics methods to quantitatively analyze H&E images, which is crucial for improving the accuracy of muscle atrophy assessment. Firstly, a weightless muscle atrophy model was established by laying macaques in bed, and H&E images of the shank muscle fibers of the control and bed rest (BR) macaques were collected. Muscle fibers were accurately segmented by designing a semi-supervised segmentation framework based on contrastive learning. Then, 77 radiomics features were extracted from the segmented muscle fibers, and a stable subset of features was selected through the LASSO method. Finally, the correlation between radiomics features and muscle atrophy was analyzed using a support vector machine (SVM) classifier. The semi-supervised segmentation results show that the proposed method had an average Spearman's and intra-class correlation coefficient (ICC) of 88% and 86% compared to manually extracted features, respectively. Radiomics analysis showed that the AUC of the muscle atrophy evaluation model based on H&E images was 96.87%. For individual features, GLSZM_SZE outperformed other features in terms of AUC (91.5%) and ACC (84.4%). In summary, the feature extraction based on the semi-supervised segmentation method is feasible and reliable for subsequent radiomics research. Texture features have greater advantages in evaluating muscle atrophy compared to other features. This study provides important biomarkers for accurate diagnosis of muscle atrophy.

Skeletal muscle immobilisation-induced atrophy: mechanistic insights from human studies

Periods of skeletal muscle disuse lead to rapid declines in muscle mass (atrophy), which is fundamentally underpinned by an imbalance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). The complex interplay of molecular mechanisms contributing to the altered regulation of muscle protein balance during disuse have been investigated but rarely synthesised in the context of humans. This narrative review discusses human models of muscle disuse and the ensuing inversely exponential rate of muscle atrophy. The molecular processes contributing to altered protein balance are explored, with a particular focus on growth and breakdown signalling pathways, mitochondrial adaptations and neuromuscular dysfunction. Finally, key research gaps within the disuse atrophy literature are highlighted providing future avenues to enhance our mechanistic understanding of human disuse atrophy.

Muscle Fiber Cross-Sectional Area Is Associated With Quadriceps Strength and Rate of Torque Development After ACL Injury

ABSTRACT

Graham, MC, Thompson, KL, Hawk, GS, Fry, CS, and Noehren, B. Muscle fiber cross-sectional area is associated with quadriceps strength and rate of torque development after ACL injury. J Strength Cond Res 38(6): e273-e279, 2024-The purpose of this study was to investigate the relationship between muscle fiber type-specific properties of the vastus lateralis and quadriceps muscle performance in individuals after an anterior cruciate ligament (ACL) tear. 26 subjects (22.0 ± 5.4 years) were included in this cross-sectional study, and all data were collected before ACL reconstruction. Quadriceps peak torque (QPT) and early (0-100 ms) and late (100-200 ms) rate of torque development (RTD) were obtained from maximal voluntary isometric quadriceps strength testing. Muscle fiber cross-sectional area (fCSA) and percent fiber type distribution (FT%) were evaluated through immunohistochemical analysis of a muscle biopsy. Between-limb differences in fiber characteristics were assessed using paired t-tests (with α-level 0.05). Relationships between fiber-specific properties and quadriceps muscle performance were determined using separate multiple linear regression analyses for ACL-injured and noninjured limbs. There were significant differences in fCSA between ACL-injured and noninjured limbs across all fiber types, but no differences in FT%. Type 1 fCSA, type 2a fCSA, and their interaction effect were the explanatory variables with the strongest relationship to all performance outcomes for the ACL-injured limb. The explanatory variables in the ACL-injured limb had a significant relationship to QPT and late RTD, but not early RTD. These findings suggest that QPT and late RTD are more heavily influenced by fCSA than FT% in ACL-injured limbs. This work serves as a foundation for the development of more specific rehabilitation strategies aimed at improving quadriceps muscle function before ACL reconstruction or for individuals electing nonsurgical management.

Dose-Response Relationship Between Exercise Duration and Enhanced Function and Cognition in Acutely Hospitalized Older Adults: A Secondary Analysis of a Randomized Clinical Trial

Background and Objectives

Exercise may reverse functional decline in hospitalized older adults, but the optimal duration is unclear. This study examined the potential relationship between in-hospital multicomponent exercise program duration and changes in physical function, cognition, and muscle function to maximize exercise-related health benefits in acutely hospitalized older patients.

Research Design and Methods

This secondary analysis of a multicenter randomized controlled trial examined the relationship between the duration of an in-hospital multicomponent exercise program and changes in physical function, cognition, and muscle strength in 570 acutely hospitalized older adults. Participants completed 3, 4, or 5-7 consecutive days of exercise based on the progression of their acute medical illness. The acute clinical condition of the older patients was similar across the study groups (i.e., 3/4/5-7 days) at admission. Outcomes included the Short Physical Performance Battery (SPPB) for functional capacity, Gait Velocity Test for gait speed, handgrip for muscle strength, and cognitive tests.

Results

Of the 570 patients included in the analysis, 298 were women (52.3%), and the mean (SD) age was 87.3 (4.8) years. Exercise groups increased SPPB scores compared with controls, with gains of 1.09 points after three days, 1.97 points after four days, and 2.02 points after 5-7 days (p < .001). The 4-day program showed the most significant benefit for functional capacity. Gait velocity increased by 0.11 m/s after 4 and 5-7 days (p = .032). Similar dose-response relationships were seen for handgrip strength and cognition, with 5-7 days showing more significant gains than three days (p < .05).

Discussion and Implications

Multicomponent exercise programs enhance physical and cognitive function in hospitalized older adults, regardless of exercise dosage. A 4-day program significantly boosts functional capacity, although 5-7 days improves handgrip strength and cognition, highlighting the importance of exercise dosage in countering functional decline. Implementing evidence-based inpatient exercise prescriptions can help reverse muscle weakness and improve cognitive and physical function.Clinical Trial Registration: NCT04600453.

The role of muscle disuse in muscular and cardiovascular fitness: A systematic review and meta-regression

We aimed to assess the effects of muscle disuse on muscle strength (MS), muscle mass (MM) and cardiovascular fitness. Databases were scrutinized to identify human studies assessing the effects of muscle disuse on both (1) MM and (2) maximal oxygen uptake (VO2max) and/or MS. Random-effects meta-analysis and meta-regression with initial physical fitness and length of the protocol as a priori determined moderators were performed. We quantitatively analyzed 51 different studies, and the level of significance was set at p < 0.05. Data from the participants in 14 studies showed a decline in both VO2max (SMD: -0.93; 95% CI: -1.27 to -0.58) and MM (SMD: -0.34; 95% CI: -0.57 to -0.10). Data from 47 studies showed a decline in strength (-0.88; 95% CI: -1.04 to -0.73) and mass (SMD: -0.47; 95% CI: -0.58 to -0.36). MS loss was twice as high as MM loss, but differences existed between anatomical regions. Notably, meta-regression analysis revealed that initial MS was inversely associated with MS decline. VO2max and MS decline to a higher extent than MM during muscle disuse. We reported a more profound strength loss in subjects with high muscular strength. This is physiologically relevant for athletes because their required muscular strength can profoundly decline during a period of muscle disuse. It should however be noted that a period of muscle disuse can have devastating consequences in old subjects with low muscular strength.

Control of muscle satellite cell function by specific exercise-induced cytokines and their applications in muscle maintenance

Exercise is recognized to play an observable role in improving human health, especially in promoting muscle hypertrophy and intervening in muscle mass loss-related diseases, including sarcopenia. Recent rapid advances have demonstrated that exercise induces the release of abundant cytokines from several tissues (e.g., liver, muscle, and adipose tissue), and multiple cytokines improve the functions or expand the numbers of adult stem cells, providing candidate cytokines for alleviating a wide range of diseases. Muscle satellite cells (SCs) are a population of muscle stem cells that are mitotically quiescent but exit from the dormancy state to become activated in response to physical stimuli, after which SCs undergo asymmetric divisions to generate new SCs (stem cell pool maintenance) and commit to later differentiation into myocytes (skeletal muscle replenishment). SCs are essential for the postnatal growth, maintenance, and regeneration of skeletal muscle. Emerging evidence reveals that exercise regulates muscle function largely via the exercise-induced cytokines that govern SC potential, but this phenomenon is complicated and confusing. This review provides a comprehensive integrative overview of the identified exercise-induced cytokines and the roles of these cytokines in SC function, providing a more complete picture regarding the mechanism of SC homeostasis and rejuvenation therapies for skeletal muscle.

Sex Differences in Quadriceps Atrophy After Anterior Cruciate Ligament Tear

BACKGROUND

Female athletes lag behind their male counterparts in recovery from anterior cruciate ligament (ACL) injury. Quadriceps muscle size and strength are crucial factors for regaining function after ACL injury, but little is known about how these metrics vary due to biological sex.

HYPOTHESIS

Female patients have reduced vastus lateralis fiber cross-sectional area (CSA) and lower quadriceps strength after ACL injury than male patients.

STUDY DESIGN

Cross-sectional study.

LEVEL OF EVIDENCE

Level 4.

METHODS

A total of 60 participants with recent ACL tear were evaluated for vastus lateralis muscle fiber CSA, isometric quadriceps peak torque, and quadriceps rate of torque development. Linear mixed models were fit to determine differences across sex and limb for each variable of interest.

RESULTS

The female group averaged almost 20% atrophy between limbs (P < 0.01), while the male group averaged just under 4% (P = 0.05). Strength deficits between limbs were comparable between female and male groups.

CONCLUSION

Immediately after ACL injury, female patients have greater between-limb differences in muscle fiber CSA but between-limb strength deficits comparable with those of male patients.

CLINICAL RELEVANCE

These results indicate that the underpinnings of strength loss differ based on biological sex, and thus individual patients could benefit from a sex-specific treatment approach to ACL injury.

Cosmic chronometers: Is spaceflight a catalyst for biological ageing?

Astronauts returning from space missions often exhibit health issues mirroring age-related conditions, suggesting spaceflight as a potential driver of biological ageing and age-related diseases. To unravel the underlying mechanisms of these conditions, this comprehensive review explores the impact of the space "exposome" on the twelve hallmarks of ageing. Through a meticulous analysis encompassing both space environments and terrestrial analogs, we aim to decipher how different conditions influence ageing hallmarks. Utilizing PubMed, we identified 189 studies and 60 meet screening criteria. Research on biological ageing in space has focused on genomic instability, chronic inflammation, and deregulated nutrient sensing. Spaceflight consistently induces genomic instability, linked to prolonged exposure to ionizing radiation, triggers pro-inflammatory and immune alterations, resembling conditions in isolated simulations. Nutrient sensing pathways reveal increased systemic insulin-like growth-factor-1. Microbiome studies indicate imbalances favoring opportunistic species during spaceflight. Telomere dynamics present intriguing patterns, with lengthening during missions and rapid shortening upon return. Despite a pro-ageing trend, some protective mechanisms emerge. Countermeasures, encompassing dietary adjustments, prebiotics, postbiotics, symbiotics, tailored exercises, meditation, and anti-inflammatory supplements, exhibit potential. Spaceflight's impact on ageing is intricate, with diverse findings challenging established beliefs. Multidisciplinary studies provide guidance for future research in this field.

Differences in Functional Capacity between Oncologic and Non-Oncologic Populations: Reference Values

This study is focused on the fact that in the context of increasing global aging and cancer diagnoses, additional challenges arise in clinical care. Adequate functionality and body composition are key to coping with antineoplastic treatment, which can lead to better treatment tolerance, survival, and quality of life. This is a cross-sectional comparative study focused on the assessment and comparison of body composition and functionality between cancer patients and a reference population, with the aim of establishing meaningful baseline values. Techniques such as manual dynamometry, the Five-Times Sit-to-Stand test, and bioimpedance were used to collect data from 374 oncologic patients and 1244 reference individuals. The results reveal significant disparities in functionality and body composition among participants, and provide age group-specific adjusted baseline values for those diagnosed with cancer. These findings may have crucial clinical implications for applying particular cut-off points designed for this population group, which makes the assessment process faster and more accurate, enhances the capacity of medical personnel to act quickly, and improves the management of frailty in cancer patients.

The Structural Adaptations That Mediate Disuse-Induced Atrophy of Skeletal Muscle

The maintenance of skeletal muscle mass plays a fundamental role in health and issues associated with quality of life. Mechanical signals are one of the most potent regulators of muscle mass, with a decrease in mechanical loading leading to a decrease in muscle mass. This concept has been supported by a plethora of human- and animal-based studies over the past 100 years and has resulted in the commonly used term of 'disuse atrophy'. These same studies have also provided a great deal of insight into the structural adaptations that mediate disuse-induced atrophy. For instance, disuse results in radial atrophy of fascicles, and this is driven, at least in part, by radial atrophy of the muscle fibers. However, the ultrastructural adaptations that mediate these changes remain far from defined. Indeed, even the most basic questions, such as whether the radial atrophy of muscle fibers is driven by the radial atrophy of myofibrils and/or myofibril hypoplasia, have yet to be answered. In this review, we thoroughly summarize what is known about the macroscopic, microscopic, and ultrastructural adaptations that mediated disuse-induced atrophy and highlight some of the major gaps in knowledge that need to be filled.

Long-Term Declines in Physical Fitness and Physical Activity for Individuals With Post-Liver Transplantation Compared to Healthy Controls

Functional changes are essential determinants of mortality and morbidity in individuals with chronic liver disease. However, there is limited information about whether these changes persist long-term after liver transplantation (LT). We aimed to compare physical fitness, physical activity, balance, kinesiophobia, and fatigue between patients with LT and healthy controls. All participants underwent evaluation with the Senior Fitness Test (SFT) for exercise capacity and physical fitness, the International Physical Activity Questionnaire (IPAQ) for physical activity, the Timed Up-and-Go Test (TUG) and the Berg Balance Scale (BBS) for balance, the Tampa Scale for Kinesiophobia (TSK) for kinesiophobia, and the Fatigue Severity Scale (FSS) and Fatigue Impact Scale (FIS) for fatigue. We studied 16 persons with LT (M age = 40.56, SD = 15.73 years; M time since LT = 66.81, SD = 72.05 months) and 16 control participants (M age = 39.87, SD = 13.98 years). Compared to controls, participants with LT showed significantly poorer performance on the SFT components assessing upper and lower body strength, aerobic endurance, agility, and dynamic balance (p < .001 for all), significantly lower IPAQ physical activity scores (p = .002) and BBS score (p = .017), and significantly higher TUG time (p < .001) and TSK, FSS, and FIS scores (p = .001, p = .001, and p = .004, respectively). Individuals with post-LT had lower exercise capacity, physical fitness, balance, and physical activity, and higher kinesiophobia and fatigue levels in the long-term compared to their peers. Future studies should focus on frailty in individuals in the long term after LT.

Effects of short-term multicomponent exercise intervention on muscle power in hospitalized older patients: A secondary analysis of a randomized clinical trial

BACKGROUND

Bed rest during hospitalization can negatively impact functional independence and clinical status of older individuals. Strategies focused on maintaining and improving muscle function may help reverse these losses. This study investigated the effects of a short-term multicomponent exercise intervention on maximal strength and muscle power in hospitalized older patients.

METHODS

This secondary analysis of a randomized clinical trial was conducted in an acute care unit in a tertiary public hospital. Ninety (39 women) older patients (mean age 87.7 ± 4.8 years) undergoing acute-care hospitalization [median (IQR) duration 8 (1.75) and 8 (3) days for intervention and control groups, respectively]) were randomly assigned to an exercise intervention group (n = 44) or a control group (n = 46). The control group received standard care hospital including physical rehabilitation as needed. The multicomponent exercise intervention was performed for 3 consecutive days during the hospitalization, consisting of individualized power training, balance, and walking exercises. Outcomes assessed at baseline and discharge were maximal strength through 1 repetition maximum test (1RM) in the leg press and bench press exercises, and muscle power output at different loads (≤30% of 1RM and between 45% and 55% of 1RM) in the leg press exercise. Mean peak power during 10 repetitions was assessed at loads between 45% and 55% of 1RM.

RESULTS

At discharge, intervention group increased 19.2 kg (Mean Δ% = 40.4%) in leg press 1RM [95% confidence interval (CI): 12.1, 26.2 kg; P < 0.001] and 2.9 kg (Mean Δ% = 19.7%) in bench press 1RM (95% CI: 0.6, 5.2 kg; P < 0.001). The intervention group also increased peak power by 18.8 W (Mean Δ% = 69.2%) (95% CI: 8.4, 29.1 W; P < 0.001) and mean propulsive power by 9.3 (Mean Δ% = 26.8%) W (95% CI: 2.5, 16.1 W; P = 0.002) at loads ≤30% of 1RM. The intervention group also increased peak power by 39.1 W (Mean Δ% = 60.0%) (95% CI: 19.2, 59.0 W; P < 0.001) and mean propulsive power by 22.9 W (Mean Δ% = 64.1%) (95% CI: 11.7, 34.1 W; P < 0.001) at loads between 45% and 55% of 1RM. Mean peak power during the 10 repetitions improved by 20.8 W (Mean Δ% = 36.4%) (95% CI: 3.0, 38.6 W; P = 0.011). No significant changes were observed in the control group for any endpoint.

CONCLUSIONS

An individualized multicomponent exercise program including progressive power training performed over 3 days markedly improved muscle strength and power in acutely hospitalized older patients.

Exploring cellular senescence in the musculoskeletal system: Any insights for biomarkers discovery?

The locomotor system comprises skeletal muscles and bones with active metabolism and cellular turnover. Chronic locomotor system disorders gradually arising with aging are inversely associated with the correct function of bone and muscles. Senescent cells appear more frequently in advanced ages or pathological conditions, and the accumulation of senescent cells in muscle tissue negatively correlates with muscle regeneration, which is crucial for maintaining strength and preventing frailty. Senescence in the bone microenvironment, osteoblasts, and osteocytes affects bone turnover favoring osteoporosis. It is likely that in response to injury and age-related damage over the lifetime, a subset of niche cells accumulates oxidative stress and DNA damage beyond the threshold that primes the onset of cellular senescence. These senescent cells may acquire resistance to apoptosis that, combined with the weakened immune system, results in impaired clearance of senescent cells and their accumulation. The secretory profile of senescent cells causes local inflammation, further spreading senescence in neighboring niche cells and impairing tissue homeostasis. The resulting impairment of turnover/tissue repair in the musculoskeletal system reduces the efficiency of the organ in response to environmental needs that finally lead to functional decline. Management of the musculoskeletal system at the cellular level can benefit the quality of life and reduce early aging. This work discusses current knowledge of cellular senescence of musculoskeletal tissues to conclude with biologically active biomarkers effective enough to reveal the underlying mechanisms of tissue flaws at the earliest possible.

Human and African ape myosin heavy chain content and the evolution of hominin skeletal muscle

Humans are unique among terrestrial mammals in our manner of walking and running, reflecting 7 to 8 Ma of musculoskeletal evolution since diverging with the genus Pan. One component of this is a shift in our skeletal muscle biology towards a predominance of myosin heavy chain (MyHC) I isoforms (i.e. slow fibers) across our pelvis and lower limbs, which distinguishes us from chimpanzees. Here, new MyHC data from 35 pelvis and hind limb muscles of a Western gorilla (Gorilla gorilla) are presented. These data are combined with a similar chimpanzee dataset to assess the MyHC I content of humans in comparison to African apes (chimpanzees and gorillas) and other terrestrial mammals. The responsiveness of human skeletal muscle to behavioral interventions is also compared to the human-African ape differential. Humans are distinct from African apes and among a small group of terrestrial mammals whose pelvis and hind/lower limb muscle is slow fiber dominant, on average. Behavioral interventions, including immobilization, bed rest, spaceflight and exercise, can induce modest decreases and increases in human MyHC I content (i.e. -9.3% to 2.3%, n = 2033 subjects), but these shifts are much smaller than the mean human-African ape differential (i.e. 31%). Taken together, these results indicate muscle fiber content is likely an evolvable trait under selection in the hominin lineage. As such, we highlight potential targets of selection in the genome (e.g. regions that regulate MyHC content) that may play an important role in hominin skeletal muscle evolution.

Upregulation of Sarcolemmal Hemichannels and Inflammatory Transcripts with Neuromuscular Junction Instability during Lower Limb Unloading in Humans

Human skeletal muscle atrophy and a disproportionate force loss occur within a few days of unloading in space and on Earth, but the underlying mechanisms are not fully understood. Disruption of neuromuscular junction homeostasis has been proposed as one of the possible causes. Here, we investigated the potential mechanisms involved in this neuromuscular disruption induced by a 10-day unilateral lower limb suspension (ULLS) in humans. Specifically, we investigated hemichannels’ upregulation, neuromuscular junction and axonal damage, neurotrophins’ receptor downregulation and inflammatory transcriptional signatures. Biomarkers were evaluated at local and systemic levels. At the sarcolemmal level, changes were found to be associated with an increased expression of connexin 43 and pannexin-1. Upregulation of the inflammatory transcripts revealed by deep transcriptomics was found after 10 days of ULLS. The destabilisation of the neuromuscular junction was not accompanied by changes in the secretion of the brain-derived neurotrophic factor and neurotrophin-4, while their receptor, BDNF/NT growth factors receptor (TrkB), decreased. Furthermore, at 5 days of ULLS, there was already a significant upregulation of the serum neurofilament light chain concentration, an established clinical biomarker of axonal injury. At 10 days of ULLS, other biomarkers of early denervation processes appeared. Hence, short periods of muscle unloading induce sarcolemmal hemichannels upregulation, inflammatory transcripts upregulation, neuromuscular junction instability and axonal damage.

Resident muscle stem cell myogenic characteristics in postnatal muscle growth impairments in children with cerebral palsy

Children with cerebral palsy (CP), a perinatal brain alteration, have impaired postnatal muscle growth, with some muscles developing contractures. Functionally, children are either able to walk or primarily use wheelchairs. Satellite cells are muscle stem cells (MuSCs) required for postnatal development and source of myonuclei. Only MuSC abundance has been previously reported in contractured muscles, with myogenic characteristics assessed only in vitro. We investigated whether MuSC myogenic, myonuclear, and myofiber characteristics in situ differ between contractured and noncontractured muscles, across functional levels, and compared with typically developing (TD) children with musculoskeletal injury. Open muscle biopsies were obtained from 36 children (30 CP, 6 TD) during surgery; contracture correction for adductors or gastrocnemius, or from vastus lateralis [bony surgery in CP, anterior cruciate ligament (ACL) repair in TD]. Muscle cross sections were immunohistochemically labeled for MuSC abundance, activation, proliferation, nuclei, myofiber borders, type-1 fibers, and collagen content in serial sections. Although MuSC abundance was greater in contractured muscles, primarily in type-1 fibers, their myogenic characteristics (activation, proliferation) were lower compared with noncontractured muscles. Overall, MuSC abundance, activation, and proliferation appear to be associated with collagen content. Myonuclear number was similar between all muscles, but only in contractured muscles were there associations between myonuclear number, MuSC abundance, and fiber cross-sectional area. Puzzlingly, MuSC characteristics were similar between ambulatory and nonambulatory children. Noncontractured muscles in children with CP had a lower MuSC abundance compared with TD-ACL injured children, but similar myogenic characteristics. Contractured muscles may have an intrinsic deficiency in developmental progression for postnatal MuSC pool establishment, needed for lifelong efficient growth and repair.

Hearing of Otof-deficient mice restored by trans-splicing of N- and C-terminal otoferlin

Mutations to the OTOF gene are among the most common reasons for auditory neuropathy. Although cochlear implants are often effective in restoring sound transduction, there are currently no biological treatments for individuals with variants of OTOF. Previous studies have reported the rescue of hearing in DFNB9 mice using OTOF gene replacement although the efficacy needs improvement. Here, we developed a novel dual-AAV-mediated gene therapy system based on the principles of protein trans-splicing, and we show that this system can reverse bilateral deafness in Otof ^-/- mice after a single unilateral injection. The system effectively expressed exogenous mouse or human otoferlin after injection on postnatal day 0-2. Human otoferlin restored hearing to near wild-type levels for at least 6 months and restored the release of synaptic vesicles in inner hair cells. Our study not only provides a preferential clinical strategy for the treatment of OTOF-related auditory neuropathies, but also describes a route of development for other large-gene therapies and protein engineering techniques.

Sedentary behavior and the biological hallmarks of aging

While the benefits of physical exercise for a healthy aging are well-recognized, a growing body of evidence shows that sedentary behavior has deleterious health effects independently, to some extent, of physical activity levels. Yet, the increasing prevalence of sedentariness constitutes a major public health issue that contributes to premature aging but the potential cellular mechanisms through which prolonged immobilization may accelerate biological aging remain unestablished. This narrative review summarizes the impact of sedentary behavior using different models of extreme sedentary behaviors including bedrest, unilateral limb suspension and space travel studies, on the hallmarks of aging such as genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. We further highlight the remaining knowledge gaps that need more research in order to promote healthspan extension and to provide future contributions to the field of geroscience.

Impact of 10-day bed rest on serum levels of irisin and markers of musculoskeletal metabolism

The bed rest (BR) is a ground-based model to simulate microgravity mimicking skeletal-muscle alterations as in spaceflight. Molecular coupling between bone and muscle might be involved in physiological and pathological conditions. Thus, the new myokine irisin and bone-muscle turnover markers have been studied during and after 10 days of BR. Ten young male individuals were subjected to 10 days of horizontal BR. Serum concentrations of irisin, myostatin, sclerostin, and haptoglobin were assessed, and muscle tissue gene expression on vastus lateralis biopsies was determined. During 10-days BR, we observed no significant fluctuation levels of irisin, myostatin, and sclerostin. Two days after BR (R+2), irisin serum levels significantly decreased while myostatin, sclerostin, and haptoglobin were significantly increased compared with BR0. Gene expression of myokines, inflammatory molecules, transcription factors, and markers of muscle atrophy and senescence on muscle biopsies were not altered, suggesting that muscle metabolism of young, healthy subjects is able to adapt to the hypomobility condition during 10-day BR. However, when subjects were divided according to irisin serum levels at BR9, muscle ring finger-1 mRNA expression was significantly lower in subjects with higher irisin serum levels, suggesting that this myokine may prevent the triggering of muscle atrophy. Moreover, the negative correlation between p21 mRNA and irisin at BR9 indicated a possible inhibitory effect of the myokine on the senescence marker. In conclusion, irisin could be a prognostic marker of hypomobility-induced muscle atrophy, and its serum levels could protect against muscle deterioration by preventing and/or delaying the expression of atrophy and senescence cellular markers.

Loss of neuromuscular junction integrity and muscle atrophy in skeletal muscle disuse

Physical inactivity (PI) is a major risk factor of chronic diseases. A major aspect of PI is loss of muscle mass and strength. The latter phenomenon significantly impacts daily life and represent a major issue for global health. Understandably, skeletal muscle itself has been the major focus of studies aimed at understanding the mechanisms underlying loss of mass and strength. Relatively lesser attention has been given to the contribution of alterations in somatomotor control, despite the fact that these changes can start very early and can occur at multiple levels, from the cortex down to the neuromuscular junction (NMJ). It is well known that exposure to chronic inactivity or immobilization causes a disproportionate loss of force compared to muscle mass, i.e. a loss of specific or intrinsic whole muscle force. The latter phenomenon may be partially explained by the loss of specific force of individual muscle fibres, but several other players are very likely to contribute to such detrimental phenomenon. Irrespective of the length of the disuse period, the loss of force is, in fact, more than two-fold greater than that of muscle size. It is very likely that somatomotor alterations may contribute to this loss in intrinsic muscle force. Here we review evidence that alterations of one component of somatomotor control, namely the neuromuscular junction, occur in disuse. We also discuss some of the novel players in NMJ stability (e.g., homer, bassoon, pannexin) and the importance of new established and emerging molecular markers of neurodegenerative processes in humans such as agrin, neural-cell adhesion molecule and light-chain neurofilaments.

Acute effect of electrical stimulation on muscle protein synthesis and break-down in the soleus muscle of hindlimb unloaded rats

Electrical stimulation (ES) is effective for disuse-induced muscle atrophy. However, the acute effect of ES on muscle protein synthesis (MPS) and muscle protein breakdown (MPB) remains unclear. We investigated the effect of a single-session ES treatment on mTORC1 signaling, MPS, and MPB in the soleus muscle of 2-week hindlimb unloaded rats. Sprague Dawley rats (n = 12 male) were randomly divided into control (CON) and hindlimb unloaded (HU) groups. After 2 weeks, the right soleus muscle was percutaneously stimulated and underwent supramaximal isometric contractions. The left soleus muscle served as an internal control. We collected soleus muscle samples 6 h after ES. Two weeks of HU decreased p70S6K and S6rp activation, downstream factors for mTORC1 signaling, and SUnSET method-assessed MPS, but increased the LC3-II/I ratio, an indicator of autophagy. ES on disused muscle successfully activated mTORC1 signaling but did not affect MPS. Contrary, ES decreased ubiquitinated proteins expression and LC3B-II/I ratio. HU might affect mTORC1 activation and MPS differently in response to acute ES possibly due to excessive ROS production caused by ES. Our findings suggest that ES applied to disused skeletal muscles may suppress MPB, but its effect on MPS appears to be attenuated.

Murine Models of Tenotomy-Induced Mechanical Overloading and Tail-Suspension-Induced Mechanical Unloading

Skeletal muscle is a highly plastic tissue that can alter its mass and strength in response to mechanical stimulation, such as overloading and unloading, which lead to muscle hypertrophy and atrophy, respectively. Mechanical loading in the muscle influences muscle stem cell dynamics, including activation, proliferation, and differentiation. Although experimental models of mechanical overloading and unloading have been widely used for the investigation of the molecular mechanisms regulating muscle plasticity and stem cell function, few studies have described the methods in detail. Here, we describe the appropriate procedures for tenotomy-induced mechanical overloading and tail-suspension-induced mechanical unloading, which are the most common and simple methods to induce muscle hypertrophy and atrophy in mouse models.

Capillary rarefaction during bed rest is proportionally less than fibre atrophy and loss of oxidative capacity

BACKGROUND

Muscle disuse from bed rest or spaceflight results in losses in muscle mass, strength and oxidative capacity. Capillary rarefaction may contribute to muscle atrophy and the reduction in oxidative capacity during bed rest. Artificial gravity may attenuate the negative effects of long-term space missions or bed rest. The aim of the present study was to assess (1) the effects of bed rest on muscle fibre size, fibre type composition, capillarization and oxidative capacity in the vastus lateralis and soleus muscles after 6 and 55 days of bed rest and (2) the effectiveness of artificial gravity in mitigating bed-rest-induced detriments to these parameters.

METHODS

Nineteen participants were assigned to a control group (control, n = 6) or an intervention group undergoing 30 min of centrifugation (n = 13). All underwent 55 days of head-down tilt bed rest. Vastus lateralis and soleus biopsies were taken at baseline and after 6 and 55 days of bed rest. Fibre type composition, fibre cross-sectional area, capillarization indices and oxidative capacity were determined.

RESULTS

After just 6 days of bed rest, fibre atrophy (-23.2 ± 12.4%, P < 0.001) and reductions in capillary-to-fibre ratio (C:F; 1.97 ± 0.57 vs. 1.56 ± 0.41, P < 0.001) were proportional in both muscles as reflected by a maintained capillary density. Fibre atrophy proceeded at a much slower rate between 6 and 55 days of bed rest (-11.6 ± 12.1% of 6 days, P = 0.032) and was accompanied by a 19.1% reduction in succinate dehydrogenase stain optical density (P < 0.001), without any further significant decrements in C:F (1.56 ± 0.41 vs. 1.49 ± 0.37, P = 0.459). Consequently, after 55 days of bed rest, the capillary supply-oxidative capacity ratio of a fibre had increased by 41.9% (P < 0.001), indicating a capillarization in relative excess of oxidative capacity. Even though the heterogeneity of capillary spacing (Log_R SD) was increased after 55 days by 12.7% (P = 0.004), tissue oxygenation at maximal oxygen consumption of the fibres was improved after 55 days bed rest. Daily centrifugation failed to blunt the bed-rest-induced reductions in fibre size and oxidative capacity and capillary rarefaction.

CONCLUSIONS

The relationship between fibre size and oxidative capacity with the capillary supply of a fibre is uncoupled during prolonged bed rest as reflected by a rapid loss of muscle mass and capillaries, followed at later stages by a more than proportional loss of mitochondria without further capillary loss. The resulting excessive capillary supply of the muscle after prolonged bed rest is advantageous for the delivery of substrates needed for subsequent muscle recovery.

Lactate Activates AMPK Remodeling of the Cellular Metabolic Profile and Promotes the Proliferation and Differentiation of C2C12 Myoblasts

Lactate is a general compound fuel serving as the fulcrum of metabolism, which is produced from glycolysis and shuttles between different cells, tissues and organs. Lactate is usually accumulated abundantly in muscles during exercise. It remains unclear whether lactate plays an important role in the metabolism of muscle cells. In this research, we assessed the effects of lactate on myoblasts and clarified the underlying metabolic mechanisms through NMR-based metabonomic profiling. Lactate treatment promoted the proliferation and differentiation of myoblasts, as indicated by significantly enhanced expression levels of the proteins related to cellular proliferation and differentiation, including p-AKT, p-ERK, MyoD and myogenin. Moreover, lactate treatment profoundly regulated metabolisms in myoblasts by promoting the intake and intracellular utilization of lactate, activating the TCA cycle, and thereby increasing energy production. For the first time, we found that lactate treatment evidently promotes AMPK signaling as reflected by the elevated expression levels of p-AMPK and p-ACC. Our results showed that lactate as a metabolic regulator activates AMPK, remodeling the cellular metabolic profile, and thereby promoting the proliferation and differentiation of myoblasts. This study elucidates molecular mechanisms underlying the effects of lactate on skeletal muscle in vitro and may be of benefit to the exploration of lactate acting as a metabolic regulator.

Muscle dysfunction in the long coronavirus disease 2019 syndrome: Pathogenesis and clinical approach

In long coronavirus disease 2019 (long COVID-19), involvement of the musculoskeletal system is characterised by the persistence or appearance of symptoms such as fatigue, muscle weakness, myalgia, and decline in physical and functional performance, even at 4 weeks after the onset of acute symptoms of COVID-19. Muscle injury biomarkers are altered during the acute phase of the disease. The cellular damage and hyperinflammatory state induced by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection may contribute to the persistence of symptoms, hypoxaemia, mitochondrial damage, and dysregulation of the renin-angiotensin system. In addition, the occurrence of cerebrovascular diseases, involvement of the peripheral nervous system, and harmful effects of hospitalisation, such as the use of drugs, immobility, and weakness acquired in the intensive care unit, all aggravate muscle damage. Here, we review the multifactorial mechanisms of muscle tissue injury, aggravating conditions, and associated sequelae in long COVID-19.

Motor unit dysregulation following 15 days of unilateral lower limb immobilisation

Disuse atrophy, caused by situations of unloading such as limb immobilisation, causes a rapid yet diverging reduction in skeletal muscle function when compared to muscle mass. While mechanistic insight into the loss of mass is well studied, deterioration of muscle function with a focus towards the neural input to muscle remains underexplored. This study aimed to determine the role of motor unit adaptation in disuse-induced neuromuscular deficits. Ten young, healthy male volunteers underwent 15 days of unilateral lower limb immobilisation with intramuscular electromyography (iEMG) bilaterally recorded from the vastus lateralis (VL) during knee extensor contractions normalised to maximal voluntary contraction (MVC), pre and post disuse. Muscle cross-sectional area was determined by ultrasound. Individual MUs were sampled and analysed for changes in motor unit (MU) discharge and MU potential (MUP) characteristics. VL CSA was reduced by approximately 15% which was exceeded by a two-fold decrease of 31% in muscle strength in the immobilised limb, with no change in either parameter in the non-immobilised limb. Parameters of MUP size were reduced by 11% to 24% with immobilisation, while neuromuscular junction (NMJ) transmission instability remained unchanged, and MU firing rate decreased by 8% to 11% at several contraction levels. All adaptations were observed in the immobilised limb only. These findings highlight impaired neural input following immobilisation reflected by suppressed MU firing rate which may underpin the disproportionate reductions of strength relative to muscle size. KEY POINTS: Muscle mass and function decline rapidly in situations of disuse such as bed rest and limb immobilisation. The reduction in muscle function commonly exceeds that of muscle mass, which may be associated with the dysregulation of neural input to muscle. We have used intramuscular electromyography to sample individual motor unit and near fibre potentials from the vastus lateralis following 15 days of unilateral limb immobilisation. Following disuse, the disproportionate loss of muscle strength when compared to size coincided with suppressed motor unit firing rate. These motor unit adaptations were observed at multiple contraction levels and in the immobilised limb only. Our findings demonstrate neural dysregulation as a key component of functional loss following muscle disuse in humans.

Effects of short-term unloading and active recovery on human motor unit properties, neuromuscular junction transmission and transcriptomic profile

Electrophysiological alterations of the neuromuscular junction (NMJ) and motor unit potential (MUP) with unloading are poorly studied. We aimed to investigate these aspects and the underlying molecular mechanisms with short-term unloading and active recovery (AR). Eleven healthy males underwent a 10-day unilateral lower limb suspension (ULLS) period, followed by 21-day AR based on resistance exercise. Quadriceps femoris (QF) cross-sectional area (CSA) and isometric maximum voluntary contraction (MVC) were evaluated. Intramuscular electromyographic recordings were obtained during 10% and 25% MVC isometric contractions from the vastus lateralis (VL). Biomarkers of NMJ molecular instability (serum c-terminal agrin fragment, CAF), axonal damage (neurofilament light chain) and denervation status were assessed from blood samples and VL biopsies. NMJ and ion channel transcriptomic profiles were investigated by RNA-sequencing. QF CSA and MVC decreased with ULLS. Increased CAF and altered NMJ transcriptome with unloading suggested the emergence of NMJ molecular instability, which was not associated with impaired NMJ transmission stability. Instead, increased MUP complexity and decreased motor unit firing rates were found after ULLS. Downregulation of ion channel gene expression was found together with increased neurofilament light chain concentration and partial denervation. The AR period restored most of these neuromuscular alterations. In conclusion, the human NMJ is destabilized at the molecular level but shows functional resilience to a 10-day unloading period at least at relatively low contraction intensities. However, MUP properties are altered by ULLS, possibly due to alterations in ion channel dynamics and initial axonal damage and denervation. These changes are fully reversed by 21 days of AR. KEY POINTS: We used integrative electrophysiological and molecular approaches to comprehensively investigate changes in neuromuscular integrity and function after a 10-day unilateral lower limb suspension (ULLS), followed by 21 days of active recovery in young healthy men, with a particular focus on neuromuscular junction (NMJ) and motor unit potential (MUP) properties alterations. After 10-day ULLS, we found significant NMJ molecular alterations in the absence of NMJ transmission stability impairment. These findings suggest that the human NMJ is functionally resilient against insults and stresses induced by short-term disuse at least at relatively low contraction intensities, at which low-threshold, slow-type motor units are recruited. Intramuscular electromyography analysis revealed that unloading caused increased MUP complexity and decreased motor unit firing rates, and these alterations could be related to the observed changes in skeletal muscle ion channel pool and initial and partial signs of fibre denervation and axonal damage. The active recovery period restored these neuromuscular changes.

Effects of Exercise Training Intensity and Duration on Skeletal Muscle Capillarization in Healthy Subjects: A Meta-analysis

PURPOSE

This study aimed to investigate the effect of intensity and duration of continuous and interval exercise training on capillarization in skeletal muscle of healthy adults.

METHODS

PubMed and Web of Science were searched from inception to June 2021. Eligibility criteria for studies were endurance exercise training >2 wk in healthy adults, and the capillary to fiber ratio (C:F) and/or capillary density (CD) reported. Meta-analyses were performed, and subsequent subgroup analyses were conducted by the characteristics of participants and training scheme.

RESULTS

Fifty-seven trials from 38 studies were included (10%/90%, athletic/sedentary). C:F was measured in 391 subjects from 47 trials, whereas CD was measured in 428 subjects from 50 trials. Exercise training increased C:F (mean difference, 0.33 (95% confidence interval, 0.30-0.37)) with low heterogeneity ( I2 = 45.08%) and CD (mean difference, 49.8 (36.9-62.6) capillaries per millimeter squared) with moderate heterogeneity ( I2 = 68.82%). Compared with low-intensity training (<50% of maximal oxygen consumption (V̇O 2max )), 21% higher relative change in C:F was observed after continuous moderate-intensity training (50%-80% of V̇O 2max ) and 54% higher change after interval training with high intensity (80%-100% of V̇O 2max ) in sedentary subjects. The magnitude of capillary growth was not dependent on training intervention duration. In already trained subjects, no additional increase in capillarization was observed with various types of training.

CONCLUSIONS

In sedentary subjects, continuous moderate-intensity training and interval training with high intensity lead to increases in capillarization, whereas low-intensity training has less effect. Within the time frame studied, no effect on capillarization was established regarding training duration in sedentary subjects. The meta-analysis highlights the need for further studies in athlete groups to discern if increased capillarization can be obtained, and if so, which combination is optimal (time vs intensity).

Myonuclear permanence in skeletal muscle memory: a systematic review and meta-analysis of human and animal studies

One aspect of skeletal muscle memory is the ability of a previously trained muscle to hypertrophy more rapidly following a period of detraining. Although the molecular basis of muscle memory remains to be fully elucidated, one potential mechanism thought to mediate muscle memory is the permanent retention of myonuclei acquired during the initial phase of hypertrophic growth. However, myonuclear permanence is debated and would benefit from a meta-analysis to clarify the current state of the field for this important aspect of skeletal muscle plasticity. The objective of this study was to perform a meta-analysis to assess the permanence of myonuclei associated with changes in physical activity and ageing. When available, the abundance of satellite cells (SCs) was also considered given their potential influence on changes in myonuclear abundance. One hundred forty-seven peer-reviewed articles were identified for inclusion across five separate meta-analyses; (1-2) human and rodent studies assessed muscle response to hypertrophy; (3-4) human and rodent studies assessed muscle response to atrophy; and (5) human studies assessed muscle response with ageing. Skeletal muscle hypertrophy was associated with higher myonuclear content that was retained in rodents, but not humans, with atrophy (SMD = -0.60, 95% CI -1.71 to 0.51, P = 0.29, and MD = 83.46, 95% CI -649.41 to 816.32, P = 0.82; respectively). Myonuclear and SC content were both lower following atrophy in humans (MD = -11, 95% CI -0.19 to -0.03, P = 0.005, and SMD = -0.49, 95% CI -0.77 to -0.22, P = 0.0005; respectively), although the response in rodents was affected by the type of muscle under consideration and the mode of atrophy. Whereas rodent myonuclei were found to be more permanent regardless of the mode of atrophy, atrophy of ≥30% was associated with a reduction in myonuclear content (SMD = -1.02, 95% CI -1.53 to -0.51, P = 0.0001). In humans, sarcopenia was accompanied by a lower myonuclear and SC content (MD = 0.47, 95% CI 0.09 to 0.85, P = 0.02, and SMD = 0.78, 95% CI 0.37-1.19, P = 0.0002; respectively). The major finding from the present meta-analysis is that myonuclei are not permanent but are lost during periods of atrophy and with ageing. These findings do not support the concept of skeletal muscle memory based on the permanence of myonuclei and suggest other mechanisms, such as epigenetics, may have a more important role in mediating this aspect of skeletal muscle plasticity.

Implementation of exercise countermeasures during spaceflight and microgravity analogue studies: Developing countermeasure protocols for bedrest in older adults (BROA)

Significant progress has been made in the development of countermeasures to attenuate the negative consequences of prolonged exposure to microgravity on astronauts’ bodies. Deconditioning of several organ systems during flight includes losses to cardiorespiratory fitness, muscle mass, bone density and strength. Similar deconditioning also occurs during prolonged bedrest; any protracted time immobile or inactive, especially for unwell older adults (e.g., confined to hospital beds), can lead to similar detrimental health consequences. Due to limitations in physiological research in space, the six-degree head-down tilt bedrest protocol was developed as ground-based analogue to spaceflight. A variety of exercise countermeasures have been tested as interventions to limit detrimental changes and physiological deconditioning of the musculoskeletal and cardiovascular systems. The Canadian Institutes of Health Research and the Canadian Space Agency recently provided funding for research focused on Understanding the Health Impact of Inactivity to study the efficacy of exercise countermeasures in a 14-day randomized clinical trial of six-degree head-down tilt bedrest study in older adults aged 55–65 years old (BROA). Here we will describe the development of a multi-modality countermeasure protocol for the BROA campaign that includes upper- and lower-body resistance exercise and head-down tilt cycle ergometry (high-intensity interval and continuous aerobic exercise training). We provide reasoning for the choice of these modalities following review of the latest available information on exercise as a countermeasure for inactivity and spaceflight-related deconditioning. In summary, this paper sets out to review up-to-date exercise countermeasure research from spaceflight and head-down bedrest studies, whilst providing support for the proposed research countermeasure protocols developed for the bedrest study in older adults.

The Contribution of Genetics to Muscle Disuse, Retraining, and Aging

Genetic background may partly explain differences in muscle responses to internal or external stimuli. Muscle disuse involves various degrees of skeletal muscle atrophy due to inactivity and mechanical unloading. Whether and to which extent genetic background impacts disuse atrophy and retraining in individuals of different ages are currently unclear. Here, we provide a brief overview of relevant literature on the contribution of genetics to muscle disuse, retraining, and aging, and offer a perspective on unanswered questions on the subject that may open new venues for research.

No effect of five days of bed rest or short-term resistance exercise prehabilitation on markers of skeletal muscle mitochondrial content and dynamics in older adults

Bed rest (BR) results in significant impairments in skeletal muscle metabolism. Mitochondrial metabolism is reportedly highly sensitive to disuse, with dysregulated fission-fusion events and impaired oxidative function previously reported. The effects of clinically relevant short-term BR (≤5 days) on mitochondrial protein expression are presently unclear, as are the effects of exercise prehabilitation as a potential counteractive intervention. The present study examined the effects of a 5-day period of BR and short-term resistance exercise prehabilitation (ST-REP) on mitochondrial-protein content. Ten older men (71 ± 4 years) underwent 5 days of BR, completing four sessions of high-volume unilateral resistance exercise prehabilitation over 7 days beforehand. Muscle biopsies were obtained from the vastus lateralis in the non-exercised control and exercised legs, both pre- and post-prehabilitation and pre- and post-BR, to determine changes in citrate synthase enzyme activity and the expression of key proteins in the mitochondrial electron transport chain and molecular regulators of fission-fusion dynamics, biosynthesis, and mitophagy. We observed no significant effect of either BR or ST-REP on citrate synthase protein content, enzyme activity, or ETC complex I-V protein content. Moreover, we observed no significant changes in markers of mitochondrial fission and fusion (p-DRP1S616 , p-DRP1S637 , p-DRP1S616/S637 ratio, p-MFFS146 , Mitofillin, OPA1, or MFN2 (p > 0.05 for all). Finally, we observed no differences in markers of biosynthesis (p-AMPKT172 , p-ACCS79 , PGC1a, TFAM) or mitophagy-related signaling (ULK-1, BNIP3/NIX, LC3B I/II) (p > 0.05 for all). In contrast to previous longer-term periods of musculoskeletal disuse (i.e., 7-14 days), a clinically relevant, 5-day period of BR resulted in no significant perturbation in muscle mitochondrial protein signaling in healthy older adults, with no effect of ST-REP in the week prior to BR. Accordingly, disuse-induced muscle atrophy may precede alterations in mitochondrial content.

The State of the Art of Piezo1 Channels in Skeletal Muscle Regeneration

Piezo1 channels are highly mechanically-activated cation channels that can sense and transduce the mechanical stimuli into physiological signals in different tissues including skeletal muscle. In this focused review, we summarize the emerging evidence of Piezo1 channel-mediated effects in the physiology of skeletal muscle, with a particular focus on the role of Piezo1 in controlling myogenic precursor activity and skeletal muscle regeneration and vascularization. The disclosed effects reported by pharmacological activation of Piezo1 channels with the selective agonist Yoda1 indicate a potential impact of Piezo1 channel activity in skeletal muscle regeneration, which is disrupted in various muscular pathological states. All findings reported so far agree with the idea that Piezo1 channels represent a novel, powerful molecular target to develop new therapeutic strategies for preventing or ameliorating skeletal muscle disorders characterized by an impairment of tissue regenerative potential.

The Pathological Links between Adiposity and the Carpal Tunnel Syndrome

An association between obesity and carpal tunnel syndrome is found in many epidemiological studies. Therefore, there is a need to evaluate the physiopathological links that could explain the association between these two entities. Ectopic adipose tissue is responsible for metabolic syndrome and inflammation, and is a major risk factor for diabetes and cardiovascular diseases. Taking these elements into consideration, we conducted an extensive literature revision of the subject, considering as ectopic fat-related mechanisms the following: (a) the direct compression and the association with the metabolic syndrome of the fat deposition around the wrist, (b) the insulin resistance, dyslipidemia, inflammatory, and oxidative mechanisms related to the central deposition of the fat, (c) the impaired muscle contraction and metabolism related to myosteatosis. Each section presents the cellular pathways which are modified by the ectopic deposition of the adipose tissue and the impact in the pathogeny of the carpal tunnel syndrome. In conclusion, the experimental and clinical data support the epidemiological findings. Efforts to reduce the obesity epidemics will improve not only cardio-metabolic health but will reduce the burden of the disability-free life expectancy due to the carpal tunnel syndrome.

Severe Muscle Deconditioning Triggers Early Extracellular Matrix Remodeling and Resident Stem Cell Differentiation into Adipocytes in Healthy Men

Besides the loss of muscle mass and strength, increased intermuscular adipose tissue (IMAT) is now a well-recognized consequence of muscle deconditioning as experienced in prolonged microgravity. IMAT content may alter the muscle stem cell microenvironment. We hypothesized that extracellular matrix structure alterations and microenvironment remodeling induced by fast and severe muscle disuse could modulate fibro-adipogenic progenitor fate and behavior. We used the dry immersion (DI) model that rapidly leads to severe muscle deconditioning due to drastic hypoactivity. We randomly assigned healthy volunteers (n = 18 men) to the control group (only DI, n = 9; age = 33.8 ± 4) or to the DI + thigh cuff group (n = 9; age = 33.4 ± 7). Participants remained immersed in the supine position in a thermo-neutral water bath for 5 days. We collected vastus lateralis biopsies before (baseline) and after DI. 5 days of DI are sufficient to reduce muscle mass significantly, as indicated by the decreased myofiber cross-sectional area in vastus lateralis samples (−18% vs. baseline, p < 0.05). Early and late adipogenic differentiation transcription factors protein levels were upregulated. Platelet-derived growth Factors alpha (PDGFR⍺) protein level and PDGFR⍺-positive cells were increased after 5 days of DI. Extracellular matrix structure was prone to remodeling with an altered ECM composition with 4 major collagens, fibronectin, and Connective Tissue Growth Factor mRNA decreases (p < 0.001 vs. baseline). Wearing thigh cuffs did not have any preventive effect on the measured variable. Our results show that altered extracellular matrix structure and signaling pathways occur early during DI, a severe muscle wasting model, favoring fibro-adipogenic progenitor differentiation into adipocytes.

Disuse-induced skeletal muscle atrophy in disease and non-disease states in humans: mechanisms, prevention, and recovery strategies

Decreased skeletal muscle contractile activity (disuse) or unloading leads to muscle mass loss, also known as muscle atrophy. The balance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB) is the primary determinant of skeletal muscle mass. A reduced mechanical load on skeletal muscle is one of the main external factors leading to muscle atrophy. However, endocrine and inflammatory factors can act synergistically in catabolic states, amplifying the atrophy process and accelerating its progression. Additionally, older individuals display aging-induced anabolic resistance, which can predispose this population to more pronounced effects when exposed to periods of reduced physical activity or mechanical unloading. Different cellular mechanisms contribute to the regulation of muscle protein balance during skeletal muscle atrophy. This review summarizes the effects of muscle disuse on muscle protein balance and the molecular mechanisms involved in muscle atrophy in the absence or presence of disease. Finally, a discussion of the current literature describing efficient strategies to prevent or improve the recovery from muscle atrophy is also presented.

Cardiovasomobility: an integrative understanding of how disuse impacts cardiovascular and skeletal muscle health

Cardiovasomobility is a novel concept that encompasses the integration of cardiovascular and skeletal muscle function in health and disease with critical modification by physical activity, or lack thereof. Compelling evidence indicates that physical activity improves health while a sedentary, or inactive, lifestyle accelerates cardiovascular and skeletal muscle dysfunction and hastens disease progression. Identifying causative factors for vascular and skeletal muscle dysfunction, especially in humans, has proven difficult due to the limitations associated with cross-sectional investigations. Therefore, experimental models of physical inactivity and disuse, which mimic hospitalization, injury, and illness, provide important insight into the mechanisms and consequences of vascular and skeletal muscle dysfunction. This review provides an overview of the experimental models of disuse and inactivity and focuses on the integrated responses of the vasculature and skeletal muscle in response to disuse/inactivity. The time course and magnitude of dysfunction evoked by various models of disuse/inactivity are discussed in detail, and evidence in support of the critical roles of mitochondrial function and oxidative stress are presented. Lastly, strategies aimed at preserving vascular and skeletal muscle dysfunction during disuse/inactivity are reviewed. Within the context of cardiovasomobility, experimental manipulation of physical activity provides valuable insight into the mechanisms responsible for vascular and skeletal muscle dysfunction that limit mobility, degrade quality of life, and hasten the onset of disease.

Preserved stem cell content and innervation profile of elderly human skeletal muscle with lifelong recreational exercise

Abstract

Muscle fibre denervation and declining numbers of muscle stem (satellite) cells are defining characteristics of ageing skeletal muscle. The aim of this study was to investigate the potential for lifelong recreational exercise to offset muscle fibre denervation and compromised satellite cell content and function, both at rest and under challenged conditions. Sixteen elderly lifelong recreational exercisers (LLEX) were studied alongside groups of age‐matched sedentary (SED) and young subjects. Lean body mass and maximal voluntary contraction were assessed, and a strength training bout was performed. From muscle biopsies, tissue and primary myogenic cell cultures were analysed by immunofluorescence and RT‐qPCR to assess myofibre denervation and satellite cell quantity and function. LLEX demonstrated superior muscle function under challenged conditions. When compared with SED, the muscle of LLEX was found to contain a greater content of satellite cells associated with type II myofibres specifically, along with higher mRNA levels of the beta and gamma acetylcholine receptors (AChR). No difference was observed between LLEX and SED for the proportion of denervated fibres or satellite cell function, as assessed in vitro by myogenic cell differentiation and fusion index assays. When compared with inactive counterparts, the skeletal muscle of lifelong exercisers is characterised by greater fatigue resistance under challenged conditions in vivo, together with a more youthful tissue satellite cell and AChR profile. Our data suggest a little recreational level exercise goes a long way in protecting against the emergence of classic phenotypic traits associated with the aged muscle.

Key points

The detrimental effects of ageing can be partially offset by lifelong self‐organized recreational exercise, as evidence by preserved type II myofibre‐associated satellite cells, a beneficial muscle innervation status and greater fatigue resistance under challenged conditions.

Satellite cell function (in vitro), muscle fibre size and muscle fibre denervation determined by immunofluorescence were not affected by recreational exercise.

Individuals that are recreationally active are far more abundant than master athletes, which sharply increases the translational perspective of the present study. Future studies should further investigate recreational activity in relation to muscle health, while also including female participants.

Bariatric Surgery and COVID-19: What We Have Learned from the Pandemic in Iran: a Retrospective Observational Cohort Study

PURPOSE

Little is known about the symptoms of coronavirus disease 2019 (COVID-19) on patients with morbid obesity following bariatric surgery (BS) in Iran. Thus, we sought to investigate the symptoms and effect of COVID-19 in patients with morbid obesity following, or candidates for, BS in Iran.

MATERIALS AND METHODS

In this retrospective observational cohort study, we enrolled 236 morbid obese patients following (surgical group) or candidates (nonsurgical group) for bariatric surgery. Demographics, probable COVID-19 incidence, acute and persistent COVID-19 symptoms, and clinical outcome parameters of bariatric patients and candidates for BS were compared. The incidence of probable COVID-19 was assessed including the clinical definition of probable case, according to World Health Organization criteria.

RESULTS

The incidence of probable COVID-19 among surgical and nonsurgical groups was significantly different (20.6% vs 26.08%, respectively, p = 0.046). The probable case of surgical patients had a shorter length of symptoms and hospitalization duration, and a lower proportion of admission in ICUs and hospitals with respect to nonsurgical patients (p < 0.001). Surgical patients had a greater prevalence of persistent symptoms including anorexia, food intolerance, and anosmia-hyposmia than nonsurgical patients. Moreover, surgical patients with probable COVID-19 had a significantly higher proportion of diabetic patients than surgical patients without probable COVID-19 (20% vs 9.3%).

CONCLUSION

These findings highlight the need to evaluate the persistent symptoms of COVID-19 and the importance of nutritional support for at least several weeks after COVID-19 symptom onset. Moreover, it seems that COVID-19 incidence in post-bariatric surgery patients could reduce the effectiveness of bariatric surgery in the resolution of diabetes.

Effects of Various Muscle Disuse States and Countermeasures on Muscle Molecular Signaling

Skeletal muscle is capable of changing its structural parameters, metabolic rate and functional characteristics within a wide range when adapting to various loading regimens and states of the organism. Prolonged muscle inactivation leads to serious negative consequences that affect the quality of life and work capacity of people. This review examines various conditions that lead to decreased levels of muscle loading and activity and describes the key molecular mechanisms of muscle responses to these conditions. It also details the theoretical foundations of various methods preventing adverse muscle changes caused by decreased motor activity and describes these methods. A number of recent studies presented in this review make it possible to determine the molecular basis of the countermeasure methods used in rehabilitation and space medicine for many years, as well as to identify promising new approaches to rehabilitation and to form a holistic understanding of the mechanisms of gravity force control over the muscular system.

Time course of capillary regression and an expression balance between vascular endothelial growth factor-A and thrombospondin-1 in the soleus muscle of hindlimb unloaded rats

INTRODUCTION/AIMS

Skeletal muscle capillaries regress with disuse; however, information on time-dependent changes in the expression of pro- and anti-angiogenic factors in disused muscle is limited. This study aimed to clarify time-dependent changes in skeletal muscle capillarization, pro-angiogenic vascular endothelial growth factor-A (VEGF-A), and anti-angiogenic thrombospondin-1 (TSP-1) in the soleus muscle of hindlimb unloaded rat.

METHODS

Eight-week-old male Sprague Dawley rats were randomly divided into four groups corresponding to different hindlimb unloading (HU) duration at 0, 1, 2, and 3 weeks.

RESULTS

Muscle atrophy and capillary regression worsened in the soleus muscle with longer periods of HU. The VEGF-A protein expression level was lower at week 1 than at week 0. In addition, the value at week 3 was also lower than those at weeks 0, 1, and 2. The TSP-1 protein expression level was higher at week 1 than that at week 0 but was similar at weeks 2 and 3. Moreover, reactive oxygen species, assessed by dihydroethidium fluorescence intensity on cryosection, were higher at weeks 2 and 3 than that at week 0.

DISCUSSION

Depending on the HU period, VEGF-A and TSP-1 showed different expression patterns. In the early HU phase, TSP-1 may play an important role in capillary regression. However, when HU extends for a longer period, decreased VEGF-A, and/or increased oxidative stress may be more involved in capillary regression. This article is protected by copyright. All rights reserved.

Metabolomics as an Important Tool for Determining the Mechanisms of Human Skeletal Muscle Deconditioning

Muscle deconditioning impairs both locomotor function and metabolic health, and is associated with reduced quality life and increased mortality rates. Despite an appreciation of the existence of phenomena such as muscle anabolic resistance, mitophagy, and insulin resistance with age and disease in humans, little is known about the mechanisms responsible for these negative traits. With the complexities surrounding these unknowns and the lack of progress to date in development of effective interventions, there is a need for alternative approaches. Metabolomics is the study of the full array of metabolites within cells or tissues, which collectively constitute the metabolome. As metabolomics allows for the assessment of the cellular metabolic state in response to physiological stimuli, any chronic change in the metabolome is likely to reflect adaptation in the physiological phenotype of an organism. This, therefore, provides a holistic and unbiased approach that could be applied to potentially uncover important novel facets in the pathophysiology of muscle decline in ageing and disease, as well as identifying prognostic markers of those at risk of decline. This review will aim to highlight the current knowledge and potential impact of metabolomics in the study of muscle mass loss and deconditioning in humans and will highlight key areas for future research.

Histomorphometrical evaluation of extensor digitorum longus muscle in sciatic nerve regeneration using tissue engineering in rats

Skeletal muscle atrophy induced by denervation is one of the common disorders in traumatic nerve injuries. The aim of this study was the evaluation of histomorphometrical changes of extensor digitorum longus muscle after denervation and its regeneration by tissue engineering. Ninety adult male Wistar rats were randomly divided into six main groups (n = 15) in three time periods (2, 4 and 8 weeks; n = 5). Control group was treated without surgery, in transection (Tr) group left sciatic nerve was transected, in scaffold (S) group only collagen gel scaffold was used, in mast cell (MC) group mast cells were used, mesenchymal stem cell (MSC) group was treated with mesenchymal stem cells and in MC+MSC group, mast cells along with mesenchymal stem cells were used. In the cellular groups, the scaffold and cells were mixed and placed in the transected nerve gap. The average diameter of muscle fibers, ratio of the muscle fibers nuclei to the fibrocytes nuclei (mn/fn), ratio of the muscle fibers nuclei number to the muscle fibers number (mn/mf), the average ratio of blood vessels to muscle fibers number (v/mf) and muscles weight in Tr group were the lowest compared to the other groups; but, in cellular and S groups, amelioration was observed according to the time period. However, in MC+MSC group, there were the highest ameliorative results. This study revealed that simultaneous use of MCs and MSCs mixed with collagen gel scaffold can be considered as a suitable approach to improve denervated skeletal muscle atrophy associated with sciatic nerve injury.

Muscle fibrosis and maladaptation occur progressively in CKD and are rescued by dialysis

BACKGROUND

Skeletal muscle maladaptation accompanies chronic kidney disease (CKD) and negatively impacts physical function. Emphasis in CKD has historically been placed on muscle fiber intrinsic deficits, such as altered protein metabolism and atrophy. However, targeted treatment of fiber intrinsic dysfunction has produced limited improvement, whereas alterations within the fiber extrinsic environment have scarcely been examined.

METHODS

We investigated alterations to the skeletal muscle interstitial environment with deep cellular phenotyping of biopsies from patients with CKD compared to age-matched control participants and performed transcriptome profiling to define the molecular underpinnings of CKD-associated muscle impairments. We further examined changes in the observed muscle maladaptation following initiation of dialysis therapy for kidney failure.

RESULTS

Patients with CKD exhibited a progressive fibrotic muscle phenotype, which was associated with impaired regenerative capacity and lower vascular density. The severity of these deficits was strongly associated with the degree of kidney dysfunction. Consistent with these profound deficits, CKD was associated with broad alterations to the muscle transcriptome, including altered extracellular matrix organization, downregulated angiogenesis, and altered expression of pathways related to stem cell self-renewal. Remarkably, despite the seemingly advanced nature of this fibrotic transformation, dialysis treatment rescued these deficits, restoring a healthier muscle phenotype. Furthermore, after accounting for muscle atrophy, strength and endurance improved after dialysis initiation.

CONCLUSION

These data identify a dialysis-responsive muscle fibrotic phenotype in CKD and suggest that the early dialysis window presents a unique opportunity of improved muscle regenerative capacity during which targeted interventions may achieve maximal impact.

TRIAL REGISTRATION

NCT01452412FUNDING. NIH.