Prolonged exercise training improves the acute type II muscle fibre satellite cell response in healthy older men

Exercise and nutrition benefit skeletal muscle: From influence factor and intervention strategy to molecular mechanism

Sarcopenia is a progressive systemic skeletal muscle disease induced by various physiological and pathological factors, including aging, malnutrition, denervation, and cardiovascular diseases, manifesting as the decline of skeletal muscle mass and function. Both exercise and nutrition produce beneficial effects on skeletal muscle growth and are viewed as feasible strategies to prevent sarcopenia. Mechanisms involve regulating blood flow, oxidative stress, inflammation, apoptosis, protein synthesis and degradation, and satellite cell activation through exerkines and gut microbiomes. In this review, we summarized and discussed the latest progress and future development of the above mechanisms for providing a theoretical basis and ideas for the prevention and treatment of sarcopenia.

High-intensity interval training, but not Spirulina supplementation, changes muscle regeneration signaling proteins in aged rats with obesity and diabetes

This study aimed to investigate changes in protein signaling associated with muscle regeneration in aged rats with obesity and diabetes following high-intensity interval training (HIIT) and SP supplementation. Forty male Wistar rats weighting 280-325 g were used in this study. Obesity was induced by eight weeks of a high-fat diet, and diabetes was induced by intraperitoneal injection of 40 mg/kg streptozocin. Rats were randomly divided into control (CON), sham, SP, HIIT, and HIIT+SP groups. HIIT was performed five times per week during the 8-week period. SP dose was 50 mg/kg. Real-time PCR was used to evaluate the expression of myogenin, MyoD1, and Pax7. The decreases in body mass in the HIIT, HIIT+SP and SP groups were significantly higher than those in the sham and CON groups (p=0.0001). The soleus muscle mass increased significantly only in the HIIT and HIIT+SP groups (p<0.01). HIIT+SP improved fasting blood glucose and insulin levels more than HIIT alone and SP (p<0.05), while HIIT increased the expression levels of myogenic factors more than other groups (p=0.0001). In conclusion HIIT alone had a significant impact on myogenic factors, whereas Spirulina had an effect only when combined with HIIT.

Muscle fibre satellite cells are located at a greater distance from capillaries in patients with COPD compared with healthy controls

Background

COPD is a disease characterised by skeletal muscle dysfunction. A spatial relationship exists between satellite cells and muscle fibre capillaries, which has been suggested to be of major importance for satellite cell function. In the present study we compared the spatial relationship between satellite cells and capillaries in patients with COPD and age-matched healthy older adults.

Methods

Muscle biopsies were obtained from the vastus lateralis of n=18 patients with COPD (8 female, 10 male; age 66±5 years, mild-to-severe airflow obstruction) and n=18 age-, sex- and body mass index-matched healthy control adults (8 female, 10 male; age 68±5 years). Immunohistochemistry was used to assess type I/II muscle fibre size, distribution, myonuclear content, satellite cell number and fibre capillarisation. In addition, type I/II muscle fibre satellite cell distance to its nearest capillary was assessed.

Results

The percentage of type II muscle fibres was significantly greater in patients with COPD (62±10%) compared with controls (50±12%, p<0.05). Muscle fibre capillarisation was significantly lower in patients with COPD compared with controls (p<0.05). While satellite cell content was not different between groups, type I and type II satellite cell distance to its nearest capillary was significantly greater in patients with COPD (type I: 21.3±4.8 µm; type II: 26.7±9.3 µm) compared with controls (type I: 16.1±3.5 µm; type II: 22.7±5.8 µm; p<0.05).

Conclusion

Satellite cells are located at a greater distance from their nearest capillary in patients with COPD compared with age-matched controls. This increased distance could play a role in impaired satellite cell function in patients with COPD.

Type 2 diabetes mellitus related sarcopenia: a type of muscle loss distinct from sarcopenia and disuse muscle atrophy

Muscle loss is a significant health concern, particularly with the increasing trend of population aging, and sarcopenia has emerged as a common pathological process of muscle loss in the elderly. Currently, there has been significant progress in the research on sarcopenia, including in-depth analysis of the mechanisms underlying sarcopenia caused by aging and the development of corresponding diagnostic criteria, forming a relatively complete system. However, as research on sarcopenia progresses, the concept of secondary sarcopenia has also been proposed. Due to the incomplete understanding of muscle loss caused by chronic diseases, there are various limitations in epidemiological, basic, and clinical research. As a result, a comprehensive concept and diagnostic system have not yet been established, which greatly hinders the prevention and treatment of the disease. This review focuses on Type 2 Diabetes Mellitus (T2DM)-related sarcopenia, comparing its similarities and differences with sarcopenia and disuse muscle atrophy. The review show significant differences between the three muscle-related issues in terms of pathological changes, epidemiology and clinical manifestations, etiology, and preventive and therapeutic strategies. Unlike sarcopenia, T2DM-related sarcopenia is characterized by a reduction in type I fibers, and it differs from disuse muscle atrophy as well. The mechanism involving insulin resistance, inflammatory status, and oxidative stress remains unclear. Therefore, future research should further explore the etiology, disease progression, and prognosis of T2DM-related sarcopenia, and develop targeted diagnostic criteria and effective preventive and therapeutic strategies to better address the muscle-related issues faced by T2DM patients and improve their quality of life and overall health.

Making the case for resistance training in improving vascular function and skeletal muscle capillarization

Through decades of empirical data, it has become evident that resistance training (RT) can improve strength/power and skeletal muscle hypertrophy. Yet, until recently, vascular outcomes have historically been underemphasized in RT studies, which is underscored by several exercise-related reviews supporting the benefits of endurance training on vascular measures. Several lines of evidence suggest large artery diameter and blood flow velocity increase after a single bout of resistance exercise, and these events are mediated by vasoactive substances released from endothelial cells and myofibers (e.g., nitric oxide). Weeks to months of RT can also improve basal limb blood flow and arterial diameter while lowering blood pressure. Although several older investigations suggested RT reduces skeletal muscle capillary density, this is likely due to most of these studies being cross-sectional in nature. Critically, newer evidence from longitudinal studies contradicts these findings, and a growing body of mechanistic rodent and human data suggest skeletal muscle capillarity is related to mechanical overload-induced skeletal muscle hypertrophy. In this review, we will discuss methods used by our laboratories and others to assess large artery size/function and skeletal muscle capillary characteristics. Next, we will discuss data by our groups and others examining large artery and capillary responses to a single bout of resistance exercise and chronic RT paradigms. Finally, we will discuss RT-induced mechanisms associated with acute and chronic vascular outcomes.

Type II Muscle Fiber Capillarization Is an Important Determinant of Post-Exercise Microvascular Perfusion in Older Adults

INTRODUCTION

Microvascular perfusion is essential for post-exercise skeletal muscle recovery to ensure adequate delivery of nutrients and growth factors. This study assessed the relationship between various indices of muscle fiber capillarization and microvascular perfusion assessed by contrast-enhanced ultrasound (CEUS) at rest and during recovery from a bout of resistance exercise in older adults.

METHODS

Sixteen older adults (72 ± 6 y, 5/11 male/female) participated in an experimental test day during which a muscle biopsy was collected from the vastus lateralis and microvascular perfusion was determined by CEUS at rest and at 10 and 40 min following a bout of resistance exercise. Immunohistochemistry was performed on muscle tissue samples to determine various indices of both mixed and fiber-type-specific muscle fiber capillarization.

RESULTS

Microvascular blood volume at t = 10 min was higher compared with rest and t = 40 min (27.2 ± 4.7 vs. 3.9 ± 4.0 and 7.0 ± 4.9 AU, respectively, both p < 0.001). Microvascular blood volume at t = 40 min was higher compared with rest (p < 0.001). No associations were observed between different indices of mixed muscle fiber capillarization and microvascular blood volume at rest and following exercise. A moderate (r = 0.59, p < 0.05) and strong (r = 0.81, p < 0.001) correlation was observed between type II muscle fiber capillary-to-fiber ratio and the microvascular blood volume increase from rest to t = 10 and t = 40 min, respectively. In addition, type II muscle fiber capillary contacts and capillary-to-fiber perimeter exchange index were strongly correlated with the microvascular blood volume increase from rest to t = 40 min (r = 0.66, p < 0.01 and r = 0.64, p < 0.01, respectively).

CONCLUSION

Resistance exercise strongly increases microvascular blood volume for at least 40 min after exercise cessation in older adults. This resistance exercise-induced increase in microvascular blood volume is strongly associated with type II muscle fiber capillarization in older adults.

Therapeutic Consequences of Targeting the IGF-1/PI3K/AKT/FOXO3 Axis in Sarcopenia: A Narrative Review

The high prevalence of sarcopenia in an aging population has an underestimated impact on quality of life by increasing the risk of falls and subsequent hospitalization. Unfortunately, the application of the major established key therapeutic-physical activity-is challenging in the immobile and injured sarcopenic patient. Consequently, novel therapeutic directions are needed. The transcription factor Forkhead-Box-Protein O3 (FOXO3) may be an option, as it and its targets have been observed to be more highly expressed in sarcopenic muscle. In such catabolic situations, Foxo3 induces the expression of two muscle specific ubiquitin ligases (Atrogin-1 and Murf-1) via the PI3K/AKT pathway. In this review, we particularly evaluate the potential of Foxo3-targeted gene therapy. Foxo3 knockdown has been shown to lead to increased muscle cross sectional area, through both the AKT-dependent and -independent pathways and the reduced impact on the two major downstream targets Atrogin-1 and Murf-1. Moreover, a Foxo3 reduction suppresses apoptosis, activates satellite cells, and initiates their differentiation into muscle cells. While this indicates a critical role in muscle regeneration, this mechanism might exhaust the stem cell pool, limiting its clinical applicability. As systemic Foxo3 knockdown has also been associated with risks of inflammation and cancer progression, a muscle-specific approach would be necessary. In this review, we summarize the current knowledge on Foxo3 and conceptualize a specific and targeted therapy that may circumvent the drawbacks of systemic Foxo3 knockdown. This approach presumably would limit the side effects and enable an activity-independent positive impact on skeletal muscle.

Treadmill training does not enhance skeletal muscle recovery following disuse atrophy in older male mice

Introduction: A hallmark of aging is poor muscle recovery following disuse atrophy. Efficacious strategies to enhance muscle recovery following disuse atrophy in aging are non-existent. Prior exercise training could result in favorable muscle morphological and cellular adaptations that may promote muscle recovery in aging. Here, we characterized the impact of exercise training on skeletal muscle inflammatory and metabolic profiles and cellular remodeling and function, together with femoral artery reactivity prior to and following recovery from disuse atrophy in aged male mice. We hypothesized that 12 weeks of treadmill training in aged male mice would improve skeletal muscle cellular remodeling at baseline and during recovery from disuse atrophy, resulting in improved muscle regrowth. Methods: Physical performance, ex vivo muscle and vascular function, tissue and organ mass, hindlimb muscle cellular remodeling (macrophage, satellite cell, capillary, myofiber size, and fibrosis), and proteolytic, inflammatory, and metabolic muscle transcripts were evaluated in aged exercise-trained and sedentary mice. Results: We found that at baseline following exercise training (vs. sedentary mice), exercise capacity and physical function increased, fat mass decreased, and endothelial function improved. However, exercise training did not alter tibialis anterior or gastrocnemius muscle transcriptional profile, macrophage, satellite cell, capillarity or collagen content, or myofiber size and only tended to increase tibialis mass during recovery from disuse atrophy. Conclusion: While exercise training in old male mice improved endothelial function, physical performance, and whole-body tissue composition as anticipated, 12 weeks of treadmill training had limited impact on skeletal muscle remodeling at baseline or in response to recovery following disuse atrophy.

Decellularized extracellular matrix in the treatment of spinal cord injury

Functional limitation caused by spinal cord injury (SCI) has the problem of significant clinical and economic burden. Damaged spinal axonal connections and an inhibitory environment severely hamper neuronal function. Regenerative biomaterials can fill the cavity and produce an optimal microenvironment at the site of SCI, inhibiting apoptosis, inflammation, and glial scar formation while promoting neurogenesis, axonal development, and angiogenesis. Decellularization aims to eliminate cells from the ultrastructure of tissues while keeping tissue-specific components that are similar to the structure of real tissues, making decellularized extracellular matrix (dECM) a suitable scaffold for tissue engineering. dECM has good biocompatibility, it can be widely obtained from natural organs of different species, and can be co-cultured with cells for 3D printing to obtain the target scaffold. In this paper, we reviewed the pathophysiology of SCI, the characteristics of dECM and its preparation method, and the application of dECM in the treatment of SCI. Although dECM has shown its therapeutic effect at present, there are still many indicators that need to be taken into account, such as the difficulty in obtaining materials and standardized production mode for large-scale use, the effect of decellularization on the physical and chemical properties of dECM, and the study on the synergistic effect of dECM and cells.

Resistance Exercise Counteracts the Impact of Androgen Deprivation Therapy on Muscle Characteristics in Cancer Patients

CONTEXT

Androgen deprivation therapy (ADT) forms the cornerstone in prostate cancer (PCa) treatment. However, ADT also lowers skeletal muscle mass.

OBJECTIVE

To identify the impact of ADT with and without resistance exercise training on muscle fiber characteristics in PCa patients.

METHODS

Twenty-one PCa patients (72 ± 6 years) starting ADT were included. Tissue samples from the vastus lateralis muscle were assessed at baseline and after 20 weeks of usual care (n = 11) or resistance exercise training (n = 10). Type I and II muscle fiber distribution, fiber size, and myonuclear and capillary contents were determined by immunohistochemistry.

RESULTS

Significant decreases in type I (from 7401 ± 1183 to 6489 ± 1293 μm2, P < .05) and type II (from 6225 ± 1503 to 5014 ± 714 μm2, P < .05) muscle fiber size were observed in the usual care group. In addition, type I and type II individual capillary-to-fiber ratio (C/Fi) declined (-12% ± 12% and -20% ± 21%, respectively, P < .05). In contrast, significant increases in type I (from 6700 ± 1464 to 7772 ± 1319 μm2, P < .05) and type II (from 5248 ± 892 to 6302 ± 1385 μm2, P < .05) muscle fiber size were observed in the training group, accompanied by an increase in type I and type II muscle fiber myonuclear contents (+24% ± 33% and +21% ± 23%, respectively, P < .05) and type I C/Fi (+18% ± 14%, P < .05).

CONCLUSION

The onset of ADT is followed by a decline in both type I and type II muscle fiber size and capillarization in PCa patients. Resistance exercise training offsets the negative impact of ADT and increases type I and II muscle fiber size and type I muscle fiber capillarization in these patients.

The Resistance Training Effects on Skeletal Muscle Stem Cells in Older Adult: A Systematic Review and Meta-Analysis

The objective of this systematic review and meta-analysis is to examine the effects of resistance exercise training on muscle stem cells in older adults. A database search was performed (PubMed, Scopus, Web of Science and Google Scholar) to identify controlled clinical trials in English language. The mean difference (MD) with 95% confidence intervals (CIs) and overall effect size were calculated for all comparisons. The PEDro scale was used to assess the methodological quality. Nineteen studies were included in the review. The meta-analysis found a significant effect of resistance training (RT) on muscle stem cells in the elderly (difference in means=-0.008, Z=-3.415, P=0.001). Also, muscle stem cells changes were similar in men and women (difference in means=-0.004, Z=-1.558, P=0.119) and significant changes occur in type II muscle fibers (difference in means=-0.017, Z=-7.048, P=0.000). Resistance-type exercise training significantly increased muscle stem cells content in intervention group that this result is similar in men and womenthis increase occurred more in type II muscle fibers.

Resistance exercise: a mighty tool that adapts, destroys, rebuilds and modulates the molecular and structural environment of skeletal muscle

PURPOSE

Skeletal muscle regulates health and performance by maintaining or increasing strength and muscle mass. Although the molecular mechanisms in response to resistance exercise (RE) significantly target the activation of protein synthesis, a plethora of other mechanisms and structures must be involved in orchestrating the communication, repair, and restoration of homeostasis after RE stimulation. In practice, RE can be modulated by variations in intensity, continuity and volume, which affect molecular responses and skeletal muscle adaptation. Knowledge of these aspects is important with respect to planning of training programs and assessing the impact of RE training on skeletal muscle.

METHODS

In this narrative review, we introduce general aspects of skeletal muscle substructures that adapt in response to RE. We further highlighted the molecular mechanisms that control human skeletal muscle anabolism, degradation, repair and memory in response to acute and repeated RE and linked these aspects to major training variables.

RESULTS

Although RE is a key stimulus for the activation of skeletal muscle anabolism, it also induces myofibrillar damage. Nevertheless, to increase muscle mass accompanied by a corresponding adaptation of the essential substructures of the sarcomeric environment, RE must be continuously repeated. This requires the permanent engagement of molecular mechanisms that re-establish skeletal muscle integrity after each RE-induced muscle damage.

CONCLUSION

Various molecular regulators coordinately control the adaptation of skeletal muscle after acute and repeated RE and expand their actions far beyond muscle growth. Variations of key resistance training variables likely affect these mechanisms without affecting muscle growth.

Sarcopenia and Cardiovascular Diseases

Sarcopenia is the loss of muscle strength, mass, and function, which is often exacerbated by chronic comorbidities including cardiovascular diseases, chronic kidney disease, and cancer. Sarcopenia is associated with faster progression of cardiovascular diseases and higher risk of mortality, falls, and reduced quality of life, particularly among older adults. Although the pathophysiologic mechanisms are complex, the broad underlying cause of sarcopenia includes an imbalance between anabolic and catabolic muscle homeostasis with or without neuronal degeneration. The intrinsic molecular mechanisms of aging, chronic illness, malnutrition, and immobility are associated with the development of sarcopenia. Screening and testing for sarcopenia may be particularly important among those with chronic disease states. Early recognition of sarcopenia is important because it can provide an opportunity for interventions to reverse or delay the progression of muscle disorder, which may ultimately impact cardiovascular outcomes. Relying on body mass index is not useful for screening because many patients will have sarcopenic obesity, a particularly important phenotype among older cardiac patients. In this review, we aimed to: (1) provide a definition of sarcopenia within the context of muscle wasting disorders; (2) summarize the associations between sarcopenia and different cardiovascular diseases; (3) highlight an approach for a diagnostic evaluation; (4) discuss management strategies for sarcopenia; and (5) outline key gaps in knowledge with implications for the future of the field.

Pax7+ Satellite Cells in Human Skeletal Muscle After Exercise: A Systematic Review and Meta-analysis

BACKGROUND

Skeletal muscle has extraordinary regenerative capabilities against challenge, mainly owing to its resident muscle stem cells, commonly identified by Pax7⁺, which expediently donate nuclei to the regenerating multinucleated myofibers. This local reserve of stem cells in damaged muscle tissues is replenished by undifferentiated bone marrow stem cells (CD34⁺) permeating into the surrounding vascular system.

OBJECTIVE

The purpose of the study was to provide a quantitative estimate for the changes in Pax7⁺ muscle stem cells (satellite cells) in humans following an acute bout of exercise until 96 h, in temporal relation to circulating CD34⁺ bone marrow stem cells. A subgroup analysis of age was also performed.

METHODS

Four databases (Web of Science, PubMed, Scopus, and BASE) were used for the literature search until February 2022. Pax7⁺ cells in human skeletal muscle were the primary outcome. Circulating CD34⁺ cells were the secondary outcome. The standardized mean difference (SMD) was calculated using a random-effects meta-analysis. Subgroup analyses were conducted to examine the influence of age, training status, type of exercise, and follow-up time after exercise.

RESULTS

The final search identified 20 studies for Pax7⁺ cells comprising a total of 370 participants between the average age of 21 and 74 years and 26 studies for circulating CD34⁺ bone marrow stem cells comprising 494 participants between the average age of 21 and 67 years. Only one study assessed Pax7⁺ cells immediately after aerobic exercise and showed a 32% reduction in exercising muscle followed by a fast repletion to pre-exercise level within 3 h. A large effect on increasing Pax7⁺ cell content in skeletal muscles was observed 24 h after resistance exercise (SMD = 0.89, p < 0.001). Pax7⁺ cells increased to ~ 50% above pre-exercise level 24-72 h after resistance exercise. For a subgroup analysis of age, a large effect (SMD = 0.81, p < 0.001) was observed on increasing Pax7⁺ cells in exercised muscle among adults aged > 50 years, whereas adults at younger age presented a medium effect (SMD = 0.64, p < 0.001). Both resistance exercise and aerobic exercise showed a medium overall effect in increasing circulating CD34⁺ cells (SMD = 0.53, p < 0.001), which declined quickly to the pre-exercise baseline level after exercise within 6 h.

CONCLUSIONS

An immediate depletion of Pax7⁺ cells in exercising skeletal muscle concurrent with a transient release of CD34⁺ cells suggest a replenishment of the local stem cell reserve from bone marrow. A protracted Pax7⁺ cell expansion in the muscle can be observed during 24-72 h after resistance exercise. This result provides a scientific basis for exercise recommendations on weekly cycles allowing for adequate recovery time. Exercise-induced Pax7⁺ cell expansion in muscle remains significant at higher age, despite a lower stem cell reserve after age 50 years. More studies are required to confirm whether Pax7⁺ cell increment can occur after aerobic exercise.

CLINICAL TRIAL REGISTRATION

Registered at the International Prospective Register of Systematic Reviews (PROSPERO) [identification code CRD42021265457].

PCM1 labeling reveals myonuclear and nuclear dynamics in skeletal muscle across species

Myonuclei transcriptionally regulate muscle fibers during homeostasis and adaptation to exercise. Their subcellular location and quantity are important when characterizing phenotypes of myopathies, the effect of treatments, and understanding the roles of satellite cells in muscle adaptation and muscle "memory." Difficulties arise in identifying myonuclei due to their proximity to the sarcolemma and closely residing interstitial cell neighbors. We aimed to determine to what extent (pericentriolar material-1) PCM1 is a specific marker of myonuclei in vitro and in vivo. Single isolated myofibers and cross sections from mice and humans were studied from several models including wild-type and Lamin A/C mutant mice after functional overload and damage and recovery in humans following forced eccentric contractions. Fibers were immunolabeled for PCM1, Pax7, and DNA. C2C12 myoblasts were also studied to investigate changes in PCM1 localization during myogenesis. PCM1 was detected at not only the nuclear envelope of myonuclei in mature myofibers and in newly formed myotubes but also centrosomes in proliferating myogenic precursors, which may or may not fuse to join the myofiber syncytium. PCM1 was also detected in nonmyogenic nuclei near the sarcolemma, especially in regenerating areas of the Lmna^+/ΔK32 mouse and damaged human muscle. Although PCM1 is not completely specific to myonuclei, the impact that PCM1+ macrophages and interstitial cells have on myonuclei counts would be small in healthy muscle. PCM1 may prove useful as a marker of satellite cell dynamics due to the distinct change in localization during differentiation, revealing satellite cells in their quiescent (PCM1-), proliferating (PCM1+ centrosome), and prefusion states (PCM1+ nuclear envelope).

Sex-based comparisons of muscle cellular adaptations after 10 weeks of progressive resistance training in middle-aged adults

Resistance training combined with adequate protein intake supports skeletal muscle strength and hypertrophy. These adaptations are supported by the action of muscle stem cells (MuSCs), which are regulated, in part, by fibro-adipogenic progenitors (FAPs) and circulating factors delivered through capillaries. It is unclear if middle-aged males and females have similar adaptations to resistance training at the cellular level. To address this gap, 27 (13 males, 14 females) middle-aged (40-64 yr) adults participated in 10 wk of whole body resistance training with dietary counseling. Muscle biopsies were collected from the vastus lateralis pre- and posttraining. Type II fiber cross-sectional area increased similarly with training in both sexes (P = 0.014). MuSC content was not altered with training; however, training increased PDGFRα⁺/CD90⁺ FAP content (P < 0.0001) and reduced PDGFRα⁺/CD90^- FAP content (P = 0.044), independent of sex. The number of CD31⁺ capillaries per fiber also increased similarly in both sexes (P < 0.05). These results suggest that muscle fiber hypertrophy, stem/progenitor cell, and capillary adaptations are similar between middle-aged males and females in response to whole body resistance training.NEW & NOTEWORTHY We demonstrate that resistance training-induced increases in fiber hypertrophy, FAP content, and capillarization are similar between middle-aged males and females.

Twist2-expressing cells reside in human skeletal muscle and are responsive to aging and resistance exercise training

Skeletal muscle is maintained and repaired by sub-laminar, Pax7-expressing satellite cells. However, recent mouse investigations have described a second myogenic progenitor population that resides within the myofiber interstitium and expresses the transcription factor Twist2. Twist2-expressing cells exclusively repair and maintain type IIx/b muscle fibers. Currently, it is unknown if Twist2-expressing cells are present in human skeletal muscle and if they function as myogenic progenitors. Here, we perform a combination of single-cell RNA sequencing analysis and immunofluorescence staining to demonstrate the identity and localization of Twist2-expressing cells in human skeletal muscle. Twist2-expressing cells were identified to be anatomically and transcriptionally comparable to fibro-adipogenic progenitors (FAPs) and lack expression of typical satellite cell markers such as Pax7. Comparative analysis revealed that human and mouse Twist2-expressing cells were highly transcriptionally analogous and resided within the same anatomical structures in vivo. Examination of young and aged skeletal muscle biopsy samples revealed that Twist2-positive cells are more prevalent in aged muscle and increase following 12-weeks of resistance exercise training (RET) in humans. However, the quantity of Twist2-positive cells was not correlated with indices of muscle mass or muscle fiber cross-sectional area (CSA) in young or older muscle, and their abundance was surprisingly, negatively correlated with CSA and myonuclear domain size following RET. Taken together, we have identified cells expressing Twist2 in human skeletal muscle which are responsive to aging and exercise. Further examination of their myogenic potential is warranted.

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).

Impact of Different Training Modalities on Molecular Alterations in Skeletal Muscle of Patients With Heart Failure With Preserved Ejection Fraction: A Substudy of the OptimEx Trial

Background:

Exercise intolerance is a cardinal feature of heart failure with preserved ejection fraction and so far exercise training (ET) is the most effective treatment. Since the improvement in exercise capacity is only weakly associated with changes in diastolic function other mechanisms, like changes in the skeletal muscle, contribute to improvement in peak oxygen consumption. The aim of the present study was to analyze molecular changes in skeletal muscle of patients with heart failure with preserved ejection fraction performing different ET modalities.

Methods:

Skeletal muscle biopsies were taken at study begin and after 3 and 12 months from patients with heart failure with preserved ejection fraction randomized either into a control group (guideline based advice for ET), a high-intensity interval training group (HIIT) or a moderate continuous training group. The first 3 months of ET were supervised in-hospital followed by 9 months home-based ET. Protein and mRNA expression of atrophy-related proteins, enzyme activities of enzymes linked to energy metabolism and satellite cells (SCs) were quantified.

Results:

Exercise capacity improved 3 months after moderate continuous exercise training and HIIT. This beneficial effect was lost after 12 months. HIIT mainly improved markers of energy metabolism and the amount and function of SC, with minor changes in markers for muscle atrophy. Only slight changes were observed after moderate continuous exercise training. The molecular changes were no longer detectable after 12 months.

Conclusions:

Despite similar improvements in exercise capacity by HIIT and moderate continuous exercise training after 3 months, only HIIT altered proteins related to energy metabolism and amount/function of SC. These effects were lost after switching from in-hospital to at-home-based ET.

Registration:

URL: https://www.clinicaltrials.gov ; Unique identifier: NCT02078947.

Sex Hormones and Satellite Cell Regulation in Women

Recent years have seen growing scholarly interest in female physiology in general. Moreover, particular attention has been devoted to how concentrations of female sex hormones vary during the menstrual cycle and menopausal transition and how hormonal contraception and hormonal therapy influence skeletal muscle tissue. While much effort has been paid to macro outcomes, such as muscle function or mass, rather less attention has been paid to mechanistic work that may help explain the underlying mechanism through which sex hormones regulate skeletal muscle tissue. Evidence from animal studies shows a strong relationship between the female sex hormone estrogen and satellite cells (SCs), a population of muscle stem cells involved in skeletal muscle regulation. A few human studies investigating this relationship have been published only recently. Thus, the purpose of this study was to bring an updated review on female sex hormones and their role in SC regulation. First, we describe how SCs regulate skeletal muscle maintenance and repair and introduce sex hormone signaling within the muscle. Second, we present evidence from animal studies elucidating how estrogen deficiency and supplementation influence SCs. Third, we present results from investigations from human trials including women whose concentrations of female hormones differ due to menopause, hormone therapy, hormonal contraceptives, and the menstrual cycle. Finally, we discuss research and methodological recommendations for future studies aiming at elucidating the link between female sex hormones and SCs with respect to aging and training.

GAPDH S-nitrosation contributes to age-related sarcopenia through mediating apoptosis

The age-related loss of muscle mass and muscle function known as sarcopenia is a major public health problem among older people. Recent research suggests that activation of apoptotic signaling is a critical aspect of the pathogenesis of age-related sarcopenia. However, little information exists in the literature about the apoptotic mechanism of sarcopenia in aging. Herein, we report that elevated glyceraldehyde-3-phosphate dehydrogenase (GAPDH) S-nitrosation and apoptosis occur in sarcopenia during natural aging and that translocation of S-nitrosated GAPDH to the nucleus and S-nitrosated GAPDH-mediated apoptosis contributed to sarcopenia. The levels and sites of GAPDH S-nitrosation in muscle tissues of young, adult and old mice were studied with a quantitative S-nitrosation proteomic analysis approach. GAPDH S-nitrosation increased with aging, and the GAPDH modification sites Cys150, Cys154 and Cys245 were identified. The upregulated S-nitrosation of GAPDH relies on inducible nitric oxide synthase (iNOS) rather than enzymes involved in denitrosylation. Treatment with the iNOS inhibitor 1400W or mutation of GAPDH S-nitrosation sites alleviated apoptosis of C2C12 cells, further demonstrating that GAPDH S-nitrosation in aging contributes to sarcopenia. Taken together, these findings reveal a new cellular mechanism underlying age-related sarcopenia, and the demonstration of muscle loss mediated by iNOS-induced GAPDH S-nitrosation suggests a potential therapeutic strategy for sarcopenia.

Implication of satellite cell behaviors in capillary growth via VEGF expression-independent mechanism in response to mechanical loading in HeyL-null mice

Angiogenesis and muscle satellite cell (SC)-mediated myonuclear accretion are considered essential for the robust response of contraction-induced muscle hypertrophy. Moreover, both myonucleus and SCs are physically adjacent to capillaries and are the major sites for the expression of proangiogenic factors, such as VEGF, in the skeletal muscle. Thus, events involving the addition of new myonuclei via activation of SCs may play an important role in angiogenesis during muscle hypertrophy. However, the relevance among myonuclei number, capillary supply, and angiogenesis factor is not demonstrated. The Notch effector HeyL is specifically expressed in SCs in the skeletal muscle and is crucial for SC proliferation by inhibiting MyoD in overload-induced muscle hypertrophy. Here, we tested whether the addition of new myonuclei by SC in overloaded muscle is associated with angiogenic adaptation by reanalyzing skeletal muscle from HeyL-knockout (KO) mice, which show blunted responses of SC proliferation, myonucleus addition, and overload-induced muscle hypertrophy. Reanalysis confirmed blunted SC proliferation and myonuclear accretion in the plantaris muscle of HeyL-KO mice 9 wk after synergist ablation. Interestingly, the increase in capillary-to-fiber ratio observed in wild-type (WT) mice was impaired in HeyL-KO mice. In both WT and HeyL-KO mice, the expression of VEGFA and VEGFB was similarly increased in response to overload. In addition, the expression pattern of TSP-1, a negative regulator of angiogenesis, was also not changed between WT and HeyL-KO mice. Collectively, these results suggest that SCs activation-myonuclear accretion plays a crucial role in angiogenesis during overload-induced muscle hypertrophy via independent of angiogenesis regulators.

The role and therapeutic potential of stem cells in skeletal muscle in sarcopenia

Sarcopenia is a common age-related skeletal muscle disorder featuring the loss of muscle mass and function. In regard to tissue repair in the human body, scientists always consider the use of stem cells. In skeletal muscle, satellite cells (SCs) are adult stem cells that maintain tissue homeostasis and repair damaged regions after injury to preserve skeletal muscle integrity. Muscle-derived stem cells (MDSCs) and SCs are the two most commonly studied stem cell populations from skeletal muscle. To date, considerable progress has been achieved in understanding the complex associations between stem cells in muscle and the occurrence and treatment of sarcopenia. In this review, we first give brief introductions to sarcopenia, SCs and MDSCs. Then, we attempt to untangle the differences and connections between these two types of stem cells and further elaborate on the interactions between sarcopenia and stem cells. Finally, our perspectives on the possible application of stem cells for the treatment of sarcopenia in future are presented. Several studies emerging in recent years have shown that changes in the number and function of stem cells can trigger sarcopenia, which in turn leads to adverse influences on stem cells because of the altered internal environment in muscle. A better understanding of the role of stem cells in muscle, especially SCs and MDSCs, in sarcopenia will facilitate the realization of novel therapy approaches based on stem cells to combat sarcopenia.

The Importance of Muscle Capillarization for Optimizing Satellite Cell Plasticity

Satellite cells are essential for skeletal muscle regeneration, repair, and adaptation. The activity of satellite cells is influenced by their interactions with muscle-resident endothelial cells. We postulate that the microvascular network between muscle fibers plays a critical role in satellite cell function. Exercise-induced angiogenesis can mitigate the decline in satellite cell function with age.

Aging, obesity, sarcopenia and the effect of diet and exercise intervention

The number of adults 65 years and older is increasing worldwide and will represent the 20% of the population by 2030. Half of them will suffer from obesity. The decline in muscle mass and strength, known as sarcopenia, is very common among older adults with obesity (sarcopenic obesity). Sarcopenic obesity is strongly associated with frailty, cardiometabolic dysfunction, physical disability, and mortality. Increasing efforts have been hence made to identify effective strategies able to promote healthy aging and curb the obesity pandemic. Among these, lifestyle interventions consisting of diet and exercise protocols have been extensively explored. Importantly, diet-induced weight loss is associated with fat, muscle, and bone mass losses, and may further exacerbate age-related sarcopenia and frailty outcomes in older adults. Successful approaches to induce fat mass loss while preserving lean and bone mass are critical to reduce the aging- and obesity-related physical and metabolic complications and at the same time ameliorate frailty. In this review article, we discuss the most recent evidence on the age-related alterations in adipose tissue and muscle health and on the effect of calorie restriction and exercise approaches for older adults with obesity and sarcopenia, emphasizing the existing gaps in the literature that need further investigation.

Protein Requirements for Master Athletes: Just Older Versions of Their Younger Selves

It is established that protein requirements are elevated in athletes to support their training and post-exercise recovery and adaptation, especially within skeletal muscle. However, research on the requirements for this macronutrient has been performed almost exclusively in younger athletes, which may complicate their translation to the growing population of Master athletes (i.e. > 35 years old). In contrast to older (> 65 years) untrained adults who typically demonstrate anabolic resistance to dietary protein as a primary mediator of the ‘normal’ age-related loss of muscle mass and strength, Master athletes are generally considered successful models of aging as evidenced by possessing similar body composition, muscle mass, and aerobic fitness as untrained adults more than half their age. The primary physiology changes considered to underpin the anabolic resistance of aging are precipitated or exacerbated by physical inactivity, which has led to higher protein recommendations to stimulate muscle protein synthesis in older untrained compared to younger untrained adults. This review puts forth the argument that Master athletes have similar muscle characteristics, physiological responses to exercise, and protein metabolism as young athletes and, therefore, are unlikely to have protein requirements that are different from their young contemporaries. Recommendations for protein amount, type, and pattern will be discussed for Master athletes to enhance their recovery from and adaptation to resistance and endurance training.

Background Inactivity Blunts Metabolic Adaptations to Intense Short-Term Training

PURPOSE

This study determined if the level of background physical inactivity (steps per day) influences the acute and short-term adaptations to intense aerobic training.

METHODS

Sixteen untrained participants (23.6 ± 1.7 yr) completed intense (80%-90% V˙O2peak) short-term training (5 bouts of exercise over 9 d) while taking either 4767 ± 377 steps per day (n = 8; low step) or 16,048 ± 725 steps per day (n = 8; high step). At baseline and after 1 d of acute exercise and then after the short-term training (posttraining), resting metabolic responses to a high-fat meal (i.e., plasma triglyceride concentration and fat oxidation) were assessed during a 6-h high-fat tolerance test. In addition, responses during submaximal exercise were recorded both before and after training during 15 min of cycling (~79% of pretraining V˙O2peak).

RESULTS

High step displayed a reduced incremental area under the curve for postprandial plasma triglyceride concentrations by 31% after acute exercise and by 27% after short-term training compared with baseline (P < 0.05). This was accompanied by increased whole-body fat oxidation (24% and 19%; P < 0.05). Furthermore, stress during submaximal exercise as reflected by heart rate, blood lactate, and deoxygenated hemoglobin were all reduced in high step (P < 0.05), indicating classic training responses. Despite completing the same training regimen, low step showed no significant improvements in postprandial fat metabolism or any markers of stress during submaximal exercise after training (P > 0.05). However, the two groups showed a similar 7% increase in V˙O2peak (P < 0.05).

CONCLUSION

When completing an intense short-term exercise training program, decreasing daily background steps from 16,000 to approximately 5000 steps per day blunts some of the classic cardiometabolic adaptations to training. The blunting might be more pronounced regarding metabolic factors (i.e., fat oxidation and blood lactate concentration) compared with cardiovascular factors (i.e., V˙O2peak).

High Intensity Interval Training (HIIT) as a Potential Countermeasure for Phenotypic Characteristics of Sarcopenia: A Scoping Review

Background: Sarcopenia is defined as a progressive and generalized loss of skeletal muscle quantity and function associated predominantly with aging. Physical activity appears the most promising intervention to attenuate sarcopenia, yet physical activity guidelines are rarely met. In recent years high intensity interval training (HIIT) has garnered interested in athletic populations, clinical populations, and general population alike. There is emerging evidence of the efficacy of HIIT in the young old (i.e. seventh decade of life), yet data concerning the oldest old (i.e., ninth decade of life onwards), and those diagnosed with sarcopenic are sparse. Objectives: In this scoping review of the literature, we aggregated information regarding HIIT as a potential intervention to attenuate phenotypic characteristics of sarcopenia. Eligibility Criteria: Original investigations concerning the impact of HIIT on muscle function, muscle quantity or quality, and physical performance in older individuals (mean age ≥60 years of age) were considered. Sources of Evidence: Five electronic databases (Medline, EMBASE, Web of Science, Scopus, and the Cochrane Central Register of Controlled Trials [CENTRAL]) were searched. Methods: A scoping review was conducted using the Arksey and O'Malley methodological framework (2005). Review selection and characterization were performed by two independent reviewers using pretested forms. Results: Authors reviewed 1,063 titles and abstracts for inclusion with 74 selected for full text review. Thirty-two studies were analyzed. Twenty-seven studies had a mean participant age in the 60s, two in the 70s, and three in the 80s. There were 20 studies which examined the effect of HIIT on muscle function, 22 which examined muscle quantity, and 12 which examined physical performance. HIIT was generally effective in Improving muscle function and physical performance compared to non-exercised controls, moderate intensity continuous training, or pre-HIIT (study design-dependent), with more ambiguity concerning muscle quantity. Conclusions: Most studies presented herein utilized outcome measures defined by the European Working Group on Sarcopenia in Older People (EWGSOP). However, there are too few studies investigating any form of HIIT in the oldest old (i.e., ≥80 years of age), or those already sarcopenic. Therefore, more intervention studies are needed in this population.

Role of endocrine factors and stem cells in skeletal muscle regeneration

The process of reconstructing damaged skeletal muscles involves degeneration, inflammatory and immune responses, regeneration and reorganization, which are regulated by a number of immune-endocrine factors affecting muscle cells and satellite cells (SCs). One of these molecules is testosterone (T), which binds to the androgen receptor (AR) to initiate the expression of the muscle isoform of insulin-like growth factor 1 (IGF-1Ec). The interaction between T and IGF-1Ec stimulates the growth and regeneration of skeletal muscles by inhibiting apoptosis, enhancement of SCs proliferation and myoblasts differentiation. As a result of sarcopenia, muscle dystrophy or wasting diseases, the SCs population is significantly reduced. Regular physical exercise attenuates a decrease in SCs count, and thus elevates the regenerative potential of muscles in both young and elderly people. One of the challenges of modern medicine is the application of SCs and extracellular matrix scaffolds in regenerative and molecular medicine, especially in the treatment of degenerative diseases and post-traumatic muscle reconstruction. The aim of the study is to present current information on the molecular and cellular mechanisms of skeletal muscle regenera,tion, the role of testosterone and growth factors in the activation of SCs and the possibility of their therapeutic use in stimulating the reconstruction of damaged muscle fibers.

Can High-Intensity Interval Training Promote Skeletal Muscle Anabolism?

Exercise training in combination with optimal nutritional support is an effective strategy to maintain or increase skeletal muscle mass. A single bout of resistance exercise undertaken with adequate protein availability increases rates of muscle protein synthesis and, when repeated over weeks and months, leads to increased muscle fiber size. While resistance-based training is considered the 'gold standard' for promoting muscle hypertrophy, other modes of exercise may be able to promote gains in muscle mass. High-intensity interval training (HIIT) comprises short bouts of exercise at or above the power output/speed that elicits individual maximal aerobic capacity, placing high tensile stress on skeletal muscle, and somewhat resembling the demands of resistance exercise. While HIIT induces rapid increases in skeletal muscle oxidative capacity, the anabolic potential of HIIT for promoting concurrent gains in muscle mass and cardiorespiratory fitness has received less scientific inquiry. In this review, we discuss studies that have determined muscle growth responses after HIIT, with a focus on molecular responses, that provide a rationale for HIIT to be implemented among populations who are susceptible to muscle loss (e.g. middle-aged or older adults) and/or in clinical settings (e.g. pre- or post-surgery).

Myostatin Inhibition-Induced Increase in Muscle Mass and Strength Was Amplified by Resistance Exercise Training, and Dietary Essential Amino Acids Improved Muscle Quality in Mice

It has been frequently reported that myostatin inhibition increases muscle mass, but decreases muscle quality (i.e., strength/muscle mass). Resistance exercise training (RT) and essential amino acids (EAAs) are potent anabolic stimuli that synergistically increase muscle mass through changes in muscle protein turnover. In addition, EAAs are known to stimulate mitochondrial biogenesis. We have investigated if RT amplifies the anabolic potential of myostatin inhibition while EAAs enhance muscle quality through stimulations of mitochondrial biogenesis and/or muscle protein turnover. Mice were assigned into ACV (myostatin inhibitor), ACV+EAA, ACV+RT, ACV+EAA +RT, or control (CON) over 4 weeks. RT, but not EAA, increased muscle mass above ACV. Despite differences in muscle mass gain, myofibrillar protein synthesis was stimulated similarly in all vs. CON, suggesting a role for changes in protein breakdown in muscle mass gains. There were increases in MyoD expression but decreases in Atrogin-1/MAFbx expression in ACV+EAA, ACV+RT, and ACV+EAA+RT vs. CON. EAA increased muscle quality (e.g., grip strength and maximal carrying load) without corresponding changes in markers of mitochondrial biogenesis and neuromuscular junction stability. In conclusion, RT amplifies muscle mass and strength through changes in muscle protein turnover in conjunction with changes in implicated signaling, while EAAs enhance muscle quality through unknown mechanisms.

Analysis of the aging-induced changes in the motor ability structure using large population fitness test results

Owing to confounding factors influencing the effect of aging, systematic analyses of age-related changes in motor ability are mostly limited to the use of animals whose diets and genetics can be controlled or the use of datasets of athletes who share similar lifestyles. However, we lack systematic methods for analyzing the effect of aging on the motor ability structure of the general public. We propose that principal component analysis (PCA) on fitness test results of a large sample may provide information on the aging-induced change in the motor ability structure of the general public. We complied the fitness test records of 7402 Koreans between the ages of 20 and 64, and performed PCA on the records of gripping, 50m dash, sit-ups, and shuttle runs, which indicate strength, speed, muscular endurance, and aerobic endurance, respectively. Our analysis shows the structural changes in motor ability around the age of 40 and 60 in Korea. We expect that the proposed approach can be applied to similar datasets from other countries or local communities to quantify any age-induced change in motor ability structure in each specific group.

Muscle fiber capillarization is associated with various indices of skeletal muscle mass in healthy, older men

INTRODUCTION

Muscle fiber capillarization plays a fundamental role in the regulation of skeletal muscle mass maintenance. However, it remains unclear to what extent capillarization is related to various other skeletal muscle characteristics. In this study we determined whether muscle fiber capillarization is independently associated with measures of skeletal muscle mass, both on a whole-body and cellular level, and post-absorptive muscle protein synthesis rates in healthy older men.

METHODS

Forty-six healthy older (70 ± 4 y) men participated in a trial during which basal muscle protein synthesis rates were assessed using stable isotope tracer methodology. Blood and muscle biopsy samples were collected to assess post-absorptive muscle protein synthesis rates over a 3-hour period. Immunohistochemistry was performed to determine various indices of muscle fiber capillarization, size, type distribution, and myonuclear content/domain size. Dual energy x-ray absorptiometry scans were performed to determine whole-body and appendicular lean tissue mass.

RESULTS

Capillary-to-fiber ratio (C/Fi) and perimeter exchange (CFPE) index correlated with whole-body lean tissue mass (r = 0.43, P < 0.01 and r = 0.25, P < 0.10, respectively), appendicular lean tissue mass (r = 0.52, P < 0.001 and r = 0.37, P < 0.05, respectively) as well as appendicular lean tissue mass divided by body mass index (r = 0.65, P < 0.001 and r = 0.62, P < 0.001, respectively). Muscle fiber size correlated with C/Fi (r = 0.45, P < 0.01), but not with CFPE index. No associations were observed between different indices of muscle fiber capillarization and post-absorptive muscle protein synthesis rates in healthy, older men.

CONCLUSION

The present study provides further evidence that muscle fiber capillarization may be a critical factor in the regulation of skeletal muscle maintenance in healthy older men.

Capillary facilitation of skeletal muscle function in health and disease

Skeletal muscle is highly vascularized, with perfusion being tightly regulated to meet wide-ranging metabolic demands. For decades, the capillary supply has been explored mainly in terms of evaluating the capillary numbers and their function in the supply of oxygen and substrates and the removal of metabolic byproducts. This review will focus on recent discoveries concerning the role played by capillaries in facilitating other aspects of cell regulation and maintenance, in health and disease, as well as alterations during the aging process.

Novelty Capillaries play a central role in the coordination of the vascular response that controls blood flow during contraction and the cellular responses to which they feed into. Nitric oxide is an important regulatory compound within the cardiovascular system, and a significant contributor to skeletal muscle capillary angiogenesis and vasodilatory response to agonists. The microvascular network between muscle fibres may play a critical role in the distribution of signalling factors necessary for optimal muscle satellite cell function.

Fiber type-specific hypertrophy and increased capillarization in skeletal muscle following testosterone administration in young women

It is well established that testosterone administration induces muscle fiber hypertrophy and myonuclear addition in men; however, it remains to be determined whether similar morphological adaptations can be achieved in women. The aim of the present study was therefore to investigate whether exogenously administered testosterone alters muscle fiber morphology in skeletal muscle of young healthy, physically active women. Thirty-five young (20-35 yr), recreationally trained women were randomly assigned to either 10-wk testosterone administration (10 mg daily) or placebo. Before and after the intervention, hormone concentrations and body composition were assessed, and muscle biopsies were obtained from the vastus lateralis. Fiber type composition, fiber size, satellite cell and myonuclei content, as well as muscle capillarization were assessed in a fiber type-specific manner by immunohistochemistry. After the intervention, testosterone administration elevated serum testosterone concentration (5.1-fold increase, P = 0.001) and induced significant accretion of total lean mass (+1.9%, P = 0.002) and leg lean mass (+2.4%, P = 0.001). On the muscle fiber level, testosterone increased mixed-fiber cross-sectional area (+8.2%, P = 0.001), an effect primarily driven by increases in type II fiber size (9.2%, P = 0.006). Whereas myonuclei content remained unchanged, a numerical increase (+30.8%) was found for satellite cells associated with type II fibers in the Testosterone group. In parallel with fiber hypertrophy, testosterone significantly increased capillary contacts (+7.5%, P = 0.015) and capillary-to-fiber ratio (+9.2%, P = 0.001) in type II muscle fibers. The present study provides novel insight into fiber type-specific adaptations present already after 10 wk of only moderately elevated testosterone levels in women.NEW & NOTEWORTHY We have recently demonstrated performance-enhancing effects of moderately elevated testosterone concentrations in young women. Here we present novel evidence that testosterone alters muscle morphology in these women, resulting in type II fiber hypertrophy and improved capillarization. Our findings suggest that low doses of testosterone potently impact skeletal muscle after only 10 wk. These data provide unique insights into muscle adaptation and support the performance-enhancing role of testosterone in women on the muscle fiber level.

The Effects of Concurrent Training Order on Satellite Cell-Related Markers, Body Composition, Muscular and Cardiorespiratory Fitness in Older Men with Sarcopenia

OBJECTIVES

Concurrent Training (CT) is described as a combination of resistance training (RT) and endurance training (ET) in a periodized program to maximize all aspects of physical performance. To date, effects of CT order on muscular and cardiorespiratory fitness adaptations are controversial. Owing to the age-related decrement in satellite cells (SC) which are critical for fiber repair, conservation, muscle hypertrophy as well as cardiorespiratory fitness, the present study examined the response of SC related markers to CT order in older sarcopenic men.

PARTICIPANTS

Thirty older men (age= 64.3 ± 3.5 years) were randomly assigned into one of 3 groups, ET followed by RT (E+R; n=10), RT followed by ET (R+E; n= 10) or a control (C; n=10).

INTERVENTION

The training protocol consisted of 3 exercise sessions per week for 8 weeks. Blood samples were obtained at baseline and 48 hours after the final training session.

RESULTS

Weight, skeletal muscle mass, lower and upper body power, maximal oxygen consumption (VO2max), Paired Box 7 (Pax7), and Myogenic factor 5 (Myf5) significantly increased, while were percent body fat significantly decreased following E+R and R+E compared to C. Importantly, the improvement in skeletal muscle mass, lower and upper body power, Myf5 and Pax7 in the E+R was significantly greater than the R+E group. Myogenin (Myog) and Paired Box 3 (Pax3) significantly increased (P < 0.01) in both training groups compared to no changes in C.

CONCLUSION

An 8-week CT intervention improves SC related markers, body composition and enhances power and VO2max in older sarcopenic participants, regardless of the order of RT and ET. However, performing ET before RT may be more effective at enhancing skeletal muscle mass, Myf5 and Pax7, in addition to both lower and upper body power. While both CT programs produced notable physiological and performance benefits, performing ET before RT during CT may provide the greatest therapeutic benefits for aging individuals.

Lack of muscle fibre hypertrophy, myonuclear addition, and satellite cell pool expansion with resistance training in 83-94-year-old men and women

AIMS

To examine satellite cell and myonuclear content in very old (≥83 years) individuals, and the response to heavy resistance training.

METHODS

A group of very old men and women (Old, 83-94 years, n = 29) was randomized to 12 weeks of heavy resistance training or untrained controls. A group of young men who did not resistance train (Young, 19-27 years, n = 9) were included for comparison.

RESULTS

Compared to young men, prior to training the old men had smaller type II fibres (-38%, P < 0.001), lower satellite cell content (-52%, P < 0.001), smaller myonuclear domain (-30%, P < 0.001), and a trend for lower myonuclear content (-13%, P = 0.09). Old women were significantly different from old men for these parameters, except for satellite cell content. Resistance training had no effect on these parameters in these old men and women. Fibre-size specific analysis showed strong correlations between fibre size and myonuclei per fibre and between fibre size and myonuclear domain for both fibre types (r = 0.94-0.99, P < 0.0001). In contrast, muscle fibre perimeter per myonucleus seemed to be constant across the range in fibre size, particularly in type I fibres (r = -0.31, P = 0.17).

CONCLUSIONS

The present data demonstrate that type II fibre size, satellite cell content and myonuclear domain is significantly smaller in very old men compared to young men, while myonuclear content is less affected. These parameters were not improved with heavy resistance training at the most advanced stage of ageing.

The role of the microcirculation in muscle function and plasticity

It is widely acknowledged that maintenance of muscle, size, strength and endurance is necessary for quality of life and the role that skeletal muscle microcirculation plays in muscle health is becoming increasingly clear. Here we discuss the role that skeletal muscle microcirculation plays in muscle function and plasticity. Besides the density of the capillary network, also the distribution of capillaries is crucial for adequate muscle oxygenation. While capillaries are important for oxygen delivery, the capillary supply to a fibre is related to fibre size rather than oxidative capacity. This link between fibre size and capillary supply is also reflected by the similar time course of hypertrophy and angiogenesis, and the cross-talk between capillaries and satellite cells. A dense vascular network may in fact be more important for a swift repair of muscle damage than the abundance of satellite cells and a lower capillary density may also attenuate the hypertrophic response. Capillary rarefaction does not only occur during ageing, but also during conditions as chronic heart failure, where endothelial apoptosis has been reported to precede muscle atrophy. It has been suggested that capillary rarefaction precedes sarcopenia. If so, stimulation of angiogenesis by for instance endurance training before a hypertrophic stimulus may enhance the hypertrophic response. The microcirculation may thus well be a little-explored target to improve muscle function and the success of rehabilitation programmes during ageing and chronic diseases.

Integrated Myofibrillar Protein Synthesis in Recovery From Unaccustomed and Accustomed Resistance Exercise With and Without Multi-ingredient Supplementation in Overweight Older Men

Background: We previously showed that daily consumption of a multi-ingredient nutritional supplement increased lean mass in older men, but did not enhance lean tissue gains during a high-intensity interval training (HIIT) plus resistance exercise training (RET) program. Here, we aimed to determine whether these divergent observations aligned with the myofibrillar protein synthesis (MyoPS) response to acute unaccustomed and accustomed resistance exercise. Methods: A sub-sample of our participants were randomly allocated (n = 15; age: 72 ± 7 years; BMI: 26.9 ± 3.1 kg/m2 [mean ± SD]) to ingest an experimental supplement (SUPP, n = 8: containing whey protein, creatine, vitamin D, and n-3 PUFA) or control beverage (CON, n = 7: 22 g maltodextrin) twice per day for 21 weeks. After 7 weeks of consuming the beverage alone (Phase 1: SUPP/CON only), subjects completed 12 weeks of RET (twice per week) + HIIT (once per week) (Phase 2: SUPP/CON + EX). Orally administered deuterated water was used to measure integrated rates of MyoPS over 48 h following a single session of resistance exercise pre- (unaccustomed) and post-training (accustomed). Results: Following an acute bout of accustomed resistance exercise, 0-24 h MyoPS was 30% higher than rest in the SUPP group (effect size: 0.86); however, in the CON group, 0-24 h MyoPS was 0% higher than rest (effect size: 0.04). Nonetheless, no within or between group changes in MyoPS were statistically significant. When collapsed across group, rates of MyoPS in recovery from acute unaccustomed resistance exercise were positively correlated with training-induced gains in whole body lean mass (r = 0.63, p = 0.01). Conclusion: There were no significant between-group differences in MyoPS pre- or post-training. Integrated rates of MyoPS post-acute exercise in the untrained state were positively correlated with training-induced gains in whole body lean mass. Our finding that supplementation did not alter 0-48 h MyoPS following 12 weeks of training suggests a possible adaptive response to longer-term increased protein intake and warrants further investigation. This study was registered at ClinicalTrials.gov. Clinical Trial Registration: www.ClinicalTrials.gov, identifier: NCT02281331.