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

Constitutional thinness might be characterized by physiologically adapted and not impaired muscle function and architecture: new results from the NUTRILEAN study

PURPOSE

While muscle mass and skeletal muscle fibers phenotype have been shown atypical in constitutional thinness (CT), force production capacities and its architectural determinants have never been explored. The present study compared muscle functionality and architecture between participants with CT and their normal-weight (NW) counterparts.

METHODS

Anthropometry, body composition (Dual-X-ray Absorptiometry), physical activity/sedentary behavior (ActiGraph wGT3X-BT), ultrasound recording of the Vastus Lateralis (2D-ultrasound system), and functional capacities at maximal isometric and isokinetic voluntary contractions (MVCISO and MVCCON) during knee extension (isokinetic dynamometer chair Biodex) have been measured in 18 women with CT (body mass index < 17.5 kg/m2) and 17 NW women.

RESULTS

A lower fat-free mass (ES: -1.94, 95%CI: -2.76 to -1.11, p < 0.001), a higher sedentary time, and a trend for a lower time spent at low-intensity physical activity, were observed in CT vs NW participants. While absolute MVCISO, MVCCON, rate of torque development (RTD), and torque work were all markedly lower in CT, these differences disappeared when normalized to body or muscle mass. Muscle thickness and fascicle length were found lower in CT (ES: -1.29, 95%CI: -2.03 to -0.52, p < 0.001; and ES: -0.87, 95%CI: -1.58 to -0.15, p = 0.02, respectively), while pennation angle was found similar.

CONCLUSION

Despite lower absolute strength capacities observed in CT, present findings support the hypothesis of physiological adaptations to the low body and muscle mass than to some intrinsic contractile impairments. These results call for further studies exploring hypertrophy-targeted strategies in the management of CT.

Evaluation of the Anatomical Cross-Sectional Area of Psoas Major Muscle Using an Ultrasound Imaging System Combined With an Inertial Measurement Unit: Improved Reliability in the US Using IMU-Based Positioning Techniques

Introduction: Recently, ultrasound (US) imaging has been used to estimate the cross-sectional area of skeletal muscle, but the reliability is uncertain. To improve the reliability of the US, we investigated skeletal muscle thickness measurement using an inertial measurement unit (IMU) to determine the direction of US beam incidence based on posture angle information. In addition, we examined whether the anatomical cross-sectional area (ACSA) of muscle can be estimated from the muscle thickness measured using the US with the IMU. Methods: In Experiment 1, two examiners measured the right psoas major at the fourth lumbar vertebra level in 10 university students using the US with and without an IMU. The intraclass correlation coefficient (ICC) was used to examine intra- and inter-rater variability. In Experiment 2, the two examiners measured the muscle thickness of the right psoas major in 31 male subjects using the US with an IMU. In addition, the ACSA of this muscle was measured using MRI. Pearson's correlation coefficient was used to examine the relationship between muscle thickness and ACSA, and a single regression analysis was performed. Results: Both intrarater reliability ICC (1, 2) and inter-rater reliability ICC (2, 2) were higher when US was used with IMU compared to without IMU (Experiment 1). A significant positive correlation (r = 0.84, p < 0.01) was observed between muscle thickness and ACSA (Experiment 2). The regression equation was significant at R 2 = 0.71 (p < 0.01). Conclusion: Using an IMU during US measurement of the psoas major improves intra- and interexaminer reliability and can be used to estimate the ACSA of the muscle.

Ultrastructural alterations and mitochondrial dysfunction in skeletal muscle of peripheral artery disease patients: implications for early therapeutic interventions

Peripheral artery disease (PAD) is an atherosclerotic condition that impairs blood flow to the lower extremities, resulting in myopathy in affected skeletal muscles. Improving our understanding of PAD and developing novel treatment strategies necessitates a comprehensive examination of cellular structural alterations that occur in the muscles with disease progression. Here we aimed to employ electron microscopy to quantify skeletal muscle ultrastructural alterations responsible for the myopathy of PAD. Fifty-two participants (22 controls, 10 PAD Stage II, and 20 PAD Stage IV) were enrolled. Gastrocnemius biopsies were obtained to determine mitochondrial respiration and oxidative stress. Skeletal muscle sarcomere, mitochondria, lipid droplets, and sarcoplasm were assessed using transmission electron microscopy and focused ion beam scanning electron microscopy. Controls and PAD Stage II patients underwent walking performance tests: 6-minute walking test, 4-minute walking velocity, and maximum graded treadmill test. We identified several prominent ultrastructural modifications in PAD gastrocnemius, including reduced sarcomere dimensions, alterations in mitochondria number and localization, myofibrillar disorientation, changes in lipid droplets, and modifications in mitochondria-lipid droplet contact area. These changes correlated with impaired mitochondrial respiration and increased ROS production. We observed progressive deterioration in mitochondrial parameters across PAD stages. Stage II PAD showed impaired mitochondrial function and structure, while stage IV exhibited further deterioration, more pronounced structural alterations, and a decrease in mitochondrial content. The walking performance of Stage II PAD patients was significantly reduced. Our findings suggest that pathological mitochondria play a key role in the skeletal muscle dysfunction of PAD patients and represent an important target for therapeutic interventions aimed at improving clinical and functional outcomes in this patient population. Our data indicate that treatments should be implemented early and may include therapies designed to preserve and enhance mitochondrial biogenesis and respiration, optimize mitochondrial-lipid droplet interactions, or mitigate oxidative stress. Translational Perspective: Peripheral artery disease (PAD) is characterized by skeletal muscle and mitochondrial dysfunction. Ultrastructural changes in skeletal muscle myofibers and mitochondria morphology can provide significant information on the PAD pathophysiology. Here, we investigated skeletal muscle and mitochondria morphological and functional changes at the sarcomere level and across the disease progression and have found that sarcomere lengths and mitochondria count and function are associated with disease progression, indicating loss of skeletal muscle contractile and metabolic function. Ultrastructural changes in the PAD skeletal muscle can provide significant information in the development of new treatments.

Exploring NADPH oxidases 2 and 4 in cardiac and skeletal muscle adaptations - A cross-tissue comparison

Striated muscle cells, encompassing cardiac myocytes and skeletal muscle fibers, are fundamental to athletic performance, facilitating blood circulation and coordinated movement through contraction. Despite their distinct functional roles, these muscle types exhibit similarities in cytoarchitecture, protein expression, and excitation-contraction coupling. Both muscle types also undergo molecular remodeling in energy metabolism and cell size in response to acute and repeated exercise stimuli to enhance exercise performance. Reactive oxygen species (ROS) produced by NADPH oxidase (NOX) isoforms 2 and 4 have emerged as signaling molecules that regulate exercise adaptations. This review systematically compares NOX2 and NOX4 expression, regulation, and roles in cardiac and skeletal muscle responses across exercise modalities. We highlight the many gaps in our knowledge and opportunities to let future skeletal muscle research into NOX-dependent mechanisms be inspired by cardiac muscle studies and vice versa. Understanding these processes could enhance the development of exercise routines to optimize human performance and health strategies that capitalize on the advantages of physical activity.

Hamstrings Hypertrophy Is Specific to the Training Exercise: Nordic Hamstring versus Lengthened State Eccentric Training

INTRODUCTION

The hamstring muscles play a crucial role in sprint running but are also highly susceptible to strain injuries, particularly within the biceps femoris long head (BFlh). This study compared the adaptations in muscle size and strength of the knee flexors, as well as BFlh muscle and aponeurosis size, after two eccentrically focused knee flexion training regimes: Nordic hamstring training (NHT) vs lengthened state eccentric training (LSET, isoinertial weight stack resistance in an accentuated hip-flexed position) vs habitual activity (no training controls: CON).

METHODS

Forty-two healthy young males completed 34 sessions of NHT or LSET over 12 wk or served as CON ( n = 14/group). Magnetic resonance imaging-measured muscle volume of seven individual knee flexors and BFlh aponeurosis area, and maximum knee flexion torque during eccentric, concentric, and isometric contractions were assessed pre- and post-training.

RESULTS

LSET induced greater increases in hamstrings (+18% vs +11%) and BFlh (+19% vs +5%) muscle volumes and BFlh aponeurosis area (+9% vs +3%) than NHT (all P ≤ 0.001), with no changes after CON. There were distinctly different patterns of hypertrophy between the two training regimes, largely due to the functional role of the muscles; LSET was more effective for increasing the size of knee flexors that also extend the hip (2.2-fold vs NHT), whereas NHT increased the size of knee flexors that do not extend the hip (1.9-fold vs LSET; both P ≤ 0.001). Changes in maximum eccentric torque differed only between LSET and CON (+17% vs +4%; P = 0.009), with NHT (+11%) inbetween.

CONCLUSIONS

These results suggest that LSET is superior to NHT in inducing overall hamstrings and BFlh hypertrophy, potentially contributing to better sprint performance improvements and protection against hamstring strain injuries than NHT.

Exercise-specific adaptations in human skeletal muscle: Molecular mechanisms of making muscles fit and mighty

The mechanisms leading to a predominantly hypertrophied phenotype versus a predominantly oxidative phenotype, the hallmarks of resistance training (RT) or aerobic training (AT), respectively, are being unraveled. In humans, exposure of naïve persons to either AT or RT results in their skeletal muscle exhibiting generic 'exercise stress-related' signaling, transcription, and translation responses. However, with increasing engagement in AT or RT, the responses become refined, and the phenotype typically associated with each form of exercise emerges. Here, we review some of the mechanisms underpinning the adaptations of how muscles become, through AT, 'fit' and RT, 'mighty.' Much of our understanding of molecular exercise physiology has arisen from targeted analysis of post-translational modifications and measures of protein synthesis. Phosphorylation of specific residue sites has been a dominant focus, with canonical signaling pathways (AMPK and mTOR) studied extensively in the context of AT and RT, respectively. These alone, along with protein synthesis, have only begun to elucidate key differences in AT and RT signaling. Still, key yet uncharacterized differences exist in signaling and regulation of protein synthesis that drive unique adaptation to AT and RT. Omic studies are required to better understand the divergent relationship between exercise and phenotypic outcomes of training.

Long-Term Resistance Trained Human Muscles Have More Fibers, More Myofibrils, and Tighter Myofilament Packing Than Untrained

INTRODUCTION

Increases in skeletal muscle size occur in response to prolonged exposure to resistance training that is typically ascribed to increased muscle fiber size. Whether muscle fiber number also changes remains controversial, and a paucity of data exists about myofibrillar structure. This cross-sectional study compared muscle fiber and myofibril characteristics in long-term resistance-trained (LRT) versus untrained (UNT) individuals.

METHODS

The maximal anatomical cross-sectional area (ACSAmax) of the biceps brachii muscle was measured by magnetic resonance imaging in 16 LRT (5.9 ± 3.5 yr' experience) and 13 UNT males. A muscle biopsy was taken from the biceps brachii to measure muscle fiber area, myofibril area, and myosin spacing. Muscle fiber number, and myofibril number in total and per fiber were estimated by dividing ACSAmax by muscle fiber area or myofibril area, and muscle fiber area by myofibril area, respectively.

RESULTS

Compared with UNT, LRT individuals had greater ACSAmax (+70%, P < 0.001), fiber area (+29%, P = 0.028), fiber number (+34%, P = 0.013), and myofibril number per fiber (+49%, P = 0.034) and in total (+105%, P < 0.001). LRT individuals also had smaller myosin spacing (-7%, P = 0.004; i.e., greater packing density) and a tendency toward smaller myofibril area (-16%, P = 0.074). ACSAmax was positively correlated with fiber area ( r = 0.526), fiber number ( r = 0.445), and myofibril number (in total r = 0.873 and per fiber r = 0.566), and negatively correlated with myofibril area ( r = -0.456) and myosin spacing ( r = -0.382) (all P < 0.05).

CONCLUSIONS

The larger muscles of LRT individuals exhibited more fibers in cross-section and larger muscle fibers, which contained substantially more total myofibrils and more packed myofilaments than UNT participants, suggesting plasticity of muscle ultrastructure.

Chronic Effects of Static Stretching Exercises on Skeletal Muscle Hypertrophy in Healthy Individuals: A Systematic Review and Multilevel Meta-Analysis

BACKGROUND

The chronic effect of static stretching (SS) on muscle hypertrophy is still unclear. This study aimed to examine the chronic effects of SS exercises on skeletal muscle hypertrophy in healthy individuals.

METHODS

A systematic literature search was conducted in the PubMed, Web of Science, Cochrane Library, and SPORTDiscus databases up to July 2023. Included studies examined chronic effects of SS exercise compared to an active/passive control group or the contralateral leg (i.e., utilizing between- or within-study designs, respectively) and assessed at least one outcome of skeletal muscle hypertrophy in healthy individuals with no age restriction.

RESULTS

Twenty-five studies met the inclusion criteria. Overall, findings indicated an unclear effect of chronic SS exercises on skeletal muscle hypertrophy with a trivial point estimate (standardised mean difference [SMD] = 0.118 [95% prediction interval [95% PI] = - 0.233 to 0.469; p = 0.017]) and low heterogeneity (I2 = 24%). Subgroup analyses revealed that trained individuals (β = 0.424; 95% PI = 0.095 to 0.753) displayed larger effects compared to recreationally trained (β = 0.115; 95% PI = - 0.195 to 0.425) and sedentary individuals (β = - 0.081; 95% PI = - 0.399 to 0.236). Subanalysis suggested the potential for greater skeletal muscle hypertrophy in samples with higher percentages of females (β = 0.003, [95% confidence interval [95% CI] = - 0.000 to 0.005]). However, the practical significance of this finding is questionable. Furthermore, a greater variety of stretching exercises elicited larger increases in muscle hypertrophy (β = 0.069, [95% CI = 0.041 to 0.097]). Longer durations of single stretching exercises (β = 0.006, [95% CI = 0.002 to 0.010]), time under stretching per session (β = 0.006, [95% CI = 0.003 to 0.009]), per week (β = 0.001, [95% CI = 0.000 to 0.001]) and in total (β = 0.008, [95% CI = 0.003 to 0.013]) induced larger muscle hypertrophy. Regarding joint range of motion, there was a clear positive effect with a moderate point estimate (β = 0.698; 95% PI = 0.147 to 1.249; p < 0.001) and moderate heterogeneity (I2 = 43%). Moreover, findings indicated no significant association between the gains in joint range of motion and the increase in muscle hypertrophy (β = 0.036, [95% CI = - 0.123 to 0.196]; p = 0.638).

CONCLUSIONS

This study revealed an overall unclear chronic effect of SS on skeletal muscle hypertrophy, although interpretation across the range of PI suggests a potential modest beneficial effect. Subgroup analysis indicated larger stretching-induced muscle gains in trained individuals, a more varied selection of SS exercises, longer mean duration of single stretching exercise, increased time under SS per session, week, and in total, and possibly in samples with a higher proportion of females. From a practical perspective, it appears that SS exercises may not be highly effective in promoting skeletal muscle hypertrophy unless a higher duration of training is utilized. PROSPERO registration number: CRD42022331762.

Rethinking the physiological cross-sectional area of skeletal muscle reveals the mechanical advantage of pennation

The shape of skeletal muscle varies remarkably-with important implications for locomotor performance. In many muscles, the fibres are arranged at an angle relative to the tendons' line of action, termed the pennation angle. These pennate muscles allow more sarcomeres to be packed side by side, enabling the muscle to generate higher maximum forces for a given muscle size. Historically, the physiological cross-sectional area (PCSA) has been used to capture both the size and arrangement of muscle fibres, and is one of the best predictors of a muscles capacity to produce force. However, the anatomical and mechanical implications of PCSA remain ambiguous as misinterpretations have limited our ability to understand the mechanical advantage of pennate muscle designs. We developed geometric models to resolve the mechanistic and functional impacts of pennation angle across a range of muscle shapes and sizes. Comparisons among model predictions and empirical data on human lower limb muscles demonstrated how a pennate arrangement of fibres allows muscles to produce up to six times more isometric force when compared with non-pennate muscles of the same volume. We show that in muscles much longer than thick, an optimal pennation angle exists at which isometric force is maximized. Using empirically informed geometric models we demonstrate the functional significance of a pennate muscle design and provide a new parameter, pennation mechanical advantage, which quantifies this performance improvement.

Satellite cell-derived TRIM28 is pivotal for mechanical load- and injury-induced myogenesis

Satellite cells are skeletal muscle stem cells that contribute to postnatal muscle growth, and they endow skeletal muscle with the ability to regenerate after a severe injury. Here we discover that this myogenic potential of satellite cells requires a protein called tripartite motif-containing 28 (TRIM28). Interestingly, different from the role reported in a previous study based on C2C12 myoblasts, multiple lines of both in vitro and in vivo evidence reveal that the myogenic function of TRIM28 is not dependent on changes in the phosphorylation of its serine 473 residue. Moreover, the functions of TRIM28 are not mediated through the regulation of satellite cell proliferation or differentiation. Instead, our findings indicate that TRIM28 regulates the ability of satellite cells to progress through the process of fusion. Specifically, we discover that TRIM28 controls the expression of a fusogenic protein called myomixer and concomitant fusion pore formation. Collectively, the outcomes of this study expose the framework of a novel regulatory pathway that is essential for myogenesis.

The utility-and limitations-of the rodent synergist ablation model in examining mechanisms of skeletal muscle hypertrophy

In this issue, Burke et al. discuss the utility of the rodent synergist ablation (SA) model for examining mechanisms associated with skeletal muscle hypertrophy. In this invited perspective, we aim to complement their original perspective by discussing limitations to the model along with alternative mechanical overload models that have strengths and limitations.

Functional specialisation and coordination of myonuclei

Myofibres serve as the functional unit for locomotion, with the sarcomere as fundamental subunit. Running the entire length of this structure are hundreds of myonuclei, located at the periphery of the myofibre, juxtaposed to the plasma membrane. Myonuclear specialisation and clustering at the centre and ends of the fibre are known to be essential for muscle contraction, yet the molecular basis of this regionalisation has remained unclear. While the 'myonuclear domain hypothesis' helped explain how myonuclei can independently govern large cytoplasmic territories, novel technologies have provided granularity on the diverse transcriptional programs running simultaneously within the syncytia and added a new perspective on how myonuclei communicate. Building upon this, we explore the critical cellular and molecular sources of transcriptional and functional heterogeneity within myofibres, discussing the impact of intrinsic and extrinsic factors on myonuclear programs. This knowledge provides new insights for understanding muscle development, repair, and disease, but also opens avenues for the development of novel and precise therapeutic approaches.

Dietary Protein and Physical Exercise for the Treatment of Sarcopenia

Sarcopenia is a multifactorial age-related disorder that causes a decrease in muscle mass, strength, and function, leading to alteration of movement, risk of falls, and hospitalization. This article aims to review recent findings on the factors underlying sarcopenia and the strategies required to delay and counteract its symptoms. We focus on molecular factors linked to ageing, on the role of low-grade chronic and acute inflammatory conditions such as cancer, which contributes to the onset of sarcopenia, and on the clinical criteria for its diagnosis. The use of drugs against sarcopenia is still subject to debate, and the suggested approaches to restore muscle health are based on adequate dietary protein intake and physical exercise. We also highlight the difference in the amount and quality of amino acids within animal- and plant-based diets, as studies have often shown varying results regarding their effect on sarcopenia in elderly people. In addition, many studies have reported that non-pharmacological approaches, such as an optimization of dietary protein intake and training programs based on resistance exercise, can be effective in preventing and delaying sarcopenia. These approaches not only improve the maintenance of skeletal muscle function, but also reduce health care costs and improve life expectancy and quality in elderly people.

Identification of constitutive materials of bi-layer soft tissues from multimodal indentations

The personalisation of finite element models is an important problem in the biomechanical fields where subject-specific analyses are fundamental, particularly in studying soft tissue mechanics. The personalisation includes the choice of the constitutive law of the model's material, as well as the choice of the material parameters. In vivo identification of the material properties of soft tissues is challenging considering the complex behaviour of soft tissues that are, among other things, non-linear hyperelastic and heterogeneous. Hybrid experimental-numerical methods combining in vivo indentations and inverse finite element analyses are common to identify these material parameters. Yet, the uniqueness and the uncertainty of the multi-material hyperelastic model have not been evaluated. This study presents a sensitivity analysis performed on synthetic indentation data to investigate the identification uncertainties of the material parameters in a bi-material thigh phantom. Synthetic numerical data, used to replace experimental measurements, considered several measurement modalities: indenter force and displacement, stereo-camera 3D digital image correlation of the indented surface, and ultrasound B-mode images. A finite element model of the indentation was designed with either Ogden-Moerman or Mooney-Rivlin constitutive laws for both materials. The parameters' identifiability (i.e. the possibility of converging to a unique parameter set within an acceptable margin of error) was assessed with various cost functions formulated using the different synthetic data sets. The results underline the need for multiple experimental modalities to reduce the uncertainty of the identified parameters. Additionally, the experimental error can impede the identification of a unique parameter set, and the cost function depends on the constitutive law. This study highlights the need for sensitivity analyses before the design of the experimental protocol. Such studies can also be used to define the acceptable range of errors in the experimental measurement. Eventually, the impact of the evaluated uncertainty of the identified parameters should be further investigated according to the purpose of the finite element modelling.

The relationship between muscle thickness and pennation angle is mediated by fascicle length in the muscles of the lower extremities

Muscle morphological architecture, a crucial determinant of muscle function, has fascinated researchers since the Renaissance. Imaging techniques enable the assessment of parameters such as muscle thickness (MT), pennation angle (PA), and fascicle length (FL), which may vary with growth, sex, and physical activity. Despite known interrelationships, robust mathematical models like causal mediation analysis have not been extensively applied to large population samples. We recruited 109 males and females, measuring knee flexor and extensor, and plantar flexor MT, PA, and FL using real-time ultrasound imaging at rest. A mixed-effects model explored sex, leg (dominant vs. non-dominant), and muscle region differences. Males exhibited greater MT in all muscles (0.1 to 2.1 cm, p < 0.01), with no sex differences in FL. Dominant legs showed greater rectus femoris (RF) MT (0.1 cm, p = 0.01) and PA (1.5°, p = 0.01), while vastus lateralis (VL) had greater FL (1.2 cm, p < 0.001) and PA (0.6°, p = 0.02). Regional differences were observed in VL, RF, and biceps femoris long head (BFlh). Causal mediation analyses highlighted MT's influence on PA, mediated by FL. Moderated mediation occurred in BFlh, with FL differences. Gastrocnemius medialis and lateralis exhibited FL-mediated MT and PA relationships. This study unveils the intricate interplay of MT, FL, and PA in muscle architecture.

Post-natal muscle growth and protein turnover: a narrative review of current understanding

A model explaining the dietary-protein-driven post-natal skeletal muscle growth and protein turnover in the rat is updated, and the mechanisms involved are described, in this narrative review. Dietary protein controls both bone length and muscle growth, which are interrelated through mechanotransduction mechanisms with muscle growth induced both from stretching subsequent to bone length growth and from internal work against gravity. This induces satellite cell activation, myogenesis and remodelling of the extracellular matrix, establishing a growth capacity for myofibre length and cross-sectional area. Protein deposition within this capacity is enabled by adequate dietary protein and other key nutrients. After briefly reviewing the experimental animal origins of the growth model, key concepts and processes important for growth are reviewed. These include the growth in number and size of the myonuclear domain, satellite cell activity during post-natal development and the autocrine/paracrine action of IGF-1. Regulatory and signalling pathways reviewed include developmental mechanotransduction, signalling through the insulin/IGF-1-PI3K-Akt and the Ras-MAPK pathways in the myofibre and during mechanotransduction of satellite cells. Likely pathways activated by maximal-intensity muscle contractions are highlighted and the regulation of the capacity for protein synthesis in terms of ribosome assembly and the translational regulation of 5-TOPmRNA classes by mTORC1 and LARP1 are discussed. Evidence for and potential mechanisms by which volume limitation of muscle growth can occur which would limit protein deposition within the myofibre are reviewed. An understanding of how muscle growth is achieved allows better nutritional management of its growth in health and disease.

A novel imaging method (FIM-ID) reveals that myofibrillogenesis plays a major role in the mechanically induced growth of skeletal muscle

An increase in mechanical loading, such as that which occurs during resistance exercise, induces radial growth of muscle fibers (i.e. an increase in cross-sectional area). Muscle fibers are largely composed of myofibrils, but whether radial growth is mediated by an increase in the size of the myofibrils (i.e. myofibril hypertrophy) and/or the number of myofibrils (i.e. myofibrillogenesis) is not known. Electron microscopy (EM) can provide images with the level of resolution that is needed to address this question, but the acquisition and subsequent analysis of EM images is a time- and cost-intensive process. To overcome this, we developed a novel method for visualizing myofibrils with a standard fluorescence microscope (fluorescence imaging of myofibrils with image deconvolution [FIM-ID]). Images from FIM-ID have a high degree of resolution and contrast, and these properties enabled us to develop pipelines for automated measurements of myofibril size and number. After extensively validating the automated measurements, we used both mouse and human models of increased mechanical loading to discover that the radial growth of muscle fibers is largely mediated by myofibrillogenesis. Collectively, the outcomes of this study offer insight into a fundamentally important topic in the field of muscle growth and provide future investigators with a time- and cost-effective means to study it.

Beneficial effects of resistance training on both mild and severe mouse dystrophic muscle function as a preclinical option for Duchenne muscular dystrophy

Mechanical overloading (OVL) resulting from the ablation of muscle agonists, a supra-physiological model of resistance training, reduces skeletal muscle fragility, i.e. the immediate maximal force drop following lengthening contractions, and increases maximal force production, in mdx mice, a murine model of Duchene muscular dystrophy (DMD). Here, we further analyzed these beneficial effects of OVL by determining whether they were blocked by cyclosporin, an inhibitor of the calcineurin pathway, and whether there were also observed in the D2-mdx mice, a more severe murine DMD model. We found that cyclosporin did not block the beneficial effect of 1-month OVL on plantaris muscle fragility in mdx mice, nor did it limit the increases in maximal force and muscle weight (an index of hypertrophy). Fragility and maximal force were also ameliorated by OVL in the plantaris muscle of D2-mdx mice. In addition, OVL increased the expression of utrophin, cytoplamic γ-actin, MyoD, and p-Akt in the D2-mdx mice, proteins playing an important role in fragility, maximal force gain and muscle growth. In conclusion, OVL reduced fragility and increased maximal force in the more frequently used mild mdx model but also in D2-mdx mice, a severe model of DMD, closer to human physiopathology. Moreover, these beneficial effects of OVL did not seem to be related to the activation of the calcineurin pathway. Thus, this preclinical study suggests that resistance training could have a potential benefit in the improvement of the quality of life of DMD patients.

Effects of low-load resistance training with blood flow restriction on muscle fiber myofibrillar and extracellular area

Blood flow restriction applied during low-load resistance training (LL-BFR) induces a similar increase in the cross-sectional area of muscle fibers (fCSA) compared to traditional high-load resistance training (HL-RT). However, it is unclear whether LL-BFR leads to differential changes in myofibrillar spacing in muscle fibers and/or extracellular area compared to HL-RT. Therefore, this study aimed to investigate whether the hypertrophy of type I and II fibers induced by LL-BFR or HL-RT is accompanied by differential changes in myofibrillar and non-myofibrillar areas. In addition, we examined if extracellular spacing was differentially affected between these two training protocols. Twenty recreationally active participants were assigned to LL-BFR or HL-RT groups and underwent a 6-week training program. Muscle biopsies were taken before and after the training period. The fCSA of type I and II fibers, the area occupied by myofibrillar and non-myofibrillar components, and extracellular spacing were analyzed using immunohistochemistry techniques. Despite the significant increase in type II and mean (type I + II) fCSA (p < 0.05), there were no significant changes in the proportionality of the myofibrillar and non-myofibrillar areas [∼86% and ∼14%, respectively (p > 0.05)], indicating that initial adaptations to LL-BFR are primarily characterized by conventional hypertrophy rather than disproportionate non-myofibrillar expansion. Additionally, extracellular spacing was not significantly altered between protocols. In summary, our study reveals that LL-BFR, like HL-RT, induces skeletal muscle hypertrophy with proportional changes in the areas occupied by myofibrillar, non-myofibrillar, and extracellular components.

Exercise induces tissue-specific adaptations to enhance cardiometabolic health

The risk associated with multiple cancers, cardiovascular disease, diabetes, and all-cause mortality is decreased in individuals who meet the current recommendations for physical activity. Therefore, regular exercise remains a cornerstone in the prevention and treatment of non-communicable diseases. An acute bout of exercise results in the coordinated interaction between multiple tissues to meet the increased energy demand of exercise. Over time, the associated metabolic stress of each individual exercise bout provides the basis for long-term adaptations across tissues, including the cardiovascular system, skeletal muscle, adipose tissue, liver, pancreas, gut, and brain. Therefore, regular exercise is associated with a plethora of benefits throughout the whole body, including improved cardiorespiratory fitness, physical function, and glycemic control. Overall, we summarize the exercise-induced adaptations that occur within multiple tissues and how they converge to ultimately improve cardiometabolic health.

The Plateau in Muscle Growth with Resistance Training: An Exploration of Possible Mechanisms

It is hypothesized that there is likely a finite ability for muscular adaptation. While it is difficult to distinguish between a true plateau following a long-term training period and short-term stalling in muscle growth, a plateau in muscle growth has been attributed to reaching a genetic potential, with limited discussion on what might physiologically contribute to this muscle growth plateau. The present paper explores potential physiological factors that may drive the decline in muscle growth after prolonged resistance training. Overall, with chronic training, the anabolic signaling pathways may become more refractory to loading. While measures of anabolic markers may have some predictive capabilities regarding muscle growth adaptation, they do not always demonstrate a clear connection. Catabolic processes may also constrain the ability to achieve further muscle growth, which is influenced by energy balance. Although speculative, muscle cells may also possess cell scaling mechanisms that sense and regulate their own size, along with molecular brakes that hinder growth rate over time. When considering muscle growth over the lifespan, there comes a point when the anabolic response is attenuated by aging, regardless of whether or not individuals approach their muscle growth potential. Our goal is that the current review opens avenues for future experimental studies to further elucidate potential mechanisms to explain why muscle growth may plateau.

The role of satellite cell-derived TRIM28 in mechanical load- and injury-induced myogenesis

Satellite cells are skeletal muscle stem cells that contribute to postnatal muscle growth, and they endow skeletal muscle with the ability to regenerate after a severe injury. Here we discovered that this myogenic potential of satellite cells requires a protein called tripartite motif-containing 28 (TRIM28). Unexpectedly, multiple lines of both in vitro and in vivo evidence revealed that the myogenic function of TRIM28 is not dependent on changes in the phosphorylation of its serine 473 residue. Moreover, the functions of TRIM28 were not mediated through the regulation of satellite cell proliferation or differentiation. Instead, our findings indicate that TRIM28 regulates the ability of satellite cells to progress through the process of fusion. Specifically, we discovered that TRIM28 controls the expression of a fusogenic protein called myomixer and concomitant fusion pore formation. Collectively, the outcomes of this study expose the framework of a novel regulatory pathway that is essential for myogenesis.

A Novel Imaging Method (FIM-ID) Reveals that Myofibrillogenesis Plays a Major Role in the Mechanically Induced Growth of Skeletal Muscle

An increase in mechanical loading, such as that which occurs during resistance exercise, induces radial growth of muscle fibers (i.e., an increase in cross-sectional area). Muscle fibers are largely composed of myofibrils, but whether radial growth is mediated by an increase in the size of the myofibrils (i.e., myofibril hypertrophy) and/or the number of myofibrils (i.e., myofibrillogenesis) is not known. Electron microscopy (EM) can provide images with the level of resolution that is needed to address this question, but the acquisition and subsequent analysis of EM images is a time- and cost-intensive process. To overcome this, we developed a novel method for visualizing myofibrils with a standard fluorescence microscope (FIM-ID). Images from FIM-ID have a high degree of resolution and contrast, and these properties enabled us to develop pipelines for automated measurements of myofibril size and number. After extensively validating the automated measurements, we used both mouse and human models of increased mechanical loading to discover that the radial growth of muscle fibers is largely mediated by myofibrillogenesis. Collectively, the outcomes of this study offer insight into a fundamentally important topic in the field of muscle growth and provide future investigators with a time- and cost-effective means to study it.

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.

Differences in Skeletal Muscle Fiber Characteristics Between Affected and Nonaffected Limbs in Individuals With Stroke: A Scoping Review

OBJECTIVE

The objective of this scoping review was to characterize and identify knowledge gaps about the changes in skeletal muscle fiber type proportion and cross-sectional area (CSA) after stroke.

METHODS

This scoping review followed previously proposed frameworks. A systematic search was conducted for articles examining muscle fiber type proportion and CSA in individuals with stroke in EMBASE, MEDLINE, PsycINFO, CINAHL, SPORTDiscus, and Web of Science databases from inception to December 20, 2022. Two independent authors screened and extracted the data. Results were discussed using theories proposed by the authors of the included studies.

RESULTS

Of 13 studies (115 participants), 6 (46%) were case studies or case series, 6 (46%) were cross-sectional studies, and 1 (8%) was an experimental study. Studies had small sample sizes (1-23 participants) and various muscle sampling sites (6 different muscles). All 13 studies examined muscle fiber type distributions, and 6 (46%) examined CSA. Ten (77%) studies examined differences between paretic and nonparetic muscles, and 5 (38%) compared people with stroke to people without stroke. Results from 9 of 13 studies (69%) supported a greater proportion of type II muscle fibers in the paretic limb. Of those, 4 studies (42 participants), 3 studies (17 participants), and 1 study (1 participant) saw no differences, preferential type II and type I CSA loss between limbs, respectively.

CONCLUSION

Of the limited available evidence, stroke appears to result in a shift to a higher proportion of type II muscle fibers in the paretic muscles. There are mixed results for effects on muscle fiber CSA, but there is some evidence of specific atrophy of type II muscle fibers.

IMPACT

Changes in paretic skeletal muscle fibers of individuals with stroke may explain, in part, the substantial losses in strength and power in this population. Interventions to restore type II muscle fiber size may benefit people with stroke.

The role of sympathetic innervation in neonatal muscle growth and neuromuscular contractures

Neonatal brachial plexus injury (NBPI), a leading cause of pediatric upper limb paralysis, results in disabling and incurable muscle contractures that are driven by impaired longitudinal growth of denervated muscles. A rare form of NBPI, which maintains both afferent and sympathetic muscle innervation despite motor denervation, protects against contractures. We have previously ruled out a role for NRG/ErbB signaling, the predominant pathway governing antegrade afferent neuromuscular transmission, in modulating the formation of contractures. Our current study therefore investigated the contributions of sympathetic innervation of skeletal muscle in modulating NBPI-induced contractures. Through chemical sympathectomy and pharmacologic modification with a β2 -adrenergic agonist, we discovered that sympathetic innervation alone is neither required nor sufficient to modulate contracture formation in neonatal mice. Despite this, sympathetic innervation plays an intriguing sex-specific role in mediating neonatal muscle growth, as the cross-sectional area (CSA) and volume of normally innervated male muscles were diminished by ablation of sympathetic neurons and increased by β-adrenergic stimulation. Intriguingly, the robust alterations in CSA occurred with minimal changes to normal longitudinal muscle growth as determined by sarcomere length. Instead, β-adrenergic stimulation exacerbated sarcomere overstretch in denervated male muscles, indicating potentially discrete regulation of muscle width and length. Future investigations into the mechanistic underpinnings of these distinct aspects of muscle growth are thus essential for improving clinical outcomes in patients affected by muscle disorders in which both length and width are affected.

Oral administration of a locally isolated Lactobacillus rhamnosus (NR_113332.1) improves regeneration of extensor digitorum longus muscle in mice

OBJECTIVE

The aim of this study was to examine the effects of probiotic supplementation on extensor digitorum longus (EDL) regeneration after grafting in mice.

METHODS

EDL muscles were ortho-transplanted in mice. The experimental group was given 1 ×  108 colony-forming units/g body weight of Lactobacillus rhamnosus daily after EDL muscle transplantation surgeries. EDL muscle transplants were recovered after 3, 5, 7, and 14 d post-transplantation from the control as well as the experimental animals and processed for histologic analysis.

RESULTS

At day 3 post-transplantation, the inflammatory cells had infiltrated into the grafted EDL muscles and the central section of the grafted tissue contained necrotic fibers. At day 5 post-transplantation, the concentration of inflammatory cells increased further and degenerative muscle fibers were being replaced with centrally nucleated muscle cells. The average cross-sectional area non-grafted EDL and grafted muscle in the probiotic supplemented mice at day 7 increased to 48% and 23% (P = 0.002), respectively, compared with the respective values in the control animals. Whereas in non-grafted and grafted EDL muscle it could approach 8% and 36% (P = 0.008), respectively at 14 d compared with the corresponding values of the control EDL muscle transplants. The number of muscle fibers in the non-grafted and grafted probiotic-supplemented groups increased to12% and 20% (P = 0.045) at day 7 compared with the control EDL muscle. In non-grafted and grafted EDL muscle, the number of regenerated muscle fibers increased to 73% and 64% (P = 0.110) at day 14 compared with control EDL grafted muscle.

CONCLUSION

Results of the present study regarding better regeneration of skeletal muscle fibers in the probiotic-supplemented mice than the control grafts warrant further molecular-level investigation to understand the underlying mechanism mediating the process of skeletal muscle fiber regeneration. Probiotics possibly modulate the process of muscle fiber regeneration by adjusting the composition of gut microbiota.

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

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

Regenerating Myofibers after an Acute Muscle Injury: What Do We Really Know about Them?

Injury to skeletal muscle through trauma, physical activity, or disease initiates a process called muscle regeneration. When injured myofibers undergo necrosis, muscle regeneration gives rise to myofibers that have myonuclei in a central position, which contrasts the normal, peripheral position of myonuclei. Myofibers with central myonuclei are called regenerating myofibers and are the hallmark feature of muscle regeneration. An important and underappreciated aspect of muscle regeneration is the maturation of regenerating myofibers into a normal sized myofiber with peripheral myonuclei. Strikingly, very little is known about processes that govern regenerating myofiber maturation after muscle injury. As knowledge of myofiber formation and maturation during embryonic, fetal, and postnatal development has served as a foundation for understanding muscle regeneration, this narrative review discusses similarities and differences in myofiber maturation during muscle development and regeneration. Specifically, we compare and contrast myonuclear positioning, myonuclear accretion, myofiber hypertrophy, and myofiber morphology during muscle development and regeneration. We also discuss regenerating myofibers in the context of different types of myofiber necrosis (complete and segmental) after muscle trauma and injurious contractions. The overall goal of the review is to provide a framework for identifying cellular and molecular processes of myofiber maturation that are unique to muscle regeneration.

Can muscle typology explain the inter-individual variability in resistance training adaptations?

Considerable inter-individual heterogeneity exists in the muscular adaptations to resistance training. It has been proposed that fast-twitch fibres are more sensitive to hypertrophic stimuli and thus that variation in muscle fibre type composition is a contributing factor to the magnitude of training response. This study investigated if the inter-individual variability in resistance training adaptations is determined by muscle typology and if the most appropriate weekly training frequency depends on muscle typology. In strength-training novices, 11 slow (ST) and 10 fast typology (FT) individuals were selected by measuring muscle carnosine with proton magnetic resonance spectroscopy. Participants trained both upper arm and leg muscles to failure at 60% of one-repetition maximum (1RM) for 10 weeks, whereby one arm and leg trained 3×/week and the contralateral arm and leg 2×/week. Muscle volume (MRI-based 3D segmentation), maximal dynamic strength (1RM) and fibre type-specific cross-sectional area (vastus lateralis biopsies) were evaluated. The training response for total muscle volume (+3 to +14%), fibre size (-19 to +22%) and strength (+17 to +47%) showed considerable inter-individual variability, but these could not be attributed to differences in muscle typology. However, ST individuals performed a significantly higher training volume to gain these similar adaptations than FT individuals. The limb that trained 3×/week had generally more pronounced hypertrophy than the limb that trained 2×/week, and there was no interaction with muscle typology. In conclusion, muscle typology cannot explain the high variability in resistance training adaptations when training is performed to failure at 60% of 1RM. KEY POINTS: This study investigated the influence of muscle typology (muscle fibre type composition) on the variability in resistance training adaptations and on its role in the individualization of resistance training frequency. We demonstrate that an individual's muscle typology cannot explain the inter-individual variability in resistance training-induced increases in muscle volume, maximal dynamic strength and fibre cross-sectional area when repetitions are performed to failure. Importantly, slow typology individuals performed a significantly higher training volume to obtain similar adaptations compared to fast typology individuals. Muscle typology does not determine the most appropriate resistance training frequency. However, regardless of muscle typology, an additional weekly training (3×/week vs. 2×/week) increases muscle hypertrophy but not maximal dynamic strength. These findings expand on our understanding of the underlying mechanisms for the large inter-individual variability in resistance training adaptations.

Cancer cachexia: molecular mechanisms and treatment strategies

Muscle wasting is a consequence of physiological changes or a pathology characterized by increased catabolic activity that leads to progressive loss of skeletal muscle mass and strength. Numerous diseases, including cancer, organ failure, infection, and aging-associated diseases, are associated with muscle wasting. Cancer cachexia is a multifactorial syndrome characterized by loss of skeletal muscle mass, with or without the loss of fat mass, resulting in functional impairment and reduced quality of life. It is caused by the upregulation of systemic inflammation and catabolic stimuli, leading to inhibition of protein synthesis and enhancement of muscle catabolism. Here, we summarize the complex molecular networks that regulate muscle mass and function. Moreover, we describe complex multi-organ roles in cancer cachexia. Although cachexia is one of the main causes of cancer-related deaths, there are still no approved drugs for cancer cachexia. Thus, we compiled recent ongoing pre-clinical and clinical trials and further discussed potential therapeutic approaches for cancer cachexia.

Changes in vastus lateralis fibre cross-sectional area, pennation angle and fascicle length do not predict changes in muscle cross-sectional area

NEW FINDINGS

What is the central question of this study? Do changes in myofibre cross-sectional area, pennation angle and fascicle length predict vastus lateralis whole-muscle cross-sectional area changes following resistance training? What is the main finding and its importance? Changes in vastus lateralis mean myofibre cross-sectional area, fascicle length and pennation angle following a period of resistance training did not collectively predict changes in whole-muscle cross-sectional area. Despite the limited sample size in this study, these data reiterate that it remains difficult to generalize the morphological adaptations that predominantly drive tissue-level vastus lateralis muscle hypertrophy.

ABSTRACT

Myofibre hypertrophy during resistance training (RT) poorly associates with tissue-level surrogates of hypertrophy. However, it is underappreciated that, in pennate muscle, changes in myofibre cross-sectional area (fCSA), fascicle length (Lf ) and pennation angle (PA) likely coordinate changes in whole-muscle cross-sectional area (mCSA). Therefore, we determined if changes in fCSA, PA and Lf predicted vastus lateralis (VL) mCSA changes following RT. Thirteen untrained college-aged males (23 ± 4 years old, 25.4 ± 5.2 kg/m2 ) completed 7 weeks of full-body RT (twice weekly). Right leg VL ultrasound images and biopsies were obtained prior to (PRE) and 72 h following (POST) the last training bout. Regression was used to assess if training-induced changes in mean fCSA, PA and Lf predicted VL mCSA changes. Correlations were also performed between PRE-to-POST changes in obtained variables. Mean fCSA (+18%), PA (+8%) and mCSA (+22%) increased following RT (P < 0.05), but not Lf (0.1%, P = 0.772). Changes in fCSA, Lf and PA did not collectively predict changes in mCSA (R2 = 0.282, adjusted R2 = 0.013, F3,8  = 1.050, P = 0.422). Moderate negative correlations existed for percentage changes in PA and Lf (r = -0.548, P = 0.052) and changes in fCSA and Lf (r = -0.649, P = 0.022), and all other associations were weak (|r| < 0.500). Although increases in mean fCSA, PA and VL mCSA were observed, inter-individual responses for each variable and limitations for each technique make it difficult to generalize the morphological adaptations that predominantly drive tissue-level VL muscle hypertrophy. However, the small subject pool is a significant limitation, and more research in this area is needed.

Multi-scale mechanobiological model for skeletal muscle hypertrophy

Skeletal muscle adaptation is correlated to training exercise by triggering different signaling pathways that target many functions; in particular, the IGF1-AKT pathway controls protein synthesis and degradation. These two functions regulate the adaptation in size and strength of muscles. Computational models for muscle adaptation have focused on: the biochemical description of signaling pathways or the mechanical description of muscle function at organ scale; however, an interrelation between these two models should be considered to understand how an adaptation in muscle size affects the protein synthesis rate. In this research, a dynamical model for the IGF1-AKT signaling pathway is linked to a continuum-mechanical model describing the active and passive mechanical response of a muscle; this model is used to study the impact of the adaptive muscle geometry on the protein synthesis at the fiber scale. This new computational model links the signaling pathway to the mechanical response by introducing a growth tensor, and links the mechanical response to the signaling pathway through the evolution of the protein synthesis rate. The predicted increase in cross sectional area (CSA) due to an 8 weeks training protocol excellently agreed with experimental data. Further, our results show that muscle growth rate decreases, if the correlation between protein synthesis and CSA is negative. The outcome of this study suggests that multi-scale models coupling continuum mechanical properties and molecular functions may improve muscular therapies and training protocols.

Independent pathways control muscle tissue size and sarcomere remodeling

Cell growth and proliferation must be balanced during development to attain a final adult size with the appropriate proportions of internal organs to maximize fitness and reproduction. While multiple signaling pathways coordinate Drosophila development, it is unclear how multi-organ communication within and between tissues converge to regulate systemic growth. One such growth pathway, mediated by insulin-like peptides that bind to and activate the insulin receptor in multiple target tissues, is a primary mediator of organismal size. Here we uncover a signaling role for the NUAK serine/threonine kinase in muscle tissue that impinges upon insulin pathway activity to limit overall body size, including a reduction in the growth of individual organs. In skeletal muscle tissue, manipulation of NUAK or insulin pathway components influences sarcomere number concomitant with modulation of thin and thick filament lengths, possibly by modulating the localization of Lasp, a nebulin repeat protein known to set thin filament length. This mode of sarcomere remodeling does not occur in other mutants that also exhibit smaller muscles, suggesting that a sensing mechanism exists in muscle tissue to regulate sarcomere growth that is independent of tissue size control.

Biceps femoris long head morphology in youth competitive alpine skiers is associated with age, biological maturation and traumatic lower extremity injuries

Lower extremity injuries are common in competitive alpine skiers, and the knee and lower leg are often affected. The hamstring muscles, especially the biceps femoris long head (BFlh), can stabilize the knee and the hip and may counteract various adverse loading patterns during typical mechanisms leading to severe lower extremity injuries. The aim of the present study was to describe BFlh morphology in youth competitive alpine skiers in relation to sex, age and biological maturation and to investigate its association with the occurrence of traumatic lower extremity injuries in the upcoming season. 95 youth skiers underwent anthropometric measurements, maturity offset estimations and ultrasound assessment, followed by 12-months prospective injury surveillance. Unpaired t tests showed that the two sexes did not differ in BFlh morphology, including fascicle length (Lf), pennation angle (PA), muscle thickness (MT) and average anatomical cross-sectional area (ACSA_avg). In contrast, U16 skiers had longer fascicles than U15 skiers (9.5 ± 1.3 cm vs 8.9 ± 1.3 cm, p < 0.05). Linear regression analyses revealed that maturity offset was associated with Lf (R² = 0.129, p < 0.001), MT (R² = 0.244, p < 0.001) and ACSA_avg (R² = 0.065, p = 0.007). No association was found between maturity offset and PA (p = 0.524). According to a binary logistic regression analysis, ACSA_avg was significantly associated with the occurrence of traumatic lower extremity injuries (Chi-square = 4.627, p = 0.031, R_Nagelkerke² = 0.064, Cohen f = 0.07). The present study showed that BFlh morphology is age- and biological maturation-dependent and that BFlh ACSA_avg can be considered a relevant modifiable variable associated with lower extremity injuries in youth competitive alpine skiers.

A glitch in the matrix: the pivotal role for extracellular matrix remodeling during muscle hypertrophy

Multinuclear muscle fibers are the most voluminous cells in skeletal muscle and the primary drivers of growth in response to loading. Outside the muscle fiber, however, is a diversity of mononuclear cell types that reside in the extracellular matrix (ECM). These muscle-resident cells are exercise-responsive and produce the scaffolding for successful myofibrillar growth. Without proper remodeling and maintenance of this ECM scaffolding, the ability to mount an appropriate response to resistance training in adult muscles is severely hindered. Complex cellular choreography takes place in muscles following a loading stimulus. These interactions have been recently revealed by single-cell explorations into muscle adaptation with loading. The intricate ballet of ECM remodeling involves collagen production from fibrogenic cells and ECM modifying signals initiated by satellite cells, immune cells, and the muscle fibers themselves. The acellular collagen-rich ECM is also a mechanical signal-transducer and rich repository of growth factors that may directly influence muscle fiber hypertrophy once liberated. Collectively, high levels of collagen expression, deposition, and turnover characterize a well-trained muscle phenotype. The purpose of this review is to highlight the most recent evidence for how the ECM and its cellular components affect loading-induced muscle hypertrophy. We also address how the muscle fiber may directly take part in ECM remodeling, and whether ECM dynamics are rate limiting for muscle fiber growth.

Effects of in ovo feeding of methionine and/or disaccharide on post-hatching breast development, glycogen reserves, nutrients absorption parameters, and jejunum antioxidant indices in geese

We investigated the effects of in ovo injection of methionine (Met) and/or disaccharide (DS) on breast muscle and small intestine development, and the aspect of the glycogen contents, digestive enzymes activities, and jejunal antioxidant parameters in geese after incubation. A total of 600 fertilized eggs were used in this study to be employed in a 2 × 2 factorial experiment. Eggs were randomly assigned to 4 groups, 6 replicates per group, and 25 eggs per replicate. Factors in four groups included non-injection, Met injection (5 g/L Met dissolved in 7.5 g/L NaCl), DS injection (25 g/L maltose and 25 g/L sucrose dissolved in 7.5 g/L NaCl), and DS plus Met injection (25 g/L maltose, 25 g/L sucrose, and 5 g/L Met dissolved in 7.5 g/L NaCl). As a result, birth weight, relative weight of breast muscle, diameter of myofiber, glycogen contents, jejunal villus and surface area, and jejunal digestive enzymes activities improved, while liver glucose-6-phosphatase activity decreased, by DS injection. Additionally, DS administration upregulated the expression of myogenic factor-5 (Myf-5) from breast muscle and sodium/glucose cotransporter protein-1 (SGLT-1) from jejunum. In ovo delivery of DS has long-term effects on the improvement of jejunal glucose transporter-2 (GLUT-2) and sucrase-isomaltase expression. In ovo feeding of Met improved the relative weight of breast muscle and small intestine, diameter of myofiber, length of small intestine, jejunal villus width, jejunal sucrase, Na⁺/K⁺ATPase and alkaline phosphatase activities, and jejunal glutathione (GSH) concentration, and decreased the jejunal glutathione disulfide (GSSH) and the ratio of GSSG to GSH, in early-life post-hatching. The breast muscle Myf-5 and myostatin expression, jejunal villus height and surface area, jejunal glutathione peroxidase concentration, and the expression of GLUT-2 in jejunum long-term improved by in ovo delivery of Met. Moreover, in ovo feeding of DS plus Met mixture synergistically improved the diameter of myofiber, jejunal villus height and width, jejunal sucrase, and alkaline phosphatase activities in early-life post-hatching, but long-term upregulated the expression of jejunal GLUT-2. Therefore, we concluded that in ovo injection of Met plus DS is an effective way to improve the development of gosling during post-hatching stages.

Appropriate Reporting of Exercise Variables in Resistance Training Protocols: Much more than Load and Number of Repetitions

Manipulating resistance training variables is crucial to plan the induced stimuli correctly. When reporting the exercise variables in resistance training protocols, sports scientists and practitioners often refer to the load lifted and the total number of repetitions. The present conceptual review explores all within-exercise variables that may influence the strength and hypertrophic gains, and the changes in muscle architecture. Together with the (1) load and (2) the number of repetitions, (3) performing repetitions to failure or not to failure, (4) the displacement of the load or the range of movement (full or partial), (5) the portion of the partial movement to identify the muscle length at which the exercise is performed, (6) the total time under tension, the duration of each phase and the position of the two isometric phases, (7) whether the concentric, eccentric or concentric-eccentric phase is performed, (8) the use of internal or external focus and (9) the inter-set rest may all have repercussions on the adaptations induced by each resistance exercise. Manipulating one or more variable allows to increase, equalize or decrease the stimuli related to each exercise. Sports scientists and practitioners are invited to list all aforementioned variables for each exercise when reporting resistance training protocols.

Influence of weighted downhill running training on serial sarcomere number and work loop performance in the rat soleus

Increased serial sarcomere number (SSN) has been observed in rats following downhill running training due to the emphasis on active lengthening contractions; however, little is known about the influence on dynamic contractile function. Therefore, we employed 4 weeks of weighted downhill running training in rats, then assessed soleus SSN and work loop performance. We hypothesised trained rats would produce greater net work output during work loops due to a greater SSN. Thirty-one Sprague-Dawley rats were assigned to a training or sedentary control group. Weight was added during downhill running via a custom-made vest, progressing from 5–15% body mass. Following sacrifice, the soleus was dissected, and a force-length relationship was constructed. Work loops (cyclic muscle length changes) were then performed about optimal muscle length (L_O) at 1.5–3-Hz cycle frequencies and 1–7-mm length changes. Muscles were then fixed in formalin at L_O. Fascicle lengths and sarcomere lengths were measured to calculate SSN. Intramuscular collagen content and crosslinking were quantified via a hydroxyproline content and pepsin-solubility assay. Trained rats had longer fascicle lengths (+13%), greater SSN (+8%), and a less steep passive force-length curve than controls (P<0.05). There were no differences in collagen parameters (P>0.05). Net work output was greater (+78–209%) in trained than control rats for the 1.5-Hz work loops at 1 and 3-mm length changes (P<0.05), however, net work output was more related to maximum specific force (R²=0.17-0.48, P<0.05) than SSN (R²=0.03-0.07, P=0.17-0.86). Therefore, contrary to our hypothesis, training-induced sarcomerogenesis likely contributed little to the improvements in work loop performance.

This article has an associated First Person interview with the first author of the paper.

Summary: An investigation of adaptations in mechanical function induced by a novel method of weighted downhill running training in rats, and the connections to adaptations in muscle architecture.

Comparisons between skeletal muscle imaging techniques and histology in tracking midthigh hypertrophic adaptations following 10 weeks of resistance training

This study had two aims. AIM1 was to determine the agreement between mid-thigh vastus lateralis (VL) cross-sectional area measured by ultrasound (mCSA_US) versus magnetic resonance imaging (mCSA_MRI) at a single time point, and the ability of each to detect hypertrophic changes. AIM2 was to assess the relationships between pre-to-post training changes in thigh lean mass determined by DXA, VL mCSA_US, ultrasound-determined VL thickness (VL_Thick), and VL mean myofiber cross-sectional area (fCSA) with changes in VL mCSA_MRI. Twelve untrained males (Age: 20±1 y, BMI: 26.9±5.4 kg/m²; n=12) engaged in a 10-week resistance training program (2x/week) where right mid-thigh images and VL biopsies were obtained prior to and 72-hours following the last training bout. Participants' VL mCSA_MRI (p=0.005), DXA thigh lean mass (p=0.015), and VL_Thick (p=0.001) increased following training, whereas VL mCSA_US and fCSA did not. For AIM1, mCSA_US demonstrated excellent concordance (CCC = 0.830) with mCSA_MRI, albeit mCSA_US values were systematically lower compared to mCSA_MRI (mean bias: -2.29 cm²). Additionally, PRE-to-POST VL mCSA changes between techniques exhibited good agreement (CCC = 0.700; mean bias: -1.08 cm²). For AIM2, moderate, positive correlations existed for PRE-to-POST changes in VL mCSA_MRI and DXA thigh lean mass (r=0.580, p=0.048), mCSA_US (r=0.622, p=0.031), and VL_Thick (r=0.520, p=0.080). A moderate, negative correlation existed between mCSA_MRI and fCSA (r=-0.569, p=0.054). Our findings have multiple implications: i) resistance training-induced hypertrophy was dependent on the quantification method, ii) ultrasound-determined mCSA shows good agreement with MRI, and iii) tissue-level changes poorly agreed with mean fCSA changes and this requires further research.

The influence of longitudinal muscle fascicle growth on mechanical function

Skeletal muscle has the remarkable ability to remodel and adapt, such as the increase in serial sarcomere number (SSN) or fascicle length (FL) observed after overstretching a muscle. This type of remodelling is termed longitudinal muscle fascicle growth, and its impact on biomechanical function has been of interest since the 1960s due to its clinical applications in muscle strain injury, muscle spasticity, and sarcopenia. Despite simplified hypotheses on how longitudinal muscle fascicle growth might influence mechanical function, existing literature presents conflicting results partly due to a breadth of methodologies. The purpose of this review is to outline what is currently known about the influence of longitudinal muscle fascicle growth on mechanical function and suggest future directions to address current knowledge gaps and methodological limitations. Various interventions indicate longitudinal muscle fascicle growth can increase the optimal muscle length for active force, but whether the whole force-length relationship widens has been less investigated. Future research should also explore the ability for longitudinal fascicle growth to broaden the torque-angle relationship's plateau region, and the relation to increased force during shortening. Without a concurrent increase in intramuscular collagen, longitudinal muscle fascicle growth also reduces passive tension at long muscle lengths; further research is required to understand whether this translates to increased joint range of motion. Lastly, some evidence suggests longitudinal fascicle growth can increase maximum shortening velocity and peak isotonic power, however, there has yet to be direct assessment of these measures in a neurologically intact model of longitudinal muscle fascicle growth.

Heterogeneous Skeletal Muscle Cell and Nucleus Populations Identified by Single-Cell and Single-Nucleus Resolution Transcriptome Assays

Single-cell RNA-seq (scRNA-seq) has revolutionized modern genomics, but the large size of myotubes and myofibers has restricted use of scRNA-seq in skeletal muscle. For the study of muscle, single-nucleus RNA-seq (snRNA-seq) has emerged not only as an alternative to scRNA-seq, but as a novel method providing valuable insights into multinucleated cells such as myofibers. Nuclei within myofibers specialize at junctions with other cell types such as motor neurons. Nuclear heterogeneity plays important roles in certain diseases such as muscular dystrophies. We survey current methods of high-throughput single cell and subcellular resolution transcriptomics, including single-cell and single-nucleus RNA-seq and spatial transcriptomics, applied to satellite cells, myoblasts, myotubes and myofibers. We summarize the major myonuclei subtypes identified in homeostatic and regenerating tissue including those specific to fiber type or at junctions with other cell types. Disease-specific nucleus populations were found in two muscular dystrophies, FSHD and Duchenne muscular dystrophy, demonstrating the importance of performing transcriptome studies at the single nucleus level in muscle.

Effects of High-Volume Versus High-Load Resistance Training on Skeletal Muscle Growth and Molecular Adaptations

We evaluated the effects of higher-load (HL) versus (lower-load) higher-volume (HV) resistance training on skeletal muscle hypertrophy, strength, and muscle-level molecular adaptations. Trained men (n = 15, age: 23 ± 3 years; training experience: 7 ± 3 years) performed unilateral lower-body training for 6 weeks (3× weekly), where single legs were randomly assigned to HV and HL paradigms. Vastus lateralis (VL) biopsies were obtained prior to study initiation (PRE) as well as 3 days (POST) and 10 days following the last training bout (POSTPR). Body composition and strength tests were performed at each testing session, and biochemical assays were performed on muscle tissue after study completion. Two-way within-subject repeated measures ANOVAs were performed on most dependent variables, and tracer data were compared using dependent samples t-tests. A significant interaction existed for VL muscle cross-sectional area (assessed via magnetic resonance imaging; interaction p = 0.046), where HV increased this metric from PRE to POST (+3.2%, p = 0.018) whereas HL training did not (-0.1%, p = 0.475). Additionally, HL increased leg extensor strength more so than HV training (interaction p = 0.032; HV < HL at POST and POSTPR, p < 0.025 for each). Six-week integrated non-myofibrillar protein synthesis (iNon-MyoPS) rates were also higher in the HV versus HL condition, while no difference between conditions existed for iMyoPS rates. No interactions existed for other strength, VL morphology variables, or the relative abundances of major muscle proteins. Compared to HL training, 6 weeks of HV training in previously trained men optimizes VL hypertrophy in lieu of enhanced iNon-MyoPS rates, and this warrants future research.

Scaling of fibre area and fibre glycogen concentration in the hindlimb musculature of monitor lizards: implications for locomotor performance with increasing body size

A considerable biomechanical challenge faces larger terrestrial animals as the demands of body support scale with body mass (Mb), while muscle force capacity is proportional to muscle cross-sectional area, which scales with Mb2/3. How muscles adjust to this challenge might be best understood by examining varanids, which vary by five orders of magnitude in size without substantial changes in posture or body proportions. Muscle mass, fascicle length and physiological cross-sectional area all scale with positive allometry, but it remains unclear, however, how muscles become larger in this clade. Do larger varanids have more muscle fibres, or does individual fibre cross-sectional area (fCSA) increase? It is also unknown if larger animals compensate by increasing the proportion of fast-twitch (higher glycogen concentration) fibres, which can produce higher force per unit area than slow-twitch fibres. We investigated muscle fibre area and glycogen concentration in hindlimb muscles from varanids ranging from 105 g to 40,000 g. We found that fCSA increased with modest positive scaling against body mass (Mb0.197) among all our samples, and ∝Mb0.278 among a subset of our data consisting of never-frozen samples only. The proportion of low-glycogen fibres decreased significantly in some muscles but not others. We compared our results with the scaling of fCSA in different groups. Considering species means, fCSA scaled more steeply in invertebrates (∝Mb0.575), fish (∝Mb0.347) and other reptiles (∝Mb0.308) compared with varanids (∝Mb0.267), which had a slightly higher scaling exponent than birds (∝Mb0.134) and mammals (∝Mb0.122). This suggests that, while fCSA generally increases with body size, the extent of this scaling is taxon specific, and may relate to broad differences in locomotor function, metabolism and habitat between different clades.

CCN2 participates in overload-induced skeletal muscle hypertrophy

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

A new framework for analysis of three-dimensional shape and architecture of human skeletal muscles from in vivo imaging data

A new framework is presented for comprehensive analysis of the three-dimensional shape and architecture of human skeletal muscles from magnetic resonance and diffusion tensor imaging data. The framework comprises three key features: (1) identification of points on the surface of and inside a muscle that have a correspondence to points on and inside another muscle, (2) reconstruction of average muscle shape and average muscle fibre orientations, and (3) utilization of data on between-muscle variation to visualize and make statistical inferences about changes or differences in muscle shape and architecture. The general use of the framework is demonstrated by its application to three case studies. Analysis of data obtained before and after eight weeks of strength training revealed there was little regional variation in hypertrophy of the vastus medialis and vastus lateralis, and no systematic change in pennation angle. Analysis of passive muscle lengthening revealed heterogeneous changes in shape of the medial gastrocnemius, and confirmed the ability of the methods to detect subtle changes in muscle fibre orientation. Analysis of the medial gastrocnemius of children with unilateral cerebral palsy showed that muscles in the more-affected limb were shorter, thinner and less wide than muscles in the less-affected limb, and had slightly more pennate muscle fibres in the central and proximal part of the muscle. Amongst other applications, the framework can be used to explore the mechanics of muscle contraction, investigate adaptations of muscle architecture, build anatomically realistic computational models of skeletal muscles, and compare muscle shape and architecture between species.