Effects of aging, exercise, and disease on force transfer in skeletal muscle

One Week of Single-Leg Immobilization Lowers Muscle Connective Protein Synthesis Rates in Healthy, Young Adults

PURPOSE

Short periods of limb immobilization lower myofibrillar protein synthesis rates. Within skeletal muscle, the extracellular matrix of connective proteins is recognized as an important factor determining the capacity to transmit contractile force. Little is known regarding the impact of immobilization and subsequent recovery on muscle connective protein synthesis rates. This study examined the impact of 1 wk of leg immobilization and 2 wk of subsequent ambulant recovery on daily muscle connective protein synthesis rates.

METHODS

Thirty healthy, young (24 ± 5 yr) men were subjected to 7 d of one-legged knee immobilization followed by 14 d of ambulant recovery. Deuterium oxide ingestion was applied over the entire period, and muscle biopsy samples were collected before immobilization, after immobilization, and after recovery to measure muscle connective protein synthesis rates and mRNA expression of key extracellular matrix proteins (collagen I, collagen III), glycoproteins (fibronectin, tenascin-C), and proteoglycans (fibromodulin, and decorin). A two-way repeated-measures (time-leg) ANOVA was used to compare changes in muscle connective protein synthesis rates during immobilization and recovery.

RESULTS

During immobilization, muscle connective protein synthesis rates were lower in the immobilized (1.07 ± 0.30%·d -1 ) compared with the nonimmobilized (1.48 ± 0.44%·d -1 ; P < 0.01) leg. When compared with the immobilization period, connective protein synthesis rates in the immobilized leg increased during subsequent recovery (1.48 ± 0.64%·d -1 ; P < 0.01). After recovery, skeletal muscle collagen I, collagen III, fibronectin, fibromodulin, and decorin mRNA expression increased when compared with the postimmobilization time point (all P < 0.001).

CONCLUSIONS

One week of leg immobilization lowers muscle connective protein synthesis rates. Muscle connective protein synthesis rates increase during subsequent ambulant recovery, which is accompanied by increased mRNA expression of key extracellular matrix proteins.

A critical discussion on the relationship between E3 ubiquitin ligases, protein degradation, and skeletal muscle wasting: it's not that simple

Ubiquitination is an important post-translational modification (PTM) for protein substrates, whereby ubiquitin is added to proteins through the coordinated activity of activating (E1), ubiquitin-conjugating (E2), and ubiquitin ligase (E3) enzymes. The E3s provide key functions in the recognition of specific protein substrates to be ubiquitinated and aid in determining their proteolytic or nonproteolytic fates, which has led to their study as indicators of altered cellular processes. MuRF1 and MAFbx/Atrogin-1 were two of the first E3 ubiquitin ligases identified as being upregulated in a range of different skeletal muscle atrophy models. Since their discovery, the expression of these E3 ubiquitin ligases has often been studied as a surrogate measure of changes to bulk protein degradation rates. However, emerging evidence has highlighted the dynamic and complex regulation of the ubiquitin proteasome system (UPS) in skeletal muscle and demonstrated that protein ubiquitination is not necessarily equivalent to protein degradation. These observations highlight the potential challenges of quantifying E3 ubiquitin ligases as markers of protein degradation rates or ubiquitin proteasome system (UPS) activation. This perspective examines the usefulness of monitoring E3 ubiquitin ligases for determining specific or bulk protein degradation rates in the settings of skeletal muscle atrophy. Specific questions that remain unanswered within the skeletal muscle atrophy field are also identified, to encourage the pursuit of new research that will be critical in moving forward our understanding of the molecular mechanisms that govern protein function and degradation in muscle.

Effects of dietary nitrate supplementation on peak power output: Influence of supplementation strategy and population

Increasing evidence indicates that dietary nitrate supplementation has the potential to increase muscular power output during skeletal muscle contractions. However, there is still a paucity of data characterizing the impact of different nitrate dosing regimens on nitric oxide bioavailability and its potential ergogenic effects across various population groups. This review discusses the potential influence of different dietary nitrate supplementation strategies on nitric oxide bioavailability and muscular peak power output in healthy adults, athletes, older adults and some clinical populations. Effect sizes were calculated for peak power output and absolute and/or relative nitrate doses were considered where applicable. There was no relationship between the effect sizes of peak power output change following nitrate supplementation and when nitrate dosage when considered in absolute or relative terms. Areas for further research are also recommended including a focus on nitrate dosing regimens that optimize nitric oxide bioavailability for enhancing peak power at times of increased muscular work in a variety of healthy and disease populations.

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.

Effects of dietary nitrate supplementation on muscular power output: Influence of supplementation strategy and population

Increasing evidence indicates that dietary nitrate supplementation has the potential to increase muscular power output during skeletal muscle contractions. However, there is still a paucity of data characterizing the impact of different nitrate dosing regimens on nitric oxide bioavailability its potential ergogenic effects across various population groups. This narrative review discusses the potential influence of different dietary nitrate supplementation strategies on nitric oxide bioavailability and muscular power output in healthy adults, athletes, older adults and some clinical populations. Areas for further research are also recommended including a focus individualized nitrate dosing regimens to optimize nitric oxide bioavailability and to promote muscular power enhancements in different populations.

The relationship between single muscle fibre and voluntary rate of force development in young and old males

PURPOSE

It is suggested that the early phase (< 50 ms) of force development during a muscle contraction is associated with intrinsic contractile properties, while the late phase (> 50 ms) is associated with maximal force. There are no direct investigations of single muscle fibre rate of force development (RFD) as related to joint-level RFD METHODS: Sixteen healthy, young (n = 8; 26.4 ± 1.5 yrs) and old (n = 8; 70.1 ± 2.8 yrs) males performed maximal voluntary isometric contractions (MVC) and electrically evoked twitches of the knee extensors to assess RFD. Then, percutaneous muscle biopsies were taken from the vastus lateralis and chemically permeabilized, to assess single fibre function.

RESULTS

At the joint level, older males were ~ 30% weaker and had ~ 43% and ~ 40% lower voluntary RFD values at 0-100 and 0-200 ms, respectively, than the younger ones (p ≤ 0.05). MVC torque was related to every voluntary RFD epoch in the young (p ≤ 0.001), but only the 0-200 ms epoch in the old (p ≤ 0.005). Twitch RFD was ~ 32% lower in the old compared to young (p < 0.05). There was a strong positive relationship between twitch RFD and voluntary RFD during the earliest time epochs in the young (≤ 100 ms; p ≤ 0.01). While single fibre RFD was unrelated to joint-level RFD in the young, older adults trended (p = 0.052-0.055) towards significant relationships between joint-level RTD and Type I single fibre RFD at the 0-30 ms (r² = 0.48) and 0-50 ms (r² = 0.49) time epochs.

CONCLUSION

Electrically evoked twitches are good predictors of early voluntary RFD in young, but not older adults. Only the older adults showed a potential relationship between single fibre (Type I) and joint-level rate of force development.

Anti-inflammatory therapy enables robot-actuated regeneration of aged muscle

Robot-actuated mechanical loading (ML)-based therapies ("mechanotherapies") can promote regeneration after severe skeletal muscle injury, but the effectiveness of such approaches during aging is unknown and may be influenced by age-associated decline in the healing capacity of skeletal muscle. To address this knowledge gap, this work used a noninvasive, load-controlled robotic device to impose highly defined tissue stresses to evaluate the age dependence of ML on muscle repair after injury. The response of injured muscle to robot-actuated cyclic compressive loading was found to be age sensitive, revealing not only a lack of reparative benefit of ML on injured aged muscles but also exacerbation of tissue inflammation. ML alone also disrupted the normal regenerative processes of aged muscle stem cells. However, these negative effects could be reversed by introducing anti-inflammatory therapy alongside ML application, leading to enhanced skeletal muscle regeneration even in aged mice.

Physiological Assessment of Muscle, Heart, and Whole Body Function in the Canine Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a lethal muscle disease caused by dystrophin deficiency. Patients gradually lose motor function, become wheelchair-bound, and die from respiratory and/or cardiac muscle failure. Dystrophin-null dogs have been used as a large animal model for DMD since 1988 and are considered an excellent bridge between rodent models and human patients. While numerous protocols have been published for studying muscle and heart physiology in mice, few such protocols exist for studying skeletal muscle contractility, heart function, and whole-body activity in dogs. Over the last 20 years, we have developed and adapted an array of assays to evaluate whole-body movement, gait, single muscle force, whole limb torque, cardiac electrophysiology, and hemodynamic function in normal and dystrophic dogs. In this chapter, we present detailed working protocols for these assays and lessons we learned during the development and use of these protocols.

Role of Muscle LIM Protein in Mechanotransduction Process

The induction of protein synthesis is crucial to counteract the deconditioning of neuromuscular system and its atrophy. In the past, hormones and cytokines acting as growth factors involved in the intracellular events of these processes have been identified, while the implications of signaling pathways associated with the anabolism/catabolism ratio in reference to the molecular mechanism of skeletal muscle hypertrophy have been recently identified. Among them, the mechanotransduction resulting from a mechanical stress applied to the cell appears increasingly interesting as a potential pathway for therapeutic intervention. At present, there is an open question regarding the type of stress to apply in order to induce anabolic events or the type of mechanical strain with respect to the possible mechanosensing and mechanotransduction processes involved in muscle cells protein synthesis. This review is focused on the muscle LIM protein (MLP), a structural and mechanosensing protein with a LIM domain, which is expressed in the sarcomere and costamere of striated muscle cells. It acts as a transcriptional cofactor during cell proliferation after its nuclear translocation during the anabolic process of differentiation and rebuilding. Moreover, we discuss the possible opportunity of stimulating this mechanotransduction process to counteract the muscle atrophy induced by anabolic versus catabolic disorders coming from the environment, aging or myopathies.

Ubiquitin Ligases in Longevity and Aging Skeletal Muscle

The development and prevalence of diseases associated with aging presents a global health burden on society. One hallmark of aging is the loss of proteostasis which is caused in part by alterations to the ubiquitin-proteasome system (UPS) and lysosome-autophagy system leading to impaired function and maintenance of mass in tissues such as skeletal muscle. In the instance of skeletal muscle, the impairment of function occurs early in the aging process and is dependent on proteostatic mechanisms. The UPS plays a pivotal role in degradation of misfolded and aggregated proteins. For the purpose of this review, we will discuss the role of the UPS system in the context of age-related loss of muscle mass and function. We highlight the significant role that E3 ubiquitin ligases play in the turnover of key components (e.g., mitochondria and neuromuscular junction) essential to skeletal muscle function and the influence of aging. In addition, we will briefly discuss the contribution of the UPS system to lifespan. By understanding the UPS system as part of the proteostasis network in age-related diseases and disorders such as sarcopenia, new discoveries can be made and new interventions can be developed which will preserve muscle function and maintain quality of life with advancing age.

CORP: Gene delivery into murine skeletal muscle using in vivo electroporation

The strategy of gene delivery into skeletal muscles has provided exciting avenues in identifying new potential therapeutics towards muscular disorders and addressing basic research questions in muscle physiology through overexpression and knockdown studies. In vivo electroporation methodology offers a simple, rapidly effective technique for the delivery of plasmid DNA into post-mitotic skeletal muscle fibers and the ability to easily explore the molecular mechanisms of skeletal muscle plasticity. The purpose of this review is to describe how to robustly electroporate plasmid DNA into different hindlimb muscles of rodent models. Further, key parameters (e.g., voltage, hyaluronidase, plasmid concentration) which contribute to the successful introduction of plasmid DNA into skeletal muscle fibers will be discussed. In addition, details on processing tissue for immunohistochemistry and fiber cross-sectional area (CSA) analysis will be outlined. The overall goal of this review is to provide the basic and necessary information needed for successful implementation of in vivo electroporation of plasmid DNA and thus open new avenues of discovery research in skeletal muscle physiology.

The beneficial effect of chronic muscular exercise on muscle fragility is increased by Prox1 gene transfer in dystrophic mdx muscle

Purpose

Greater muscle fragility is thought to cause the exhaustion of the muscle stem cells during successive degeneration/repair cycles, leading to muscle wasting and weakness in Duchenne muscular dystrophy. Chronic voluntary exercise can partially reduce the susceptibility to contraction induced-muscle damage, i.e., muscle fragility, as shown by a reduced immediate maximal force drop following lengthening contractions, in the dystrophic mdx mice. Here, we studied the effect of Prospero-related homeobox factor 1 gene (Prox1) transfer (overexpression) using an AAV on fragility in chronically exercised mdx mice, because Prox1 promotes slower type fibres in healthy mice and slower fibres are less fragile in mdx muscle.

Methods

Both tibialis anterior muscles of the same mdx mouse received the transfer of Prox1 and PBS and the mice performed voluntary running into a wheel during 1 month. We also performed Prox1 transfer in sedentary mdx mice. In situ maximal force production of the muscle in response to nerve stimulation was assessed before, during and after 10 lengthening contractions. Molecular muscle parameters were also evaluated.

Results

Interestingly, Prox1 transfer reduced the isometric force drop following lengthening contractions in exercised mdx mice (p < 0.05 to 0.01), but not in sedentary mdx mice. It also increased the muscle expression of Myh7 (p < 0.001), MHC-2x (p < 0.01) and Trpc1 (p < 0.01), whereas it reduced that one of Myh4 (p < 0.001) and MHC-2b (p < 0.01) in exercised mdx mice. Moreover, Prox1 transfer decreased the absolute maximal isometric force (p < 0.01), but not the specific maximal isometric force, before lengthening contraction in exercised (p < 0.01) and sedentary mdx mice.

Conclusion

Our results indicate that Prox1 transfer increased the beneficial effect of chronic exercise on muscle fragility in mdx mice, but reduced absolute maximal force. Thus, the potential clinical benefit of the transfer of Prox1 into exercised dystrophic muscle can merit further investigation.

Effect of epicatechin on skeletal muscle

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Loss of skeletal muscle (SkM) quality is associated with different clinical conditions such as aging, diabetes, obesity, cancer and heart failure. Nutritional research has focused on identifying naturally occurring molecules that mitigate the loss of SkM quality induced by a pathology or syndrome. In this context, although few human studies have been conducted, Epicatechin (Epi) is a prime candidate that may positively affect SkM quality by its potential ability to mitigate muscle mass loss. This seems to be a consequence of its antioxidant, anti-inflammatory properties, and its stimulation of mitochondrial biogenesis to increase myogenic differentiation, as well as its modulation of key proteins involved in SkM structure, function, metabolism, and growth. In conclusion, the Epi could prevent, mitigate, delay, and even treat muscle-related disorders caused by aging and diseases, however, studies in humans are needed.

Chronic Systemic Curcumin Administration Antagonizes Murine Sarcopenia and Presarcopenia

Curcumin administration attenuates muscle disuse atrophy, but its effectiveness against aging-induced, selective loss of mass or force (presarcopenia or asthenia/dynopenia), or combined loss (sarcopenia), remains controversial. A new systemic curcumin treatment was developed and tested in 18-month-old C57BL6J and C57BL10ScSn male mice. The effects on survival, liver toxicity, loss of muscle mass and force, and satellite cell responsivity and commitment were evaluated after 6-month treatment. Although only 24-month-old C57BL10ScSn mice displayed age-related muscle impairment, curcumin significantly increased survival of both strains (+20–35%), without signs of liver toxicity. Treatment prevented sarcopenia in soleus and presarcopenia in EDL of C57BL10ScSn mice, whereas it did not affect healthy-aged muscles of C57BL6J. Curcumin-treated old C57BL10ScSn soleus preserved type-1 myofiber size and increased type-2A one, whereas EDL maintained adult values of total myofiber number and fiber-type composition. Mechanistically, curcumin only partially prevented the age-related changes in protein level and subcellular distribution of major costamere components and regulators. Conversely, it affected satellite cells, by maintaining adult levels of myofiber maturation in old regenerating soleus and increasing percentage of isolated, MyoD-positive satellite cells from old hindlimb muscles. Therefore, curcumin treatment successfully prevents presarcopenia and sarcopenia development by improving satellite cell commitment and recruitment.

Extensor carpi ulnaris muscle shows unexpected slow-to-fast fiber type switch in Duchenne muscular dystrophy dogs

Aged dystrophin-null canines are excellent models for studying experimental therapies for Duchenne muscular dystrophy, a lethal muscle disease caused by dystrophin deficiency. To establish the baseline, we studied the extensor carpi ulnaris (ECU) muscle in 15 terminal age (3-year-old) male affected dogs and 15 age/sex-matched normal dogs. Affected dogs showed histological and anatomical hallmarks of dystrophy, including muscle inflammation and fibrosis, myofiber size variation and centralized myonuclei, as well as a significant reduction of muscle weight, muscle-to-body weight ratio and muscle cross-sectional area. To rigorously characterize the contractile properties of the ECU muscle, we developed a novel in situ assay. Twitch and tetanic force, contraction and relaxation rate, and resistance to eccentric contraction-induced force loss were significantly decreased in affected dogs. Intriguingly, the time-to-peak tension and half-relaxation time were significantly shortened in affected dogs. Contractile kinetics predicted an unforeseen slow-to-fast myofiber-type switch, which we confirmed at the protein and transcript level. Our study establishes a foundation for studying long-term and late-stage therapeutic interventions in dystrophic canines. The unexpected myofiber-type switch highlights the complexity of muscle remodeling in dystrophic large mammals.

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

Summary: A slow-to-fast fiber-type switch in dystrophic canine ECU muscle is revealed by contraction kinetics and myosin protein and transcript expression. This highlights the complexity of muscle remodeling in Duchenne muscular dystrophy.

Biomechanical Properties of the Sarcolemma and Costameres of Skeletal Muscle Lacking Desmin

Intermediate filaments (IFs), composed primarily by desmin and keratins, link the myofibrils to each other, to intracellular organelles, and to the sarcolemma. There they may play an important role in transfer of contractile force from the Z-disks and M-lines of neighboring myofibrils to costameres at the membrane, across the membrane to the extracellular matrix, and ultimately to the tendon (“lateral force transmission”). We measured the elasticity of the sarcolemma and the connections it makes at costameres with the underlying contractile apparatus of individual fast twitch muscle fibers of desmin-null mice. By positioning a suction pipet to the surface of the sarcolemma and applying increasing pressure, we determined the pressure at which the sarcolemma separated from nearby sarcomeres, P_separation, and the pressure at which the isolated sarcolemma burst, P_bursting. We also examined the time required for the intact sarcolemma-costamere-sarcomere complex to reach equilibrium at lower pressures. All measurements showed the desmin-null fibers to have slower equilibrium times and lower P_separation and P_bursting than controls, suggesting that the sarcolemma and its costameric links to nearby contractile structures were weaker in the absence of desmin. Comparisons to earlier values determined for muscles lacking dystrophin or synemin suggest that the desmin-null phenotype is more stable than the former and less stable than the latter. Our results are consistent with the moderate myopathy seen in desmin-null muscles and support the idea that desmin contributes significantly to sarcolemmal stability and lateral force transmission.

Effects of aging on basement membrane-related gene expression of the skeletal muscle in rats

The basement membrane (BM), with collagen IV as a major component, plays an important role in the maintenance of muscle structure and its robustness. To investigate the effects of aging on factors related to BM construction, we compared the expression status of these factors in 3- and 20-month-old male Wistar rats. The expression levels of Col4a1 and Col4a2 (encoding collagen IV), Sparc (involved in collagen IV functionalization), and Mmp14 (a collagen IV degradation factor) were decreased. These results suggest that aging suppresses collagen IV synthetic and degradative factors and affects BM-related factors in the steady state.

Effect of a 12-week endurance training program on force transfer and membrane integrity proteins in lean, obese, and type 2 diabetic subjects

The mechanisms accounting for the loss of muscle function with obesity and type 2 diabetes are likely the result of a combination of neural and muscular factors. One muscular factor that is important, yet has received little attention, is the protein machinery involved in longitudinal and lateral force transmission. The purpose of this study was to compare the levels of force transfer and membrane integrity proteins before and after a 12‐week endurance training program in lean, obese, and obese type 2 diabetic adults. Nineteen sedentary subjects (male = 8 and female = 11) were divided into three groups: Lean (n = 7; 50.3 ± 4.1 y; 69.1 ± 7.2 kg); Obese (n = 6; 49.8 ± 4.1 y; 92.9 ± 19.5 kg); and Obese with type 2 diabetes (n = 6; 51.5 ± 7.9 years; 88.9 ± 15.1 kg). Participants trained 150 min/week between 55% and 75% of VO_2max for 12 weeks. Skeletal muscle biopsies were taken before and after the training intervention. Baseline dystrophin and muscle LIM protein levels were higher (~50% p < .01) in lean compared to obese and type 2 diabetic adults, while the protein levels of the remaining force transfer and membrane integrity proteins were similar between groups. After training, obese individuals decreased (−53%; p < .01) the levels of the muscle ankyrin repeat protein and lean individuals decreased dystrophin levels (−45%; p = .01), while the levels of the remaining force transfer and membrane integrity proteins were not affected by training. These results suggest that there are modest changes to force transfer and membrane integrity proteins in middle‐aged individuals as a result of 12 weeks of lifestyle and training interventions.

Identification of the MuRF1 Skeletal Muscle Ubiquitylome Through Quantitative Proteomics

MuRF1 (TRIM63) is a muscle-specific E3 ubiquitin ligase and component of the ubiquitin proteasome system. MuRF1 is transcriptionally upregulated under conditions that cause muscle loss, in both rodents and humans, and is a recognized marker of muscle atrophy. In this study, we used in vivo electroporation to determine whether MuRF1 overexpression alone can cause muscle atrophy and, in combination with ubiquitin proteomics, identify the endogenous MuRF1 substrates in skeletal muscle. Overexpression of MuRF1 in adult mice increases ubiquitination of myofibrillar and sarcoplasmic proteins, increases expression of genes associated with neuromuscular junction instability, and causes muscle atrophy. A total of 169 ubiquitination sites on 56 proteins were found to be regulated by MuRF1. MuRF1-mediated ubiquitination targeted both thick and thin filament contractile proteins, as well as, glycolytic enzymes, deubiquitinases, p62, and VCP. These data reveal a potential role for MuRF1 in not only the breakdown of the sarcomere but also the regulation of metabolism and other proteolytic pathways in skeletal muscle.

A prolonged hiatus in postmenopausal HRT, does not nullify the therapy's positive impact on ageing related sarcopenia

Background

Previous work suggest a positive skeletal muscle effect of hormone replacement therapy (HRT) on skeletal muscle characteristics This study aimed to quantify any continued positive effect of HRT even after a sustained hiatus in treatment, controlling for two key muscle modulation hormones: Estradiol (E2) and Tri-iodo-thyronine (T3).

Method and findings

In 61 untrained women (18-78yrs) stratified as pre-menopausal, post-menopausal without (No_HRT) and post-menopausal with (Used_HRT) HRT history, body composition, physical activity, serum E2 and T3 were assessed by dual energy x-ray absorptiometry, Baecke questionnaire and ELISA. Gastrocnemius medialis (GM) and tibialis anterior (TA) electromyographic profiles (mean power frequency (mPowerF)), isometric plantar-flexion (PF) and dorsi-flexion (DF) maximum voluntary contraction (MVC), rate of torque development (RTD), isokinetic MVC and muscle volume, were assessed using surface electromyography, dynamometry and ultrasonography. Muscle quality was quantified as MVC per unit muscle size. E2 and E2:T3 ratio were significantly lower in postmenopausal participants, and were positively correlated with RTD even after controlling for adiposity and/or age. Pre-menopausal females had greater MVC in 8/8 PF and 2/5 DF (23.7–98.1%; P<0.001–0.049) strength measures compared to No_HRT, but only 6/8 PF (17.4–42.3%; P<0.001–0.046) strength measures compared to Used_HRT. Notably, Used_HRT had significant higher MVC in 7 PF MVC (30.0%-37.7%; P = 0.006–0.031) measures than No_HRT, while premenopausal and Used_HRT had similar uncorrected muscle size or quality. In addition, this cross-sectional data suggest an annual reduction in GM muscle volume corrected for intra-muscular fat by 1.3% in No_HRT and only 0.5% in Used_HRT.

Conclusion

Even years after cessation of the therapy, a history of HRT is positively associated with negating the expected post-menopausal drop in muscle quantity and quality. Whilst mPowerF did not differ between groups, our work highlights positive associations between RTD against E2 and E2:T3. Notwithstanding our study limitation of single time point for blood sampling, our work is the first to illustrate an HRT attenuation of ageing-related decline in RTD. We infer from these data that high E2, even in the absence of high T3, may help maintain muscle contractile speed and quality. Thus our work is the first to points to markedly larger physiological reserves in women with a past history of HRT.

The Human Muscle Size and Strength Relationship: Effects of Architecture, Muscle Force, and Measurement Location

PURPOSE

This study aimed to determine the best muscle size index of muscle strength by establishing if incorporating muscle architecture measurements improved the human muscle size-strength relationship. The influence of calculating muscle force, and the location of anatomical cross-sectional area (ACSA) measurements on this relationship were also examined.

METHODS

Fifty-two recreationally active males completed unilateral isometric knee extension strength assessments and MRI scans of the dominant thigh and knee to determine quadriceps femoris (QF) size variables (ACSA along the length of the femur, maximum ACSA [ACSAMAX] and volume [VOL]) and patellar tendon moment arm. Ultrasound images (2 sites per constituent muscle) were analyzed to quantify muscle architecture (fascicle length, pennation angle), and when combined with VOL (from MRI), facilitated calculation of QF effective PCSA (EFFPCSA) as potentially the best muscle size determinant of strength. Muscle force was calculated by dividing maximum voluntary torque (MVT) by the moment arm and addition of antagonist torque (derived from hamstring EMG).

RESULTS

The associations of EFFPCSA (r=0.685), ACSAMAX (r=0.697), or VOL (r=0.773) with strength did not differ, although qualitatively VOL explained 59.8% of the variance in strength, ~11-13% greater than EFFPCSA or ACSAMAX. All muscle size variables had weaker associations with muscle force than MVT. The association of strength-ACSA at 65% of femur length (r=0.719) was greater than for ACSA measured between 10-55% and 75-90% (r=-0.042-0.633) of femur length.

CONCLUSIONS

In conclusion, using contemporary methods to assess muscle architecture and calculate EFFPCSA did not enhance the muscle strength-size association. For understanding/monitoring muscle size, the major determinant of strength, these findings support the assessment of muscle volume, that is independent of architecture measurements, and was most highly correlated to strength.

The Dystrophin Node as Integrator of Cytoskeletal Organization, Lateral Force Transmission, Fiber Stability and Cellular Signaling in Skeletal Muscle

The systematic bioanalytical characterization of the protein product of the DMD gene, which is defective in the pediatric disorder Duchenne muscular dystrophy, led to the discovery of the membrane cytoskeletal protein dystrophin. Its full-length muscle isoform Dp427-M is tightly linked to a sarcolemma-associated complex consisting of dystroglycans, sarcoglyans, sarcospan, dystrobrevins and syntrophins. Besides these core members of the dystrophin-glycoprotein complex, the wider dystrophin-associated network includes key proteins belonging to the intracellular cytoskeleton and microtubular assembly, the basal lamina and extracellular matrix, various plasma membrane proteins and cytosolic components. Here, we review the central role of the dystrophin complex as a master node in muscle fibers that integrates cytoskeletal organization and cellular signaling at the muscle periphery, as well as providing sarcolemmal stabilization and contractile force transmission to the extracellular region. The combination of optimized tissue extraction, subcellular fractionation, advanced protein co-purification strategies, immunoprecipitation, liquid chromatography and two-dimensional gel electrophoresis with modern mass spectrometry-based proteomics has confirmed the composition of the core dystrophin complex at the sarcolemma membrane. Importantly, these biochemical and mass spectrometric surveys have identified additional members of the wider dystrophin network including biglycan, cavin, synemin, desmoglein, tubulin, plakoglobin, cytokeratin and a variety of signaling proteins and ion channels.

Master Regulators of Muscle Atrophy: Role of Costamere Components

The loss of muscle mass and force characterizes muscle atrophy in several different conditions, which share the expression of atrogenes and the activation of their transcriptional regulators. However, attempts to antagonize muscle atrophy development in different experimental contexts by targeting contributors to the atrogene pathway showed partial effects in most cases. Other master regulators might independently contribute to muscle atrophy, as suggested by our recent evidence about the co-requirement of the muscle-specific chaperone protein melusin to inhibit unloading muscle atrophy development. Furthermore, melusin and other muscle mass regulators, such as nNOS, belong to costameres, the macromolecular complexes that connect sarcolemma to myofibrils and to the extracellular matrix, in correspondence with specific sarcomeric sites. Costameres sense a mechanical load and transduce it both as lateral force and biochemical signals. Recent evidence further broadens this classic view, by revealing the crucial participation of costameres in a sarcolemmal “signaling hub” integrating mechanical and humoral stimuli, where mechanical signals are coupled with insulin and/or insulin-like growth factor stimulation to regulate muscle mass. Therefore, this review aims to enucleate available evidence concerning the early involvement of costamere components and additional putative master regulators in the development of major types of muscle atrophy.

Remodeling the Skeletal Muscle Extracellular Matrix in Older Age-Effects of Acute Exercise Stimuli on Gene Expression

With advancing age, the skeletal muscle extracellular matrix (ECM) undergoes fibrotic changes that may lead to increased muscle stiffness, injury susceptibility and strength loss. This study tested the potential of different exercises to counter these changes by stimulating the activity of genes associated with ECM remodeling. Twenty-six healthy men (66.9 ± 3.9 years) were stratified to two of four groups, performing unilateral (i) conventional resistance exercise, (ii) conventional resistance exercise followed by self-myofascial release (CEBR), (iii) eccentric-only exercise (ECC) or (iv) plyometric jumps (PLY). The non-trained leg served as control. Six hours post-exercise, vastus lateralis muscle biopsy samples were analyzed for the expression of genes associated with ECM collagen synthesis (COL1A1), matrix metallopeptidases (collagen degradation; MMPs) and peptidase inhibitors (TIMP1). Significant between-group differences were found for MMP3, MMP15 and TIMP1, with the greatest responses in MMP3 and TIMP1 seen in CEBR and in MMP15 in ECC. MMP9 (3.24–3.81-fold change) and COL1A1 (1.47–2.40-fold change) were increased in CEBR and PLY, although between-group differences were non-significant. The expression of ECM-related genes is exercise-specific, with CEBR and PLY triggering either earlier or stronger remodeling than other stimuli. Training studies will test whether execution of such exercises may help counter age-associated muscle fibrosis.

Identification and characterization of Fbxl22, a novel skeletal muscle atrophy-promoting E3 ubiquitin ligase

Muscle-specific E3 ubiquitin ligases have been identified in muscle atrophy-inducing conditions. The purpose of the current study was to explore the functional role of F-box and leucine-rich protein 22 (Fbxl22), and a newly identified splice variant (Fbxl22-193), in skeletal muscle homeostasis and neurogenic muscle atrophy. In mouse C₂C₁₂ muscle cells, promoter fragments of the Fbxl22 gene were cloned and fused with the secreted alkaline phosphatase reporter gene to assess the transcriptional regulation of Fbxl22. The tibialis anterior muscles of male C57/BL6 mice (12-16 wk old) were electroporated with expression plasmids containing the cDNA of two Fbxl22 splice variants and tissues collected after 7, 14, and 28 days. Gastrocnemius muscles of wild-type and muscle-specific RING finger 1 knockout (MuRF1 KO) mice were electroporated with an Fbxl22 RNAi or empty plasmid and denervated 3 days posttransfection, and tissues were collected 7 days postdenervation. The full-length gene and novel splice variant are transcriptionally induced early (after 3 days) during neurogenic muscle atrophy. In vivo overexpression of Fbxl22 isoforms in mouse skeletal muscle leads to evidence of myopathy/atrophy, suggesting that both are involved in the process of neurogenic muscle atrophy. Knockdown of Fbxl22 in the muscles of MuRF1 KO mice resulted in significant additive muscle sparing 7 days after denervation. Targeting two E3 ubiquitin ligases appears to have a strong additive effect on protecting muscle mass loss with denervation, and these findings have important implications in the development of therapeutic strategies to treat muscle atrophy.

Desmin prevents muscle wasting, exaggerated weakness and fragility, and fatigue in dystrophic mdx mouse

KEY POINTS

Desmin, similar to dystrophin, is associated with costameric structures bridging sarcomeres to the extracellular matrix. Deletion of the desmin gene in mdx mice [double knockout (DKO) mice] induces marked muscle weakness and fatigue resistance compared to mdx mice. Muscle fragility (higher susceptibility to contraction-induced injury) was also aggravated in DKO mice compared to mdx mice. By contrast to mdx mice, the DKO mice did not undergo muscle hypertrophy. Desmin cDNA transfer with adeno-associated virus in newborn mdx mice reduced muscle weakness. Overall, desmin plays important and beneficial roles in muscle wasting, performance and fragility in dystrophic muscle.

ABSTRACT

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disease caused by dystrophin deficiency. Desmin, similar to dystrophin, is associated with costameric structures bridging sarcomeres to the extracellular matrix that contributes to muscle function. In the present study, we attempted to provide further insight into the roles of desmin, for which the expression is increased in the muscle from the mouse mdx DMD model. We show that a deletion of the desmin gene (Des) in mdx mice [double knockout (DKO) mice, mdx:desmin-/-] induces a marked muscle weakness; namely, a reduced absolute maximal force production and increased fatigue compared to that in mdx mice. Fragility (i.e. higher susceptibility to contraction-induced injury) was also aggravated in DKO mice compared to mdx mice, despite the promotion of supposedly less fragile muscle fibres in DKO mice, and this worsening of fragility was related to a decreased muscle excitability. Moreover, in contrast to mdx mice, the DKO mice did not undergo muscle hypertrophy, as indicated by smaller and fewer fibres, with a reduced percentage of centronucleated fibres, potentially explaining the severe muscle weakness. Notably, Desmin cDNA transfer with adeno-associated virus in newborn mdx mice improved specific maximal force normalized to muscle weight. Overall, desmin plays important and beneficial roles in muscle wasting, performance and fragility in dystrophic mdx mice, which differ, at least in part, from those observed in healthy muscle.

Aerobic and resistance exercise training reverses age-dependent decline in NAD+ salvage capacity in human skeletal muscle

Aging decreases skeletal muscle mass and strength, but aerobic and resistance exercise training maintains skeletal muscle function. NAD⁺ is a coenzyme for ATP production and a required substrate for enzymes regulating cellular homeostasis. In skeletal muscle, NAD⁺ is mainly generated by the NAD⁺ salvage pathway in which nicotinamide phosphoribosyltransferase (NAMPT) is rate‐limiting. NAMPT decreases with age in human skeletal muscle, and aerobic exercise training increases NAMPT levels in young men. However, whether distinct modes of exercise training increase NAMPT levels in both young and old people is unknown. We assessed the effects of 12 weeks of aerobic and resistance exercise training on skeletal muscle abundance of NAMPT, nicotinamide riboside kinase 2 (NRK2), and nicotinamide mononucleotide adenylyltransferase (NMNAT) 1 and 3 in young (≤35 years) and older (≥55 years) individuals. NAMPT in skeletal muscle correlated negatively with age (r² = 0.297, P < 0.001, n = 57), and VO₂peak was the best predictor of NAMPT levels. Moreover, aerobic exercise training increased NAMPT abundance 12% and 28% in young and older individuals, respectively, whereas resistance exercise training increased NAMPT abundance 25% and 30% in young and in older individuals, respectively. None of the other proteins changed with exercise training. In a separate cohort of young and old people, levels of NAMPT, NRK1, and NMNAT1/2 in abdominal subcutaneous adipose tissue were not affected by either age or 6 weeks of high‐intensity interval training. Collectively, exercise training reverses the age‐dependent decline in skeletal muscle NAMPT abundance, and our findings highlight the value of exercise training in ameliorating age‐associated deterioration of skeletal muscle function.

Is Tensiomyography-Derived Velocity of Contraction a Sensitive Marker to Detect Acute Performance Changes in Elite Team-Sport Athletes?

PURPOSE

To examine the variations in the velocity of contraction (Vc) assessed using tensiomyography, vertical jumping ability, and sprinting speed induced by 4 different exercise protocols (ie, strength, sprint, plyometric, and technical training sessions) in 14 male national-team rugby players (age 21.8 [2.6] y, weight 83.6 [8.5] kg, and height 177.4 [6.7] cm).

METHODS

Physical tests were conducted immediately before and after 4 distinct workouts in the following order: tensiomyography in the rectus femoris and biceps femoris muscles, squat and countermovement jumps, and 30-m sprint velocity. To analyze the differences in the assessed variables before and after each training session, the differences based on magnitudes were calculated.

RESULTS

After strength and plyometric workouts, the players presented possible to almost certain impairments in sprint and jump performance and in the Vc of the rectus femoris (effect sizes 0.26-0.64). After the sprint-training session, possible to very likely decreases were observed in the squat jump, 30-m sprint, and Vc of the biceps femoris (effect sizes 0.21-0.44). By contrast, after the technical training, athletes demonstrated a possible increase in the squat jump and Vc in both muscles examined (effect sizes 0.13-0.20).

CONCLUSIONS

The main finding of this research is that, for the vast majority of results, the direction of changes observed in Vc were the same as those observed in performance assessments. This suggests that Vc might be used as a sensitive marker of acute variations in speed and power performance of elite team-sport athletes.

Tensiomyography Derived Parameters Reflect Skeletal Muscle Architectural Adaptations Following 6-Weeks of Lower Body Resistance Training

Measurement of muscle specific contractile properties in response to resistance training (RT) can provide practitioners valuable information regarding physiological status of individuals. Field based measurements of such contractile properties within specific muscle groups, could be beneficial when monitoring efficacy of training or rehabilitation interventions. Tensiomyography (TMG) quantifies contractile properties of individual muscles via an electrically stimulated twitch contraction and may serve as a viable option in the aforementioned applications. Thus, aims of this study were; (i) to investigate the potential use of TMG to quantify training adaptations and differences, in response to exercise specific lower limb RT; and (ii) investigate any associations between TMG parameters and accompanying muscle architectural measures. Non-resistance trained male participants (n = 33) were randomly assigned to 1 of 3 single-exercise intervention groups (n = 11 per group); back squat (BS), deadlift (DL), or hip thrust (HT). Participants completed a 6-week linearized training program (2× per week), where the assigned exercise was the sole method of lower body training. Pre- and post-intervention testing of maximal dynamic strength was assessed by one repetition maximum (1RM) of BS, DL, and HT. Radial muscle belly displacement (Dm) and contraction time (Tc) were obtained via TMG from the rectus femoris (RF) and vastus lateralis (VL) pre- and post-intervention, alongside muscle architectural measures (pennation angle and muscle thickness). All three groups displayed significant increases all 1RM strength tests (p < 0.001; pη2 = 0.677-0.753). Strength increases were accompanied by significant overall increases in RF muscle thickness (p < 0.001, pη2 = 0.969), and pennation angle (p = 0.007, pη2 = 0.220). Additionally, an overall reduction in RF Dm (p < 0.001, pη2 = 0.427) was observed. Significant negative relationships were observed between RF Dm and pennation angle (p = 0.003, r = -0.36), and with RF Dm and muscle thickness (p < 0.001, r = -0.50). These findings indicate that TMG is able to detect improved contractile properties, alongside improvements in muscle function within an untrained population. Furthermore, the observed associations between Dm and muscle architecture suggest that TMG contractile property assessments could be used to obtain information on muscle geometry.

Discovery proteomics in aging human skeletal muscle finds change in spliceosome, immunity, proteostasis and mitochondria

A decline of skeletal muscle strength with aging is a primary cause of mobility loss and frailty in older persons, but the molecular mechanisms of such decline are not understood. Here, we performed quantitative proteomic analysis from skeletal muscle collected from 58 healthy persons aged 20 to 87 years. In muscle from older persons, ribosomal proteins and proteins related to energetic metabolism, including those related to the TCA cycle, mitochondria respiration, and glycolysis, were underrepresented, while proteins implicated in innate and adaptive immunity, proteostasis, and alternative splicing were overrepresented. Consistent with reports in animal models, older human muscle was characterized by deranged energetic metabolism, a pro-inflammatory environment and increased proteolysis. Changes in alternative splicing with aging were confirmed by RNA-seq analysis. We propose that changes in the splicing machinery enables muscle cells to respond to a rise in damage with aging.

As humans age, their muscles become weaker, making it increasingly harder for them to move, a condition known as sarcopenia. Analyzing old muscles in other animals revealed that they produce energy inefficiently, they destroy more proteins than younger muscles, and they have high levels of molecules that cause inflammation. These characteristics may be involved in causing muscle weakness.

Proteomics is the study of proteins, the molecules that play many roles in keeping the body working: for example, they accelerate chemical reactions, participate in copying DNA and help cells respond to stimuli. Using proteomics, it is possible to examine a large number of the different proteins in a tissue, which can provide information about the state of that tissue. Ubaida-Mohien et al. used this approach to answer the question of why muscles become weaker with age.

First, they analyzed the levels of all the proteins found in skeletal muscle collected from 58 healthy volunteers between 20 and 87 years of age. This revealed that the muscles of older people have fewer copies of the proteins that make up ribosomes – the cellular machines that produce new proteins – and fewer proteins involved in providing the cell with chemical energy. In contrast, proteins implicated in the immune system, in the maintenance of existing proteins, and in processing other molecules called RNAs were more abundant in older muscles.

Ubaida-Mohien et al. then looked more closely at changes involving RNA processing. Cells make proteins by copying DNA sequences into an RNA template and using this template to instruct the ribosomes on how to make the specific protein. Before the RNA can be ‘read’ by a ribosome, however, some parts must be cut out and others added, which can lead to different versions of the final RNA, also known as alternative transcripts.

In order to check whether the difference in the levels of proteins that process RNAs was affecting the RNAs being produced, Ubaida-Mohien et al. extracted the RNAs from older and younger muscles and compared them. This showed that the RNA in older people had more alternative transcripts, confirming that the change in protein levels was having downstream effects.

Currently, it is not possible to prevent or delay the loss of muscle strength associated with aging. Understanding how the protein make-up of muscles changes as humans grow older may help find new ways to prevent and perhaps even reverse this decline.

Alterations in the muscle force transfer apparatus in aged rats during unloading and reloading: impact of microRNA-31

KEY POINTS

Force transfer is integral for maintaining skeletal muscle structure and function. One important component is dystrophin. There is limited understanding of how force transfer is impacted by age and loading. Here, we investigate the force transfer apparatus in muscles of adult and old rats exposed to periods of disuse and reloading. Our results demonstrate an increase in dystrophin protein during the reloading phase in the adult tibialis anterior muscle that is delayed in the old muscle. The consequence of this delay is an increased susceptibility towards contraction-induced muscle injury. Central to the lack of dystrophin protein is an increase in miR-31, a microRNA that inhibits dystrophin translation. In vivo electroporation with a miR-31 sponge led to increased dystrophin protein and decreased contraction-induced muscle injury in old skeletal muscle. Overall, our results detail the importance of the force transfer apparatus and provide new mechanisms for contraction-induced injury in ageing skeletal muscle.

ABSTRACT

In healthy muscle, the dystrophin-associated glycoprotein complex (DGC), the integrin/focal adhesion complex, intermediate filaments and Z-line proteins transmit force from the contractile proteins to the extracellular matrix. How loading and age affect these proteins is poorly understood. The experiments reported here sought to determine the effect of ageing on the force transfer apparatus following muscle unloading and reloading. Adult (9 months) and old (28 months) rats were subjected to 14 days of hindlimb unloading and 1, 3, 7 and 14 days of reloading. The DGC complex, intermediate filament and Z-line protein and mRNA levels, as well as dystrophin-targeting miRNAs (miR-31, -146b and -374) were examined in the tibialis anterior (TA) and medial gastrocnemius muscles at both ages. There was a significant increase in dystrophin protein levels (2.79-fold) upon 3 days of reloading in the adult TA muscle that did not occur in the old rats (P ≤ 0.05), and the rise in dystrophin protein occurred independent of dystrophin mRNA. The disconnect between dystrophin protein and mRNA levels can partially be explained by age-dependent differences in miR-31. The impaired dystrophin response in aged muscle was followed by an increase in other force transfer proteins (β-dystroglycan, desmuslin and LIM) that was not sufficient to prevent membrane disruption and muscle injury early in the reloading period. Inserting a miR-31 sponge increased dystrophin protein and decreased contraction-induced injury in the TA (P ≤ 0.05). Collectively, these data suggest that increased miR-31 with age contributes to an impaired dystrophin response and increased muscle injury after disuse.

An equine-assisted therapy intervention to improve pain, range of motion, and quality of life in adults and older adults with arthritis: A randomized controlled trial

RESEARCH AIM

To compare equine-assisted therapy to exercise education on pain, range of motion, and quality of life in adults and older adults with arthritis.

BACKGROUND

Quality of life for adults and older adults is negatively impacted by arthritis pain, stiffness, and decreased function. Equine-assisted therapy provides unique movements to the rider's joints and muscles improving pain, range of motion, and quality of life and has improved outcomes in balance, gait, strength, functional mobility, and spasticity for older adults, stroke, spinal cord injury, and multiple sclerosis patients. No research has investigated the effects on adults and older adults with arthritis.

METHODS

Twenty adults and older adults with arthritis recruited from rheumatology clinics participated in a randomized controlled trial for six weeks. Participants and research assistants were blinded to assignment. Standardized valid and reliable instruments were used to measure pain, range of motion, and quality of life targeting back, knees, shoulders, and hips.

RESULTS

Mean age was 63.85 (SD 6.885, 53-75) years. Pain significantly improved in shoulders (p = 0.007), hips (p = 0.027), and back (p = 0.006), not knees (p = 0.061). Range of motion improved for back (p = 0.02), hips (p = 0.04), shoulders (p = 0.005) and not knees. Quality of life improved for upper limb (p = 0.002), lower limb (p = 0.021), and affect (p = 0.030), not social interaction and symptoms.

CONCLUSION

This randomized controlled trial provides evidence that equine-assisted therapy decreases pain, and improves range of motion, and quality of life for adults and older adults with arthritis. Further fully powered research with cost/benefit outcomes would be beneficial.

Optimal length, calcium sensitivity and twitch characteristics of skeletal muscles from mdm mice with a deletion in N2A titin

During isometric contractions, the optimal length of skeletal muscles increases with decreasing activation. The underlying mechanism for this phenomenon is thought to be linked to length dependence of Ca²⁺ sensitivity. Muscular dystrophy with myositis (mdm), a recessive titin mutation in mice, was used as a tool to study the role of titin in activation dependence of optimal length and length dependence of Ca²⁺ sensitivity. We measured the shift in optimal length between tetanic and twitch stimulation in mdm and wild-type muscles, and the length dependence of Ca²⁺ sensitivity at short and long sarcomere lengths in mdm and wild-type fiber bundles. The results indicate that the mdm mutation leads to a loss of activation dependence of optimal length without the expected change in length dependence of Ca²⁺ sensitivity, demonstrating that these properties are not linked, as previously suggested. Furthermore, mdm muscles produced maximum tetanic stress during sub-optimal filament overlap at lengths similar to twitch contractions in both genotypes, but the difference explains less than half of the observed reduction in active force of mdm muscles. Mdm muscles also exhibited increased electromechanical delay, contraction and relaxation times, and decreased rate of force development in twitch contractions. We conclude that the small deletion in titin associated with mdm in skeletal muscles alters force production, suggesting an important regulatory role for titin in active force production. The molecular mechanisms for titin's role in regulating muscle force production remain to be elucidated.

The skeletal muscle fiber: a mechanically sensitive cell

The plasticity of skeletal muscle, whether an increase in size, change in metabolism, or alteration in structural properties, is in a continuous state of flux largely dependent upon physical activity. Much of the past research has expounded upon these ever-changing aspects of the muscle fiber following exercise. Specifically, endocrine and paracrine signaling have been heavily investigated lending to much of the past literature comprised of such endocrinological dynamics following muscle activity. Mechanotransduction, the ability of a cell to convert a mechanical stimulus into an intracellular biochemical response, has garnered much less attention. Recent work, however, has demonstrated the physical continuity of the muscle fiber, specifically demonstrating a continuous physical link between the extracellular matrix (ECM), cytoskeleton, and nuclear matrix as a means to rapidly regulate gene expression following a mechanical stimulus. Similarly, research has shown mechanical stimuli to directly influence cytoplasmic signaling whether through oxidative adaptations, increased muscle size, or enhanced muscle integrity. Regrettably, minimal research has investigated the role that exercise may play within the mechanotransducing signaling cascades. This proposed line of study may prove paramount as muscle-related diseases greatly impact one's ability to lead an independent lifestyle along with contributing a substantial burden upon the economy. Thus, this review explores both biophysical and biochemical mechanotransduction, and how these signaling pathways may be influenced following exercise.

Skeletal Muscle Adaptations and Passive Muscle Stiffness in Cerebral Palsy: A Literature Review and Conceptual Model

This literature review focuses on the primary morphological and structural characteristics, and mechanical properties identified in muscles affected by spastic cerebral palsy (CP). CP is a non-progressive neurological disorder caused by brain damage and is commonly diagnosed at birth. Although the brain damage is not progressive, subsequent neuro-physiological developmental adaptations may initiate changes in muscle structure, function, and composition, causing abnormal muscle activity and coordination. The symptoms of CP vary among patients. However, muscle spasticity is commonly present and is one of the most debilitating effects of CP. Here, we present the current knowledge regarding the mechanical properties of skeletal tissue affected by spastic CP. An increase in sarcomere length, collagen content, and fascicle diameter, and a reduction in the number of satellite cells within spastic CP muscle were consistent findings in the literature. Studies differed, however, in changes in fascicle lengths and fiber diameters. We also present a conceptual mechanical model of fascicle force transmission that incorporates mechanisms that impact both serial and lateral force production, highlighting the connections between the macro and micro structures of muscle to assist in deducing specific mechanisms for property changes and reduced force production.

Molecular Regulation of Exercise-Induced Muscle Fiber Hypertrophy

Skeletal muscle hypertrophy is a widely sought exercise adaptation to counteract the muscle atrophy of aging and disease, or to improve athletic performance. While this desired muscle enlargement is a well-known adaptation to resistance exercise training (RT), the mechanistic underpinnings are not fully understood. The purpose of this review is thus to provide the reader with a summary of recent advances in molecular mechanisms-based on the most current literature-that are thought to promote RT-induced muscle hypertrophy. We have therefore focused this discussion on the following areas of fertile investigation: ribosomal function and biogenesis, muscle stem (satellite) cell activity, transcriptional regulation, mechanotransduction, and myokine signaling.

Adaptations to Endurance and Strength Training

The capacity for human exercise performance can be enhanced with prolonged exercise training, whether it is endurance- or strength-based. The ability to adapt through exercise training allows individuals to perform at the height of their sporting event and/or maintain peak physical condition throughout the life span. Our continued drive to understand how to prescribe exercise to maximize health and/or performance outcomes means that our knowledge of the adaptations that occur as a result of exercise continues to evolve. This review will focus on current and new insights into endurance and strength-training adaptations and will highlight important questions that remain as far as how we adapt to training.

Shear strain rate from phase contrast velocity encoded MRI: Application to study effects of aging in the medial gastrocnemius muscle

BACKGROUND

Strain rate (SR) is a measure of the rate of regional deformation that can be computed by analyzing velocity-encoded phase-contrast 2D images. Recent studies have explored the changes in normal components of the strain tensor in aging muscle, while shear strain may also provide valuable information.

PURPOSE

To compute the shear SR from velocity-encoded MRI of the lower leg and to study the correlation of SR parameters measured in the medial gastrocnemius (MG) to muscle force in a cohort of young and senior subjects.

STUDY TYPE

Prospective cohort study.

SUBJECTS

Six young (26.1 ± 2.3 years) and six senior (76.7 ± 8.3 years) healthy females; two other subjects were scanned on three separate occasions for repeatability studies.

FIELD STRENGTH/SEQUENCE

1.5T using a single oblique sagittal slice with velocity-encoding in three directions (velocity-encoded phase contrast gradient echo sequence).

ASSESSMENT

Age-related and regional differences in the SR eigenvalues (SR_fiber , SR_in-plane ), normal SRs (SR_ff , SR_cc ), and shear SRs (SR_fc , SR_{fc_max} ) were statistically analyzed.

STATISTICAL TESTS

Difference between young and senior cohorts were assessed using two-way analysis of variance (ANOVAs). The coefficient of variation and repeatability coefficient were calculated from repeat studies. Univariate and stepwise multivariable linear regression was performed to identify predictors of force.

RESULTS

During isometric plantarflexion contraction, SRs in the principal basis (SR_fiber , SR_in-plane ) and maximum shear SR (SR_{fc_max} ) was significantly lower in the senior cohort (P < 0.05). On multiple variable regression, maximum shear SR (SR_{fc_max} ) and normal SR in the fiber cross-section (SR_cc ) were significantly associated with force (R = 0.681, F = 14.034, P < 0.001).

DATA CONCLUSION

This study establishes that computation of shear strain is feasible and is a significant predictor of force variability with age.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;47:1351-1357.

Relationship between skeletal muscle contractile properties and power production capacity in female Olympic rugby players

This study aimed to determine which contractile properties measured by tensiomyography (TMG) could better differentiate athletes with high- and low-power values, as well as to analyse the relationship between contractile properties and power production capacity. The contractile properties of the vastus medialis (VM), rectus femoris (RF) and vastus lateralis (VL) of an Olympic women's Rugby Sevens team (n = 14) were analysed before a Wingate test in which their peak power output (PPO) was determined. Athletes were then divided into a high-power (HP) and a low-power (LP) group. HP presented an almost certainly higher PPO (9.8 ± 0.3 vs. 8.9 ± 0.4 W kg^-1, ES = 3.00) than LP, as well as a very likely lower radial displacement (3.39 ± 1.16 vs. 5.65 ± 1.50 mm, ES = 1.68) and velocity of deformation (0.08 ± 0.02 vs. 0.13 ± 0.03 mm ms^-1, ES = 1.87) of the VL. A likely lower time of delay was observed in HP for all analysed muscles (ES > 0.60). PPO was very largely related to the radial displacement (r = -0.75, 90% CI = -0.90 to -0.44) and velocity of deformation (r = -0.70, 90% CI = -0.87 to -0.34) of the VL. A large correlation was found between PPO and the time of delay of the VL (r = -0.61, 90% CI = -0.84 to -0.22). No correlations were found for the contractile properties of RF or VM. These results highlight the importance of VL contractile properties (but not so much those of RF and VM) for maximal power production and suggest TMG as a practical technique for its evaluation.

Effect of speed endurance training and reduced training volume on running economy and single muscle fiber adaptations in trained runners

The aim of the present study was to examine whether improved running economy with a period of speed endurance training and reduced training volume could be related to adaptations in specific muscle fibers. Twenty trained male (n = 14) and female (n = 6) runners (maximum oxygen consumption (VO2 -max): 56.4 ± 4.6 mL/min/kg) completed a 40-day intervention with 10 sessions of speed endurance training (5-10 × 30-sec maximal running) and a reduced (36%) volume of training. Before and after the intervention, a muscle biopsy was obtained at rest, and an incremental running test to exhaustion was performed. In addition, running at 60% vVO2 -max, and a 10-km run was performed in a normal and a muscle slow twitch (ST) glycogen-depleted condition. After compared to before the intervention, expression of mitochondrial uncoupling protein 3 (UCP3) was lower (P < 0.05) and dystrophin was higher (P < 0.05) in ST muscle fibers, and sarcoplasmic reticulum calcium ATPase 1 (SERCA1) was lower (P < 0.05) in fast twitch muscle fibers. Running economy at 60% vVO2 -max (11.6 ± 0.2 km/h) and at v10-km (13.7 ± 0.3 km/h) was ~2% better (P < 0.05) after the intervention in the normal condition, but unchanged in the ST glycogen-depleted condition. Ten kilometer performance was improved (P < 0.01) by 3.2% (43.7 ± 1.0 vs. 45.2 ± 1.2 min) and 3.9% (45.8 ± 1.2 vs. 47.7 ± 1.3 min) in the normal and the ST glycogen-depleted condition, respectively. VO2 -max was the same, but vVO2 -max was 2.0% higher (P < 0.05; 19.3 ± 0.3 vs. 18.9 ± 0.3 km/h) after than before the intervention. Thus, improved running economy with intense training may be related to changes in expression of proteins linked to energy consuming processes in primarily ST muscle fibers.

Age-related Differences in Dystrophin: Impact on Force Transfer Proteins, Membrane Integrity, and Neuromuscular Junction Stability

The loss of muscle strength with age has been studied from the perspective of a decline in muscle mass and neuromuscular junction (NMJ) stability. A third potential factor is force transmission. The purpose of this study was to determine the changes in the force transfer apparatus within aging muscle and the impact on membrane integrity and NMJ stability. We measured an age-related loss of dystrophin protein that was greatest in the flexor muscles. The loss of dystrophin protein occurred despite a twofold increase in dystrophin mRNA. Importantly, this disparity could be explained by the four- to fivefold upregulation of the dystromir miR-31. To compensate for the loss of dystrophin protein, aged muscle contained increased α-sarcoglycan, syntrophin, sarcospan, laminin, β1-integrin, desmuslin, and the Z-line proteins α-actinin and desmin. In spite of the adaptive increase in other force transfer proteins, over the 48 hours following lengthening contractions, the old muscles showed more signs of impaired membrane integrity (fourfold increase in immunoglobulin G-positive fibers and 70% greater dysferlin mRNA) and NMJ instability (14- to 96-fold increases in Runx1, AchRδ, and myogenin mRNA). Overall, these data suggest that age-dependent alterations in dystrophin leave the muscle membrane and NMJ more susceptible to contraction-induced damage even before changes in muscle mass are obvious.

Skeletal muscle and resistance exercise training; the role of protein synthesis in recovery and remodeling

Exercise results in the rapid remodeling of skeletal muscle. This process is underpinned by acute and chronic changes in both gene and protein synthesis. In this short review we provide a brief summary of our current understanding regarding how exercise influences these processes as well as the subsequent impact on muscle protein turnover and resultant shift in muscle phenotype. We explore concepts of ribosomal biogenesis and the potential role of increased translational capacity vs. translational efficiency in contributing to muscular hypertrophy. We also examine whether high-intensity sprinting-type exercise promotes changes in protein turnover that lead to hypertrophy or merely a change in mitochondrial content. Finally, we propose novel areas for future study that will fill existing knowledge gaps in the fields of translational research and exercise science.

Genetic variation and exercise-induced muscle damage: implications for athletic performance, injury and ageing

Prolonged unaccustomed exercise involving muscle lengthening (eccentric) actions can result in ultrastructural muscle disruption, impaired excitation-contraction coupling, inflammation and muscle protein degradation. This process is associated with delayed onset muscle soreness and is referred to as exercise-induced muscle damage. Although a certain amount of muscle damage may be necessary for adaptation to occur, excessive damage or inadequate recovery from exercise-induced muscle damage can increase injury risk, particularly in older individuals, who experience more damage and require longer to recover from muscle damaging exercise than younger adults. Furthermore, it is apparent that inter-individual variation exists in the response to exercise-induced muscle damage, and there is evidence that genetic variability may play a key role. Although this area of research is in its infancy, certain gene variations, or polymorphisms have been associated with exercise-induced muscle damage (i.e. individuals with certain genotypes experience greater muscle damage, and require longer recovery, following strenuous exercise). These polymorphisms include ACTN3 (R577X, rs1815739), TNF (-308 G>A, rs1800629), IL6 (-174 G>C, rs1800795), and IGF2 (ApaI, 17200 G>A, rs680). Knowing how someone is likely to respond to a particular type of exercise could help coaches/practitioners individualise the exercise training of their athletes/patients, thus maximising recovery and adaptation, while reducing overload-associated injury risk. The purpose of this review is to provide a critical analysis of the literature concerning gene polymorphisms associated with exercise-induced muscle damage, both in young and older individuals, and to highlight the potential mechanisms underpinning these associations, thus providing a better understanding of exercise-induced muscle damage.

Mitochondrial impairment induced by postnatal ActRIIB blockade does not alter function and energy status in exercising mouse glycolytic muscle in vivo

Because it leads to a rapid and massive muscle hypertrophy, postnatal blockade of the activin type IIB receptor (ActRIIB) is a promising therapeutic strategy for counteracting muscle wasting. However, the functional consequences remain very poorly documented in vivo. Here, we have investigated the impact of 8-wk ActRIIB blockade with soluble receptor (sActRIIB-Fc) on gastrocnemius muscle anatomy, energy metabolism, and force-generating capacity in wild-type mice, using totally noninvasive magnetic resonance imaging (MRI) and dynamic(31)P-MRS. Compared with vehicle (PBS) control, sActRIIB-Fc treatment resulted in a dramatic increase in body weight (+29%) and muscle volume (+58%) calculated from hindlimb MR imaging, but did not alter fiber type distribution determined via myosin heavy chain isoform analysis. In resting muscle, sActRIIB-Fc treatment induced acidosis and PCr depletion, thereby suggesting reduced tissue oxygenation. During an in vivo fatiguing exercise (6-min repeated maximal isometric contraction electrically induced at 1.7 Hz), maximal and total absolute forces were larger in sActRIIB-Fc treated animals (+26 and +12%, respectively), whereas specific force and fatigue resistance were lower (-30 and -37%, respectively). Treatment with sActRIIB-Fc further decreased the maximal rate of oxidative ATP synthesis (-42%) and the oxidative capacity (-34%), but did not alter the bioenergetics status in contracting muscle. Our findings demonstrate in vivo that sActRIIB-Fc treatment increases absolute force-generating capacity and reduces mitochondrial function in glycolytic gastrocnemius muscle, but this reduction does not compromise energy status during sustained activity. Overall, these data support the clinical interest of postnatal ActRIIB blockade.