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Decline in mitochondrial function is linked to decreased muscle mass and strength in conditions like sarcopenia and type 2 diabetes. Despite therapeutic opportunities, there is limited and equivocal data regarding molecular cues controlling muscle mitochondrial plasticity. Here we uncovered that the mitochondrial mRNA-stabilizing protein SLIRP, in complex with LRPPRC, is a PGC-1α target that regulates mitochondrial structure, respiration, and mtDNA-encoded-mRNA pools in skeletal muscle. Exercise training effectively counteracts mitochondrial defects caused by genetically-induced LRPPRC/SLIRP loss, despite sustained low mtDNA-encoded-mRNA pools, by increasing mitoribosome translation capacity and mitochondrial quality control. In humans, exercise training robustly increases muscle SLIRP and LRPPRC protein across exercise modalities and sexes, yet less prominently in individuals with type 2 diabetes. SLIRP muscle loss reduces Drosophila lifespan. Our data points to a mechanism of post-transcriptional mitochondrial regulation in muscle via mitochondrial mRNA stabilization, offering insights into how exercise enhances mitoribosome capacity and mitochondrial quality control to alleviate defects., Competing Interests: Competing interests Since the study concluded, Solvejg Hansen, Jens Frey Halling and Anders Krogh Lemminger have become employees and shareholders of Novo Nordisk A/S, Denmark. The remaining authors declare no competing interests., (© 2024. The Author(s).)

To investigate the effect of moderate or heavy slow resistance training on structural and mechanical properties of patellar and Achilles tendons in older men, in vivo. Healthy older men (n = 27) undertook a 12-week resistance training program (3 times/week) of triceps surae and quadriceps muscle-tendon complexes. Participants were randomly assigned to either a moderate load of 55% 1RM (O55, n = 13, age: 70.0 ± 4.6 years) or a progressively heavier load from 55% up to 80% 1RM (O80, n = 14, age: 69.8 ± 4.4 years). Tendon morphology was investigated using a 1.5 T MRI scanner. Mechanical properties of the free Achilles tendon (AT) and medial gastrocnemius (MG) aponeurosis, patellar tendon (PT), and deep aponeurosis of the vastus lateralis (VL) muscle were assessed using ultrasound imaging. Both training programs led to an increase in mean CSA of the PT (O55; +2.7%, O80; +1.7%, p = 0.002) and AT (O55; +2.8%, O80; +2.5%, p < 0.001). Force and stress were increased for all the four regions investigated, while stiffness and Young's modulus assessed at a common force level were only increased for the AT and VL aponeurosis. No interaction between group and time was demonstrated for any of the outcomes. No interaction between time and the four tendon sites was observed for any of the outcomes at common force. A supervised short-term heavy or moderate slow resistance training of equal volume yielded an increase in tendon size and altered mechanical properties of tendinous tissue., (© 2024 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

It is well known that a training intervention leads to mitochondrial adaptations with increased skeletal muscle mitochondrial biogenesis and function. Studies have recently indicated that skeletal muscle mitochondrial function is important for athletic performance. During exercise reactive oxygen species are released from skeletal muscle potentially leading to adaptations but maybe also to fatigue. Focus has been on how chronic antioxidant supplementation affects a training adaptation, where some studies are reporting an abolished adaptation. Whether acute antioxidant supplementation could have a positive effect on fatigue and performance is interesting and highly relevant in sports where athletes are competing over several consecutive days or on the same day, with preliminary competitions in the morning and finals in the afternoon, where it is important for the athletes to recover fast. This review provides an overview of the effects of acute antioxidant supplementation and whether it leads to improved performance and/or faster recovery in humans., Competing Interests: Declaration of competing interest No competing interest are related to this review., (Copyright © 2024 The Author. Published by Elsevier Inc. All rights reserved.)

Objective: This study investigated the effectiveness of supramaximal high-intensity interval training (supramaximal HIT) on muscle capacities and physical function compared to moderate-intensity training (MIT) for older adults., Methods: Sixty-eight older adults (66-79 years, 56% women), not engaged in regular exercise, were randomised to 3 months of twice-weekly supramaximal HIT (20 minutes including 10 × 6-second intervals) or MIT (40 minutes including 3 × 8-minute intervals). Both groups performed the training on stationary bicycles in a group setting. Target intensity was watt-controlled, with standardised cadence and individualised resistance. Outcomes analysed with linear-mixed models included leg power (Nottingham Power Rig), hand grip strength (Jamar dynamometer), static and dynamic balance (One leg stance, 30-second step test), chair stand (30-second chair stand), and anaerobic cycling performance (modified Borg Cycle Strength Test)., Results: Baseline values were (supramaximal HIT/MIT, mean ± SD) leg power 198 ± 60/189 ± 53 W, hand grip strength 4.2 ± 1.0/4.3 ± 1.1 N/kg, static balance 64 ± 41/62 ± 41 s, dynamic balance 39 ± 7/38 ± 5 steps, chair stands 22 ± 6/22 ± 6 and anaerobic cycling performance 224 ± 60/217 ± 55 W. At 3-month follow-up, a between-group difference in favour of supramaximal HIT [95% CI] was observed in anaerobic cycling performance of 19[3;35] W. Within-group mean changes for supramaximal HIT/MIT were for leg power 8.4[0.9;15.8]/6.0[-1.3;13.3] W, hand grip strength 0.14[0.00;0.27]/0.13[-0.01;0.26] N/kg, static balance 11[3;20]/10[1;18] s, dynamic balance 1.6[0.3;2.8]/2.3[1.1;3.6] steps, 2.1[1.1;3.1]/1.4[0.4;2.3] chair stands and anaerobic cycling performance 31.3[19.6;43.0]/12.0[0.4;23.5] W., Conclusion: Supramaximal HIT showed superior effect on anaerobic cycling performance when compared to MIT. Additionally, the results indicate that supramaximal HIT is comparably beneficial as MIT in terms of effects on muscle capacity and physical function for older adults., (© The Author(s) 2024. Published by Oxford University Press on behalf of the British Geriatrics Society.)

It is not clear if fat oxidation is attenuated at higher exercise intensities, when exercising with a small muscle mass, and therefore, we studied leg fat oxidation during graded one-leg exercise. Ten males (age: 27 ± 2 years, body mass: 82 ± 3 kg, BMI: 24 ± 1 kg m -2 , V̇O 2max : 49 ± 2 mL min -1  kg -1 ) performed one-leg exercise at 25% of maximal workload (W max ) for 30 min, followed by 120-min exercise at 55% W max with the contralateral leg, and finally 30-min exercise at 85% W max with the first leg. Blood was sampled from an artery and both femoral veins, and blood flow was determined using Doppler ultrasound. Muscle biopsies were obtained before and after 30 min at each workload. One-way RM ANOVA was applied to determine the impact of exercise intensity. Data are expressed as mean ± SEM. From rest through exercise average blood flow (0.4 ± 0.1, 2.1 ± 0.1, 2.6 ± 0.2, 3.7 ± 0.2 L min -1 ) and oxygen uptake across the leg (0.03 ± 0.01, 0.23 ± 0.02, 0.35 ± 0.03, 0.53 ± 0.04 L min -1 ) increased with exercise intensity (p < 0.001). Leg RQ (0.76 ± 0.04, 0.86 ± 0.02,0.87 ± 0.01, 0.92 ± 0.01, p < 0.001), leg plasma FA uptake (2 ± 2, 46 ± 8,83 ± 9, 114 ± 16 μmol min -1 ; p < 0.001) and rate of leg fat oxidation (0.016 ± 0.005, 0.062 ± 0.012, 0.075 ± 0.011, 0.084 ± 0.018 g min -1 , p < 0.007) increased with exercise intensity. Muscle-free carnitine content was unchanged from rest at 25% W max and decreased after 30 min exercise at 55% and 85% W max (17.4 ± 1.6, 16.6 ± 0.7, 14.5 ± 1.2, 10.5 ± 1.0 mmol/kg dry muscle, respectively; p < 0.006). During incremental one-leg exercise, the rate of leg fat oxidation was not attenuated with increasing exercise intensity, probably due to an insufficient muscle metabolic stress response., (© 2024 The Author(s). Scandinavian Journal of Medicine & Science In Sports published by John Wiley & Sons Ltd.)

Prematurity has physical consequences, such as lower birth weight, decreased muscle mass and increased risk of adult-onset metabolic disease. Insulin-like growth factor 1 (IGF-1) has therapeutic potential to improve the growth and quality of muscle and tendon in premature births, and thus attenuate some of these sequalae. We investigated the effect of IGF-1 on extensor carpi radialis muscle and biceps brachii tendon of preterm piglets. The preterm group consisted of 19-day-old preterm (10 days early) piglets, treated with either IGF-1 or vehicle. Term controls consisted of groups of 9-day-old piglets (D9) and 19-day-old piglets (D19). Muscle samples were analysed by immunofluorescence to determine the cross-sectional area (CSA) of muscle fibres, fibre type composition, satellite cell content and central nuclei-containing fibres in the muscle. Tendon samples were analysed for CSA, collagen content and maturation, and vascularization. Gene expression of the tendon was measured by RT-qPCR. Across all endpoints, we found no significant effect of IGF-1 treatment on preterm piglets. Preterm piglets had smaller muscle fibre CSA compared to D9 and D19 control group. Satellite cell content was similar across all groups. For tendon, we found an effect of age on tendon CSA, and mRNA levels of COL1A1, tenomodulin and scleraxis. Immunoreactivity for elastin and CD31, and several markers of tendon maturation, were increased in D9 compared to the preterm piglets. Collagen content was similar across groups. IGF-1 treatment of preterm-born piglets does not influence the growth and maturation of skeletal muscle and tendon., (© 2024 The Authors. Experimental Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

Traumatic muscle injury represents a collection of skeletal muscle pathologies caused by trauma to the muscle tissue and is defined as damage to the muscle tissue that can result in a functional deficit. Traumatic muscle injury can affect people across the lifespan and can result from high stresses and strains to skeletal muscle tissue, often due to muscle activation while the muscle is lengthening, resulting in indirect and non-contact muscle injuries (strains or ruptures), or from external impact, resulting in direct muscle injuries (contusion or laceration). At a microscopic level, muscle fibres can repair focal damage but must be completely regenerated after full myofibre necrosis. The diagnosis of muscle injury is based on patient history and physical examination. Imaging may be indicated to eliminate differential diagnoses. The management of muscle injury has changed within the past 5 years from initial rest, immobilization and (over)protection to early activation and progressive loading using an active approach. One challenge of muscle injury management is that numerous medical treatment options, such as medications and injections, are often used or proposed to try to accelerate muscle recovery despite very limited efficacy evidence. Another challenge is the prevention of muscle injury owing to the multifactorial and complex nature of this injury., (© 2023. Springer Nature Limited.)

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.

Background: The occurrence of hyperplasia, through myofibre splitting, remains a widely debated phenomenon. Structural alterations and fibre typing of skeletal muscle fibres, as seen during regeneration and in certain muscle diseases, can be challenging to interpret. Neuromuscular electrical stimulation can induce myofibre necrosis followed by changes in spatial and temporal cellular processes. Thirty days following electrical stimulation, remnants of regeneration can be seen in the myofibre and its basement membrane as the presence of small myofibres and encroachment of sarcolemma and basement membrane (suggestive of myofibre branching/splitting). The purpose of this study was to investigate myofibre branching and fibre type in a systematic manner in human skeletal muscle undergoing adult regenerative myogenesis., Methods: Electrical stimulation was used to induce myofibre necrosis to the vastus lateralis muscle of one leg in 5 young healthy males. Muscle tissue samples were collected from the stimulated leg 30 days later and from the control leg for comparison. Biopsies were sectioned and stained for dystrophin and laminin to label the sarcolemma and basement membrane, respectively, as well as ATPase, and antibodies against types I and II myosin, and embryonic and neonatal myosin. Myofibre branches were followed through 22 serial Sects. (264 μm). Single fibres and tissue blocks were examined by confocal and electron microscopy, respectively., Results: Regular branching of small myofibre segments was observed (median length 144 μm), most of which were observed to fuse further along the parent fibre. Central nuclei were frequently observed at the point of branching/fusion. The branch commonly presented with a more immature profile (nestin + , neonatal myosin + , disorganised myofilaments) than the parent myofibre, together suggesting fusion of the branch, rather than splitting. Of the 210 regenerating muscle fibres evaluated, 99.5% were type II fibres, indicating preferential damage to type II fibres with our protocol. Furthermore, these fibres demonstrated 7 different stages of "fibre-type" profiles., Conclusions: By studying the regenerating tissue 30 days later with a range of microscopy techniques, we find that so-called myofibre branching or splitting is more likely to be fusion of myotubes and is therefore explained by incomplete regeneration after a necrosis-inducing event., (© 2023. BioMed Central Ltd., part of Springer Nature.)

Permanent loss of muscle function seen after an Achilles tendon rupture may partly be explained by tendon elongation and accompanying shortening of the muscle. Muscle fascicle length shortens, serial sarcomere number is reduced, and the sarcomere length is unchanged after Achilles tendon transection (ATT), and these changes are mitigated with suturing. The method involved in this study was a controlled laboratory study. Two groups of rats underwent ATT on one side with a contralateral control (CTRL): A ) ATT with 3 mm removal of the Achilles tendon and no suturing (substantial tendon elongation), and B ) ATT with suture repair (minimal tendon elongation). The operated limb was immobilized for 2 wk to reduce load. Four weeks after surgery the rats were euthanized, and hindlimbs were analyzed for tendon length, gastrocnemius medialis (GM) muscle mass, length, fascicle length, sarcomere number and length. No differences were observed between the groups, and in both groups the Achilles tendon length was longer (15.2%, P < 0.001), GM muscle mass was smaller (17.5%, P < 0.001), and muscle length was shorter (8.2%, P < 0.001) on the ATT compared with CTRL side. GM fascicle length was shorter (11.2%, P < 0.001), and sarcomere number was lower (13.8%, P < 0.001) on the ATT side in all regions. Sarcomere length was greater in the proximal (5.8%, P < 0.001) and mid (4.2%, P = 0.003), but not distal region on the ATT side. In this animal model, regardless of suturing, ATT resulted in tendon elongation, loss of muscle mass and length, and reduced serial sarcomere number, which resulted in an "overshoot" lengthening of the sarcomeres. NEW & NOTEWORTHY Following acute Achilles tendon rupture, patients are often left with functional deficits. The specific reason remains largely unknown. The shortened muscle leads to reduced fascicle length, in turn leading to adaptation by reduced serial sarcomere numbers. Surprisingly, this adaptation appears to "overshoot" and lead to increased sarcomere length. The present animal model advances understanding of how muscle sarcomeres, which are difficult to measure in humans, are affected when undue elongation takes place after tendon rupture.

Mitochondria are the cellular organelles responsible for resynthesising the majority of ATP. In skeletal muscle, there is an increased ATP turnover during resistance exercise to sustain the energetic demands of muscle contraction. Despite this, little is known regarding the mitochondrial characteristics of chronically strength-trained individuals and any potential pathways regulating the strength-specific mitochondrial remodelling. Here, we investigated the mitochondrial structural characteristics in skeletal muscle of strength athletes and age-matched untrained controls. The mitochondrial pool in strength athletes was characterised by increased mitochondrial cristae density, decreased mitochondrial size, and increased surface-to-volume ratio, despite similar mitochondrial volume density. We also provide a fibre-type and compartment-specific assessment of mitochondria morphology in human skeletal muscle, which reveals across groups a compartment-specific influence on mitochondrial morphology that is largely independent of fibre type. Furthermore, we show that resistance exercise leads to signs of mild mitochondrial stress, without an increase in the number of damaged mitochondria. Using publicly available transcriptomic data we show that acute resistance exercise increases the expression of markers of mitochondrial biogenesis, fission and mitochondrial unfolded protein responses (UPR mt ). Further, we observed an enrichment of the UPR mt in the basal transcriptome of strength-trained individuals. Together, these findings show that strength athletes possess a unique mitochondrial remodelling, which minimises the space required for mitochondria. We propose that the concurrent activation of markers of mitochondrial biogenesis and mitochondrial remodelling pathways (fission and UPR mt ) with resistance exercise may be partially responsible for the observed mitochondrial phenotype of strength athletes. KEY POINTS: Untrained individuals and strength athletes possess comparable skeletal muscle mitochondrial volume density. In contrast, strength athletes' mitochondria are characterised by increased cristae density, decreased size and increased surface-to-volume ratio. Type I fibres have an increased number of mitochondrial profiles with minor differences in the mitochondrial morphological characteristics compared with type II fibres. The mitochondrial morphology is distinct across the subcellular compartments in both groups, with subsarcolemmal mitochondria being bigger in size when compared with intermyofibrillar. Acute resistance exercise leads to signs of mild morphological mitochondrial stress accompanied by increased gene expression of markers of mitochondrial biogenesis, fission and mitochondrial unfolded protein response (UPR mt )., (© 2023 The Authors. The Journal of Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

Skeletal muscle possesses adaptability to mechanical loading and regenerative potential following muscle injury due to muscle stem cell activity. So far, it is known that muscle stem cell activity is supported by the roles of several interstitial cells within skeletal muscle in response to muscle damage. The adjacent tendon is also exposed to repetitive mechanical loading and possesses plasticity like skeletal muscle. However, the interplay between the skeletal muscle and adjacent tendon tissue has not been fully investigated. In this study, we tested whether factors released by three-dimensional engineered human tendon constructs in response to uniaxial tensile loading can stimulate the proliferation and differentiation of human-derived myogenic cells (myoblasts). Tendon constructs were subjected to repetitive mechanical loading (4% strain at 0.5 Hz for 4 h) and nonrepetitive loading (0% strain at 0 Hz for 4 h), and the conditioned media from mechanically loaded and nonmechanically loaded control constructs were applied to myoblasts. Immunofluorescence analysis revealed both an increase of myotube fusion index (≥5 nuclei within one desmin+ myotube) and the myotube diameter when conditioned medium from mechanically loaded tendon constructs was applied. Myostatin, myosin heavy chain 7, and AXIN2 gene expressions were downregulated in myotubes treated with conditioned medium from mechanically loaded tendon constructs. However, proliferative potential (number of Ki67+ and bromodeoxyuridine+ myoblasts) did not differ between the two groups. These results indicate that tendon fibroblasts enhance myotube formation by mechanical loading-induced factors. Our finding suggests that mechanical loading affects the signaling interplay between skeletal muscle and tendon tissue and is thus important for musculoskeletal tissue development and regeneration in humans. Impact statement The interplay between satellite cells and various types of resident cells within the skeletal muscle for muscle regeneration has been extensively studied. However, even though tendon tissue is located adjacent to skeletal muscle tissue and cells in these tissues are exposed to repetitive mechanical loading together, the interaction between muscle and tendon tissues for muscle regeneration remains to be elucidated. In this study, we report that the conditioned media from engineered human tendon tissues undergoing repetitive tensile mechanical loading enhanced myotube formation. Our in vitro findings extend the fundamental understanding of the crosstalk between adjacent tissues of the muscle-tendon unit.

Skeletal muscle injury in aged rodents is characterized by an asynchronous infiltration of pro- and anti-inflammatory macrophage waves, leading to improper and incomplete regeneration. It is unclear whether this aberration also occurs in aged human muscle. In this study, we quantified the macrophage responses in a human model of muscle damage and regeneration induced by electrical stimulation in 7 young and 21 older adults. At baseline, total resident macrophage (CD68 + /DAPI + ) content was not different between young and old subjects, but pro-inflammatory (CD206 - /CD68 + /DAPI + ) macrophage content was lower in the old. Following damage, muscle Infiltration of CD206 - /CD68 + /DAPI + macrophages was lower in old relative to young subjects. Further, only the increase in CD206 - /CD68 + macrophages correlated with the change in muscle satellite cell content. Our data show that older individuals have a compromised macrophage response during muscle regeneration, pointing to an altered inflammatory response as a potential mechanism for reduced muscle regenerative efficacy in aged humans., Competing Interests: Declaration of competing interest None., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

Mechanical inputs give rise to p38 and JNK activation, which mediate adaptive physiological responses in various tissues. In skeletal muscle, contraction-induced p38 and JNK signaling ensure adaptation to exercise, muscle repair, and hypertrophy. However, the mechanisms by which muscle fibers sense mechanical load to activate this signaling have remained elusive. Here, we show that the upstream MAP3K ZAKβ is activated by cellular compression induced by osmotic shock and cyclic compression in vitro, and muscle contraction in vivo. This function relies on ZAKβ's ability to recognize stress fibers in cells and Z-discs in muscle fibers when mechanically perturbed. Consequently, ZAK-deficient mice present with skeletal muscle defects characterized by fibers with centralized nuclei and progressive adaptation towards a slower myosin profile. Our results highlight how cells in general respond to mechanical compressive load and how mechanical forces generated during muscle contraction are translated into MAP kinase signaling., (© 2022 The Authors. Published under the terms of the CC BY NC ND 4.0 license.)

Aim & Methods: Extreme endurance exercise provides a valuable research model for understanding the adaptive metabolic response of older and younger individuals to intense physical activity. Here, we compare a wide range of metabolic and physiologic parameters in two cohorts of seven trained men, age 30 ± 5 years or age 65 ± 6 years, before and after the participants travelled ≈3000 km by bicycle over 15 days., Results: Over the 15-day exercise intervention, participants lost 2-3 kg fat mass with no significant change in body weight. V̇O 2 max did not change in younger cyclists, but decreased (p = 0.06) in the older cohort. The resting plasma FFA concentration decreased markedly in both groups, and plasma glucose increased in the younger group. In the older cohort, plasma LDL-cholesterol and plasma triglyceride decreased. In skeletal muscle, fat transporters CD36 and FABPm remained unchanged. The glucose handling proteins GLUT4 and SNAP23 increased in both groups. Mitochondrial ROS production decreased in both groups, and ADP sensitivity increased in skeletal muscle in the older but not in the younger cohort., Conclusion: In summary, these data suggest that older but not younger individuals experience a negative adaptive response affecting cardiovascular function in response to extreme endurance exercise, while a positive response to the same exercise intervention is observed in peripheral tissues in younger and older men. The results also suggest that the adaptive thresholds differ in younger and old men, and this difference primarily affects central cardiovascular functions in older men after extreme endurance exercise., (© 2022 The Authors. Acta Physiologica published by John Wiley & Sons Ltd on behalf of Scandinavian Physiological Society.)

Muscle fiber denervation is a major contributor to the decline in muscle mass and function during aging. Heavy resistance exercise is an effective tool for increasing muscle mass and strength, but whether it can rescue denervated muscle fibers remains unclear. Therefore, the purpose of this study was to investigate the potential of heavy resistance exercise to modify indices of denervation in healthy elderly individuals. Thirty-eight healthy elderly men (72 ± 5 yr) underwent 16 wk of heavy resistance exercise, whereas 20 healthy elderly men (72 ± 6 yr) served as nonexercising sedentary controls. Muscle biopsies were obtained pre and post training, and midway at 8 wk. Biopsies were analyzed by immunofluorescence for the prevalence of myofibers expressing embryonic myosin [embryonic myosin heavy chain (MyHCe)], neonatal myosin [neonatal myosin heavy chain (MyHCn)], nestin, and neural cell adhesion molecule (NCAM), and by RT-qPCR for gene expression levels of acetylcholine receptor (AChR) subunits, MyHCn, MyHCe, p16, and Ki67. In addition to increases in strength and type II fiber hypertrophy, heavy resistance exercise training led to a decrease in AChR α1 and ε subunit messenger RNA (mRNA; at 8 wk). Changes in gene expression levels of the α1 and ε AChR subunits with 8 wk of heavy resistance exercise supports the role of this type of exercise in targeting stability of the neuromuscular junction. The number of fibers positive for NCAM, nestin, and MyHCn was not affected, suggesting that a longer timeframe is needed for adaptations to manifest at the protein level.

Objective: NAD + is a co-factor and substrate for enzymes maintaining energy homeostasis. Nicotinamide phosphoribosyltransferase (NAMPT) controls NAD + synthesis, and in skeletal muscle, NAD + is essential for muscle integrity. However, the underlying molecular mechanisms by which NAD + synthesis affects muscle health remain poorly understood. Thus, the objective of the current study was to delineate the role of NAMPT-mediated NAD + biosynthesis in skeletal muscle development and function., Methods: To determine the role of Nampt in muscle development and function, we generated skeletal muscle-specific Nampt KO (SMNKO) mice. We performed a comprehensive phenotypic characterization of the SMNKO mice, including metabolic measurements, histological examinations, and RNA sequencing analyses of skeletal muscle from SMNKO mice and WT littermates., Results: SMNKO mice were smaller, with phenotypic changes in skeletal muscle, including reduced fiber area and increased number of centralized nuclei. The majority of SMNKO mice died prematurely. Transcriptomic analysis identified that the gene encoding the mitochondrial permeability transition pore (mPTP) regulator Cyclophilin D (Ppif) was upregulated in skeletal muscle of SMNKO mice from 2 weeks of age, with associated increased sensitivity of mitochondria to the Ca 2+ -stimulated mPTP opening. Treatment of SMNKO mice with the Cyclophilin D inhibitor, Cyclosporine A, increased membrane integrity, decreased the number of centralized nuclei, and increased survival., Conclusions: Our study demonstrates that NAMPT is crucial for maintaining cellular Ca 2+ homeostasis and skeletal muscle development, which is vital for juvenile survival., (Copyright © 2021 The Author(s). Published by Elsevier GmbH.. All rights reserved.)

Objective: Skeletal muscle is an attractive target for blood glucose-lowering pharmacological interventions. Oral dosing of small molecule direct pan-activators of AMPK that bind to the allosteric drug and metabolite (ADaM) site, lowers blood glucose through effects in skeletal muscle. The molecular mechanisms responsible for this effect are not described in detail. This study aimed to illuminate the mechanisms by which ADaM-site activators of AMPK increase glucose uptake in skeletal muscle. Further, we investigated the consequence of co-stimulating muscles with two types of AMPK activators i.e., ADaM-site binding small molecules and the prodrug AICAR., Methods: The effect of the ADaM-site binding small molecules (PF739 and 991), AICAR or co-stimulation with PF739 or 991 and AICAR on muscle glucose uptake was investigated ex vivo in m. extensor digitorum longus (EDL) excised from muscle-specific AMPKα1α2 as well as whole-body AMPKγ3-deficient mouse models. In vitro complex-specific AMPK activity was measured by immunoprecipitation and molecular signaling was assessed by western blotting in muscle lysate. To investigate the transferability of these studies, we treated diet-induced obese mice in vivo with PF739 and measured complex-specific AMPK activation in skeletal muscle., Results: Incubation of skeletal muscle with PF739 or 991 increased skeletal muscle glucose uptake in a dose-dependent manner. Co-incubating PF739 or 991 with a maximal dose of AICAR increased glucose uptake to a greater extent than any of the treatments alone. Neither PF739 nor 991 increased AMPKα2β2γ3 activity to the same extent as AICAR, while co-incubation led to potentiated effects on AMPKα2β2γ3 activation. In muscle from AMPKγ3 KO mice, AICAR-stimulated glucose uptake was ablated. In contrast, the effect of PF739 or 991 on glucose uptake was not different between WT and AMPKγ3 KO muscles. In vivo PF739 treatment lowered blood glucose levels and increased muscle AMPKγ1-complex activity 2-fold, while AMPKα2β2γ3 activity was not affected., Conclusions: ADaM-site binding AMPK activators increase glucose uptake independently of AMPKγ3. Co-incubation with PF739 or 991 and AICAR potentiates the effects on muscle glucose uptake and AMPK activation. In vivo, PF739 lowers blood glucose and selectively activates muscle AMPKγ1-complexes. Collectively, this suggests that pharmacological activation of AMPKγ1-containing complexes in skeletal muscle can increase glucose uptake and can lead to blood glucose lowering., (Copyright © 2021 The Authors. Published by Elsevier GmbH.. All rights reserved.)

This study explores if unhealthy lipoprotein distribution (LPD) impairs the anabolic and amino acid sensing responses to whey-protein feeding. Thus, if impairment of such anabolic response to protein consumption is seen by the LPD this may negatively affect the skeletal muscle mass. Muscle protein synthesis (MPS) was measured by puromycin labeling in Apolipoprotein E knockout (Apoe KO), characterized by an unhealthy LPD, and wild type mice post-absorptive at 10 and 20 weeks, and post-prandial after whey-protein feeding at 20 weeks. Hypertrophy signaling and amino acid sensing mechanisms were studied and gut microbiome diversity explored. Surprisingly, whey-protein feeding did not affect MPS. p-mTOR and p-4E-BP1 was increased 2 h after whey-protein feeding in both genotypes, but with general lower levels in Apoe KO compared to wild type. At 20 weeks of age, Apoe KO had a greater mRNA-expression for SNAT2, CD98, ATF4 and GCN2 compared to wild type. These responses were not associated with gut microbiota compositional differences. Regardless of LPD status, MPS was similar in Apoe KO and wild type. Surprisingly, whey-protein did not stimulate MPS. However, Apoe KO had lower levels of hypertrophy signaling, was amino acid deprived, and had impaired amino acid sensing mechanisms., (© 2021. The Author(s).)

Key Points: Tamoxifen-inducible skeletal muscle-specific AXIN1 knockout (AXIN1 imKO) in mouse does not affect whole-body energy substrate metabolism. AXIN1 imKO does not affect AICAR or insulin-stimulated glucose uptake in adult skeletal muscle. AXIN1 imKO does not affect adult skeletal muscle AMPK or mTORC1 signalling during AICAR/insulin/amino acid incubation, contraction and exercise. During exercise, α2/β2/γ3AMPK and AMP/ATP ratio show greater increases in AXIN1 imKO than wild-type in gastrocnemius muscle., Abstract: AXIN1 is a scaffold protein known to interact with >20 proteins in signal transduction pathways regulating cellular development and function. Recently, AXIN1 was proposed to assemble a protein complex essential to catabolic-anabolic transition by coordinating AMPK activation and inactivation of mTORC1 and to regulate glucose uptake-stimulation by both AMPK and insulin. To investigate whether AXIN1 is permissive for adult skeletal muscle function, a phenotypic in vivo and ex vivo characterization of tamoxifen-inducible skeletal muscle-specific AXIN1 knockout (AXIN1 imKO) mice was conducted. AXIN1 imKO did not influence AMPK/mTORC1 signalling or glucose uptake stimulation at rest or in response to different exercise/contraction protocols, pharmacological AMPK activation, insulin or amino acids stimulation. The only genotypic difference observed was in exercising gastrocnemius muscle, where AXIN1 imKO displayed elevated α2/β2/γ3 AMPK activity and AMP/ATP ratio compared to wild-type mice. Our work shows that AXIN1 imKO generally does not affect skeletal muscle AMPK/mTORC1 signalling and glucose metabolism, probably due to functional redundancy of its homologue AXIN2., (© 2021 The Authors. The Journal of Physiology © 2021 The Physiological Society.)

Sarcopenia worsens survival in patients with advanced liver disease including cirrhosis. In this study, we aimed to characterize skeletal muscle status by dual-energy X-ray absorptiometry (DXA) in patients with cirrhosis and examine the association between different skeletal muscle compartments and mortality. We included 231 men and 84 women ( Child A , B , and C ) with cirrhosis and 315 healthy matched controls (231 men and 84 women). Body composition was assessed with DXA. Appendicular skeletal muscle index (ASMI), arms index (AI), and legs index (LI) were calculated by normalizing lean mass to height squared. Low ASMI was defined as ASMI < 7.0 kg/m 2 in men and <5.5 kg/m 2 in women. Biochemical and hemodynamic data were recorded for cirrhotic patients and mortality data retrieved from registers. Low ASMI was more prevalent in both men (49%) and women (43%) with cirrhosis compared with healthy men (8%) and women (5%) ( P < 0.001). ASMI and LI were lowest in Child B , whereas AI decreased gradually with advancing Child class. ASMI was inversely associated with mortality in men [HR = 0.74 (0.59-0.93), P < 0.01], and this was mainly driven by AI [HR = 0.37 (0.18-0.71), P < 0.01]. AI showed closer association than ASMI or LI to both the severity of liver disease and to mortality, which may be due to increasing prevalence of leg edema with disease progression in this population. Determination of arm lean mass may add information on survival in patients with cirrhosis. NEW & NOTEWORTHY Sarcopenia increases mortality in patients with end-stage liver disease. We show that arm lean mass determined by dual-energy X-ray absorptiometry is a better marker than the traditional appendicular skeletal muscle mass when predicting sarcopenia-related mortality in patients with cirrhosis of different severity. The findings add to the dispute about the optimal method for repeated assessments of skeletal muscle status in patients with cirrhosis and may have implications for clinical decision making.

Cerebral palsy (CP) is a non-progressive motor disorder that affects posture and gait due to contracture development. The purpose of this study is to analyze a possible relation between muscle stiffness and gene expression levels in muscle tissue of children with CP. Next-generation sequencing (NGS) of gene transcripts was carried out in muscle biopsies from gastrocnemius muscle (n = 13 children with CP and n = 13 typical developed (TD) children). Passive stiffness of the ankle plantarflexors was measured. Structural changes of the basement membranes and the sarcomere length were measured. Twelve pre-defined gene target sub-categories of muscle function, structure and metabolism showed significant differences between muscle tissue of CP and TD children. Passive stiffness was significantly correlated to gene expression levels of HSPG2 (p = 0.02; R 2  = 0.67), PRELP (p = 0.002; R 2  = 0.84), RYR3 (p = 0.04; R 2  = 0.66), C COL5A3 (p = 0.0007; R 2  = 0.88), ASPH (p = 0.002; R 2  = 0.82) and COL4A6 (p = 0.03; R 2  = 0.97). Morphological differences in the basement membrane were observed between children with CP and TD children. The sarcomere length was significantly increased in children with CP when compared with TD (p = 0.04). These findings show that gene targets in the categories: calcium handling, basement membrane and collagens, were significantly correlated to passive muscle stiffness. A Reactome pathway analysis showed that pathways involved in DNA repair, ECM proteoglycans and ion homeostasis were amongst the most upregulated pathways in CP, while pathways involved in collagen fibril crosslinking, collagen fibril assembly and collagen turnover were amongst the most downregulated pathways when compared with TD children. These results underline that contracture formation and motor impairment in CP is an interplay between multiple factors.

Background: Protein supplementation alone or combined with resistance training has been proposed to be effective in counteracting age-related losses of muscle mass and strength., Objectives: To investigate the effect of protein supplementation alone or combined with light-intensity or heavy-load resistance exercise on muscle size, strength, and function in older adults., Methods: In a 1-y randomized controlled trial, 208 healthy older adults (>65 y) were randomly assigned to 1 of 5 interventions: 1) carbohydrate supplementation (CARB); 2) collagen protein supplementation (COLL); 3) whey protein supplementation (WHEY); 4) light-intensity resistance training 3-5 times/wk with whey protein supplementation (LITW); and 5) heavy resistance training 3 times weekly with whey protein supplementation (HRTW). Protein supplements contained 20 g protein + 10 g carbohydrate, whereas CARB contained 30 g of carbohydrates. All intervention groups received the supplement twice daily. The primary outcome was change in the quadriceps cross-sectional area (qCSA). Secondary outcomes included measures of lower extremity strength and power, functional capabilities, and body composition., Results: There were 184 participants who completed the study. COLL and WHEY did not affect any measured parameter compared to CARB. Compared to WHEY, HRTW improved the qCSA size (between-group difference, +1.68 cm2; 95% CI, +0.41 to +2.95 cm2; P = 0.03), as well as dynamic (+18.4 Nm; 95% CI, +10.1 to +26.6 Nm; P < 10-4) and isometric knee extensor strength (+23.9 Nm; 95% CI, +14.2 to +33.6 Nm; P < 10-5). LITW did not improve the qCSA size, but increased dynamic knee extensor strength compared to WHEY (+13.7 Nm; 95% CI, +5.3 and +22.1 Nm; P = 0.01)., Conclusions: Recommending protein supplementation as a stand-alone intervention for healthy older individuals seems ineffective in improving muscle mass and strength. Only HRTW was effective in both preserving muscle mass and increasing strength. Thus, we recommend that future studies investigate strategies to increase long-term compliance to heavy resistance exercise in healthy older adults. This trial was registered at clinicaltrials.gov as NCT02034760., (© The Author(s) 2021. Published by Oxford University Press on behalf of the American Society for Nutrition.)

The COVID-19 pandemic is an unprecedented health crisis as entire populations have been asked to self-isolate and live in home-confinement for several weeks to months, which in itself represents a physiological challenge with significant health risks. This paper describes the impact of sedentarism on the human body at the level of the muscular, cardiovascular, metabolic, endocrine and nervous systems and is based on evidence from several models of inactivity, including bed rest, unilateral limb suspension, and step-reduction. Data form these studies show that muscle wasting occurs rapidly, being detectable within two days of inactivity. This loss of muscle mass is associated with fibre denervation, neuromuscular junction damage and upregulation of protein breakdown, but is mostly explained by the suppression of muscle protein synthesis. Inactivity also affects glucose homeostasis as just few days of step reduction or bed rest, reduce insulin sensitivity, principally in muscle. Additionally, aerobic capacity is impaired at all levels of the O 2 cascade, from the cardiovascular system, including peripheral circulation, to skeletal muscle oxidative function. Positive energy balance during physical inactivity is associated with fat deposition, associated with systemic inflammation and activation of antioxidant defences, exacerbating muscle loss. Importantly, these deleterious effects of inactivity can be diminished by routine exercise practice, but the exercise dose-response relationship is currently unknown. Nevertheless, low to medium-intensity high volume resistive exercise, easily implementable in home-settings, will have positive effects, particularly if combined with a 15-25% reduction in daily energy intake. This combined regimen seems ideal for preserving neuromuscular, metabolic and cardiovascular health.

Objective: Externally applied abduction and rotational loads are major contributors to the knee joint injury mechanism; yet, how muscles work together to stabilize the knee against these loads remains unclear. Our study sought to evaluate lower limb functional muscle synergies in healthy young adults such that muscle activation can be directly related to internal knee joint moments., Methods: Concatenated non-negative matrix factorization extracted muscle and moment synergies of 22 participants from electromyographic signals and joint moments elicited during a weight-bearing force matching protocol., Results: Two synergy sets were extracted: Set 1 included four synergies, each corresponding to a general anterior, posterior, medial, or lateral force direction. Frontal and transverse moments were coupled during medial and lateral force directions. Set 2 included six synergies, each corresponding to a moment type (extension/flexion, ab/adduction, internal/external rotation). Hamstrings and quadriceps dominated synergies associated with respective flexion and extension moments while quadriceps-hamstring co-activation was associated with knee abduction. Rotation moments were associated with notable contributions from hamstrings, quadriceps, gastrocnemius, and hip ab/adductors, corresponding to a general co-activation muscle synergy., Conclusion: Our results highlight the importance of muscular co-activation of all muscles crossing the knee to support it during injury-inducing loading conditions such as externally applied knee abduction and rotation. Functional muscle synergies can provide new insight into the relationship between neuromuscular control and knee joint stability by directly associating biomechanical variables to muscle activation., (Copyright © 2020. Published by Elsevier Ltd.)

Prolonged physical inactivity in young adults may lead to deficiencies in musculoskeletal fitness, and thus a need exists to develop physical activity and exercise programmes that are effective of increasing musculoskeletal fitness. The aim of this study, therefore, was to investigate the effects of small-sided team handball training on lower limb muscle strength, postural balance and body composition in young adults. Twenty-six men and twenty-eight women were stratified for peak oxygen uptake (VO2peak) and body fat percentage and randomly allocated to either 12 wks of small-sided recreational team handball training (THG: 14 men and 14 women, age 24.1±2.6 yrs (mean±SD), VO2peak 39.8±5.9 ml/kg/min and body fat percentage 32.7±8.7%) or serving as non-exercising controls (CON: 12 men and 14 women, age 24.8±3.1 yrs, VO2peak 39.7±5.0 ml/kg/min, body fat percentage 31.7±9.7%). THG trained on average 1.8 times/week for 12 wks. At 0 and 12 wks, lower limb muscle strength, rate of force development (RFD), vertical jump height and power, postural balance, body composition and muscle biopsies were assessed. No training effects were observed for maximal isokinetic or isometric knee extensor strength, maximal vertical jump height or take-off power, fibre type distribution or capillarization. Late phase (RFD) increased (+7.4%, p<0.05) and postural sway excursion length was improved after training (-9%, p<0.05) in THG with no difference from CON (p>0.05). Further, THG demonstrated a decrease in body fat percentage (-3.7%) accompanied by increases in whole-body fat free mass (FFM) (+2.2%), leg FFM (+2.5%), total bone mineral content (BMC) (+1.1%), leg BMC (+1.2%), total hip bone mineral density (+1.6%) and hip T-score (+50%) which differed from CON (all p<0.05). In conclusion, recreational small-sided team handball training appears to effectively improve rapid force capacity, postural balance, lean and fat body mass and bone health in previously untrained young adults. The study was registered at ClinicalTrials.gov (NCT04247724). ClinicalTrials.gov ID number: NCT04247724., Competing Interests: The authors declare that no competing interests exist.

Background: Fibroblasts are the powerhouses responsible for the production and assembly of extracellular matrix (ECM). Their activity needs to be tightly controlled especially within the musculoskeletal system, where changes to ECM composition affect force transmission and mechanical loading that are required for effective movement of the body. Extracellular vesicles (EVs) are a mode of cell-cell communication within and between tissues, which has been largely characterised in cancer. However, it is unclear what the role of healthy fibroblast-derived EVs is during tissue homeostasis., Methods: Here, we performed proteomic analysis of small EVs derived from primary human muscle and tendon cells to identify the potential functions of healthy fibroblast-derived EVs., Results: Mass spectrometry-based proteomics revealed comprehensive profiles for small EVs released from healthy human fibroblasts from different tissues. We found that fibroblast-derived EVs were more similar than EVs from differentiating myoblasts, but there were significant differences between tendon fibroblast and muscle fibroblast EVs. Small EVs from tendon fibroblasts contained higher levels of proteins that support ECM synthesis, including TGFβ1, and muscle fibroblast EVs contained proteins that support myofiber function and components of the skeletal muscle matrix., Conclusions: Our data demonstrates a marked heterogeneity among healthy fibroblast-derived EVs, indicating shared tasks between EVs of skeletal muscle myoblasts and fibroblasts, whereas tendon fibroblast EVs could play a fibrotic role in human tendon tissue. These findings suggest an important role for EVs in tissue homeostasis of both tendon and skeletal muscle in humans. Video abstract.

The peak fat oxidation rate (PFO) and the exercise intensity that elicits PFO (Fat max ) are associated with endurance performance during exercise primarily involving lower body musculature, but it remains elusive whether these associations are present during predominant upper body exercise. The aim was to investigate the relationship between PFO and Fat max determined during a graded exercise test on a ski-ergometer using double-poling (GET-DP) and performance in the long-distance cross-country skiing race, Vasaloppet. Forty-three healthy men completed GET-DP and Vasaloppet and were divided into two subgroups: recreational (RS, n = 35) and elite (ES, n = 8) skiers. Additionally, RS completed a cycle-ergometer GET (GET-Cycling) to elucidate whether the potential relationships were specific to exercise modality. PFO (r 2  = .10, P = .044) and Fat max (r 2  = .26, P < .001) were correlated with performance; however, V ˙ O 2 peak was the only independent predictor of performance (adj. R 2  = .36) across all participants. In ES, Fat max was the only variable associated with performance (r 2  = .54, P = .038). Within RS, DP V ˙ O 2 peak (r 2  = .11, P = .047) and ski-specific training background (r 2  = .30, P = .001) were associated with performance. Between the two GETs, Fat max (r 2  = .20, P = .006) but not PFO (r 2  = .07, P = .135) was correlated. Independent of exercise mode, neither PFO nor Fat max were associated with performance in RS (P > .05). These findings suggest that prolonged endurance performance is related to PFO and Fat max but foremost to V ˙ O 2 peak during predominant upper body exercise. Interestingly, Fat max may be an important determinant of performance among ES. Among RS, DP V ˙ O 2 peak , and skiing experience appeared as performance predictors. Additionally, whole-body fat oxidation seemed specifically coupled to exercise modality., (© 2020 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Thyroid hormones are important for homeostatic control of energy metabolism and body temperature. Although skeletal muscle is considered a key site for thyroid action, the contribution of thyroid hormone receptor signaling in muscle to whole-body energy metabolism and body temperature has not been resolved. Here, we show that T3-induced increase in energy expenditure requires thyroid hormone receptor alpha 1 (TRα 1 ) in skeletal muscle, but that T3-mediated elevation in body temperature is achieved in the absence of muscle-TRα 1 . In slow-twitch soleus muscle, loss-of-function of TRα 1 (TRα HSACre ) alters the fiber-type composition toward a more oxidative phenotype. The change in fiber-type composition, however, does not influence the running capacity or motivation to run. RNA-sequencing of soleus muscle from WT mice and TRα HSACre mice revealed differentiated transcriptional regulation of genes associated with muscle thermogenesis, such as sarcolipin and UCP3, providing molecular clues pertaining to the mechanistic underpinnings of TRα 1 -linked control of whole-body metabolic rate. Together, this work establishes a fundamental role for skeletal muscle in T3-stimulated increase in whole-body energy expenditure., (© 2020 The Authors. The FASEB Journal published by Wiley Periodicals LLC on behalf of Federation of American Societies for Experimental Biology.)

This study aimed to assess the validity and functional relevance of a standardized procedure to assess lower limb muscle power by means of the 30-s sit-to-stand (STS) test when compared to leg extension power (LEP), traditional STS performance and handgrip strength. A total of 628 community-dwelling older subjects (60-93 years) from the Copenhagen Sarcopenia Study were included. Physical performance was assessed by the 30-s STS and 10-m maximal gait speed tests. Handgrip strength and LEP were recorded by a hand-held dynamometer and the Nottingham power rig, respectively. STS muscle power was calculated using the subjects' body mass and height, chair height and the number of repetitions completed in the 30-s STS test. We found a small albeit significant difference between LEP and unilateral STS power in older men (245.5 ± 88.8 vs. 223.4 ± 81.4 W; ES = 0.26; p < 0.05), but not in older women (135.9 ± 51.9 vs. 138.5 ± 49.6 W; ES = 0.05; p > 0.05). Notably, a large positive correlation was observed between both measures (r = 0.75; p < 0.001). Relative STS power was more strongly related with maximal gait speed than handgrip strength, repetition-based STS performance and relative LEP after adjusting for age (r = 0.53 vs 0.35-0.45; p < 0.05). In conclusion, STS power obtained from the 30-s STS test appeared to provide a valid measure of bilateral lower limb power and was more strongly related with physical performance than maximal handgrip strength, repetition-based STS performance and LEP.

Objective: Evidence for AMP-activated protein kinase (AMPK)-mediated regulation of skeletal muscle metabolism during exercise is mainly based on transgenic mouse models with chronic (lifelong) disruption of AMPK function. Findings based on such models are potentially biased by secondary effects related to a chronic lack of AMPK function. To study the direct effect(s) of AMPK on muscle metabolism during exercise, we generated a new mouse model with inducible muscle-specific deletion of AMPKα catalytic subunits in adult mice., Methods: Tamoxifen-inducible and muscle-specific AMPKα1/α2 double KO mice (AMPKα imdKO) were generated by using the Cre/loxP system, with the Cre under the control of the human skeletal muscle actin (HSA) promoter., Results: During treadmill running at the same relative exercise intensity, AMPKα imdKO mice showed greater depletion of muscle ATP, which was associated with accumulation of the deamination product IMP. Muscle-specific deletion of AMPKα in adult mice promptly reduced maximal running speed and muscle glycogen content and was associated with reduced expression of UGP2, a key component of the glycogen synthesis pathway. Muscle mitochondrial respiration, whole-body substrate utilization, and muscle glucose uptake and fatty acid (FA) oxidation during muscle contractile activity remained unaffected by muscle-specific deletion of AMPKα subunits in adult mice., Conclusions: Inducible deletion of AMPKα subunits in adult mice reveals that AMPK is required for maintaining muscle ATP levels and nucleotide balance during exercise but is dispensable for regulating muscle glucose uptake, FA oxidation, and substrate utilization during exercise., (Copyright © 2020 The Authors. Published by Elsevier GmbH.. All rights reserved.)

Muscle contracture development is a major complication for individuals with cerebral palsy (CP) and has lifelong implications. In order to recognize contracture development early and to follow up on preventive interventions aimed at muscle health development, non-invasive, and easy to use methods are needed. The aim of the present study was to assess whether multi-frequency Bioimpedance (mfBIA) can be used to detect differences between skeletal muscle of individuals with CP and healthy controls. The mfBIA technique was applied to the medial gastrocnemius muscle of n = 24 adults with CP and n = 20 healthy controls of both genders. The phase angle (PA) and the centre frequency (fc) were significantly lower in individuals with CP when compared to controls; PA: - 25% for women and - 31.8% for men (P < 0.0001); fc: - 5.6% for women and - 5.2% for men (P < 0.009). The reactance (Xc) and the extracellular resistance (Re) of skeletal muscle from individuals with CP were significantly higher when compared to controls; Xc: + 9.9% for women and + 28.9% for men (P < 0.0001); Re: + 39.7% for women and + 91.2% for men (P < 0.0001). The present study shows that several mfBIA parameters differ significantly between individuals with CP and healthy controls. Furthermore, these changes correlated significantly with the severity of CP, as assessed using the GMFCS scale. The present data indicate that mfBIA shows promise in terms of being a useful diagnostic tool, capable of characterizing muscle health and its development in individuals with cerebral palsy.

There is growing evidence for a link between loss of skeletal muscle, impaired muscle performance, and systemic markers of acute inflammation in hospitalized geriatric patients. The present literature suggests a negative effect of acute inflammation at the time of hospital admission upon muscle performance and the change of this during the hospital stay, particularly in patients with persistent rather than resolved inflammation. Further, a few studies have reported a positive effect of anti-inflammatory medication upon recovery of muscle function in geriatric patients, but how this is mediated (e.g. inhibition of inflammatory cytokines) is not clear. In conclusion, a negative association between the presence of acute and persistent systemic markers of inflammation and various aspects of muscle function and its recovery after bedrest is observed in geriatric patients., Competing Interests: Declaration of competing interest All authors declare that they have no conflict of interest, financial or otherwise., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Integrins are important for mechanosensation in tissue and play, together with nutrition, a role in regulating extracellular matrix (ECM) in skeletal muscle and tendon. Integrin receptors are dimers that consist of an α and β subunit and bridge extracellular and intracellular signals. The present study investigates whether the deletion of the integrin receptor α 1 subunit influences collagen and other matrix proteins in the musculotendinous tissue and whether it causes any compensatory changes in other integrin subunits in C57BL/6J mice. In addition, we study whether a high-fat diet (HFD) influences these responses in muscle or tendon. Mice on a HFD had a higher number of non-enzymatic cross-links in skeletal muscle ECM and increased gene expression of collagen and other extracellular matrix proteins. In contrast to gene expression, total collagen protein content was decreased by HFD in the muscle with no change in tendon. Integrin α 1 subunit knockout resulted in a decrease of collagen type I and III, TGF-β1 and IGF-1 gene expression in muscle of HFD mice but did not affect total collagen protein compared with wild-type (WT) littermates in either muscle or tendon. There was no compensatory increase in the genes that express other integrin subunits. In conclusion, HFD induced a significant increase in expression of ECM genes in muscle. On the protein level, HFD resulted in a lower collagen content in muscle. Tendons were unaffected by the diet. Deletion of the integrin α 1 subunit did not affect collagen protein or gene expression in muscle or tendon.

Blunted muscle hypertrophy and impaired regeneration with aging have been partly attributed to satellite cell (SC) dysfunction. However, true muscle regeneration has not yet been studied in elderly individuals. To investigate this, muscle injury was induced by 200 electrically stimulated (ES) eccentric contractions of the vastus lateralis (VL) of one leg in seven young (20-31 years) and 19 elderly men (60-73 years). This was followed by 13 weeks of resistance training (RT) for both legs to investigate the capacity for hypertrophy. Muscle biopsies were collected Pre- and Post-RT, and 9 days after ES, for immunohistochemistry and RT-PCR. Hypertrophy was assessed by MRI, DEXA, and immunohistochemistry. Overall, surprisingly comparable responses were observed between the young and elderly. Nine days after ES, Pax7+ SC number had doubled (P < .05), alongside necrosis and substantial changes in expression of genes related to matrix, myogenesis, and innervation (P < .05). Post-RT, VL cross-sectional area had increased in both legs (~15%, P < .05) and SCs/type II fiber had increased ~2-4 times more with ES+RT vs RT alone (P < .001). Together these novel findings demonstrate "youthful" regeneration and hypertrophy responses in human elderly muscle. Furthermore, boosting SC availability in healthy elderly men does not enhance the subsequent muscle hypertrophy response to RT., (© 2020 Federation of American Societies for Experimental Biology.)

Study Design: A quasi-experimental, pretest-posttest design., Objectives: To identify acute changes in the supraspinatus and biceps tendon following fatiguing wheelchair propulsion and to associate tendon changes with risk factors associated with shoulder pain in persons with spinal cord injury (SCI)., Setting: Biomechanical laboratory Swiss Paraplegic Research., Methods: A population-based sample of 50 wheelchair users with SCI at lesion level T2 or below participated. Fatigue was measured by using the rate of perceived exertion and heart rate. Linear regression techniques were used to assess the association between the dependent and independent variables. Dependent variables included absolute differences in supraspinatus and biceps tendon thickness, contrast, and echogenicity ratio assessed with ultrasound before and after a fatiguing wheelchair propulsion intervention. Independent variables included susceptibility to fatigue (Yes/No), the acromio-humeral distance, sex, time since injury, activity levels, and body weight., Results: A reduction in supraspinatus tendon thickness after fatiguing wheelchair propulsion (-1.39 mm; 95% CI: -2.28; -0.51) was identified after controlling for all potential confounders. Females who fatigued (n = 4) displayed a greater reduction in supraspinatus tendon thickness as compared with those who did not fatigue (n = 7). In contrast, higher body weight was associated with an increase in supraspinatus tendon thickness and a greater acromio-humeral distance before the intervention was associated with an increase in biceps tendon thickness., Conclusions: Acute changes in the supraspinatus and biceps tendon after fatiguing wheelchair propulsion may explain the high prevalence of tendon injuries in this population. Future research should determine the consequences of tendon changes and its relationship to tendinopathy.

Introduction : High intensity interval training (HIIT) has shown to be as effective as moderate intensity endurance training to improve metabolic health. However, the current knowledge on the effect of HIIT in older individuals is limited and it is uncertain whether the adaptations are sex specific. The aim was to investigate effects of HIIT on mitochondrial respiratory capacity and mitochondrial content in older females and males. Methods : Twenty-two older sedentary males ( n  = 11) and females ( n  = 11) completed 6 weeks of supervised HIIT 3 days per week. The training consisted of 5 × 1 min cycling (124 ± 3% of max power output at session 2-6 and 135 ± 3% of max power output at session 7-20) interspersed by 1½ min recovery. Before the intervention and 72 h after last training session a muscle biopsy was obtained and mitochondrial respiratory capacity, citrate synthase activity and proteins involved in mitochondria metabolism were assessed. Furthermore, body composition and ⩒O 2 max were measured. Results : ⩒O 2 max increased and body fat percentage decreased after HIIT in both sexes ( p  < 0.05). In addition, CS activity and protein content of MnSOD and complex I-V increased in both sexes. Coupled and uncoupled mitochondrial respiratory capacity increased only in males. Mitochondrial respiratory capacity normalised to CS activity (intrinsic mitochondrial respiratory capacity) did not change following HIIT. Conclusion : HIIT induces favourable adaptions in skeletal muscle in older subjects by increasing mitochondrial content, which may help to maintain muscle oxidative capacity and slow down the process of sarcopenia associated with ageing.

Muscle inactivity reduces muscle protein synthesis (MPS), whereas a subsequent period of rehabilitation resistance training (retraining) increases MPS. However, less is known regarding muscle protein breakdown (MPB) during such conditions. Furthermore, nonsteroidal anti-inflammatory drugs (NSAIDs) may have a dampening effect on MPB during periods of inactivity in older individuals. Thus, we measured the average MPB, by use of the deuterated water methodology, during an immobilization period and a subsequent retraining period in older individuals with and without NSAID treatment. Eighteen men (60-80 years: range) were randomly assigned to ibuprofen (1200 mg/d, Ibu) or placebo (Plc). One lower limb was immobilized in a cast for 2 weeks and retrained for 2 weeks, and 2 × 20 g of whey protein was ingested daily during both periods. Besides MPB, the protein expression of different muscle degradation signaling molecules was investigated. MPB was lower during immobilization compared to retraining (p < 0.01). NSAID treatment did not affect the MPB rate during immobilization or retraining (p > 0.05). The protein expression of muscle degradation signaling molecules changed during the study intervention but were unaffected by NSAID treatment. The finding that MPB was lower during immobilization than during retraining indicates that an increased MPB may play an important role in the muscle protein remodeling processes taking place within the initial retraining period. Moreover, NSAID treatment did not significantly influence the MPB rate during 2 weeks of lower limb immobilization or during 2 weeks of subsequent retraining in older individuals.

Purpose: Numerous daily tasks such as walking and rising from a chair involve bilateral lower limb movements. During such tasks, lower extremity function (LEF) may be compromised among older adults. LEF may be further impaired due to high degrees of between-limb asymmetry. The present study investigated the prevalence of between-limb asymmetry in muscle mass, strength, and power in a cohort of healthy older adults and examined the influence of between-limb asymmetry on LEF., Methods: Two hundred and eight healthy older adults (mean age 70.2 ± 3.9 years) were tested for LEF (400 m walking and 30-seconds chair stand). Furthermore, maximal isometric and dynamic knee extensor strength, leg extensor power, and lower limb lean tissue mass (LTM) were obtained unilaterally., Results: Mean between-limb asymmetry in maximal muscle strength and power ranged between 10% and 13%, whereas LTM asymmetry was 3 ± 2.3%. Asymmetry in dynamic knee extensor strength was larger for women compared with men (15.0 ± 11.8% vs 11.1 ± 9.5%; P = .005) Leg strength and power were positively correlated with LEF (r 2  = .43-.46, P < .001). The weakest leg was not a stronger predictor of LEF than the strongest leg. Between-limb asymmetry in LTM and isometric strength was negatively associated with LEF (LTM; r 2  = .12, P = .005, isometric peak torque; r 2  = 0.40, P = .03.) but dynamic strength and power were not., Conclusion: The present study supports the notion that in order to improve or maintain LEF, healthy older adults should participate in training interventions that increase muscle strength and power, whereas the effects of reducing between-limb asymmetry in these parameters might be of less importance., (© 2019 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Background: Exercise therapy is recommended for knee osteoarthritis (OA), but the underlying mechanisms of pain relief are not fully understood. The purpose of this study was to explore the effects of exercise on muscle perfusion assessed by dynamic contrast enhanced MRI (DCE-MRI) and its association with changes in pain in patients with knee OA., Methods: Exploratory outcome analyses of a randomised controlled study with per-protocol analyses ( ClinicalTrials.gov : NCT01545258) performed at an outpatient clinic at a public hospital in Denmark. We compared 12 weeks of supervised exercise therapy 3 times per week (ET) with a no attention control group (CG). Analyses of covariance (ANCOVA) were used to assess group mean differences in changes from baseline to week 12 in knee muscle perfusion quantified by DCE-MRI, patient-reported pain and function using the Knee Injury and Osteoarthritis Outcome Score (KOOS) questionnaire, knee extensor and flexor muscle strength tests, and the six-minute walking test (6MWT). Spearman's correlation coefficients were used to determine the correlation between changes in DCE-MRI variables, KOOS, muscle strength, and 6MWT. The potential effect mediation of the DCE-MRI perfusion variables was investigated in a post-hoc mediation analysis., Results: Of 60 participants randomised with knee osteoarthritis, 33 (ET, n = 16, CG, n = 17) adhered to the protocol and had complete DCE-MRI data. At follow-up, there were significant group differences in muscle perfusion changes and clinically relevant group differences in KOOS pain changes (10.7, 95% CI 3.3 to 18.1, P = 0.006) in favor of ET. There were no significant between-group differences on muscle strength and function. The changes in pain and muscle perfusion were significantly correlated (highest Spearman's rho = 0.42, P = 0.014). The mediation analyses were generally not statistically significant., Conclusion: The pain-reducing effects of a 12-week exercise program are associated with changes in knee muscle perfusion quantified by DCE-MRI in individuals with knee OA, but whether the effects are mediated by muscle perfusion changes remains unclear., Trial Registration: ClinicalTrials.gov: NCT01545258 , first posted March 6, 2012.

Sustaining a weight loss after a lifestyle intervention is challenging. The objective of the present study was to investigate if mitochondrial function is associated with the ability to maintain a weight loss. Sixty-eight former participants in an 11-12-week lifestyle intervention were recruited into 2 groups; weight loss maintenance (WLM; body mass index (BMI): 32 ± 1 kg/m 2 ) and weight regain (WR; BMI: 43 ± 2 kg/m 2 ) based on weight loss measured at a follow-up visit (WLM: 4.8 ± 0.4; WR: 7.6 ± 0.8 years after lifestyle intervention). Maximal oxygen consumption rate, physical activity level, and blood and muscle samples were obtained at the follow-up experiment. Mitochondrial respiratory capacity and reactive oxygen species (ROS) production were measured. Fasting blood samples were used to calculate glucose homeostasis index. WR had impaired glucose homeostasis and decreased maximal oxygen uptake and physical activity level compared with WLM. The decreased physical activity in WR was due to a lower activity level at vigorous and moderate intensities. Mitochondrial respiratory capacity and citrate synthase (CS) activity was higher in WLM, but intrinsic mitochondrial respiratory capacity (mitochondrial respiratory capacity corrected for mitochondrial content (CS activity)) was similar. ROS production was higher in WR compared with WLM, which was accompanied by a decreased content of antioxidant proteins in WR. Intrinsic mitochondrial respiratory capacity in skeletal muscle is not associated with the ability to maintain a long-term weight loss. WLM had a higher maximal oxygen uptake, physical activity level, mitochondrial respiratory capacity and CS activity compared with WR. The reduced glucose tolerance was concurrent with increased ROS production per mitochondria in WR, and could also be associated with the lower physical activity level in this group.

Traumatic strain injury in skeletal muscle is often associated with fluid accumulation at the site of rupture, but the role of this injury exudate (EX) in cellular responses and healing is unknown. We aimed to characterize the EX sampled from human hamstring or calf muscles following a strain injury ( n = 12). The cytokine and growth-factor profile, gene expression, and transcriptome analysis of EX-derived cells were compared with blood taken simultaneously from the same individuals. Cellular responses to the EX were tested in 3-dimensional (3D) culture based on primary human fibroblasts and myoblasts isolated from hamstring muscles. The EX contained a highly proinflammatory profile with a substantial expression of angiogenic factors. The proinflammatory profile was present in samples taken early postinjury and in samples aspirated several weeks postinjury, suggesting persistent inflammation. Cells derived from the EX demonstrated an increased expression of fibrogenic, adipogenic, and angiogenesis-related genes in comparison with blood cells. The injury EX stimulated fibroblast proliferation 2-fold compared with plasma, whereas such an effect was not seen for myoblasts. Finally, in 3D cell culture, the EX induced an up-regulation of connective tissue-related genes. In summary, EX formation following a muscle-strain injury stimulates fibroblast proliferation and the synthesis of connective tissue in fibroblasts. This suggests that the EX promotes an acute tissue-healing response but potentially also contributes to the formation of fibrotic tissue in the later phases of tissue repair.-Bayer, M. L., Bang, L., Hoegberget-Kalisz, M., Svensson, R. B., Olesen, J. L., Karlsson, M. M., Schjerling, P., Hellsten, Y., Hoier, B., Magnusson, S. P., Kjaer, M. Muscle-strain injury exudate favors acute tissue healing and prolonged connective tissue formation in humans.

Background: Myalgia is a common adverse effect of statin therapy, but the underlying mechanism is unknown. Statins may reduce levels of coenzyme Q10 (CoQ10), which is an essential electron carrier in the mitochondrial electron transport system, thereby impairing mitochondrial respiratory function, potentially leading to myalgia., Objectives: To investigate whether statin-induced myalgia is coupled to reduced intramuscular CoQ10 concentration and impaired mitochondrial respiratory function., Methods: Patients receiving simvastatin (i.e., statin) therapy (n = 64) were recruited, of whom 25 experienced myalgia (myalgic group) and 39 had no symptoms of myalgia (NS group). Another 20 had untreated high blood cholesterol levels (control group). Blood and muscle samples were obtained. Intramuscular CoQ10 concentration was measured, and mitochondrial respiratory function and reactive oxygen species (ROS) production were measured. Citrate synthase (CS) activity was used as a biomarker of mitochondrial content in skeletal muscle., Results: Intramuscular CoQ10 concentration was comparable among groups. Mitochondrial complex II-linked respiration was reduced in the statin-myalgic and -NS groups compared with the control group. When mitochondrial respiration was normalized to CS activity, respiration rate was higher in the myalgic group compared with the NS and control groups. Maximal ROS production was similar among groups., Conclusion: Statin therapy appeared to impair mitochondrial complex-II-linked respiration, but the mitochondrial capacity for complex I+II-linked respiration remained intact. Myalgia was not coupled to reduced intramuscular CoQ10 levels. Intrinsic mitochondrial respiratory capacity was increased with statin-induced myalgia but not accompanied by increased ROS production., (Copyright © 2019 Endocrine Society.)

Aim: Chronic inflammation increases with age and is correlated positively to visceral fat mass, but inversely to muscle mass. We investigated the hypothesis that resistance training would increase muscle mass and strength together with a concomitant drop in local and systemic inflammation level independent of any changes in visceral fat tissue in elderly., Methods: 25 subjects (mean 67, range 62-70 years) were randomized to 1 year of heavy resistance training (HRT) or control (CON), and tested at 0, 4 and 12 months for physical performance, body composition (DXA), vastus lateralis muscle area (MRI) local and systemic inflammation (blood and muscle). In addition, systemic and local muscle immunological responses to acute exercise was determined before and after the training period., Results: Increases in muscle mass (≈2%, p < 0.05), vastus lateralis area (≈9%. P < 0.05), isometric (≈15%) and dynamic (≈15%) muscle strength (p < 0.05) were found in the HRT group after 12 months training. HRT did not alter overall or visceral fat mass (p > 0.05). Blood C-Reactive Protein declined over time in both groups (p < 0.05), whereas muscle inflammation markers were unchanged to 1 year of HRT. Acute exercise increased plasma IL-6 and FGF-19 (p < 0.05), decreased FGF-21 (p < 0.05) and CCL-20 (p < 0.05), and increased GDNF in muscle (p < 0.001) similarly before and after 1 year in both groups., Conclusion: Long term resistance training increased muscle strength and improved muscle mass, but did not alter visceral fat mass and did not show any specific effect upon resting or exercise induced markers of inflammation., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Background: A growing body of evidence suggests that exercise training has beneficial effects in cancer patients. The aim of the present study was to investigate the molecular basis underlying these beneficial effects in skeletal muscle from cancer patients., Methods: We investigated expression of selected proteins involved in cellular processes known to orchestrate adaptation to exercise training by western blot. Skeletal muscle biopsies were sampled from ten cancer patients before and after 4-7 weeks of ongoing chemotherapy, and subsequently after 10 weeks of continued chemotherapy in combination with exercise training. Biopsies from ten healthy matched subjects served as reference., Results: The expression of the insulin-regulated glucose transporter, GLUT4, increased during chemotherapy and continued to increase during exercise training. A similar trend was observed for ACC, a key enzyme in the biosynthesis and oxidation of fatty acids, but we did not observe any changes in other regulators of substrate metabolism (AMPK and PDH) or mitochondrial proteins (Cyt-C, COX-IV, SDHA, and VDAC). Markers of proteasomal proteolysis (MURF1 and ATROGIN-1) decreased during chemotherapy, but did not change further during chemotherapy combined with exercise training. A similar pattern was observed for autophagy-related proteins such as ATG5, p62, and pULK1 Ser 757 , but not ULK1 and LC3BII/LC3BI. Phosphorylation of FOXO3a at Ser 318/321 did not change during chemotherapy, but decreased during exercise training. This could suggest that FOXO3a-mediated transcriptional regulation of MURF1 and ATROGIN-1 serves as a mechanism by which exercise training maintains proteolytic systems in skeletal muscle in cancer patients. Phosphorylation of proteins that regulate protein synthesis (mTOR at Ser 2448 and 4EBP1 at Thr 37/46 ) increased during chemotherapy and leveled off during exercise training. Finally, chemotherapy tended to increase the number of satellite cells in type 1 fibers, without any further change during chemotherapy and exercise training. Conversely, the number of satellite cells in type 2 fibers did not change during chemotherapy, but increased during chemotherapy combined with exercise training., Conclusions: Molecular signaling cascades involved in exercise training are disturbed during cancer and chemotherapy, and exercise training may prevent further disruption of these pathways., Trial Registration: The study was approved by the local Scientific Ethics Committee of the Central Denmark Region (Project ID: M-2014-15-14; date of approval: 01/27/2014) and the Danish Data Protection Agency (case number 2007-58-0010; date of approval: 01/28/2015). The trial was registered at http//www.clinicaltrials.gov (registration number: NCT02192216; date of registration 07/17-2014).

Aim: To examine the effect of high-intensity interval training (HIIT) on glucose clearance rates in skeletal muscle and explore the mechanism within the muscle., Methods: Ten males with type 2 diabetes mellitus (T2DM) and ten matched healthy subjects performed 2 weeks of one-legged HIIT (total of eight sessions, each comprised of 10 × 1 minute ergometer bicycle exercise at >80% of maximal heart rate, interspersed with one min of rest). Insulin sensitivity was assessed by an isoglycaemic, hyperinsulinaemic clamp combined with arteriovenous leg balance technique of the trained (T) and the untrained (UT) leg and muscle biopsies of both legs., Results: Insulin-stimulated glucose clearance in T legs was ~30% higher compared with UT legs in both groups due to increased blood flow in T vs UT legs and maintained glucose extraction. With each training session, muscle glycogen content decreased only in the training leg, and after the training, glycogen synthase and citrate synthase activities were higher in T vs UT legs. No major changes occurred in the expression of proteins in the insulin signalling cascade. Mitochondrial respiratory capacity was similar in T2DM and healthy subjects, and unchanged by HIIT., Conclusion: HIIT improves skeletal muscle insulin sensitivity. With HIIT, the skeletal muscle of patients with T2DM becomes just as insulin sensitive as untrained muscle in healthy subjects. The mechanism includes oscillations in muscle glycogen stores and a maintained ability to extract glucose from the blood in the face of increased blood flow in the trained leg., (© 2018 Scandinavian Physiological Society. Published by John Wiley & Sons Ltd.)