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The neuromuscular junction (NMJ) is a specialized synapse that bridges the motor neuron and the skeletal muscle fiber and is crucial for conversion of electrical impulses originating in the motor neuron to action potentials in the muscle fiber. The consideration of contributing factors to skeletal muscle injury, muscular dystrophy and sarcopenia cannot be restricted only to processes intrinsic to the muscle, as data show that these conditions incur denervation-like findings, such as fragmented NMJ morphology and corresponding functional changes in neuromuscular transmission. Primary defects in the NMJ also influence functional loss in motor neuron disease, congenital myasthenic syndromes and myasthenia gravis, resulting in skeletal muscle weakness and heightened fatigue. Such findings underscore the role that the NMJ plays in neuromuscular performance. Regardless of cause or effect, functional denervation is now an accepted consequence of sarcopenia and muscle disease. In this short review, we provide an overview of the pathologic etiology, symptoms, and therapeutic strategies related to the NMJ. In particular, we examine the role of the NMJ as a disease modifier and a potential therapeutic target in neuromuscular injury and disease.

Introduction: Our aim was to assess key muscle imaging and contractility parameters in the Duchenne muscular dystrophy (DMD) rat model (Dmd-KO rat), which have not yet been characterized sufficiently., Methods: We performed in-vivo magnetic resonance imaging (MRI) for thigh and leg muscles, and performed hematoxylin and eosin (H&E) staining and in-vivo muscle contractility testing in specific hindlimb muscles., Results: MRI prior to testing muscle contractility revealed multiple, unevenly distributed focal hyperintensities in the Dmd-KO rat quadriceps and tibialis anterior muscles. H&E staining showed corresponding areas of inflammation and ongoing regeneration. In-vivo contractile testing showed maximal force generated by Dmd-KO muscles was significantly lower, and susceptibility to injury was ~ two-fold greater in the Dmd-KO rats compared to wild-type (WT) rats., Discussion: Together, the MRI findings, histological findings, and the low strength and high susceptibility to injury in muscles support use of the Dmd-KO rat as an animal model of DMD., (© 2020 Wiley Periodicals LLC.)

The focus of this review is on Duchenne muscular dystrophy (DMD), which is caused by the absence of the protein dystrophin and is characterized as a neuromuscular disease in which muscle weakness, increased susceptibility to muscle injury, and inadequate repair appear to underlie the pathology. Considerable attention has been dedicated to studying muscle fiber damage, but data show that both human patients and animal models for DMD present with fragmented neuromuscular junction (NMJ) morphology. In addition to pre- and post-synaptic abnormalities, studies indicate increased susceptibility of the NMJ to contraction-induced injury, with corresponding functional changes in neuromuscular transmission and nerve-evoked electromyographic activity. Such findings suggest that alterations in the NMJ of dystrophic muscle may play a role in muscle weakness via impairment of neuromuscular transmission. Further work is needed to fully understand the role of the NMJ in the weakness, susceptibility to injury, and progressive wasting associated with DMD., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Volumetric muscle loss (VML) overwhelms the native regenerative capabilities of skeletal muscle and has few effective treatments to regain lost muscle mass and function. Tissue engineered muscle constructs designed to promote neuromuscular regeneration are a promising therapeutic avenue. To date, there has been no engineered muscle construct for VML treatment that has incorporated a pharmacologic agent to promote neuromuscular regeneration. Here, we have modified electrospun fibrin microfiber bundles, which have demonstrated muscle regenerative potential, with the heparan sulfate proteoglycan, agrin, to stimulate innervation post-VML. Myoblasts cultured on microfiber bundles with either soluble or chemically tethered agrin demonstrated statistically significant increased clustering of acetylcholine receptors (AChRs) with soluble agrin displaying AChR clusters throughout the myofiber bundles, and tethered agrin displaying AChR clusters only at 10 μm from the substrate surface. Following implantation into murine VML defects for 4 weeks, constructs pre-treated with soluble or tethered agrin resulted in statistically significant increased neuromuscular junctions, regenerating myofibers, vascular infiltration, neural infiltration, and nuclear yes-associated protein (YAP) expression within the defect site compared to the control without agrin. The agrin-tethered microfiber bundles provided sustained agrin signaling within the regenerating site during the 4-week post-implantation periods and further augmented the density of regenerating myofibers in regenerated tissue with statistical significance compared to constructs with soluble agrin. These data demonstrate the neuromuscular regenerative potential of engineered muscle constructs pre-treated to induce AChR clustering with locally delivered agrin at the site of VML regeneration., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Background: Clinical use of platelet-rich plasma (PRP) and mesenchymal stem cells (MSCs) has gained momentum as treatment for muscle injuries. Exosomes, or small cell-derived vesicles, could be helpful if they could deliver the same or better physiological effect without cell transplantation into the muscle., Hypothesis: Local delivery of exosomes derived from PRP (PRP-exos) or MSCs (MSC-exos) to injured muscles hastens recovery of contractile function., Study Design: Controlled laboratory study., Methods: In a rat model, platelets were isolated from blood, and MSCs were isolated from bone marrow and expanded in culture; exosomes from both were isolated through ultracentrifugation. The tibialis anterior muscles were injured in vivo using maximal lengthening contractions. Muscles were injected with PRP-exos or MSC-exos (immediately after injury and 5 and 10 days after injury); controls received an equal volume of saline. Histological and biochemical analysis was performed on tissues for all groups., Results: Injury resulted in a significant loss of maximal isometric torque (66% ± 3%) that gradually recovered over 2 weeks. Both PRP-exos and MSC-exos accelerated recovery, with similar faster recovery of contractile function over the saline-treated group at 5, 10, and 15 days after injury ( P < .001). A significant increase in centrally nucleated fibers was seen with both types of exosome groups by day 15 ( P < .01). Genes involved in skeletal muscle regeneration were modulated by different exosomes. Muscles treated with PRP-exos had increased expression of Myogenin gene ( P < .05), whereas muscles treated with MSC-exos had reduced expression of TGF-β ( P < .05) at 10 days after muscle injury., Conclusion: Exosomes derived from PRP or MSCs can facilitate recovery after a muscle strain injury in a small-animal model likely because of factors that can modulate inflammation, fibrosis, and myogenesis., Clinical Relevance: Given their small size, low immunogenicity, and ease with which they can be obtained, exosomes could represent a novel therapy for many orthopaedic ailments.

Intermediate filaments (IFs) contribute to force transmission, cellular integrity, and signaling in skeletal muscle. We previously identified keratin 19 (Krt19) as a muscle IF protein. We now report the presence of a second type I muscle keratin, Krt18. Krt18 mRNA levels are about half those for Krt19 and only 1:1,000th those for desmin; the protein was nevertheless detectable in immunoblots. Muscle function, measured by maximal isometric force in vivo, was moderately compromised in Krt18 -knockout ( Krt18 -KO) or dominant-negative mutant mice ( Krt18 DN), but structure was unaltered. Exogenous Krt18, introduced by electroporation, was localized in a reticulum around the contractile apparatus in wild-type muscle and to a lesser extent in muscle lacking Krt19 or desmin or both proteins. Exogenous Krt19, which was either reticular or aggregated in controls, became reticular more frequently in Krt19-null than in Krt18-null, desmin-null, or double-null muscles. Desmin was assembled into the reticulum normally in all genotypes. Notably, all three IF proteins appeared in overlapping reticular structures. We assessed the effect of Krt18 on susceptibility to injury in vivo by electroporating siRNA into tibialis anterior (TA) muscles of control and Krt19-KO mice and testing 2 wk later. Results showed a 33% strength deficit (reduction in maximal torque after injury) compared with siRNA-treated controls. Conversely, electroporation of siRNA to Krt19 into Krt18 -null TA yielded a strength deficit of 18% after injury compared with controls. Our results suggest that Krt18 plays a complementary role to Krt19 in skeletal muscle in both assembling keratin-based filaments and transducing contractile force.

Mechanical forces regulate muscle development, hypertrophy, and homeostasis. Force-transmitting structures allow mechanotransduction at the sarcolemma, cytoskeleton, and nuclear envelope. There is growing evidence that Yes-associated protein (YAP) serves as a nuclear relay of mechanical signals and can induce a range of downstream signaling cascades. Dystrophin is a sarcolemma-associated protein, and its absence underlies the pathology in Duchenne muscular dystrophy. We tested the hypothesis that the absence of dystrophin in muscle would result in reduced YAP signaling in response to loading. Following in vivo contractile loading in muscles of healthy (wild-type; WT) mice and mice lacking dystrophin ( mdx ), we performed Western blots of whole and fractionated muscle homogenates to examine the ratio of phospho (cytoplasmic) YAP to total YAP and nuclear YAP, respectively. We show that in vivo contractile loading induced a robust increase in YAP expression and its nuclear localization in WT muscles. Surprisingly, in mdx muscles, active YAP expression was constitutively elevated and unresponsive to load. Results from qRT-PCR analysis support the hyperactivation of YAP in vivo in mdx muscles, as evidenced by increased gene expression of YAP downstream targets. In vitro assays of isolated myofibers plated on substrates with high stiffness showed YAP nuclear labeling for both genotypes, indicating functional YAP signaling in mdx muscles. We conclude that while YAP signaling can occur in the absence of dystrophin, dystrophic muscles have altered mechanotransduction, whereby constitutively active YAP results in a failure to respond to load, which could be attributed to the increased state of "pre-stress" with increased cytoskeletal and extracellular matrix stiffness.

Key Points: Breast cancer 1 early onset gene codes for the DNA repair enzyme, breast cancer type 1 susceptibility protein (BRCA1). The gene is prone to mutations that cause a loss of protein function. BRCA1/Brca1 has recently been found to regulate several cellular pathways beyond DNA repair and is expressed in skeletal muscle. Skeletal muscle specific knockout of Brca1 in mice caused a loss of muscle quality, identifiable by reductions in muscle force production and mitochondrial respiratory capacity. Loss of muscle quality was associated with a shift in muscle phenotype and an accumulation of mitochondrial DNA mutations. These results demonstrate that BRCA1 is necessary for skeletal muscle function and that increased mitochondrial DNA mutations may represent a potential underlying mechanism., Abstract: Recent evidence suggests that the breast cancer 1 early onset gene (BRCA1) influences numerous peripheral tissues, including skeletal muscle. The present study aimed to determine whether induced-loss of the breast cancer type 1 susceptibility protein (Brca1) alters skeletal muscle function. We induced genetic ablation of exon 11 in the Brca1 gene specifically in the skeletal muscle of adult mice to generate skeletal muscle-specific Brca1 homozygote knockout (Brca1KO smi ) mice. Brca1KO smi exhibited kyphosis and decreased maximal isometric force in limb muscles compared to age-matched wild-type mice. Brca1KO smi skeletal muscle shifted toward an oxidative muscle fibre type and, in parallel, increased myofibre size and reduced capillary numbers. Unexpectedly, myofibre bundle mitochondrial respiration was reduced, whereas contraction-induced lactate production was elevated in Brca1KO smi muscle. Brca1KO smi mice accumulated mitochondrial DNA mutations and exhibited an altered mitochondrial morphology characterized by distorted and enlarged mitochondria, and these were more susceptible to swelling. In summary, skeletal muscle-specific loss of Brca1 leads to a myopathy and mitochondriopathy characterized by reductions in skeletal muscle quality and a consequent kyphosis. Given the substantial impact of BRCA1 mutations on cancer development risk in humans, a parallel loss of BRCA1 function in patient skeletal muscle cells would potentially result in implications for human health., (© 2018 The Authors. The Journal of Physiology © 2018 The Physiological Society.)

Assessment of skeletal muscle contractile function is an important measurement for both clinical and research purposes. Numerous conditions can negatively affect skeletal muscle. This can result in a loss of muscle mass (atrophy) and/or loss of muscle quality (reduced force per unit of muscle mass), both of which are prevalent in chronic disease, muscle-specific disease, immobilization, and aging (sarcopenia). Skeletal muscle function in animals can be evaluated by a range of different tests. All tests have limitations related to the physiological testing environment, and the selection of a specific test often depends on the nature of the experiments. Here, we describe an in vivo, non-invasive technique involving a helpful and easy assessment of force frequency-curve (FFC) in mice that can be performed on the same animal over time. This permits monitoring of disease progression and/or efficacy of a potential therapeutic treatment.

The breast cancer type 1 susceptibility protein (Brca1) is a regulator of DNA repair in mammary gland cells; however, recent cell culture evidence suggests that Brca1 influences other processes, including those in nonmammary cells. In this study, we sought to determine whether Brca1 is necessary for metabolic regulation of skeletal muscle using a novel in vivo mouse model. We developed an inducible skeletal muscle-specific Brca1knockout (BRCA1KO smi ) model to test whether Brca1 expression is necessary for maintenance of metabolic function of skeletal muscle when exposed to a high-fat diet (HFD). Our data demonstrated that deletion of Brca1 prevented HFD-induced alterations in glucose and insulin tolerance. Irrespective of diet, BRCA1KO smi mice exhibited significantly lower ADP-stimulated complex I mitochondrial respiration rates compared to age-matched wild-type (WT) mice. The data show that Brca1 has the ability to localize to the mitochondria in skeletal muscle and that BRCA1KO smi mice exhibit higher whole-body CO 2 production, respiratory exchange ratio, and energy expenditure, compared with the WT mice. Our results demonstrate that loss of Brca1 in skeletal muscle leads to dysregulated metabolic function, characterized by decreased mitochondrial respiration. Thus, any condition that results in loss of Brca1 function could induce metabolic imbalance in skeletal muscle.-Jackson, K. C., Tarpey, M. D., Valencia, A. P., Iñigo, M. R., Pratt, S. J., Patteson, D. J., McClung, J. M., Lovering, R. M., Thomson, D. M., Spangenburg, E. E. Induced Cre-mediated knockdown of Brca1 in skeletal muscle reduces mitochondrial respiration and prevents glucose intolerance in adult mice on a high-fat diet.

Tissue engineering strategies to treat patients with volumetric muscle loss (VML) aim to recover the structure and contractile function of lost muscle tissue. Here, we assessed the capacity of novel electrospun fibrin hydrogel scaffolds seeded with murine myoblasts to regenerate the structure and function of damaged muscle within VML defects to the mouse tibialis anterior muscle. The electrospun fibrin scaffolds provide pro-myogenic alignment and stiffness cues, myomimetic hierarchical structure, suturability, and scale-up capabilities. Myoblast-seeded scaffolds enabled remarkable muscle regeneration with high myofiber and vascular densities after 2 and 4 weeks, mimicking that of native skeletal muscle, while acellular scaffolds lacked muscle regeneration. Both myoblast-seeded and acellular scaffolds fully recovered muscle contractile function to uninjured values after 2 and 4 weeks. Electrospun scaffolds pre-vascularized with co-cultured human endothelial cells and human adipose-derived stem cells implanted into VML defects for 2 weeks anastomosed with host vasculature and were perfused with host red blood cells. These data demonstrate the significant potential of electrospun fibrin scaffolds seeded with myoblasts to fully regenerate the structure and function of volumetric muscle defects and these scaffolds offer a promising treatment option for patients with VML., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Duchenne muscular dystrophy (DMD), caused by the absence of the protein dystrophin, is characterized as a neuromuscular disease in which muscle weakness, increased susceptibility to muscle injury, and inadequate repair appear to underlie the pathology. Considerable attention has been dedicated to studying muscle fiber damage, but there is little information to determine if damage from contraction-induced injury also occurs at or near the nerve terminal axon. Interestingly, both human patients and the mouse model for DMD (the mdx mouse) present fragmented neuromuscular junction (NMJ) morphology. Studies of mdx mice have revealed presynaptic and postsynaptic abnormalities, nerve terminal discontinuity, as well as increased susceptibility of the NMJ to contraction-induced injury with corresponding functional changes in neuromuscular transmission and nerve-evoked electromyography. Focusing on the NMJ as a contributor to functional deficits in the muscle represents a paradigm shift from the more prevalent myocentric perspectives. Further studies are needed to determine the extent to which the nerve-muscle interaction is disrupted in DMD and the role of the NMJ in the dystrophic progression. This chapter lists the tools needed for nerve terminal and NMJ structural analysis using fluorescence imaging, and provides a step-by-step outline for how to stain, image, and analyze the NMJ in skeletal muscle, with specific attention to mdx muscle.

Neuromuscular diseases are often caused by inherited mutations that lead to progressive skeletal muscle weakness and degeneration. In diverse populations of normal healthy mice, we observed correlations between the abundance of mRNA transcripts related to mitochondrial biogenesis, the dystrophin-sarcoglycan complex, and nicotinamide adenine dinucleotide (NAD + ) synthesis, consistent with a potential role for the essential cofactor NAD + in protecting muscle from metabolic and structural degeneration. Furthermore, the skeletal muscle transcriptomes of patients with Duchene's muscular dystrophy (DMD) and other muscle diseases were enriched for various poly[adenosine 5'-diphosphate (ADP)-ribose] polymerases (PARPs) and for nicotinamide N-methyltransferase (NNMT), enzymes that are major consumers of NAD + and are involved in pleiotropic events, including inflammation. In the mdx mouse model of DMD, we observed significant reductions in muscle NAD + levels, concurrent increases in PARP activity, and reduced expression of nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme for NAD + biosynthesis. Replenishing NAD + stores with dietary nicotinamide riboside supplementation improved muscle function and heart pathology in mdx and mdx/Utr -/- mice and reversed pathology in Caenorhabditis elegans models of DMD. The effects of NAD + repletion in mdx mice relied on the improvement in mitochondrial function and structural protein expression (α-dystrobrevin and δ-sarcoglycan) and on the reductions in general poly(ADP)-ribosylation, inflammation, and fibrosis. In combination, these studies suggest that the replenishment of NAD + may benefit patients with muscular dystrophies or other neuromuscular degenerative conditions characterized by the PARP/NNMT gene expression signatures., (Copyright © 2016, American Association for the Advancement of Science.)

In patients with muscle injury or muscle disease, assessment of muscle damage is typically limited to clinical signs, such as tenderness, strength, range of motion, and more recently, imaging studies. Animal models provide unmitigated access to histological samples, which provide a "direct measure" of damage. However, even with unconstrained access to tissue morphology and biochemistry assays, the findings typically do not account for loss of muscle function. Thus, the most comprehensive measure of the overall health of the muscle is assessment of its primary function, which is to produce contractile force. The majority of animal models testing contractile force have been limited to the muscle groups moving the ankle, with advantages and disadvantages depending on the equipment. Here, we describe in vivo methods to measure torque, to produce a reliable muscle injury, and to follow muscle function within the same animal over time. We also describe in vivo methods to measure tension in the leg and thigh muscles.

Mutations in the dysferlin gene (DYSF) lead to human muscular dystrophies known as dysferlinopathies. The dysferlin-deficient A/J mouse develops a mild myopathy after 6 months of age, and when younger models the subclinical phase of the human disease. We subjected the tibialis anterior muscle of 3- to 4-month-old A/J mice to in vivo large-strain injury (LSI) from lengthening contractions and studied the progression of torque loss, myofiber damage, and inflammation afterward. We report that myofiber damage in A/J mice occurs before inflammatory cell infiltration. Peak edema and inflammation, monitored by magnetic resonance imaging and by immunofluorescence labeling of neutrophils and macrophages, respectively, develop 24 to 72 hours after LSI, well after the appearance of damaged myofibers. Cytokine profiles 72 hours after injury are consistent with extensive macrophage infiltration. Dysferlin-sufficient A/WySnJ mice show much less myofiber damage and inflammation and lesser cytokine levels after LSI than do A/J mice. Partial suppression of macrophage infiltration by systemic administration of clodronate-incorporated liposomes fails to suppress LSI-induced damage or to accelerate torque recovery in A/J mice. The findings from our studies suggest that, although macrophage infiltration is prominent in dysferlin-deficient A/J muscle after LSI, it is the consequence and not the cause of progressive myofiber damage., (Copyright © 2015 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.)

Diseases of striated muscle linked to intermediate filament (IF) proteins are associated with defects in the organization of the contractile apparatus and its links to costameres, which connect the sarcomeres to the cell membrane. Here we study the role in skeletal muscle of synemin, a type IV IF protein, by examining mice null for synemin (synm-null). Synm-null mice have a mild skeletal muscle phenotype. Tibialis anterior (TA) muscles show a significant decrease in mean fiber diameter, a decrease in twitch and tetanic force, and an increase in susceptibility to injury caused by lengthening contractions. Organization of proteins associated with the contractile apparatus and costameres is not significantly altered in the synm-null. Elastimetry of the sarcolemma and associated contractile apparatus in extensor digitorum longus myofibers reveals a reduction in tension consistent with an increase in sarcolemmal deformability. Although fatigue after repeated isometric contractions is more marked in TA muscles of synm-null mice, the ability of the mice to run uphill on a treadmill is similar to controls. Our results suggest that synemin contributes to linkage between costameres and the contractile apparatus and that the absence of synemin results in decreased fiber size and increased sarcolemmal deformability and susceptibility to injury. Thus synemin plays a moderate but distinct role in fast twitch skeletal muscle., (Copyright © 2015 the American Physiological Society.)

Introduction: We examined the possibility that tetanus toxin can prevent muscle atrophy associated with limb immobility in rats., Methods: While the knee and ankle joints were immobilized unilaterally, the tibialis anterior (TA) muscle on the immobilized side was injected with 1 μl saline or with 1 ng tetanus toxin. After 2 weeks, TA wet weights, contractile forces, and myofiber sizes from the immobilized sides were compared with those from body weight-matched normal animals., Results: Saline group wet weights decreased and produced less absolute twitch and tetanic force and normalized tetanic force compared with the toxin or normal groups. Cross-sectional areas of saline group type I, IIa, and IId myofibers, and the masses of saline group IIa, IId, IIb, and toxin group IIb myofibers, were smaller compared with the normal group., Conclusions: Tetanus toxin prevented common signs of muscle atrophy and may become a useful adjunct to current rehabilitation strategies., (© 2014 Wiley Periodicals, Inc.)

Evidence is accumulating regarding the benefits of exercise in people who are more susceptible to injury, such as the elderly, or those with a neuromuscular disease, for example Duchenne muscular dystrophy (DMD). There appears to be a consensus that exercise can be safely performed in aging and diseased muscles, but the role of eccentric exercise is not as clear. Eccentric (lengthening) contractions have risks and benefits. Eccentric contractions are commonly performed on a daily basis, and high-force voluntary eccentric contractions are often employed in strength training paradigms with excellent results; however, high-force eccentric contractions are also linked to muscle damage. This minireview examines the benefits and safety issues of using eccentric exercise in at-risk populations. A common recommendation for all individuals is difficult to achieve, and guidelines are still being established. Some form of exercise is generally recommended with aging and even with diseased muscles, but the prescription (frequency, intensity, and duration) and type (resistance vs. aerobic) of exercise requires personal attention, as there is great diversity in the functional level and comorbidities in the elderly and those with neuromuscular disease., (Copyright © 2014 the American Physiological Society.)

Introduction: The ability to view individual myofibers is possible with many histological techniques, but not yet with standard in vivo imaging. Optical coherence tomography (OCT) is an emerging technology that can generate high resolution 1-10 μm cross-sectional imaging of tissue in vivo and in real time., Methods: We used OCT to determine architectural differences of tibialis anterior muscles in situ from healthy mice (wild-type [WT], n = 4) and dystrophic mice (mdx, n = 4). After diffusion tensor imaging (DTI) and OCT, muscles were harvested, snap-frozen, and sectioned for staining with wheat germ agglutinin., Results: DTI suggested differences in pennation and OCT was used to confirm this supposition. OCT indicated a shorter intramuscular tendon (WT/mdx ratio of 1.2) and an 18% higher degree of pennation in mdx. Staining confirmed these architectural changes., Conclusions: Architectural changes in mdx muscles, which could contribute to reduction of force, are detectable with OCT., (Copyright © 2012 Wiley Periodicals, Inc.)

Disruptions of ovarian function in women are associated with increased risk of metabolic disease due to dysregulation of peripheral glucose homeostasis in skeletal muscle. Our previous evidence suggests that alterations in skeletal muscle lipid metabolism coupled with altered mitochondrial function may also develop. The objective of this study was to use an integrative metabolic approach to identify potential areas of dysfunction that develop in skeletal muscle from ovariectomized (OVX) female mice compared with age-matched ovary-intact adult female mice (sham). The OVX mice exhibited significant increases in body weight, visceral, and inguinal fat mass compared with sham mice. OVX mice also had significant increases in skeletal muscle intramyocellular lipids (IMCL) compared with the sham animals, which corresponded to significant increases in the protein content of the fatty acid transporters CD36/FAT and FABPpm. A targeted metabolic profiling approach identified significantly lower levels of specific acyl carnitine species and various amino acids in skeletal muscle from OVX mice compared with the sham animals, suggesting a potential dysfunction in lipid and amino acid metabolism, respectively. Basal and maximal mitochondrial oxygen consumption rates were significantly impaired in skeletal muscle fibers from OVX mice compared with sham animals. Collectively, these data indicate that loss of ovarian function results in increased IMCL storage that is coupled with alterations in mitochondrial function and changes in the skeletal muscle metabolic profile.

The most common and severe form of muscular dystrophy is Duchenne muscular dystrophy (DMD), a disorder caused by the absence of dystrophin, a structural protein found on the cytoplasmic surface of the sarcolemma of striated muscle fibres. Considerable attention has been dedicated to studying myofibre damage and muscle plasticity, but there is little information to determine if damage from contraction-induced injury occurs at or near the nerve terminal axon. We used α-bungarotoxin to compare neuromuscular junction (NMJ) morphology in healthy (wild-type, WT) and dystrophic (mdx) mouse quadriceps muscles and evaluated transcript levels of the post-synaptic muscle-specific kinase signalling complex. Our focus was to study changes in NMJs after injury induced with an established in vivo animal injury model. Neuromuscular transmission, electromyography (EMG), and NMJ morphology were assessed 24 h after injury. In non-injured muscle, muscle-specific kinase expression was significantly decreased in mdx compared to WT. Injury resulted in a significant loss of maximal torque in WT (39 ± 6%) and mdx (76 ± 8%) quadriceps, but significant changes in NMJ morphology, neuromuscular transmission and EMG data were found only in mdx following injury. Compared with WT mice, motor end-plates of mdx mice demonstrated less continuous morphology, more disperse acetylcholine receptor aggregates and increased number of individual acetylcholine receptor clusters, an effect that was exacerbated following injury. Neuromuscular transmission failure increased and the EMG measures decreased after injury in mdx mice only. The data show that eccentric contraction-induced injury causes morphological and functional changes to the NMJs in mdx skeletal muscle, which may play a role in excitation-contraction coupling failure and progression of the dystrophic process.

Skeletal muscle injury is often assessed by clinical findings (history, pain, tenderness, strength loss), by imaging, or by invasive techniques. The purpose of this work was to determine if in vivo proton magnetic resonance spectroscopy ((1)H MRS) could reveal metabolic changes in murine skeletal muscle after contraction-induced injury. We compared findings in the tibialis anterior muscle from both healthy wild-type (WT) muscles (C57BL/10 mice) and dystrophic (mdx mice) muscles (an animal model for human Duchenne muscular dystrophy) before and after contraction-induced injury. A mild in vivo eccentric injury protocol was used due to the high susceptibility of mdx muscles to injury. As expected, mdx mice sustained a greater loss of force (81%) after injury compared with WT (42%). In the uninjured muscles, choline (Cho) levels were 47% lower in the mdx muscles compared with WT muscles. In mdx mice, taurine levels decreased 17%, and Cho levels increased 25% in injured muscles compared with uninjured mdx muscles. Intramyocellular lipids and total muscle lipid levels increased significantly after injury but only in WT. The increase in lipid was confirmed using a permeable lipophilic fluorescence dye. In summary, loss of torque after injury was associated with alterations in muscle metabolite levels that may contribute to the overall injury response in mdx mice. These results show that it is possible to obtain meaningful in vivo (1)H MRS regarding skeletal muscle injury.

In skeletal muscle fibers, forces must be transmitted between the plasma membrane and the intracellular contractile lattice, and within this lattice between adjacent myofibrils. Based on their prevalence, biomechanical properties and localization, desmin and keratin intermediate filaments (IFs) are likely to participate in structural connectivity and force transmission. We examined the passive load-bearing response of single fibers from the extensor digitorum longus (EDL) muscles of young (3 months) and aged (10 months) wild-type, desmin-null, K19-null, and desmin/K19 double-null mice. Though fibers are more compliant in all mutant genotypes compared to wild-type, the structural response of each genotype is distinct, suggesting multiple mechanisms by which desmin and keratin influence the biomechanical properties of myofibers. This work provides additional insight into the influences of IFs on structure-function relationships in skeletal muscle. It may also have implications for understanding the progression of desminopathies and other IF-related myopathies.

Mutations in the DYSF gene that severely reduce the levels of the protein dysferlin are implicated in muscle-wasting syndromes known as dysferlinopathies. Although studies of its function in skeletal muscle have focused on its potential role in repairing the plasma membrane, dysferlin has also been found, albeit inconsistently, in the sarcoplasm of muscle fibers. The aim of this article is to study the localization of dysferlin in skeletal muscle through optimized immunolabeling methods. We studied the localization of dysferlin in control rat skeletal muscle using several different methods of tissue collection and subsequent immunolabeling. We then applied our optimized immunolabeling methods on human cadaveric muscle, control and dystrophic human muscle biopsies, and control and dysferlin-deficient mouse muscle. Our data suggest that dysferlin is present in a reticulum of the sarcoplasm, similar but not identical to those containing the dihydropyridine receptors and distinct from the distribution of the sarcolemmal protein dystrophin. Our data illustrate the importance of tissue fixation and antigen unmasking for proper immunolocalization of dysferlin. They suggest that dysferlin has an important function in the internal membrane systems of skeletal muscle, involved in calcium homeostasis and excitation-contraction coupling.

Background: Muscle strains are one of the most common complaints treated by physicians. High-force lengthening contractions can produce very high forces resulting in pain and tissue damage; such strains are the most common cause of muscle injuries. The hamstring muscles are particularly susceptible as they cross two joints and regularly perform lengthening contractions during running. We describe a patient with return to full function after a large hamstring tear., Case Description: We report the case of a 26-year-old man who presented 1 year after a noncontact, left-sided proximal hamstring tear incurred while sprinting. He received no medical treatment or formal rehabilitation. He was able to return to all sports and activities 1 to 2 months after injury, but noted a persistent deformity of the proximal thigh, which led him to seek evaluation. Physical examination, MRI functional tests, and specific muscle tests 1 year after his injury documented a major hamstring tear at the musculotendinous junction with muscle retraction, but no avulsion of the proximal tendon attachment., Literature Review: Surgery often is recommended for major proximal hamstring tendon tears, especially when more than one tendon of origin is ruptured from the ischial tuberosity. Myotendinous tears are treated nonoperatively, but may be associated with decreased strength, prolonged recovery, and recurrence., Purpose and Clinical Relevance: We describe the case of a young man who sustained a hamstring tear, with retraction, at the proximal myotendinous junction, where the biceps femoris and semitendinosus arise from the conjoint tendon. He achieved full functional recovery without medical attention, but had a persistent cosmetic deformity and slight hamstring tightness. This case suggests a benign natural history for this injury and the appropriateness of noninvasive treatment.

Objective: Stresses to skeletal muscle often result in injury. A subsequent bout of the same activity performed days or even weeks after an initial bout results in significantly less damage. The underlying causes of this phenomenon, termed the "repeated-bout effect" (RBE), are unclear. This study compared the protective effect of two different injury protocols on the ankle dorsiflexors in the rat. We hypothesized that the RBE would occur soon after the initial injury and persist for several weeks and that the RBE would occur even if the second injury was performed under different biomechanical conditions than the first., Design: In this controlled laboratory study, the dorsiflexor muscles in the left hind limbs of adult male Sprague-Dawley rats (N = 75) were subjected to ten repetitions of large-strain lengthening contractions or 150 repetitions of small-strain lengthening contractions., Results: Both protocols induced a significant (P < 0.001) and similar loss of isometric torque (approximately 50%) after the first bout of contractions. The RBE occurred as early as 2 days after the injury and remained high for 14 days (P < 0.001) but diminished by 28 days and was lost by 42 days. The small-strain contractions offered a protective effect against a subsequent large-strain contraction, but not vice versa. Although the RBE did not occur sooner than day 2, the early recovery after a second large-strain injury performed 8 hrs after the first was 2-fold greater than after a single injury., Conclusions: The RBE is both rapid in onset and prolonged, and some, but not all, injuries can protect against different types of subsequent injury.

In vitro, calmodulin (CaM) and S100A1 activate the skeletal muscle ryanodine receptor ion channel (RyR1) at submicromolar Ca(2+) concentrations, whereas at micromolar Ca(2+) concentrations, CaM inhibits RyR1. One amino acid substitution (RyR1-L3625D) has previously been demonstrated to impair CaM binding and regulation of RyR1. Here we show that the RyR1-L3625D substitution also abolishes S100A1 binding. To determine the physiological relevance of these findings, mutant mice were generated with the RyR1-L3625D substitution in exon 74, which encodes the CaM and S100A1 binding domain of RyR1. Homozygous mutant mice (Ryr1(D/D)) were viable and appeared normal. However, single RyR1 channel recordings from Ryr1(D/D) mice exhibited impaired activation by CaM and S100A1 and impaired CaCaM inhibition. Isolated flexor digitorum brevis muscle fibers from Ryr1(D/D) mice had depressed Ca(2+) transients when stimulated by a single action potential. However, during repetitive stimulation, the mutant fibers demonstrated greater relative summation of the Ca(2+) transients. Consistently, in vivo stimulation of tibialis anterior muscles in Ryr1(D/D) mice demonstrated reduced twitch force in response to a single action potential, but greater summation of force during high-frequency stimulation. During repetitive stimulation, Ryr1(D/D) fibers exhibited slowed inactivation of sarcoplasmic reticulum Ca(2+) release flux, consistent with increased summation of the Ca(2+) transient and contractile force. Peak Ca(2+) release flux was suppressed at all voltages in voltage-clamped Ryr1(D/D) fibers. The results suggest that the RyR1-L3625D mutation removes both an early activating effect of S100A1 and CaM and delayed suppressing effect of CaCaM on RyR1 Ca(2+) release, providing new insights into CaM and S100A1 regulation of skeletal muscle excitation-contraction coupling.

Intermediate filaments, composed of desmin and of keratins, play important roles in linking contractile elements to each other and to the sarcolemma in striated muscle. Our previous results show that the tibialis anterior (TA) muscles of mice lacking keratin 19 (K19) lose costameres, accumulate mitochondria under the sarcolemma, and generate lower specific tension than controls. Here we compare the physiology and morphology of TA muscles of mice lacking K19 with muscles lacking desmin or both proteins [double knockout (DKO)]. K19-/- mice and DKO mice showed a threefold increase in the levels of creatine kinase (CK) in the serum. The absence of desmin caused a larger change in specific tension (-40%) than the absence of K19 (-19%) and played the predominant role in contractile function (-40%) and decreased tolerance to exercise in the DKO muscle. By contrast, the absence of both proteins was required to obtain a significantly greater loss of contractile torque after injury (-48%) compared with wild type (-39%), as well as near-complete disruption of costameres. The DKO muscle also showed a significantly greater misalignment of myofibrils than either mutant alone. In contrast, large subsarcolemmal gaps and extensive accumulation of mitochondria were only seen in K19-null TA muscles, and the absence of both K19 and desmin yielded milder phenotypes. Our results suggest that keratin filaments containing K19- and desmin-based intermediate filaments can play independent, complementary, or antagonistic roles in the physiology and morphology of fast-twitch skeletal muscle.

Triglyceride storage is altered across various chronic health conditions necessitating various techniques to visualize and quantify lipid droplets (LDs). Here, we describe the utilization of the BODIPY (493/503) dye in skeletal muscle as a means to analyze LDs. We found that the dye was a convenient and simple approach to visualize LDs in both sectioned skeletal muscle and cultured adult single fibers. Furthermore, the dye was effective in both fixed and nonfixed cells, and the staining seemed unaffected by permeabilization. We believe that the use of the BODIPY (493/503) dye is an acceptable alternative and, under certain conditions, a simpler method for visualizing LDs stored within skeletal muscle.

The purpose of this study was to determine if variables calculated from diffusion tensor imaging (DTI) would serve as a reliable marker of damage after a muscle strain injury in dystrophic (mdx) and wild type (WT) mice. Unilateral injury to the tibialis anterior muscle (TA) was induced in vivo by 10 maximal lengthening contractions. High resolution T1- and T2-weighted structural MRI, including T2 mapping and spin echo DTI was acquired on a 7T small animal MRI system. Injury was confirmed by a significant loss of isometric torque (85% in mdx versus 42% in WT). Greater increases in apparent diffusion coefficient (ADC), axial, and radial diffusivity (AD and RD) of the injured muscle were present in the mdx mice versus controls. These changes were paralleled by decreases in fractional anisotropy (FA). Additionally, T2 was increased in the mdx mice, but the spatial extent of the changes was less than those in the DTI parameters. The data suggest that DTI is an accurate indicator of muscle injury, even at early time points where the MR signal changes are dominated by local edema.

The role of S100A1 in skeletal muscle is just beginning to be elucidated. We have previously shown that skeletal muscle fibers from S100A1 knockout (KO) mice exhibit decreased action potential (AP)-evoked Ca(2+) transients, and that S100A1 binds competitively with calmodulin to a canonical S100 binding sequence within the calmodulin-binding domain of the skeletal muscle ryanodine receptor. Using voltage clamped fibers, we found that Ca(2+) release was suppressed at all test membrane potentials in S100A1(-/-) fibers. Here we examine the role of S100A1 during physiological AP-induced muscle activity, using an integrative approach spanning AP propagation to muscle force production. With the voltage-sensitive indicator di-8-aminonaphthylethenylpyridinium, we first demonstrate that the AP waveform is not altered in flexor digitorum brevis muscle fibers isolated from S100A1 KO mice. We then use a model for myoplasmic Ca(2+) binding and transport processes to calculate sarcoplasmic reticulum Ca(2+) release flux initiated by APs and demonstrate decreased release flux and greater inactivation of flux in KO fibers. Using in vivo stimulation of tibialis anterior muscles in anesthetized mice, we show that the maximal isometric force response to twitch and tetanic stimulation is decreased in S100A1(-/-) muscles. KO muscles also fatigue more rapidly upon repetitive stimulation than those of wild-type counterparts. We additionally show that fiber diameter, type, and expression of key excitation-contraction coupling proteins are unchanged in S100A1 KO muscle. We conclude that the absence of S100A1 suppresses physiological AP-induced Ca(2+) release flux, resulting in impaired contractile activation and force production in skeletal muscle.

Insulin-like growth factor (IGF-I) is hypothesized to be a critical upstream regulator of mammalian target of rapamycin (mTOR)-regulated protein synthesis with muscle contraction. We utilized a mouse model that expresses a skeletal muscle specific dominant-negative IGF-I receptor to investigate the role of IGF-I signaling of protein synthesis in response to unilateral lengthening contractions (10 sets, 6 repetitions, 100 Hz) at 0 and 3 h following the stimulus. Our results indicate that one session of high frequency muscle contractions can activate mTOR signaling independent of signaling components directly downstream of the receptor., (Copyright 2010 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)

We studied the response of dysferlin-null and control skeletal muscle to large- and small-strain injuries to the ankle dorsiflexors in mice. We measured contractile torque and counted fibers retaining 10-kDa fluorescein dextran, necrotic fibers, macrophages, and fibers with central nuclei and expressing developmental myosin heavy chain to assess contractile function, membrane resealing, necrosis, inflammation, and myogenesis. We also studied recovery after blunting myogenesis with X-irradiation. We report that dysferlin-null myofibers retain 10-kDa dextran for 3 days after large-strain injury but are lost thereafter, following necrosis and inflammation. Recovery of dysferlin-null muscle requires myogenesis, which delays the return of contractile function compared with controls, which recover from large-strain injury by repairing damaged myofibers without significant inflammation, necrosis, or myogenesis. Recovery of control and dysferlin-null muscles from small-strain injury involved inflammation and necrosis followed by myogenesis, all of which were more pronounced in the dysferlin-null muscles, which recovered more slowly. Both control and dysferlin-null muscles also retained 10-kDa dextran for 3 days after small-strain injury. We conclude that dysferlin-null myofibers can survive contraction-induced injury for at least 3 days but are subsequently eliminated by necrosis and inflammation. Myogenesis to replace lost fibers does not appear to be significantly compromised in dysferlin-null mice.

High-force lengthening contractions are associated with muscle damage and pain, and the muscle-tendon junction is commonly cited as the primary area where myofiber damage occurs. We induced injury in the rat tibialis anterior muscle and acquired magnetic resonance imaging (MRI) images postinjury. We also assayed membrane damage and quantified the number of centrally nucleated myofibers throughout the injured muscles. Results suggest that myofiber injury occurs primarily in the middle portion of the muscle, with interstitial edema in the middle and distal portions.

We used a gait analysis system (GAS) to measure the changes in locomotion parameters of adult Sprague-Dawley rats after neuromuscular injury, induced by repeated large-strain lengthening contractions of the dorsiflexors muscles. We developed a logistic regression model from test runs of control and permanently impaired (denervation of the dorsiflexor muscles) rats and used this model to predict the probabilities of locomotory impairment in rats injured by lengthening contractions. The data showed that GAS predicts the probability of locomotory impairment with very high reliability, with values close to 100% immediately after injury and close to 0% after several weeks of recovery from injury. The six transformed locomotion parameters most effective in the model were in three domains: frequency, force, and time. We conclude that application of the GAS instrument with our predictive model accurately identifies locomotory changes due to neuromuscular deficits. Use of this technology should be valuable for monitoring the progression of a neuromuscular disease and the effects of therapeutic interventions.

Background: Standard nonoperative therapy for acute muscle strains usually involves short-term rest, ice, and nonsteroidal anti-inflammatory medications, but there is no clear consensus on how to accelerate recovery., Hypothesis: Local delivery of platelet-rich plasma to injured muscles hastens recovery of function., Study Design: Controlled laboratory study., Methods: In vivo, the tibialis anterior muscles of anesthetized Sprague-Dawley rats were injured by a single (large strain) lengthening contraction or multiple (small strain) lengthening contractions, both of which resulted in a significant injury. The tibialis anterior either was injected with platelet-rich plasma, was injected with platelet-poor plasma as a sham treatment, or received no treatment., Results: Both injury protocols yielded a similar loss of force. The platelet-rich plasma only had a beneficial effect at 1 time point after the single contraction injury protocol. However, platelet-rich plasma had a beneficial effect at 2 time points after the multiple contraction injury protocol and resulted in a faster recovery time to full contractile function. The sham injections had no effect compared with no treatment., Conclusion: Local delivery of platelet-rich plasma can shorten recovery time after a muscle strain injury in a small-animal model. Recovery of muscle from the high-repetition protocol has already been shown to require myogenesis, whereas recovery from a single strain does not. This difference in mechanism of recovery may explain why platelet-rich plasma was more effective in the high-repetition protocol, because platelet-rich plasma is rich in growth factors that can stimulate myogenesis., Clinical Relevance: Because autologous blood products are safe, platelet-rich plasma may be a useful product in clinical treatment of muscle injuries.

Recent evidence suggests that ovarian hormones contribute to altered function of skeletal muscle, however the signaling processes thought to regulate muscle function remain undefined in females. Thus, the purpose of this investigation is to determine if ovarian hormone status is critical for contraction-induced activation of AMPK or MAPK in skeletal muscle. Female mice were divided into two groups, ovariectomy (OVX) and SHAM, which were then subjected to in situ isometric contractile protocols. AMPK, ERK 1/2, p38, and JNK phosphorylation were measured in the control and contracting limb. In the in situ protocol, OVX muscles were significantly more resistant to fatigue compared to the SHAM animals. In addition, the muscles from OVX mice demonstrated significantly lower levels of normalized AMPK phosphorylation at rest. AMPK phosphorylation was not increased in the muscles from SHAM mice after the in situ contractile protocol, while the OVX demonstrated significant increases in AMPK phosphorylation. After contraction, normalized ERK2 phosphorylation was significantly higher in the OVX group compared to the SHAM group. Both p38 and JNK phosphorylation increased in response to contraction; but no group differences were detected. A second set of SHAM and OVX animals were subjected to fatigue stimulated under in vitro conditions. Significant increases in AMPK and ERK2 phosphorylation were detected, but no differences were found between groups. In conclusion, removal of the ovaries results in different responses to contraction-induced changes in phosphorylation of AMPK and ERK2 in female mice and suggests hormones secreted from the ovaries significantly impacts cellular signaling in skeletal muscle.

The paired pyramidalis muscles are small triangular-shaped muscles that lie between the anterior surface of the rectus abdominus and the posterior surface of the rectus sheath. The precise function of pyramidalis muscles is unclear, but together the muscles are thought to tense the linea alba. The muscles are not always present, or are often unilateral, and vary greatly in size. Their wider inferior margins attach to the pubic symphyses and pubic crests, whereas their narrow superior margins attach to the linea alba. The gross anatomy and innervation of the pyramidalis muscles has been described by others, but their architecture and fiber type have not been determined in previous publications. The purpose of the present paper was therefore to investigate these parameters and place the findings into context for the literature available on this muscle. An example of bilateral pyramidalis muscles was recently encountered in a male cadaver that provided ample tissue for an analysis of its architecture and fiber type. The muscle mass, muscle length, fiber length, and pennation angle of muscle fibers were measured to ascertain physiological cross-sectional area and thereby estimate force production. Fiber type composition was also examined using immunofluorescent labeling. The results show that this is a muscle of mixed fiber type composition, similar to the rectus abdominus, and that the estimated forces generated by this muscle are relatively small.

The protein, dysferlin, mediates sarcolemmal repair in vitro, implicating defective membrane repair in dysferlinopathies. To study the role of dysferlin in vivo, we assessed contractile function, sarcolemmal integrity, and myogenesis before and after injury from large-strain lengthening contractions in dysferlin-null and control mice. We report that dysferlin-null muscles produce higher contractile torque, and are equally susceptible to initial injury but recover from injury more slowly. Two weeks after injury, control muscles retain fluorescein dextran and do not show myogenesis. Dysferlin-null muscles do not retain fluorescein dextran, and show necrosis followed by myogenesis. Our data indicate that recovery of control muscles from injury primarily involves sarcolemmal repair whereas recovery of dysferlin-null muscles primarily involves myogenesis without repair and long-term survival of myofibers.

Bex1 and Calmodulin (CaM) are upregulated during skeletal muscle regeneration. We confirm this finding and demonstrate the novel finding that they interact in a calcium-dependent manner. To study the role of Bex1 and its interaction with CaM in skeletal muscle regeneration, we generated Bex1 knock out (Bex1-KO) mice. These mice appeared to develop normally and are fertile, but displayed a functional deficit in exercise performance compared to wild type (WT) mice. After intramuscular injection of cardiotoxin, which causes extensive and reproducible myotrauma followed by recovery, regenerating muscles of Bex1-KO mice exhibited elevated and prolonged cell proliferation, as well as delayed cell differentiation, compared to WT mice. Thus, our results provide the first evidence that Bex1-KO mice show altered muscle regeneration, and allow us to propose that the interaction of Bex1 with Ca(2+)/CaM may be involved in skeletal muscle regeneration.

Objective: To determine if the proliferation of myogenic cells is equally important to recovery of contractile function after 2 different types of contraction-induced muscle injuries., Design: Randomized trial., Setting: Muscle biology laboratory., Animals: Adult male Sprague-Dawley rats., Interventions: Tibialis anterior muscles were injured by a single lengthening contraction with large strain (1R) or multiple lengthening contractions with small strain (MR). The hindlimbs of some animals in each group were irradiated before injury to prevent proliferation of myogenic cells during recovery., Main Outcome Measures: Contractile tension was measured immediately after injury and 3, 7, 14, and 21 days after injury. Permeation to Evans blue dye was used to assay membrane damage. Centrally nucleated fibers and reverse transcriptase-polymerase chain reaction of myoD and myogenin were used as measures of myogenesis., Results: Inhibiting myogenesis prevented the recovery of contractile function after MR, but not after 1R. Both protocols caused Evans blue dye uptake immediately after injury, but Evans blue dye was only retained in fibers for several days after 1R. This suggests that membranes reseal after 1R, but not after MR., Conclusions: The mechanisms that underlie recovery after injuries caused by repeated lengthening contractions and injuries caused by a single lengthening contraction are different. The differences may be important when planning targeted rehabilitation strategies for each type of injury.

Muscle strains, frequently the result of a lengthening contraction, sometimes are treated with corticosteroids. We tested whether an injection of dexamethasone administered soon after muscle injury would minimize inflammation and facilitate the recovery of contractile tension. We applied one eccentric contraction on the tibialis anterior of 76 rats, which were randomly assigned to one of three groups: sham-injured plus dexamethasone, injured plus vehicle, and injured plus dexamethasone. Electrophysiology, histology, and reverse transcription-polymerase chain reaction were used to study the relation between contractile tension, inflammation, and the expression of inflammatory molecules. The single eccentric contraction led to a reversible muscle injury characterized initially by reduced contractile tension and inflammation. The dexamethasone injection reduced the expression of interleukin-1beta and transforming growth factor-beta1 compared with injured vehicle-injected controls and led to a transient improvement of contractile tension 3 days after the injury. No adverse effects were seen for as much as 3 weeks after the dexamethasone injection. The data indicate that one dose of dexamethasone administered soon after muscle strain may facilitate recovery of contractile tension without causing major adverse consequences in this experimental model.

Purpose: Some muscle injuries are the result of a single lengthening contraction. Our goal was to evaluate the contributions of angular velocity, arc of motion, and timing of contractile activation relative to the onset of joint motion in an animal model of muscle injury using a single lengthening contraction., Methods: The intact tibialis anterior (TA) muscle of rats was activated while lengthened, preceded by a maximal isometric contraction of 0, 25, 50, 100, or 200 ms. The lengthening contraction was performed at two different angular velocities (300 or 900 degrees/s) and through two different arcs of motion (90 degrees or 45 degrees)., Results: Muscle contractile function, as measured by maximal isometric tetanic tension, was significantly decreased only when the TA was activated at least 50 ms prior to the motion, regardless of angular velocity or arc of motion., Conclusion: The data indicated that the duration of an isometric contraction prior to a single lengthening contraction determined the extent of muscle injury irrespective of two different angular velocities.

Purpose: The purpose of this study was to understand the detailed architectural properties of the human flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU) muscles and their implications for tendon transfer surgery., Methods: Muscle fiber length was measured in 6 separate regions of the FCU and FCR from 10 cadaveric specimens. Sarcomere length was measured by laser diffraction for normalization. Moment arms were estimated by measuring tendon excursion with respect to joint angle. The position of entry of the motor nerve branches into each muscle also was measured to establish limits for the safe length of muscle mobilization., Results: Muscle fiber length varied significantly along both the FCU and FCR. Fiber length variability in the FCU was twice that of the FCR. Although the average fiber length for both muscles across all regions was similar (62.6 +/- 2.1 mm for the FCR and 63.1 +/- 4.0 mm for the FCU), the proximal fibers of the FCU were longer compared with the proximal fibers of the FCR and the distal fibers of the FCU were shorter compared with the distal fibers of the FCR. The 99% confidence interval for the second nerve branch entry into the muscles was located approximately 69 mm distal to the medial epicondyle for the FCU and approximately 73 mm distal for the FCR., Conclusions: These data show different designs of both the FCU and the FCR. The functional significance of fiber length variability is not clear but imply that, when used in tendon transfer, the properly mobilized FCU has a much greater excursion.

The purpose of this study was to evaluate the integrity of the muscle membrane and its associated cytoskeleton after a contraction-induced injury. A single eccentric contraction was performed in vivo on the tibialis anterior (TA) of male Sprague-Dawley rats at 900 degrees /s throughout a 90 degrees -arc of motion. Maximal tetanic tension (Po) of the TAs was assessed immediately and at 3, 7, and 21 days after the injury. To evaluate sarcolemmal integrity, we used an Evans blue dye (EBD) assay, and to assess structural changes, we used immunofluorescent labeling with antibodies against contractile (myosin, actin), cytoskeletal (alpha-actinin, desmin, dystrophin, beta-spectrin), integral membrane (alpha- and beta-dystroglycan, sarcoglycan), and extracellular (laminin, fibronectin) proteins. Immediately after injury, P0 was significantly reduced to 4.23 +/- 0.22 N, compared with 8.24 +/- 1.34 N in noninjured controls, and EBD was detected intracellularly in 54 +/- 22% of fibers from the injured TA, compared with 0% in noninjured controls. We found a significant association between EBD-positive fibers and the loss of complete dystrophin labeling. The loss of dystrophin was notable because organization of other components of the subsarcolemmal cytoskeleton was affected minimally (beta-spectrin) or not at all (alpha- and beta-dystroglycan). Labeling with specific antibodies indicated that dystrophin's COOH terminus was selectively more affected than its rod domain. Twenty-one days after injury, contractile properties were normal, fibers did not contain EBD, and dystrophin organization and protein level returned to normal. These data indicate the selective vulnerability of dystrophin after a single eccentric contraction-induced injury and suggest a critical role of dystrophin in force transduction.