Sarcomerogenesis, or the addition of sarcomeres in series within a fiber, has a profound impact on the performance of a muscle by increasing its contractile velocity and power. Sarcomerogenesis may provide a beneficial adaptation to prevent injury when a muscle consistently works at long lengths, accounting for the repeated-bout effect. The association between eccentric exercise, sarcomerogenesis and the repeated-bout effect has been proposed to depend on damage, where regeneration allows sarcomeres to work at shorter lengths for a given muscle-tendon unit length. To gain additional insight into this phenomenon, we measured fiber dynamics directly in the vastus lateralis (VL) muscle of rats during uphill and downhill walking, and we measured serial sarcomere number in the VL and vastus intermedius (VI) after chronic training on either a decline or incline grade. We found that the knee extensor muscles of uphill walking rats undergo repeated active concentric contractions, and therefore they suffer no contraction-induced injury. Conversely, the knee extensor muscles during downhill walking undergo repeated active eccentric contractions. Serial sarcomere numbers change differently for the uphill and downhill exercise groups, and for the VL and VI muscles. Short muscle lengths for uphill concentric-biased contractions result in a loss of serial sarcomeres, and long muscle lengths for downhill eccentric-biased contractions result in a gain of serial sarcomeres.

Key pointsr Muscle fibre cross sectional area is enhanced with massage in the form of cyclic compressive loading during regrowth after atrophy.r Massage enhances protein synthesis of the myofibrillar and cytosolic, but not the mitochondrial fraction, in muscle during regrowth. r Massage in the form of cyclic compressive loading is a potential anabolic intervention during muscle regrowth after atrophy.Abstract Massage, in the form of cyclic compressive loading (CCL), is associated with multiple health benefits, but its potential anabolic effect on atrophied muscle has not been investigated. We hypothesized that the mechanical activity associated with CCL induces an anabolic effect in skeletal muscle undergoing regrowth after a period of atrophy. Fischer-Brown Norway rats at 10 months of age were hindlimb unloaded for a period of 2 weeks. The rats were then allowed reambulation with CCL applied at a 4.5 N load at 0.5 Hz frequency for 30 min every other day for four bouts during a regrowth period of 8 days. Muscle fibre cross sectional area was enhanced by 18% with massage during regrowth compared to reloading alone, and this was accompanied by elevated myofibrillar and cytosolic protein as well as DNA synthesis. Focal adhesion kinase phosphorylation indicated that CCL increased mechanical stimulation, while a higher number of Pax7 + cells likely explains the elevated DNA synthesis. Surprisingly, the contralateral non-massaged limb exhibited a comparable 17% higher muscle fibre size compared to reloading alone, and myofibrillar protein synthesis, but not DNA synthesis, was also elevated. We conclude that massage in the form of CCL induces an anabolic response in muscles regrowing after an atrophy-inducing event. We suggest that massage can be used as an intervention to aid in the regrowth of muscle lost during immobilization.

Key pointsr The endogenous molecular clock in skeletal muscle is necessary for maintenance of phenotype and function.r Loss of Bmal1 solely from adult skeletal muscle (iMSBmal1 −/− ) results in reductions in specific tension, increased oxidative fibre type and increased muscle fibrosis with no change in feeding or activity.r Disruption of the molecular clock in adult skeletal muscle is sufficient to induce changes in skeletal muscle similar to those seen in the Bmal1 knockout mouse (Bmal1 −/− ), a model of advanced ageing. r This study uncovers a fundamental role for the skeletal muscle clock in musculoskeletal homeostasis with potential implications for ageing.Abstract Disruption of circadian rhythms in humans and rodents has implicated a fundamental role for circadian rhythms in ageing and the development of many chronic diseases including diabetes, cardiovascular disease, depression and cancer. The molecular clock mechanism underlies circadian rhythms and is defined by a transcription-translation feedback loop with Bmal1 encoding a core molecular clock transcription factor. Germline Bmal1 knockout (Bmal1 KO) mice have a shortened lifespan, show features of advanced ageing and exhibit significant weakness with decreased maximum specific tension at the whole muscle and single fibre levels. We tested the role of the molecular clock in adult skeletal muscle by generating mice that allow for the inducible skeletal muscle-specific deletion of Bmal1 (iMSBmal1). Here we show that disruption of the molecular clock, specifically in adult skeletal muscle, is associated with a muscle phenotype including reductions in specific tension, increased oxidative fibre type, and increased muscle fibrosis similar to that seen in the Bmal1 KO mouse. Remarkably, the phenotype observed in the iMSBmal1 −/− mice was not limited to changes in muscle. Similar to the germline Bmal1 KO mice, we observed significant bone and cartilage changes throughout the body suggesting a role for the skeletal muscle molecular clock in both the skeletal muscle niche and the systemic milieu. This emerging area of circadian rhythms and the molecular clock in skeletal muscle holds the potential to provide significant insight into intrinsic mechanisms of the maintenance of muscle quality and function as well as identifying a novel crosstalk between skeletal muscle, cartilage and bone.

Purpose To assess the biologic basis of massage therapies, we developed an experimental approach to mimic Swedish massage and evaluate this approach on recovery from eccentric exercise-induced muscle damage using a well-controlled animal model. Methods Tibialis anterior muscles of six New Zealand White rabbits were subjected to one bout of damaging, eccentric contractions. One muscle was immediately subjected to cyclic compressive loads, and the contralateral served as the exercised control. Results We found that commencing 30 min of cyclic compressive loading to the muscle, immediately after a bout of eccentric exercise, facilitated recovery of function and attenuated leukocyte infiltration. In addition, fiber necrosis and wet weight of the tissue were also reduced by compressive loading. Conclusion We conclude that subjecting muscle to compressive loads immediately after exercise leads to an enhanced recovery of muscle function and attenuation of the damaging effects of inflammation in the rabbit model. Although these observations suggest that skeletal muscle responds to cyclic compressive forces similar to those generated by clinical approaches, such as therapeutic massage, further research is needed to assess the translational efficacy of these findings.

Chronic critical illness is a global clinical issue affecting millions of sepsis survivors annually. Survivors report chronic skeletal muscle weakness and development of new functional limitations that persist for years. To delineate mechanisms of sepsis-induced chronic weakness, we first surpassed a critical barrier by establishing a murine model of sepsis with ICU-like interventions that allows for the study of survivors. We show that sepsis survivors have profound weakness for at least 1 month, even after recovery of muscle mass. Abnormal mitochondrial ultrastructure, impaired respiration and electron transport chain activities, and persistent protein oxidative damage were evident in the muscle of survivors. Our data suggest that sustained mitochondrial dysfunction, rather than atrophy alone, underlies chronic sepsis-induced muscle weakness. This study emphasizes that conventional efforts that aim to recover muscle quantity will likely remain ineffective for regaining strength and improving quality of life after sepsis until deficiencies in muscle quality are addressed.

Background Optimal strategies for massage and its use in athletes have not been conclusively demonstrated. Purpose/study design Effects of varying duration, frequency and magnitude of massage-like compressive loading (MLL) on recovery of skeletal muscle active properties (torque angle (T-Θ) relationship) following exercise-induced muscle injury were studied. Methods Twenty-four New Zealand White rabbits were surgically instrumented with bilateral peroneal nerve cuffs for stimulation of hindlimb tibialis anterior muscles. Following a bout of eccentric exercise (EEX), rabbits were randomly assigned to a MLL protocol of 0.25 or 0.5 Hz at 5 or 10 N for 15 or 30 min. T-Θ was obtained for 21 tibiotarsal joint angles pre-and post-EEX and post 4 consecutive days of MLL. Muscle wet weight and H&E sections were obtained following final treatments. Results EEX produced an average 61.8%±2.1 decrease in peak isometric torque output. Differences in torque recovery were found between magnitudes (5 and 10 N; p=0.004, n=12) and frequencies (0.25 and 0.5 Hz; p=0.012, n=12), but no difference for durations (15 and 30 min) with the 0.5 Hz, 10 N, 15 min protocol showing greatest recovery 4 days post-EEX. MLL muscle (n=12) wet weight was 3.22±0.18 g, while no MLL tissue (n=9) weighed 3.74±0.22 g ( p=0.029). Histological analysis showed a difference in torn fibres between low-parameter and high-parameter MLL (6.5±1.04 vs 0.5±0.29 per 0.59 mm 2 , p=0.005). Conclusions Results showed a dose-response effect for magnitude and frequency of MLL on recovery of active muscle properties following EEX. Future studies will investigate underlying biological mechanisms for this enhanced recovery of muscle function.

Objective. Physiotherapies are the most widely recommended conservative treatment for arthritic diseases. The present study was undertaken to examine the molecular mechanisms underlying the effects of gentle treadmill walking (GTW) on various stages of monoiodoacetate-induced arthritis (MIA) to elucidate the basis for the success or failure of such therapies in joint damage.Methods. Knees were obtained from untreated control rats, rats with MIA that did not undergo GTW, rats with MIA in which GTW regimens were started 1 day post-MIA induction, and rats with MIA in which GTW regimens were started after cartilage damage had progressed to grade 1 or grade 2. The cartilage was examined macroscopically, microscopically, and by microfocal computed tomography imaging. Transcriptome-wide gene expression analysis was performed, and microarray data were assessed by Ingenuity Pathways Analysis to identify molecular functional networks regulated by GTW.Results. GTW intervention started on day 1 post-MIA induction significantly prevented the progression of MIA, but its efficacy was reduced when implemented on knees exhibiting close to grade 1 cartilage damage.GTW accelerated cartilage damage in knees with close to grade 2 damage. Transcriptome-wide gene expression analysis revealed that GTW intervention started 1 day post-MIA inception significantly suppressed inflammation-associated genes and up-regulated matrixassociated gene networks. However, delayed GTW intervention after grade 1 damage had occurred was less effective in suppressing proinflammatory genes or upregulating matrix synthesis.Conclusion. The present findings suggest that GTW suppresses proinflammatory gene networks and up-regulates matrix synthesis to prevent progression of cartilage damage in MIA-affected knees. However, the extent of cartilage damage at the initiation of GTW may be an important determinant of the success or failure of such therapies.

Muscle strain injuries are some of the most frequent injuries in sports and command a great deal of attention in an effort to understand their etiology. These injuries may be the culmination of a series of subcellular events accumulated through repetitive lengthening (eccentric) contractions during exercise, and they may be influenced by a variety of variables including fiber strain magnitude, peak joint torque, and starting muscle length. To assess the influence of these variables on muscle injury magnitude in vivo, we measured fiber dynamics and joint torque production during repeated stretch-shortening cycles in the rabbit tibialis anterior muscle, at short and long muscle lengths, while varying the timing of activation before muscle stretch. We found that a muscle subjected to repeated stretch-shortening cycles of constant muscle-tendon unit excursion exhibits significantly different joint torque and fiber strains when the timing of activation or starting muscle length is changed. In particular, measures of fiber strain and muscle injury were significantly increased by altering activation timing and increasing the starting length of the muscle. However, we observed differential effects on peak joint torque during the cyclic stretch-shortening exercise, as increasing the starting length of the muscle did not increase torque production. We conclude that altering activation timing and muscle length before stretch may influence muscle injury by significantly increasing fiber strain magnitude and that fiber dynamics is a more important variable than muscle-tendon unit dynamics and torque production in influencing the magnitude of muscle injury.