Use it or lose it: multiscale skeletal muscle adaptation to mechanical stimuli

Age-related differences in the loss and recovery of serial sarcomere number following disuse atrophy in rats

BACKGROUND

Older adults exhibit a slower recovery of muscle mass following disuse atrophy than young adults. At a smaller scale, muscle fibre cross-sectional area (i.e., sarcomeres in parallel) exhibits this same pattern. Less is known, however, about age-related differences in the recovery of muscle fibre length, driven by increases in serial sarcomere number (SSN), following disuse. The purpose of this study was to investigate age-related differences in SSN adaptations and muscle mechanical function during and following muscle immobilization. We hypothesized that older adult rats would experience a similar magnitude of SSN loss during immobilization, however, take longer to recover SSN than young following cast removal, which would limit the recovery of muscle mechanical function.

METHODS

We casted the plantar flexors of young (8 months) and old (32 months) male rats in a shortened position for 2 weeks, and assessed recovery during 4 weeks of voluntary ambulation. Following sacrifice, legs were fixed in formalin for measurement of soleus SSN and physiological cross-sectional area (PCSA) with the un-casted soleus acting as a control. Ultrasonographic measurements of pennation angle (PA) and muscle thickness (MT) were conducted weekly. In-vivo active and passive torque-angle relationships were constructed pre-cast, post-cast, and following 4 weeks of recovery.

RESULTS

From pre- to post-cast, young and older adult rats experienced similar decreases in SSN (-20%, P < 0.001), muscle wet weight (-25%, P < 0.001), MT (-30%), PA (-15%, P < 0.001), and maximum isometric torque (-40%, P < 0.001), but there was a greater increase in passive torque in older (+ 180%, P < 0.001) compared to young adult rats (+ 68%, P = 0.006). Following cast removal, young exhibited quicker recovery of SSN and MT than old, but SSN recovered sooner than PA and MT in both young and old. PCSA nearly recovered and active torque fully recovered in young adult rats, whereas in older adult rats these remained unrecovered at ∼ 75%.

CONCLUSIONS

This study showed that older adult rats retain a better ability to recover longitudinal compared to parallel muscle morphology following cast removal, making SSN a highly adaptable target for improving muscle function in elderly populations early on during rehabilitation.

Discussing Conflicting Explanatory Approaches in Flexibility Training Under Consideration of Physiology: A Narrative Review

The mechanisms underlying range of motion enhancements via flexibility training discussed in the literature show high heterogeneity in research methodology and study findings. In addition, scientific conclusions are mostly based on functional observations while studies considering the underlying physiology are less common. However, understanding the underlying mechanisms that contribute to an improved range of motion through stretching is crucial for conducting comparable studies with sound designs, optimising training routines and accurately interpreting resulting outcomes. While there seems to be no evidence to attribute acute range of motion increases as well as changes in muscle and tendon stiffness and pain perception specifically to stretching or foam rolling, the role of general warm-up effects is discussed in this paper. Additionally, the role of mechanical tension applied to greater muscle lengths for range of motion improvement will be discussed. Thus, it is suggested that physical training stressors can be seen as external stimuli that control gene expression via the targeted stimulation of transcription factors, leading to structural adaptations due to enhanced protein synthesis. Hence, the possible role of serial sarcomerogenesis in altering pain perception, reducing muscle stiffness and passive torque, or changes in the optimal joint angle for force development is considered as well as alternative interventions with a potential impact on anabolic pathways. As there are limited possibilities to directly measure serial sarcomere number, longitudinal muscle hypertrophy remains without direct evidence. The available literature does not demonstrate the necessity of only using specific flexibility training routines such as stretching to enhance acute or chronic range of motion.

The Biomechanics of Musculoskeletal Tissues during Activities of Daily Living: Dynamic Assessment Using Quantitative Transmission-Mode Ultrasound Techniques

The measurement of musculoskeletal tissue properties and loading patterns during physical activity is important for understanding the adaptation mechanisms of tissues such as bone, tendon, and muscle tissues, particularly with injury and repair. Although the properties and loading of these connective tissues have been quantified using direct measurement techniques, these methods are highly invasive and often prevent or interfere with normal activity patterns. Indirect biomechanical methods, such as estimates based on electromyography, ultrasound, and inverse dynamics, are used more widely but are known to yield different parameter values than direct measurements. Through a series of literature searches of electronic databases, including Pubmed, Embase, Web of Science, and IEEE Explore, this paper reviews current methods used for the in vivo measurement of human musculoskeletal tissue and describes the operating principals, application, and emerging research findings gained from the use of quantitative transmission-mode ultrasound measurement techniques to non-invasively characterize human bone, tendon, and muscle properties at rest and during activities of daily living. In contrast to standard ultrasound imaging approaches, these techniques assess the interaction between ultrasound compression waves and connective tissues to provide quantifiable parameters associated with the structure, instantaneous elastic modulus, and density of tissues. By taking advantage of the physical relationship between the axial velocity of ultrasound compression waves and the instantaneous modulus of the propagation material, these techniques can also be used to estimate the in vivo loading environment of relatively superficial soft connective tissues during sports and activities of daily living. This paper highlights key findings from clinical studies in which quantitative transmission-mode ultrasound has been used to measure the properties and loading of bone, tendon, and muscle tissue during common physical activities in healthy and pathological populations.

Habitually wearing high heels may improve user walking economy in any footwear

The habitual use of high-heeled footwear may structurally remodel user leg muscle tendons, thereby altering their functional capabilities. High heels set users' ankles in relatively plantarflexed positions, causing calf muscle tendons to operate at relatively short lengths. Habitually operating muscle tendons at relatively short lengths induces structural remodeling that theoretically affects muscle metabolism. Because structural changes occur within the body, the user's locomotor metabolism may change in any footwear condition (e.g., conventional shoes, barefoot). Here, we studied the influence of habitual high-heel use on users' leg muscle-tendon structure and metabolism during walking in flat-soled footwear. We tested eight participants before and after 14 wk of agreeing to wear high heels as their daily shoes. Overall, participants who wore high heels >1,500 steps per day, experienced a 9% decrease in their net metabolic power during walking in flat-soled footwear (d = 1.66, P ≤ 0.049), whereas participants who took <1,000 daily steps in high heels did not (d = 0.44; P = 0.524). Across participants, for every 1,000 daily steps in high heels, net metabolic power during walking in flat-soled footwear decreased 5.3% (r = -0.73; P = 0.040). Metabolic findings were partially explained (r2 = 0.43; P = 0.478) by trending shorter medial gastrocnemius fascicle lengths (d = 0.500, P = 0.327) and increased Achilles tendon stiffness (d = 2.889, P = 0.088). The high-heel intervention did not alter user walking stride kinematics in flat-soled footwear (d ≤ 0.567, P ≥ 0.387). While our limited dataset is unable to establish the mechanisms underlying the high-heel-induced walking economy improvement, it appears that prescribing specific footwear use can be implemented to alter user muscle-tendon properties and augment their function in any shoes.NEW & NOTEWORTHY Habitually wearing high-heeled footwear structurally remodels leg muscle tendons and improves user walking economy, regardless of worn attire.

Chronic Effects of Inter-Set Static Stretching on Morphofunctional Outcomes in Recreationally Resistance-Trained Male and Female

Purpose: The purpose of this study was to compare the effects of resistance training (RT) with inter-set static stretching (IS) versus traditional RT (TRT) on morphofunctional outcomes in recreationally resistance-trained male and female. Methods: Twenty-two recreationally-trained subjects were allocated to IS group (n = 12) or TRT (n = 10) and completed eight weeks of RT. The only difference between the groups was that IS group included static stretching between sets, while the TRT rested between the sets. Ultrasound images, dynamic and isometric strength tests for the elbow flexors and elbow extensors were evaluated pre- and post-intervention period. Results: Total training volume (TTV) was greater in TRT than IS (p = .031). TRT and IS caused similar increases in maximal dynamic and isometric strength. Fascicle length of the brachialis increased following TRT (p = .033); muscle thickness and the pennation angle of the distal portion of the triceps brachii increased following IS (p = .035 and p = .007, respectively). There were no significant changes in thickness and architecture for biceps brachii in either group. There were no significant differences between groups for any muscle strength and morphology outcome. Conclusion: IS negatively affects TTV but does not affect muscle strength and architecture of recreationally resistance-trained male and female.

A novel computational framework for the estimation of internal musculoskeletal loading and muscle adaptation in hypogravity

Introduction: Spaceflight is associated with substantial and variable musculoskeletal (MSK) adaptations. Characterisation of muscle and joint loading profiles can provide key information to better align exercise prescription to astronaut MSK adaptations upon return-to-Earth. A case-study is presented of single-leg hopping in hypogravity to demonstrate the additional benefit computational MSK modelling has when estimating lower-limb MSK loading. Methods: A single male participant performed single-leg vertical hopping whilst attached to a body weight support system to replicate five gravity conditions (0.17, 0.25, 0.37, 0.50, 1 g). Experimental joint kinematics, joint kinetics and ground reaction forces were tracked in a data-tracking direct collocation simulation framework. Ground reaction forces, sagittal plane hip, knee and ankle net joint moments, quadriceps muscle forces (Rectus Femoris and three Vasti muscles), and hip, knee and ankle joint reaction forces were extracted for analysis. Estimated quadriceps muscle forces were input into a muscle adaptation model to predict a meaningful increase in muscle cross-sectional area, defined in (DeFreitas et al., 2011). Results: Two distinct strategies were observed to cope with the increase in ground reaction forces as gravity increased. Hypogravity was associated with an ankle dominant strategy with increased range of motion and net plantarflexor moment that was not seen at the hip or knee, and the Rectus Femoris being the primary contributor to quadriceps muscle force. At 1 g, all three joints had increased range of motion and net extensor moments relative to 0.50 g, with the Vasti muscles becoming the main muscles contributing to quadriceps muscle force. Additionally, hip joint reaction force did not increase substantially as gravity increased, whereas the other two joints increased monotonically with gravity. The predicted volume of exercise needed to counteract muscle adaptations decreased substantially with gravity. Despite the ankle dominant strategy in hypogravity, the loading on the knee muscles and joint also increased, demonstrating this provided more information about MSK loading. Discussion: This approach, supplemented with muscle-adaptation models, can be used to compare MSK loading between exercises to enhance astronaut exercise prescription.

Hamstring stiffness and injury risk factors during the handball season in female players

Monitoring the muscle mechanical properties and functions of female athletes throughout their training season is relevant to understand the relationships between these factors and to predict noncontact injuries, which are prevalent among female athletes. The first aim of this study was to determine whether female handball players' passive stiffness of the hamstring muscles is associated with hamstring extensibility, strength of knee flexors and extensors, and lower limb stiffness. Additionally, the study monitored fluctuations in these factors over 25 weeks. The study utilized an isokinetic dynamometer to record hamstring passive stiffness, extensibility, and hamstring and quadriceps strength of 18 young handball players. Lower limb stiffness was determined from a countermovement vertical jump conducted on a force plate. The countermovement jump involved the calculation of the peak force during the eccentric phase and the mean force during the concentric phase. The results showed a positive correlation between hamstring passive stiffness and lower limb stiffness (r = 0.660, p < 0.01), knee flexion and extension strength (r = 0.592, p < 0.01 and r = 0.497, p < 0.05, respectively), and eccentric peak force (r = 0.587, p < 0.01) during jumping. The strength of knee extensors increased significantly after 6 weeks, and hamstring stiffness after 12 weeks of training. In conclusion, the increased hamstring stiffness following training did not match other factors associated with injury risk. Therefore, preventing multifactorial injury risk requires a comprehensive approach, and monitoring one factor alone is insufficient to predict noncontact injuries in female handball players.

Pain-sensorimotor interactions: New perspectives and a new model

How pain and sensorimotor behavior interact has been the subject of research and debate for many decades. This article reviews theories bearing on pain-sensorimotor interactions and considers their strengths and limitations in the light of findings from experimental and clinical studies of pain-sensorimotor interactions in the spinal and craniofacial sensorimotor systems. A strength of recent theories is that they have incorporated concepts and features missing from earlier theories to account for the role of the sensory-discriminative, motivational-affective, and cognitive-evaluative dimensions of pain in pain-sensorimotor interactions. Findings acquired since the formulation of these recent theories indicate that additional features need to be considered to provide a more comprehensive conceptualization of pain-sensorimotor interactions. These features include biopsychosocial influences that range from biological factors such as genetics and epigenetics to psychological factors and social factors encompassing environmental and cultural influences. Also needing consideration is a mechanistic framework that includes other biological factors reflecting nociceptive processes and glioplastic and neuroplastic changes in sensorimotor and related brain and spinal cord circuits in acute or chronic pain conditions. The literature reviewed and the limitations of previous theories bearing on pain-sensorimotor interactions have led us to provide new perspectives on these interactions, and this has prompted our development of a new concept, the Theory of Pain-Sensorimotor Interactions (TOPSMI) that we suggest gives a more comprehensive framework to consider the interactions and their complexity. This theory states that pain is associated with plastic changes in the central nervous system (CNS) that lead to an activation pattern of motor units that contributes to the individual's adaptive sensorimotor behavior. This activation pattern takes account of the biological, psychological, and social influences on the musculoskeletal tissues involved in sensorimotor behavior and on the plastic changes and the experience of pain in that individual. The pattern is normally optimized in terms of biomechanical advantage and metabolic cost related to the features of the individual's musculoskeletal tissues and aims to minimize pain and any associated sensorimotor changes, and thereby maintain homeostasis. However, adverse biopsychosocial factors and their interactions may result in plastic CNS changes leading to less optimal, even maladaptive, sensorimotor changes producing motor unit activation patterns associated with the development of further pain. This more comprehensive theory points towards customized treatment strategies, in line with the management approaches to pain proposed in the biopsychosocial model of pain.

Role of hierarchy structure on the mechanical adaptation of self-healing hydrogels under cyclic stretching

Soft materials with mechanical adaptability have substantial potential for various applications in tissue engineering. Gaining a deep understanding of the structural evolution and adaptation dynamics of soft materials subjected to cyclic stretching gives insight into developing mechanically adaptive materials. Here, we investigate the effect of hierarchy structure on the mechanical adaptation of self-healing hydrogels under cyclic stretching training. A polyampholyte hydrogel, composed of hierarchical structures including ionic bonds, transient and permanent polymer networks, and bicontinuous hard/soft-phase networks, is adopted as a model. Conditions for effective training, mild overtraining, and fatal overtraining are demonstrated in soft materials. We further reveal that mesoscale hard/soft-phase networks dominate the long-term memory effect of training and play a crucial role in the asymmetric dynamics of compliance changes and the symmetric dynamics of hydrogel shape evolution. Our findings provide insights into the design of hierarchical structures for adaptive soft materials.

The Structural Adaptations That Mediate Disuse-Induced Atrophy of Skeletal Muscle

The maintenance of skeletal muscle mass plays a fundamental role in health and issues associated with quality of life. Mechanical signals are one of the most potent regulators of muscle mass, with a decrease in mechanical loading leading to a decrease in muscle mass. This concept has been supported by a plethora of human- and animal-based studies over the past 100 years and has resulted in the commonly used term of 'disuse atrophy'. These same studies have also provided a great deal of insight into the structural adaptations that mediate disuse-induced atrophy. For instance, disuse results in radial atrophy of fascicles, and this is driven, at least in part, by radial atrophy of the muscle fibers. However, the ultrastructural adaptations that mediate these changes remain far from defined. Indeed, even the most basic questions, such as whether the radial atrophy of muscle fibers is driven by the radial atrophy of myofibrils and/or myofibril hypoplasia, have yet to be answered. In this review, we thoroughly summarize what is known about the macroscopic, microscopic, and ultrastructural adaptations that mediated disuse-induced atrophy and highlight some of the major gaps in knowledge that need to be filled.

Postural stability, spinal alignment, mobility, and postural competency in women with unilateral lower extremity lymphedema after radical hysterectomy following gynecologic cancer: A case-control study

PURPOSE

To compare postural stability, spinal alignment, mobility, and postural competency in women with unilateral lower extremity lymphedema after radical hysterectomy following gynecologic cancer with a matched control group.

METHODS

Twenty-seven women with unilateral lower extremity lymphedema (lymphedema group, age: 54.14 ± 5.80 years) and 30 healthy women (control group, age: 51.90 ± 6.54 years) were included. The lymphedema severity was evaluated with circumferential measurements. Postural stability with the Biodex Balance System SD and the spinal alignment, mobility, and postural competency with the Spinal Mouse device were assessed.

RESULTS

In the lymphedema group, it was found that 3.7% of the women had mild lymphedema, 7.4% had moderate lymphedema, and 88.9% had severe lymphedema. Static eyes open (EO) (overall, medio-lateral and antero-posterior) and eyes closed (EC) (antero-posterior) stability scores and dynamic EO and EC stability scores (overall and antero-posterior) were detected to be higher in the lymphedema group than in the controls (p < 0.05). Spinal mobility and postural competency scores were lower in the lymphedema group than in the control group (p < 0.05). In other parameters, there were no significant differences between the groups (p > 0.05).

CONCLUSION

Decreased postural stability, spinal mobility, and postural competency were detected in women with unilateral lower extremity lymphedema; however, no difference was seen in spinal alignment. These changes should be taken into account in the assessment and the treatment of unilateral lower extremity lymphedema.

Spatiotemporal single-cell tracking analysis in 3D tissues to reveal heterogeneous cellular response to mechanical stimuli

Mechanical stimuli have been recognized as important for tissue maturation, homeostasis and constructing engineered three-dimensional (3D) tissues. However, we know little about the cellular mechanical response in tissues that could be considerably heterogeneous and spatiotemporally dynamic due to the complex structure of tissues. Here, we report a spatiotemporal single-cell tracking analysis of in vitro 3D tissues under mechanical stretch, to reveal the heterogeneous cellular behavior by using a developed stretch and optical live imaging system. The system could affect the cellular orientation and directly measure the distance of cells in in vitro 3D myoblast tissues (3DMTs) at the single-cell level. Moreover, we observed the spatiotemporal heterogeneous cellular locomotion and shape changes under mechanical stretch in 3DMTs. This single-cell tracking analysis can become a principal method to investigate the heterogeneous cellular response in tissues and provide insights that conventional analyses have not yet offered.

Ultrasonographic measurements of fascicle length overestimate adaptations in serial sarcomere number

Ultrasound-derived measurements of muscle fascicle length (FL) are often used to infer increases (chronic stretch or training) or decreases (muscle disuse or aging) in serial sarcomere number (SSN). Whether FL adaptations measured via ultrasound can truly approximate SSN adaptations has not been investigated. We casted the right hindlimb of 15 male Sprague-Dawley rats in a dorsiflexed position (i.e., stretched the plantar flexors) for 2 weeks, with the left hindlimb serving as a control. Ultrasound images of the soleus, lateral gastrocnemius (LG), and medial gastrocnemius (MG) were obtained with the ankle at 90° and full dorsiflexion for both hindlimbs pre and post-cast. Following post-cast ultrasound measurements, legs were fixed in formalin with the ankle at 90°, then muscles were dissected and fascicles were teased out for measurement of sarcomere lengths via laser diffraction and calculation of SSN. Ultrasound detected an 11% increase in soleus FL, a 12% decrease in LG FL, and an 8-11% increase in MG FL for proximal fascicles and at full dorsiflexion. These adaptations were partly reflected by SSN adaptations, with a 6% greater soleus SSN in the casted leg than the un-casted leg, but no SSN differences for the gastrocnemii. Weak relationships were observed between ultrasonographic measurements of FL and measurements of FL and SSN from dissected fascicles. Our results showed that ultrasound-derived FL measurements can overestimate an increase in SSN by ∼5%. Future studies should be cautious when concluding a large magnitude of sarcomerogenesis from ultrasound-derived FL measurements, and may consider applying a correction factor. NEW FINDINGS: What is the central question of this study? Measurements of muscle fascicle length via ultrasound are often used to infer changes in serial sarcomere number, such as increases following chronic stretch or resistance training, and decreases with ageing: does ultrasound-derived fascicle length accurately depict adaptations in serial sarcomere number? What is the main finding and its importance? Ultrasound detected an ∼11% increase in soleus fascicle length, but measurements on dissected fascicles showed the actual serial sarcomere number increase was only ∼6%; therefore, measurements of ultrasound-derived fascicle length can overestimate serial sarcomere number adaptations by as much as 5%.

The History and Future of Neuromusculoskeletal Biomechanics

The Executive Council of the International Society of Biomechanics has initiated and overseen the commemorations of the Society's 50th Anniversary in 2023. This included multiple series of lectures at the ninth World Congress of Biomechanics in 2022 and XXIXth Congress of the International Society of Biomechanics in 2023, all linked to special issues of International Society of Biomechanics' affiliated journals. This special issue of the Journal of Applied Biomechanics is dedicated to the biomechanics of the neuromusculoskeletal system. The reader is encouraged to explore this special issue which comprises 6 papers exploring the current state-of the-art, and future directions and roles for neuromusculoskeletal biomechanics. This editorial presents a very brief history of the science of the neuromusculoskeletal system's 4 main components: the central nervous system, musculotendon units, the musculoskeletal system, and joints, and how they biomechanically integrate to enable an understanding of the generation and control of human movement. This also entails a quick exploration of contemporary neuromusculoskeletal biomechanics and its future with new fields of application.

Impaired Cardiac and Skeletal Muscle Energetics Following Anthracycline Therapy for Breast Cancer

BACKGROUND

Anthracycline-related cardiac toxicity is a recognized consequence of cancer therapies. We assess resting cardiac and skeletal muscle energetics and myocyte, sarcomere, and mitochondrial integrity in patients with breast cancer receiving epirubicin.

METHODS

In a prospective, mechanistic, observational, longitudinal study, we investigated chemotherapy-naive patients with breast cancer receiving epirubicin versus sex- and age-matched healthy controls. Resting energetic status of cardiac and skeletal muscle (phosphocreatine/gamma ATP and inorganic phosphate [Pi]/phosphocreatine, respectively) was assessed with 31P-magnetic resonance spectroscopy. Cardiac function and tissue characterization (magnetic resonance imaging and 2D-echocardiography), cardiac biomarkers (serum NT-pro-BNP and high-sensitivity troponin I), and structural assessments of skeletal muscle biopsies were obtained. All study assessments were performed before and after chemotherapy.

RESULTS

Twenty-five female patients with breast cancer (median age, 53 years) received a mean epirubicin dose of 304 mg/m2, and 25 age/sex-matched controls were recruited. Despite comparable baseline cardiac and skeletal muscle energetics with the healthy controls, after chemotherapy, patients with breast cancer showed a reduction in cardiac phosphocreatine/gamma ATP ratio (2.0±0.7 versus 1.1±0.5; P=0.001) and an increase in skeletal muscle Pi/phosphocreatine ratio (0.1±0.1 versus 0.2±0.1; P=0.022). This occurred in the context of increases in left ventricular end-systolic and end-diastolic volumes (P=0.009 and P=0.008, respectively), T1 and T2 mapping (P=0.001 and P=0.028, respectively) but with preserved left ventricular ejection fraction, mass and global longitudinal strain, and no change in cardiac biomarkers. There was preservation of the mitochondrial copy number in skeletal muscle biopsies but a significant increase in areas of skeletal muscle degradation (P=0.001) in patients with breast cancer following chemotherapy. Patients with breast cancer demonstrated a reduction in skeletal muscle sarcomere number from the prechemotherapy stage compared with healthy controls (P=0.013).

CONCLUSIONS

Contemporary doses of epirubicin for breast cancer treatment result in a significant reduction of cardiac and skeletal muscle high-energy 31P-metabolism alongside structural skeletal muscle changes.

REGISTRATION

URL: https://www.

CLINICALTRIALS

gov; Unique identifier: NCT04467411.

Athlete Muscular Phenotypes Identified and Compared with High-Dimensional Clustering of Lower Limb Muscle Volume Measurements

INTRODUCTION

Athletes use their skeletal muscles to demonstrate performance. Muscle force generating capacity is correlated with volume, meaning that variations in sizes of different muscles may be indicative of how athletes meet different demands in their sports. Medical imaging enables in vivo quantification of muscle volumes; however, muscle volume distribution has not been compared across athletes of different sports.

PURPOSE

The goal of this work was to define "muscular phenotypes" in athletes of different sports and compare these using hierarchical clustering.

METHODS

Muscle volumes normalized by body mass of athletes (football, baseball, basketball, or track) were compared with control participants to quantify size differences using z -scores. z -Scores of 35 muscles described the pattern of volume deviation within each athlete's lower limb, characterizing their muscular phenotype. Data-driven high-dimensional clustering analysis was used to group athletes presenting similar phenotypes. Efficacy of clustering to identify similar phenotypes was demonstrated by grouping athletes' contralateral limbs before other athletes' limbs.

RESULTS

Analyses revealed that athletes did not tend to cluster with others competing in the same sport. Basketball players with similar phenotypes grouped by clustering also demonstrated similarities in performance. Clustering also identified muscles with similar volume variation patterns across athletes, and principal component analysis revealed specific muscles that accounted for most of the variance (gluteus maximus, sartorius, semitendinosus, vastus medialis, vastus lateralis, and rectus femoris).

CONCLUSIONS

Athletes exhibit heterogeneous lower limb muscle volumes that can be characterized and compared as individual muscular phenotypes. Clustering revealed that athletes with the most similar phenotypes do not always play the same sport such that patterns of muscular heterogeneity across a group of athletes reflect factors beyond their specific sports.

Reversible Effects of Functional Mandibular Lateral Shift on Masticatory Muscles in Growing Rats

In this study, we aimed to determine the effects of functional mandibular lateral shift (FMLS) on the muscle mass, fiber size, myosin heavy chain fiber type, and related gene expression in masticatory muscles (masseter and temporalis), as well as whether the baseline levels could be recovered after FMLS correction in growing rats. The FMLS appliance was placed to shift the mandible leftward by approximately 2 mm. After FMLS placement for 2 and 4 weeks, the muscles on the left side had significantly lower wet weight, mean cross-sectional area, and proportion of type IIa fibers than those on the right side or in the control groups (p < 0.05), with downregulation and upregulation of IGF-1 and GDF-8 gene expression, respectively (p < 0.05). Following 2 weeks devoted to recovery from FMLS, the muscle parameters in the recovery group were not significantly different to those of the control group, and IGF-1 expression in the left-side muscles was enhanced and GDF-8 expression was simultaneously suppressed. These findings indicate that the masticatory muscle changes induced via FMLS tend to revert to normal conditions if the intervention is eliminated at an early stage. Therefore, appropriate orthodontic treatment for FMLS during the growth period is advisable to prevent asymmetric alterations in masticatory muscles.

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

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

Understanding the role of physical activity on the pathway from intra-articular knee injury to post-traumatic osteoarthritis disease in young people: a scoping review protocol

INTRODUCTION

The prevalence of intra-articular knee injuries and reparative surgeries is increasing in many countries. Alarmingly, there is a risk of developing post-traumatic osteoarthritis (PTOA) after sustaining a serious intra-articular knee injury. Although physical inactivity is suggested as a risk factor contributing to the high prevalence of the condition, there is a paucity of research characterising the association between physical activity and joint health. Consequently, the primary aim of this review will be to identify and present available empirical evidence regarding the association between physical activity and joint degeneration after intra-articular knee injury and summarise the evidence using an adapted Grading of Recommendations Assessment, Development and Evaluations. The secondary aim will be to identify potential mechanistic pathways through which physical activity could influence PTOA pathogenesis. The tertiary aim will be to highlight gaps in current understanding of the association between physical activity and joint degeneration following joint injury.

METHODS

A scoping review will be conducted using the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews checklist and best-practice recommendations. The review will be guided by the following research question: what is the role of physical activity in the trajectory from intra-articular knee injury to PTOA in young men and women? We will identify primary research studies and grey literature by searching the electronic databases Scopus, Embase: Elsevier, PubMed, Web of Science: all databases, and Google Scholar. Reviewing pairs will screen abstracts, full texts and will extract data. Data will be presented descriptively using charts, graphs, plots and tables.

ETHICS AND DISSEMINATION

This research does not require ethical approval due to the data being published and publicly available. This review will be submitted for publication in a peer-reviewed sports medicine journal irrespective of discoveries and disseminated through scientific conference presentations and social media.

TRIAL REGISTRATION NUMBER

https://osf.io/84pnh/.

Understanding altered contractile properties in advanced age: insights from a systematic muscle modelling approach

Age-related alterations of skeletal muscle are numerous and present inconsistently, and the effect of their interaction on contractile performance can be nonintuitive. Hill-type muscle models predict muscle force according to well-characterised contractile phenomena. Coupled with simple, yet reasonably realistic activation dynamics, such models consist of parameters that are meaningfully linked to fundamental aspects of muscle excitation and contraction. We aimed to illustrate the utility of a muscle model for elucidating relevant mechanisms and predicting changes in output by simulating the individual and combined effects on isometric force of several known ageing-related adaptations. Simulating literature-informed reductions in free Ca2+ concentration and Ca2+ sensitivity generated predictions at odds qualitatively with the characteristic slowing of contraction speed. Conversely, incorporating slower Ca2+ removal or a fractional increase in type I fibre area emulated expected changes; the former was required to simulate slowing of the twitch measured experimentally. Slower Ca2+ removal more than compensated for force loss arising from a large reduction in Ca2+ sensitivity or moderate reduction in Ca2+ release, producing realistic age-related shifts in the force-frequency relationship. Consistent with empirical data, reductions in free Ca2+ concentration and Ca2+ sensitivity reduced maximum tetanic force only slightly, even when acting in concert, suggesting a modest contribution to lower specific force. Lower tendon stiffness and slower intrinsic shortening speed slowed and prolonged force development in a compliance-dependent manner without affecting force decay. This work demonstrates the advantages of muscle modelling for exploring sources of variation and identifying mechanisms underpinning the altered contractile properties of aged muscle.

Metabolic reprogramming underlies cavefish muscular endurance despite loss of muscle mass and contractility

Physical inactivity is a scourge to human health, promoting metabolic disease and muscle wasting. Interestingly, multiple ecological niches have relaxed investment into physical activity, providing an evolutionary perspective into the effect of adaptive physical inactivity on tissue homeostasis. One such example, the Mexican cavefish Astyanax mexicanus, has lost moderate-to-vigorous activity following cave colonization, reaching basal swim speeds ~3.7-fold slower than their river-dwelling counterpart. This change in behavior is accompanied by a marked shift in body composition, decreasing total muscle mass and increasing fat mass. This shift persisted at the single muscle fiber level via increased lipid and sugar accumulation at the expense of myofibrillar volume. Transcriptomic analysis of laboratory-reared and wild-caught cavefish indicated that this shift is driven by increased expression of pparγ-the master regulator of adipogenesis-with a simultaneous decrease in fast myosin heavy chain expression. Ex vivo and in vivo analysis confirmed that these investment strategies come with a functional trade-off, decreasing cavefish muscle fiber shortening velocity, time to maximal force, and ultimately maximal swimming speed. Despite this, cavefish displayed a striking degree of muscular endurance, reaching maximal swim speeds ~3.5-fold faster than their basal swim speeds. Multi-omic analysis suggested metabolic reprogramming, specifically phosphorylation of Pgm1-Threonine 19, as a key component enhancing cavefish glycogen metabolism and sustained muscle contraction. Collectively, we reveal broad skeletal muscle changes following cave colonization, displaying an adaptive skeletal muscle phenotype reminiscent to mammalian disuse and high-fat models while simultaneously maintaining a unique capacity for sustained muscle contraction via enhanced glycogen metabolism.

Do Repeated Sprints Affect the Biceps Femoris Long Head Architecture in Football Players with and without an Injury History?-A Retrospective Study

The aim of this study was to compare the biceps femoris long head (BFlh) architecture between football players with (twelve) and without (twenty) history of BFlh injury before and after a repeated sprint task. Fascicle length (FL), pennation angle (PA) and muscle thickness (MT) were assessed at rest and in the active condition before and after the repeated sprint protocol. Athletes with previous BFlh injury showed shorter FL at rest (p = 0.014; η2p = 0.196) and active state (p < 0.001; η2p = 0.413), and greater PA at rest (p = 0.002; η2p = 0.307) and active state (p < 0.001; η2p = 0.368) before and after the task. Intra-individual comparisons showed that injured limbs have shorter FL at rest (p = 0.012; η2p = 0.519) and in the active state (p = 0.039; η2p = 0.332), and greater PA in passive (p < 0.001; η2p = 0.732) and active conditions (p = 0.018; η2p = 0.412), when compared with contralateral limbs. Injured players, at rest and in the active condition, display shorter BFlh FL and greater PA than contralateral and healthy controls after repeated sprints. Moreover, the BFlh of injured players presented a different architectural response to the protocol compared with the healthy controls.

Intensive stretch-activated CRT-PMCA1 feedback loop promoted apoptosis of myoblasts through Ca2+ overloading

Mechanical stretch exerted pro-apoptotic effect on myoblasts, the mechanism of which is currently unknown. Intracellular Ca²⁺ accumulation has been implicated in stretch-induced apoptosis. calreticulin (CRT) and plasma membrane Ca²⁺ transporting ATPase 1 (PMCA1) are two critical components of Ca²⁺ signaling system participating in intracellular Ca²⁺ homeostasis. In this study, we explored the contribution of CRT and PMCA1 in mediating stretch-induced Ca²⁺ accumulation and apoptosis of myoblasts. Stretching stimuli elevated level of CRT while inhibited activity of PMCA1. Moreover, there were bidirectional regulations between CRT and PMCA1, which formed the positive feedback loop leading to continuous increment of CRT level and repression of PMCA1 activity, in stretched myoblasts. Specifically, increased CRT level inhibited PMCA1 activity via suppressing Calmodulin (CaM), while reduced PMCA1 activity promoted CRT expression through activating p38MAPK pathway. Thus, the CRT-CaM-PMCA1 and PMCA1-p38MAPK-CRT pathways constituted a close cycle comprising CRT, PMCA1, CaM and p38MAPK. Inhibition of both CaM and p38MAPK affected the other three factors in stretched myoblasts. Circulation of the vicious cycle resulted in escalated Ca²⁺ overloading in myoblasts under continuous stretching stimuli. CRT knock-down, PMCA1 overexpression, and p38MAPK inhibition all attenuated the raised intracellular Ca²⁺ level and ameliorated myoblast apoptosis in the stretching environment. Conversely, CRT overexpression, PMCA1 knock-down, and CaM inhibition all aggravated stretch-induced Ca²⁺ overloading and myoblast apoptosis. A positive feedback loop between CRT and PMCA1 was activated in stretched myoblasts, which contributed to intracellular Ca²⁺ accumulation and resultant myoblast apoptosis.

The influence of longitudinal muscle fascicle growth on mechanical function

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

Plasticity of the gastrocnemius elastic system in response to decreased work and power demand during growth

Elastic energy storage and release can enhance performance that would otherwise be limited by the force-velocity constraints of muscle. Although functional influence of a biological spring depends on tuning between components of an elastic system (the muscle, spring-driven mass and lever system), we do not know whether elastic systems systematically adapt to functional demand. To test whether altering work and power generation during maturation alters the morphology of an elastic system, we prevented growing guinea fowl (Numida meleagris) from jumping. We compared the jump performance of our treatment group at maturity with that of controls and measured the morphology of the gastrocnemius elastic system. We found that restricted birds jumped with lower jump power and work, yet there were no significant between-group differences in the components of the elastic system. Further, subject-specific models revealed no difference in energy storage capacity between groups, though energy storage was most sensitive to variations in muscle properties (most significantly operating length and least dependent on tendon stiffness). We conclude that the gastrocnemius elastic system in the guinea fowl displays little to no plastic response to decreased demand during growth and hypothesize that neural plasticity may explain performance variation.

Skeletal muscle regeneration with robotic actuation-mediated clearance of neutrophils

[Figure: see text].

Multiscale simulations of left ventricular growth and remodeling

Cardiomyocytes can adapt their size, shape, and orientation in response to altered biomechanical or biochemical stimuli. The process by which the heart undergoes structural changes-affecting both geometry and material properties-in response to altered ventricular loading, altered hormonal levels, or mutant sarcomeric proteins is broadly known as cardiac growth and remodeling (G&R). Although it is likely that cardiac G&R initially occurs as an adaptive response of the heart to the underlying stimuli, prolonged pathological changes can lead to increased risk of atrial fibrillation, heart failure, and sudden death. During the past few decades, computational models have been extensively used to investigate the mechanisms of cardiac G&R, as a complement to experimental measurements. These models have provided an opportunity to quantitatively study the relationships between the underlying stimuli (primarily mechanical) and the adverse outcomes of cardiac G&R, i.e., alterations in ventricular size and function. State-of-the-art computational models have shown promise in predicting the progression of cardiac G&R. However, there are still limitations that need to be addressed in future works to advance the field. In this review, we first outline the current state of computational models of cardiac growth and myofiber remodeling. Then, we discuss the potential limitations of current models of cardiac G&R that need to be addressed before they can be utilized in clinical care. Finally, we briefly discuss the next feasible steps and future directions that could advance the field of cardiac G&R.

Dentists have a high occupational risk of neck disorders with impact on somatosensory function and neck mobility

OBJECTIVES

Musculoskeletal disorders (MSDs) in the neck and shoulder region may be associated with significant impairment of quality of life and well-being. The study was to determine the prevalence of painful MSDs in Chinese dentists and evaluate somatosensory function and neck mobility compared with non-dental professional controls.

METHODS

One hundred dentists (age: 36.5 ± 9.8 years) and 102 controls (age: 36.2 ± 10.0 years) were recruited between September 2019 and December 2020. The Medical Outcome Study 36-item short-form health survey questionnaire and information of MSDs history were recorded. The cervical range of motion (CROM) with and without pain, and the pressure pain thresholds (PPTs) of the facial and neck muscles were tested. Chi-square test, Mann-Whitney U test and multiple linear regression analysis were used to analyze the data. The factors in the multiple linear regression analysis were occupation, working age, and gender.

RESULTS

The prevalence rate of neck pain was significantly higher in dentists (73.0%) compared with the controls (52.0%) (P = .002). The regression models of cervical range of posterior extension, lateral flexion and rotation were statistically significant (P ≤ .001). The regression models of PPTs of the tested facial and neck muscles were statistically significant (P < .001).

CONCLUSION

Dentists are at higher risk of neck pain. The bigger cervical range of left rotation of dentists could be related to the working posture. The lower PPTs in dentists may reflect a hypersensitivity in the facial and neck muscles. Preventive measures are needed to reduce occupational hazards in dentists.

Feasibility of simple exercise interventions for men with osteoporosis - A prospective randomized controlled pilot study

Background

Aging is associated with progressive loss of musculoskeletal performance. Exercise interventions can improve physical function in the elderly but there is a paucity of comparative assessments in order to understand what specific goals can be achieved particularly with less demanding exercise interventions readily accessible for untrained men.

Methods

Prospective randomized, controlled, single center exploratory trial to compare four distinct exercise interventions, i.e. Resistance Training (RT), Whole Body Vibration Exercise (WBV), Qi Gong (QG) and wearing a Spinal orthosis (SO) for 6 months in men at risk for osteoporosis aged 65–90 years. Primary endpoint was change in isometric one repetition maximum force trunk strength for extension (TSE) and flexion (TSF) compared to baseline, secondary endpoints covered key parameters of geriatric functional assessment, including Handgrip Strength (HS), Chair-Rise-Test (CRT), Usual Gait Speed (UGS) and Timed-Up-and-Go (TUG).

Results

Altogether 47 men (mean age 77 ±6.1 years) were randomized to RT, (n = 11) WBV (n = 13), QG (n = 10) and SO(n = 13). RT, defined as reference exercise intervention, lead to significant improvements for TSE (p = 0.009) and TSF (p = 0.013) and was significantly superior in the between-group analysis for TSE (p = 0.038). Vibration exercise caused sign. Improvements in TSE (p = 0.014) and CRT (p = 0.005), the Spinal orthosis improved CRT (p = 0.003) and Gait Speed (p = 0.027), while the QG intervention did not attain any sig. Developments.

Subgroup analyses revealed most pronounced musculoskeletal progress in vulnerable patients (age ≥ 80 years, pre-sarcopenia, multimorbidity ≥3chronic diseases). Irrespective of the type of exercise, participants ≥80 years experienced significant gains in TSE (p = 0.029) and CRT (p = 0.017). Presarcopenic subjects (Skeletal muscle Index (SMI) ≤10.75 kg/m²) improved in TSE (p = 0.003), CRT (p = 0.001) and UGS (p = 0.016). Multimorbid participants achieved sig. Gains in TSE (p < 0.001), TSF (p = 0.002), UGS (p = 0.036) and HS (p = 0.046).

Conclusions

In this exploratory trial we found that simple exercise interventions are feasible in elderly men eliciting specific benefits, i.e. improvements are attained in those tasks addressed with the respective exercise modality. While targeted resistance training is superior in increasing TSE, alternative simple exercise interventions also appear to elicit beneficial effects, even in vulnerable patients, i.e. those with low muscle mass, above 80 years of age or multimorbidity.

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Simple exercise interventions are safe and feasible in elderly man with elevated fracture risk.

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Improvements are observed in those tasks specifically addressed by the respective exercise.

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Simple exercise interventions are particularly effective in vulnerable patients (Presarcopenic, ≥80 years old, multimorbid).

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Even vulnerable patients at risk for muscular deficits can experience some benefits from exercise.

A new integrated behavioural intervention for knee osteoarthritis: development and pilot study

Background

Exercise-based approaches have been a cornerstone of physiotherapy management of knee osteoarthritis for many years. However, clinical effects are considered small to modest and the need for continued adherence identified as a barrier to clinical efficacy. While exercise-based approaches focus on muscle strengthening, biomechanical research has identified that people with knee osteoarthritis over activate their muscles during functional tasks. Therefore, we aimed to create a new behavioural intervention, which integrated psychologically informed practice with biofeedback training to reduce muscle overactivity, and which was suitable for delivery by a physiotherapist.

Methods

Through literature review, we created a framework linking theory from pain science with emerging biomechanical concepts related to overactivity of the knee muscles. Using recognised behaviour change theory, we then mapped a set of intervention components which were iteratively developed through ongoing testing and consultation with patients and physiotherapists.

Results

The underlying framework incorporated ideas related to central sensitisation, motor responses to pain and also focused on the idea that increased knee muscle overactivity could result from postural compensation. Building on these ideas, we created an intervention with five components: making sense of pain, general relaxation, postural deconstruction, responding differently to pain and functional muscle retraining. The intervention incorporated a range of animated instructional videos to communicate concepts related to pain and biomechanical theory and also used EMG biofeedback to facilitate visualization of muscle patterns. User feedback was positive with patients describing the intervention as enabling them to “create a new normal” and to be “in control of their own treatment.” Furthermore, large reductions in pain were observed from 11 patients who received a prototype version of the intervention.

Conclusion

We have created a new intervention for knee osteoarthritis, designed to empower individuals with capability and motivation to change muscle activation patterns and beliefs associated with pain. We refer to this intervention as Cognitive Muscular Therapy. Preliminary feedback and clinical indications are positive, motivating future large-scale trials to understand potential efficacy. It is possible that this new approach could bring about improvements in the pain associated with knee osteoarthritis without the need for continued adherence to muscle strengthening programmes.

Trial registration

ISRCTN51913166 (Registered 24-02-2020, Retrospectively registered).

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04389-0.

MRI and muscle imaging for idiopathic inflammatory myopathies

Although idiopathic inflammatory myopathies (IIM) are a heterogeneous group of diseases nearly all patients display muscle inflammation. Originally, muscle biopsy was considered as the gold standard for IIM diagnosis. The development of muscle imaging led to revisiting not only the IIM diagnosis strategy but also the patients' follow-up. Different techniques have been tested or are in development for IIM including positron emission tomography, ultrasound imaging, ultrasound shear wave elastography, though magnetic resonance imaging (MRI) remains the most widely used technique in routine. Whereas guidelines on muscle imaging in myositis are lacking here we reviewed the relevance of muscle imaging for both diagnosis and myositis patients' follow-up. We propose recommendations about when and how to perform MRI on myositis patients, and we describe new techniques that are under development.

Dietary reversal reverts diet-induced alterations in obstructed bladders of Wistar rats

OBJECTIVES

The aim of this study was to investigate the effects of diet reversal to standard chow on diet-induced changes in structure and function of normal and obstructed bladders in male Wistar rats.

METHODS

Eighty animals were equally divided into sham-surgery and bladder outlet obstruction (BOO) dietary groups and fed standard chow (control), high-carbohydrate, high-fat, and high-protein diets. BOO groups had surgically induced BOO, whereas sham surgery was performed on sham groups at the end of week 8. Animals were continued on the treatment diets for 4 wk after surgery, then the diets were all changed to standard chow for the remainder of the study period. Bladder weight, detrusor contractility, Rho-associated protein kinase (Rho-kinase), and myosin light chain kinase were determined. Polymerase chain reaction was used to assay for transforming growth factor-β, connecting tissue growth factor, hypoxia-inducible factor-1α, and platelet-derived growth factor subunit A levels in the bladder. C-reactive protein, insulin-like growth factor-1, nerve growth factor, and C-X-C motif chemokine ligand 12 concentrations were determined by enzyme-linked immunosorbent assay. The collagen content of the bladder was estimated by liquid chromatography/mass spectrometry.

RESULTS

Reversal of diet to standard chow resulted in reversal of diet-induced changes in all variables measured in obstructed bladders. High-fat-diet-induced alterations in normal bladders were also reversed.

CONCLUSION

The results suggested that in obstructed bladders of animals, reversal of the diet could reverse all diet-associated changes that increase inflammation and fibrosis in obstructed bladders. This is especially important in changes related to high consumption of fatty diets and associated lower urinary tract symptoms.

Remote Control of Time-Regulated Stretching of Ligand-Presenting Nanocoils In Situ Regulates the Cyclic Adhesion and Differentiation of Stem Cells

Native extracellular matrix (ECM) can exhibit cyclic nanoscale stretching and shrinking of ligands to regulate complex cell-material interactions. Designing materials that allow cyclic control of changes in intrinsic ligand-presenting nanostructures in situ can emulate ECM dynamicity to regulate cellular adhesion. Unprecedented remote control of rapid, cyclic, and mechanical stretching ("ON") and shrinking ("OFF") of cell-adhesive RGD ligand-presenting magnetic nanocoils on a material surface in five repeated cycles are reported, thereby independently increasing and decreasing ligand pitch in nanocoils, respectively, without modulating ligand-presenting surface area per nanocoil. It is demonstrated that cyclic switching "ON" (ligand nanostretching) facilitates time-regulated integrin ligation, focal adhesion, spreading, YAP/TAZ mechanosensing, and differentiation of viable stem cells, both in vitro and in vivo. Fluorescence resonance energy transfer (FRET) imaging reveals magnetic switching "ON" (stretching) and "OFF" (shrinking) of the nanocoils inside animals. Versatile tuning of physical dimensions and elements of nanocoils by regulating electrodeposition conditions is also demonstrated. The study sheds novel insight into designing materials with connected ligand nanostructures that exhibit nanocoil-specific nano-spaced declustering, which is ineffective in nanowires, to facilitate cell adhesion. This unprecedented, independent, remote, and cytocompatible control of ligand nanopitch is promising for regulating the mechanosensing-mediated differentiation of stem cells in vivo.

Response of the porcine MYH4-promoter and MYH4-expressing myotubes to known anabolic and catabolic agents in vitro

Myosin heavy chain-IIB (MyHC-IIB; encoded by MYH4 or Myh4) expression is often associated with muscle hypertrophic growth. Unlike other large mammals, domestic pig breeds express MyHC-IIB at both the mRNA and protein level.

Aim

To utilise a fluorescence-based promoter-reporter system to test the influence of anabolic and catabolic agents on increasing porcine MYH4-promoter activity and determine whether cell hypertrophy was subsequently induced.

Methods

C2C12 myoblasts were co-transfected with porcine MYH4-promoter-driven ZsGreen and CMV-driven DsRed expression plasmids. At the onset of differentiation, treatments (dibutyryl cyclic-AMP (dbcAMP), Des(1–3) Insulin-Like Growth Factor-1 (IGF-I), triiodo-l-thyronine (T3) and dexamethasone (Dex)) or appropriate vehicle controls were added and cells maintained for up to four days. At day 4 of differentiation, measurements were collected for total fluorescence and average myotube diameter, as indicators of MYH4-promoter activity and cell hypertrophy respectively.

Results

Porcine MYH4-promoter activity increased during C2C12 myogenic differentiation, with a marked increase between days 3 and 4. MYH4-promoter activity was further increased following four days of dbcAMP treatment and average myotube diameter was significantly increased by dbcAMP. Porcine MYH4-promoter activity also tended to be increased by T3 treatment, but there were no effects of Des(1–3) IGF-I or Dex treatment, whereas average myotube diameter was increased by Des(1–3) IGF-I, but not T3 or Dex.

Conclusion

Porcine MYH4-promoter activity responded to dbcAMP, Des(1–3) IGF-I and T3 treatment in vitro as observed previously in reported in vivo studies. However, we report that increased MYH4-promoter activity was not always associated with muscle cell hypertrophy. The fluorescence-based reporter system offers a useful tool to study muscle cell hypertrophic growth.

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In vitro porcine MYH4-promoter-reporter system to test anabolic & catabolic agents.

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Changes in porcine MYH4-promoter activity & myotube diameter measured in tandem.

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MYH4-promoter activity responded to dbcAMP, Des(1–3) IGF-I and T3 as seen in vivo.

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Increased MYH4-promoter activity was not always associated with cell hypertrophy.

Prolonged sitting and physical inactivity are associated with limited hip extension: A cross-sectional study

BACKGROUND

It is possible that physical inactivity and prolonged sitting could lead to changes in muscle properties or bony limitations which may reduce passive hip extension.

OBJECTIVES

This study explored the association between passive hip extension and sitting/physical activity patterns.

DESIGN

Cross sectional study.

METHOD

The modified Thomas Test is a clinical test used to characterise hip flexion contracture. This test was used to measure passive hip extension across 144 individuals. In addition, sitting behaviours and physical activity patterns were quantified using the Global Physical Activity Questionnaire. Cut off points were defined for low/high physical activity (150 min per week), prolonged sitting (>7 h per day) and minimal sitting (<4 h per day). ANOVA testing was then used to compare passive hip extension between three groups, defined using the specified thresholds: low activity & prolonged sitting, high activity & minimal sitting and high activity & prolonged sitting.

RESULTS

A total of 98 participants were allocated to one of the three groups which were shown to differ significantly in passive hip extension (P < 0.001). Importantly, there was 6.1° more passive hip extension in the high activity & minimal sitting group when compared to the low activity & prolonged sitting group.

CONCLUSION

This study is the first to demonstrate an association between passive hip extension and prolonged sitting/physical inactivity. It is possible that these findings indicate a physiological adaptation in passive muscle stiffness. Further research is required to understand whether such adaptation may play a role in the aetiology of musculoskeletal pain linked to prolonged sitting.

Modeling muscle function using experimentally determined subject-specific muscle properties

Muscle models are commonly based on intrinsic properties pooled across a number of individuals, often from a different species, and rarely validated against directly measured muscle forces. Here we use a rich data set of rat medial gastrocnemius muscle forces recorded during in-situ and in-vivo isometric, isotonic, and cyclic contractions to test the accuracy of forces predicted using Hill-type muscle models. We identified force-length and force-velocity parameters for each individual, and used either these subject-specific intrinsic properties, or population-averaged properties within the models. The modeled forces for cyclic in-vivo and in-situ contractions matched with measured muscle-tendon forces with r² between 0.70 and 0.86, and root-mean square errors (RMSE) of 0.10 to 0.13 (values normalized to the maximum isometric force). The modeled forces were least accurate at the highest movement and cycle frequencies and did not show an improvement in r² when subject-specific intrinsic properties were used; however, there was a reduction in the RMSE with fewer predictions having higher errors. We additionally recorded and tested muscle models specific to proximal and distal regions of the muscle and compared them to measures and models from the whole muscle belly: there was no improvement in model performance when using data from specific anatomical regions. These results show that Hill-type muscle models can yield very good performance for cyclic contractions typical of locomotion, with small reductions in errors when subject-specific intrinsic properties are used.

The Geometry of Incompatibility in Growing Soft Tissues: Theory and Numerical Characterization

Tissues in vivo are not stress-free. As we grow, our tissues adapt to different physiological and disease conditions through growth and remodeling. This adaptation occurs at the microscopic scale, where cells control the microstructure of their immediate extracellular environment to achieve homeostasis. The local and heterogeneous nature of this process is the source of residual stresses. At the macroscopic scale, growth and remodeling can be accurately captured with the finite volume growth framework within continuum mechanics, which is akin to plasticity. The multiplicative split of the deformation gradient into growth and elastic contributions brings about the notion of incompatibility as a plausible description for the origin of residual stress. Here we define the geometric features that characterize incompatibility in biological materials. We introduce the geometric incompatibility tensor for different growth types, showing that the constraints associated with growth lead to specific patterns of the incompatibility metrics. To numerically investigate the distribution of incompatibility measures, we implement the analysis within a finite element framework. Simple, illustrative examples are shown first to explain the main concepts. Then, numerical characterization of incompatibility and residual stress is performed on three biomedical applications: brain atrophy, skin expansion, and cortical folding. Our analysis provides new insights into the role of growth in the development of tissue defects and residual stresses. Thus, we anticipate that our work will further motivate additional research to characterize residual stresses in living tissue and their role in development, disease, and clinical intervention.

A mathematical model-based approach to optimize loading schemes of isometric resistance training sessions

Individualized resistance training is necessary to optimize training results. A model-based optimization of loading schemes could provide valuable impulses for practitioners and complement the predominant manual program design by customizing the loading schemes to the trainee and the training goals. We compile a literature overview of model-based approaches used to simulate or optimize the response to single resistance training sessions or to long-term resistance training plans in terms of strength, power, muscle mass, or local muscular endurance by varying the loading scheme. To the best of our knowledge, contributions employing a predictive model to algorithmically optimize loading schemes for different training goals are nonexistent in the literature. Thus, we propose to set up optimal control problems as follows. For the underlying dynamics, we use a phenomenological model of the time course of maximum voluntary isometric contraction force. Then, we provide mathematical formulations of key performance indicators for loading schemes identified in sport science and use those as objective functionals or constraints. We then solve those optimal control problems using previously obtained parameter estimates for the elbow flexors. We discuss our choice of training goals, analyze the structure of the computed solutions, and give evidence of their real-life feasibility. The proposed optimization methodology is independent from the underlying model and can be transferred to more elaborate physiological models once suitable ones become available.

Emphasizing Task-Specific Hypertrophy to Enhance Sequential Strength and Power Performance

While strength is indeed a skill, most discussions have primarily considered structural adaptations rather than ultrastructural augmentation to improve performance. Altering the structural component of the muscle is often the aim of hypertrophic training, yet not all hypertrophy is equal; such alterations are dependent upon how the muscle adapts to the training stimuli and overall training stress. When comparing bodybuilders to strength and power athletes such as powerlifters, weightlifters, and throwers, while muscle size may be similar, the ability to produce force and power is often inequivalent. Thus, performance differences go beyond structural changes and may be due to the muscle's ultrastructural constituents and training induced adaptations. Relative to potentiating strength and power performances, eliciting specific ultrastructural changes should be a variable of interest during hypertrophic training phases. By focusing on task-specific hypertrophy, it may be possible to achieve an optimal amount of hypertrophy while deemphasizing metabolic and aerobic components that are often associated with high-volume training. Therefore, the purpose of this article is to briefly address different types of hypertrophy and provide directions for practitioners who are aiming to achieve optimal rather than maximal hypertrophy, as it relates to altering ultrastructural muscular components, to potentiate strength and power performance.

A tissue engineering approach for repairing craniofacial volumetric muscle loss in a sheep following a 2, 4, and 6-month recovery

Volumetric muscle loss (VML) is the loss of skeletal muscle that results in significant and persistent impairment of function. The unique characteristics of craniofacial muscle compared trunk and limb skeletal muscle, including differences in gene expression, satellite cell phenotype, and regenerative capacity, suggest that VML injuries may affect craniofacial muscle more severely. However, despite these notable differences, there are currently no animal models of craniofacial VML. In a previous sheep hindlimb VML study, we showed that our lab’s tissue engineered skeletal muscle units (SMUs) were able to restore muscle force production to a level that was statistically indistinguishable from the uninjured contralateral muscle. Thus, the goals of this study were to: 1) develop a model of craniofacial VML in a large animal model and 2) to evaluate the efficacy of our SMUs in repairing a 30% VML in the ovine zygomaticus major muscle. Overall, there was no significant difference in functional recovery between the SMU-treated group and the unrepaired control. Despite the use of the same injury and repair model used in our previous study, results showed differences in pathophysiology between craniofacial and hindlimb VML. Specifically, the craniofacial model was affected by concomitant denervation and ischemia injuries that were not exhibited in the hindlimb model. While clinically realistic, the additional ischemia and denervation likely created an injury that was too severe for our SMUs to repair. This study highlights the importance of balancing the use of a clinically realistic model while also maintaining control over variables related to the severity of the injury. These variables include the volume of muscle removed, the location of the VML injury, and the geometry of the injury, as these affect both the muscle’s ability to self-regenerate as well as the probability of success of the treatment.

Relationship between tip-toe behavior and soleus - gastrocnemius muscle lengths in individuals with autism spectrum disorders

BACKGROUND

About 20% of individuals with autism spectrum disorders (ASD) showed tip-toe behavior (TTB). This behavior may be related to a decreased ankle joint range of motion (ROM) in dorsiflexion. Physiologically, gastrocnemius (GM) and soleus (SM) muscles influence ankle ROM independently. However, no studies investigated the relationship between the amount of time individuals with ASD spend in TTB and GM and SM muscle lengths.

OBJECTIVE

To evaluate the relationship between three mutually exclusive clinical patterns of TTB i.e., during standing, walking and running (TTB Class 1), or during walking and running (TTB Class 2), or only when running (TTB Class 3), and GM and SM muscle lengths.

METHODS

Sixty-nine individuals with ASD (average age: 14.1 ± 3.6 years, 56 males) were enrolled. In a clinical setting, SM and GM muscle lengths of both legs were assessed through a manual goniometer. Measurements were performed by two trained assessors blinded to TTB classifications.

RESULTS

Individuals with ASD classified as TTB Class 1 demonstrated a shortening of both GM and SM compared with NO-TTB and TTB Class 3 individuals.

CONCLUSIONS

Our results support the relationship between TTB severity and GM and SM shortening assessed by a decreased ankle joint ROM in dorsiflexion. Further studies are needed to determine the factors associated with TTB and decreased ankle ROM.

Association between ultrasound quadriceps muscle status with premorbid functional status and 60-day mortality in mechanically ventilated critically ill patient: A single-center prospective observational study

BACKGROUND & AIMS

In critically ill patients, direct measurement of skeletal muscle using bedside ultrasound (US) may identify a patient population that might benefit more from optimal nutrition practices. When US is not available, survey measures of nutrition risk and functional status that are associated with muscle status may be used to identify patients with low muscularity. This study aims to determine the association between baseline and changing ultrasound quadriceps muscle status with premorbid functional status and 60-day mortality.

METHODS

This single-center prospective observational study was conducted in a general ICU. Mechanically ventilated critically ill adult patients (age ≥18 years) without pre-existing systemic neuromuscular diseases and expected to stay for ≥96 h in the ICU were included. US measurements were performed within 48 h of ICU admission (baseline), at day 7, day 14 of ICU stay and at ICU discharge (if stay >14 days). Quadriceps muscle layer thickness (QMLT), rectus femoris cross sectional area (RFCSA), vastus intermedius pennation angle (PA) and fascicle length (FL), and rectus femoris echogenicity (mean and standard deviation [SD]) were measured. Patients' next-of-kin were interviewed by using established questionnaires for their pre-hospitalization nutritional risk (nutrition risk screening-2002) and functional status (SARC-F, clinical frailty scale [CFS], Katz activities of daily living [ADL] and Lawton Instrumental ADL).

RESULTS

Ninety patients were recruited. A total of 86, 53, 24 and 10 US measures were analyzed, which were performed at a median of 1, 7, 14 and 22 days from ICU admission, respectively. QMLT, RFCSA and PA reduced significantly over time. The overall trend of change of FL was not significant. The only independent predictor of 60-day mortality was the change of QMLT from baseline to day 7 (adjusted odds ratio 0.95 for every 1% less QMLT loss, 95% confidence interval 0.91-0.99; p = 0.02). Baseline measures of high nutrition risk (modified nutrition risk in critically ill ≥5), sarcopenia (SARC-F ≥4) and frailty (CFS ≥5) were associated with lower baseline QMLT, RFCSA and PA and higher 60-day mortality.

CONCLUSIONS

Every 1% loss of QMLT over the first week of critical illness was associated with 5% higher odds of 60-day mortality. SARC-F, CFS and mNUTRIC are associated with quadriceps muscle status and 60-day mortality and may serve as a potential simple and indirect measures of premorbid muscle status at ICU admission.

Effect of Training and Match Loads on Hamstring Passive Stiffness in Professional Soccer Players

OBJECTIVE

the purpose of this study was to identify differences in hamstring passive stiffness between the pre-season and in-season periods.

METHODS

Hamstring strength and passive stiffness were measured in professional male soccer players before and after the pre-season (4 weeks), and after the in-season (6 weeks) periods using an isokinetic dynamometer. Muscle passive stiffness was determined from the slope of the passive torque-angle relationship. External loads (acceleration and jumps) were monitored by GPS and internal loads by questionnaire.

RESULTS

Hamstring passive stiffness increased after 10 weeks of training and matches, without changes in passive peak torque and range of motion. The hamstring passive stiffness modifications were associated with the volume and intensity of accelerations and jumps. The individual data analysis also provided some support for the suppression of the biomechanical adaptation in the subjects with relatively large external load.

CONCLUSIONS

Regular training and match workouts increase hamstring passive stiffness in professional soccer players but the adaptation of muscle-tendon unit passive elements might not occur if players experience excessive mechanical stress.

Eliminating high-intensity activity during growth reduces mechanical power capacity but not submaximal metabolic cost in a bipedal animal model

Decreases in activity levels in children worldwide are feared to have long-term health repercussions. Yet, because of the difficulty of performing controlled long-term studies in humans, we do not yet understand how decreases in childhood activity influence adult functional capacity. Here, in an avian bipedal model, we evaluated the elimination of all high-intensity activity during growth on adult performance. We evaluated three alternative hypotheses: Elimination of high-intensity activity 1) does not influence adult function, 2) results in task-specific deficits in adulthood, or 3) results in deficits that generalize across locomotor tasks. We found that animals restricted from jumping and sprinting during growth showed detriments as adults in maximal jump performance in comparison to controls, but did not require more metabolic energy during steady-state running or standing. From this, we conclude that functional deficits from elimination of high-intensity exercise are task specific and do not generalize across all locomotor functions.NEW & NOTEWORTHY Decreasing childhood activity levels are feared to have long-term health repercussions, but testing this hypothesis is hampered by restrictions of human experimentation. Here, in a bipedal animal model, we examine how the elimination of high-intensity activity during all of maturation influences adult locomotor capacity. We found restricted activity during growth reduced mechanical power capacity but not submaximal metabolic cost. This suggests that reduced childhood activity may result in task-specific, rather than generalized locomotor deficits.

Growth and remodelling of living tissues: perspectives, challenges and opportunities

One of the most remarkable differences between classical engineering materials and living matter is the ability of the latter to grow and remodel in response to diverse stimuli. The mechanical behaviour of living matter is governed not only by an elastic or viscoelastic response to loading on short time scales up to several minutes, but also by often crucial growth and remodelling responses on time scales from hours to months. Phenomena of growth and remodelling play important roles, for example during morphogenesis in early life as well as in homeostasis and pathogenesis in adult tissues, which often adapt to changes in their chemo-mechanical environment as a result of ageing, diseases, injury or surgical intervention. Mechano-regulated growth and remodelling are observed in various soft tissues, ranging from tendons and arteries to the eye and brain, but also in bone, lower organisms and plants. Understanding and predicting growth and remodelling of living systems is one of the most important challenges in biomechanics and mechanobiology. This article reviews the current state of growth and remodelling as it applies primarily to soft tissues, and provides a perspective on critical challenges and future directions.

Review of Shoulder Range of Motion in the Throwing Athlete: Distinguishing Normal Adaptations from Pathologic Deficits

PURPOSE OF REVIEW

Overhand (OH) throwers demonstrate a unique motion profile of the shoulder joint complex. This manuscript reviews normal adaptations in the OH thrower and contrast findings with pathologic motion deficits.

RECENT FINDINGS

Multiple adaptations in range of motion have been associated with increased risk for arm injury. The use of a more conservative cutoff value for glenohumeral internal rotation deficit and horizontal adduction in younger throwers may help reduce injury risk. Deficits in glenohumeral internal rotation, total range of motion, shoulder flexion, and external rotation insufficiency have all been proposed as means to identify OH throwers at risk for arm injury, but conflicting evidence exists. Understanding normal adaptation due to repetitive stress of throwing is essential to effective management of these athletes. Adaptive change in bone and soft tissues is normal and contributes to the unique motion profiles expected in throwers. The causative link between normal adaptation and shoulder and elbow injury remains uncertain.