Effect of MTU length on child-adult difference in neuromuscular fatigue

Purpose The purpose of this study was to compare the development and etiology of neuromuscular fatigue of the knee extensor muscles (KE) at different muscle-tendon unit (MTU) lengths during repeated maximal voluntary isometric contractions (MVIC) between boys and men.Methods Twenty-two pre-pubertal boys (9-11 years) and 22 men (18-30 years) performed three KE fatigue protocols at short (SHORT), optimal (OPT) and long (LONG) MTU lengths, consisting of repeating 5-s MVIC interspersed with 5-s passive recovery periods until torque reached 60% of the initial MVIC torque. The etiology of neuromuscular fatigue was identified using non-invasive methods such as surface electromyography, near-infrared spectroscopy, magnetic nerve stimulation and twitch interpolation technique.Results The number of repetitions was significantly lower in men at OPT (14.8$\pm$3.2) and LONG (15.8$\pm$5.8) than boys (39.7$\pm$18.4 and 29.5 $\pm$10.2, respectively; p<0.001), while no difference was found at SHORT between both age groups (boys: 33.7$\pm$15.4, men: 40.9$\pm$14.2). At OPT and LONG boys showed a lower reduction in the single potentiated twitch (Qtwpot) and a greater decrease in the voluntary activation level (VA) than men. At SHORT, both populations displayed a moderate Qtwpot decrement and a significant VA reduction (p<0.001). The differences in maximal torque between boys and men were almost twice greater at OPT (223.9 N.m) than at SHORT (123.3 N.m) and LONG (136.5 N.m).Conclusion The differences in neuromuscular fatigue between children and adults are dependent on MTU length. Differences in maximal torque could underpin differences in neuromuscular fatigue between children and adults at OPT and SHORT. However, at LONG these differences do not seem to be explained by differences in maximal torque. The origins of this specific effect of MTU length remain to be determined

Protostellar collapse induced by compression. II: rotation and fragmentation

We investigate numerically and semi-analytically the collapse of low-mass, rotating prestellar cores. Initially, the cores are in approximate equilibrium with low rotation (the initial ratio of thermal to gravitational energy is $\alpha_0 \simeq 0.5$, and the initial ratio of rotational to gravitational energy is $\beta_0 = 0.02 {\rm to} 0.05$). They are then subjected to a steady increase in external pressure. Fragmentation is promoted -- in the sense that more protostars are formed -- both by more rapid compression, and by higher rotation (larger $\beta_0$). In general, the large-scale collapse is non-homologous, and follows the pattern described in Paper I for non-rotating clouds, viz. a compression wave is driven into the cloud, thereby increasing the density and the inflow velocity. The effects of rotation become important at the centre, where the material with low angular momentum forms a central primary protostar (CPP), whilst the material with higher angular momentum forms an accretion disc around the CPP. More rapid compression drives a stronger compression wave and delivers material more rapidly into the outer parts of the disc.Comment: 17 pages, accepted for publication in MNRA

Children Exhibit a More Comparable Neuromuscular Fatigue Profile to Endurance Athletes Than Untrained Adults

The present study compared neuromuscular fatigue profiles between children, untrained adults and adult endurance athletes during repeated maximal muscle contractions. Eighteen prepubertal boys, 19 untrained men and 13 endurance male athletes performed 5-s maximal voluntary isometric knee extensor contractions (MVICs) interspersed with 5-s recovery until MVIC reached 60% of its initial value. Single and doublet magnetic stimulations were delivered to the femoral nerve to quantify the time course of potentiated twitch amplitude (Ttw,pot), high-frequency torque (T100Hz) and the low-to-high frequency torque ratio (T10Hz/T100Hz), i.e., indicators of peripheral fatigue. M-wave-normalized EMG amplitudes (EMG/M) and the maximal voluntary activation level (VA) were calculated to quantify central fatigue. Adults (15.9 ± 3.9 repetitions) performed fewer MVICs than children (40.4 ± 19.7) and endurance athletes (51.7 ± 19.6), however, no difference was observed between children and athletes (P = 0.13). Ttw,pot (∼52%, P &lt; 0.001), T100Hz (∼39%, P &lt; 0.001) and T10Hz/T100Hz (∼23%, P &lt; 0.001) decreased only in adults. Similar decrements in vastus medialis and vastus lateralis EMG/M were observed in children and endurance athletes (range: 40–50%), and these were greater than in adults (∼15%). Whilst VA decreased more in children (-38.4 ± 22.5%, P &lt; 0.001) than endurance athletes (-20.3 ± 10.1%, P &lt; 0.001), it did not change in adults. Thus, children fatigued more slowly than adults and as much as endurance athletes. They developed less peripheral and more central fatigue than adults and, although central fatigue appeared somewhat higher in children than endurance athletes, both children and endurance athletes experienced greater decrements than adults. Therefore, children exhibit a more comparable neuromuscular fatigue profile to endurance athletes than adults

Evidence For Cloud-Cloud Collision and Parsec-Scale Stellar Feedback Within the L1641-N Region

We present high spatial resolution $^{12}$CO ($J=1-0$) images taken by the Nobeyama 45m telescope toward a $48' \times 48'$ area including the L1641-N cluster. The effective spatial resolution of the maps is $21"$, corresponding to 0.04 pc at a distance of 400 pc. A recent 1.1 mm dust continuum map reveals that the dense gas is concentrated in several thin filaments. We find that a few dust filaments are located at the parts where $^{12}$CO ($J=1-0$) emission drops sharply. Furthermore, the filaments have two-components with different velocities. The velocity difference between the two-components is about 3 km s$^{-1}$, corresponding to a Mach number of 10, significantly larger than the local turbulent velocity in the cloud. These facts imply that the collision of the two components (hereafter, the cloud-cloud collision) possibly contributed to the formation of these filaments. Since the two components appear to overlap toward the filaments on the plane of the sky, the collision may have occurred almost along the line of sight. Star formation in the L1641-N cluster was probably triggered by such a collision. We also find several parsec-scale CO shells whose centers are close to either the L1641-N cluster or V 380 Ori cluster. We propose that these shells were created by multiple winds and/or outflows from cluster YSOs, i.e., "protocluster winds." One exceptional dust filament located at the western cloud edge lies along a shell; it is presumably a part of the expanding shell. Both the cloud-cloud collision and protocluster winds are likely to influence the cloud structure and kinematics in this region.Comment: 44 pages, 12 figures, submitted to Ap

Control of star formation by supersonic turbulence

Understanding the formation of stars in galaxies is central to much of modern astrophysics. For several decades it has been thought that stellar birth is primarily controlled by the interplay between gravity and magnetostatic support, modulated by ambipolar diffusion. Recently, however, both observational and numerical work has begun to suggest that support by supersonic turbulence rather than magnetic fields controls star formation. In this review we outline a new theory of star formation relying on the control by turbulence. We demonstrate that although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28 figures, in pres

The JCMT Gould Belt Survey: evidence for dust grain evolution in Perseus star-forming clumps

The dust emissivity spectral index, β, is a critical parameter for deriving the mass and temperature of star-forming structures and, consequently, their gravitational stability. The β value is dependent on various dust grain properties, such as size, porosity, and surface composition, and is expected to vary as dust grains evolve. Here we present β, dust temperature, and optical depth maps of the star-forming clumps in the Perseus Molecular Cloud determined from fitting spectral energy distributions to combined Herschel and JCMT observations in the 160, 250, 350, 500, and 850 μm bands. Most of the derived β and dust temperature values fall within the ranges of 1.0-2.7 and 8-20 K, respectively. In Perseus, we find the β distribution differs significantly from clump to clump, indicative of grain growth. Furthermore, we also see significant localized β variations within individual clumps and find low-β regions correlate with local temperature peaks, hinting at the possible origins of low-β grains. Throughout Perseus, we also see indications of heating from B stars and embedded protostars, as well evidence of outflows shaping the local landscape

Commentaries on viewpoint : physiology and fast marathons

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Caractérisation des réponses aiguës et des adaptations chroniques à l’exercice excentrique et exploration de stratégies préventives de réduction des perturbations neuromusculaires : application au modèle de la course à pied en descente.

An eccentric muscle contraction occurs when a force applied to the muscle exceeds the momentary force produced by the muscle itself. This results in a forced elongation of the muscle-tendon complex during the contraction. Interestingly, eccentric contractions have several unique characteristics that can lead to specific adaptive responses, both during exercise and even during training. On the one hand, the high repetition of eccentric muscle loading promotes "chronic" optimisation of training capacity, i.e. the development of specific physiological adaptations to meet commonly encountered environmental stresses (e.g. during counter-resistance training). On the other hand, eccentric muscle loading during exercise, this time, favours the occurrence of neuromuscular fatigue [i.e., a decrease in the capacity to produce force and/or power, independently of the ability to continue the task] and a relatively severe state of damage and/or destructuring of the muscle fibres. Thus, this doctoral project is placed in a current and somewhat dichotomous context where eccentric loading is both a training tool of choice to induce positive physiological adaptations, both in the athletic and clinical context, but also a source of deleterious physiological failures during acute exercise that can impair the physical and motor performance capacity of individuals for up to several days after the task.Une contraction musculaire excentrique se produit lorsqu’une force appliquée au muscle dépasse la force momentanée produite par le muscle lui-même. Il en résulte alors d’un allongement forcé du complexe muscle-tendon lors de la contraction. De façon intéressante, les contractions excentriques possèdent plusieurs caractéristiques uniques qui peuvent conduire à des réponses adaptatives spécifiques, que cela soit à l’exercice ou même à l’entraînement. D’un côté, la répétition importante de sollicitations musculaires excentriques favorise l’optimisation « chronique » de la capacité de l’entraînement, c’est-à-dire la mise en place d’adaptations physiologiques spécifiques afin de répondre à des contraintes environnementales couramment rencontrées (par exemple, au cours d’un entraînement contre-résistance). De l’autre, les sollicitations musculaires excentriques au cours de l’exercice, cette fois-ci, favorisent la survenue de fatigue neuromusculaire [c.-à-d., une baisse de la capacité de production de force et/ou de puissance, indépendamment de la capacité à poursuivre la tâche] et d’un état relativement sévère d’endommagement et/ou de déstructuration des fibres musculaires. Ainsi, ce projet doctoral se place dans un contexte actuel et quelque peu dichotomique où les sollicitations excentriques constituent à la fois un outil d’entraînement de choix afin d’induire des adaptations physiologiques positives, aussi bien dans le contexte athlétique que clinique, mais également une source de défaillances physiologiques délétères à l’exercice aigu pouvant entraver la capacité de performance physique et motrice des individus jusqu’à plusieurs jours après la tâche

Commentaries on Viewpoint: Distinct modalities of eccentric exercise: different recipes, not the same dish

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River Loire levees hazard studies – CARDigues’ model principles and utilization examples on Blois levees

Along the river Loire, in order to have a homogenous method to do specific risk assessment studies, a new model named CARDigues (for Levee Breach Hazard Calculation) was developed in a partnership with DREAL Centre-Val de Loire (owner of levees), Cerema and Irstea. This model enables to approach the probability of failure on every levee sections and to integrate and cross different “stability” parameters such topography and included structures, geology and material geotechnical characteristics, hydraulic loads… and observations of visual inspections or instrumentation results considered as disorders (seepage, burrowing animals, vegetation, pipes, etc.). This model and integrated tool CARDigues enables to check for each levee section, the probability of appearance and rupture of five breaching scenarios initiated by: overflowing, internal erosion, slope instability, external erosion and uplift. It has been recently updated and has been applied on several levee systems by different contractors. The article presents the CARDigues model principles and its recent developments (version V28.00) with examples on river Loire and how it is currently used for a relevant and global levee system diagnosis and assessment. Levee reinforcement or improvement management is also a perspective of applications for this model CARDigues