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Norihide Sugisaki, Kai Kobayashi, Hiroyasu Tsuchie and Hiroaki Kanehisa

 al . Biomechanical and skeletal muscle determinants of maximum running speed with aging . Med Sci Sports Exerc . 2009 ; 41 ( 4 ): 844 – 856 . PubMed doi:10.1249/MSS.0b013e3181998366 19276848 10.1249/MSS.0b013e3181998366 19. Nakatani M , Takai Y , Akagi R , et al . Validity of muscle thickness

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Trent J. Herda, Philip M. Gallagher, Jonathan D. Miller, Matthew P. Bubak and Mandy E. Parra

obese ( 34 ), and it is a worldwide health concern ( 37 ). Adipose infiltration in skeletal muscles of the lower extremity is linked to glucose metabolism and insulin sensitivity in adults ( 5 , 19 – 21 ). In the realm of insulin resistance in children, skeletal muscle is often overlooked despite the

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Nicola Giovanelli, Lea Biasutti, Desy Salvadego, Hailu K. Alemayehu, Bruno Grassi and Stefano Lazzer

the skeletal muscle, as the exercise they analyzed was not isolated to a single muscle group (cycling at 1 and 1.5 W·kg −1 ). Thus, the purpose of the present study was to evaluate the effects of a trail-running race on muscle oxidative function by measuring pulmonary gas exchange variables and muscle

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Alif Laila Tisha, Ashley Allison Armstrong, Amy Wagoner Johnson and Citlali López-Ortiz

, we analyze the series and lateral force components separately and then return to the general conceptual model to consider the net effect of force transmission in a muscle fiber contraction. Conceptual Model The system is represented using a cross-sectional view of skeletal muscle (Figure  2A ). We

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Walter Herzog, Timothy Koh, Evelyne Hasler and Tim Leonard

We hypothesize that the neuromuscular system is designed to function effectively in accomplishing everyday movement tasks. Since everyday movement tasks may vary substantially in terms of speed and resistance, we speculate that agonistic muscles contribute differently to varying movement tasks such that the mechanical, structural, and physiological properties of the system are optimized at all times. We further hypothesize that a mechanical perturbation to the musculoskeletal system, such as the loss of an important joint ligament or the change of a muscle’s line of action, causes an adaptation of the system aimed at reestablishing effective function. Here. we demonstrate how the specificity of the cat ankle extensors is used to accommodate different locomotor tasks. We then illustrate how the loss of an important ligament in the cat knee leads to neuromuscular adaptation. Finally, we discuss the adaptability of skeletal muscle following an intervention that changes a muscle’s line of action, moment arm, and excursion.

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Emily Arentson-Lantz, Elfego Galvan, Adam Wacher, Christopher S. Fry and Douglas Paddon-Jones

et al., 2011 ). To mimic the overt physical inactivity experienced during hospitalization, while separating the catabolic, disease-related effects from the intrinsic effects of skeletal muscle disuse, we subjected a cohort of healthy, community-dwelling older adults to a 7-day bed rest (BR) protocol

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David T. Corr, Ray Vanderby Jr. and Thomas M. Best

An existing rheological model of skeletal muscle (Forcinito et al., 1998) was modified with a nonlinear Maxwell fluid element to provide a phenomenological model capable of analyzing the strain-stiffening behavior typically found in passive, and occasionally observed in active, skeletal muscle. This new model describes both active and passive muscular behavior as a combination of the behavior of each model component, without requiring prior knowledge of the force-length or force-velocity characteristics of the muscle.

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Joshua N. Farr, Deepika R. Laddu and Scott B. Going

Although primarily considered a disorder of the elderly, emerging evidence suggests the antecedents of osteoporosis are established during childhood and adolescence. A complex interplay of genetic, environmental, hormonal and behavioral factors determines skeletal development, and a greater effort is needed to identify the most critical factors that establish peak bone strength. Indeed, knowledge of modifiable factors that determine skeletal development may permit optimization of skeletal health during growth and could potentially offset reductions in bone strength with aging. The peripubertal years represent a unique period when the skeleton is particularly responsive to loading exercises, and there is now overwhelming evidence that exercise can optimize skeletal development. While this is not controversial, the most effective exercise prescription and how much investment in this prescription is needed to significantly impact bone health continues to be debated. Despite considerable progress, these issues are not easy to address, and important questions remain unresolved. This review focuses on the key determinants of skeletal development, whether exercise during childhood and adolescence should be advocated as a safe and effective strategy for optimizing peak bone strength, and whether investment in exercise early in life protects against the development of osteoporosis and fractures later in life.

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Lothar Stein, Constanze Pacht, Sibylle Junge, Tobias S. Kaeding, Momme Kück, Norbert Maassen, Torge Wittke and Vladimir Shushakov

Purpose:

Defects in the gene encoding the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) cause CF. Absence of the CFTR may result in skeletal muscle dysfunction. Here, we tested skeletal muscle function in male adolescent patients with CF.

Methods:

Ten CF and 10 control participants (age: 16.8 ± 0.6 years) performed 7 repetitive sets of maximum voluntary contractions (MVCs) and underwent an isometric fatigue test of the knee extensors. Electromyography (EMG) activity was recorded from the m. vastus lateralis (VL) and m. vastus medialis (VM).

Results:

In CF, the MVC torque was lower and correlated with the predicted forced expiratory volume in one second (r = .73, p = .012, n = 10). The M-wave in the VL was shorter in CF than in controls (18.6 ± 0.5 vs. 20.3 ± 0.5 ms, p < .028). In the VM, both the M-wave (4.96 ± 0.61 vs. 7.97 ± 0.60 mV, p = .001) and the EMG (0.29 ± 0.04 vs. 0.47 ± 0.04 mV, p = .004) amplitudes were smaller in CF.

Conclusion:

The differences in the VL and VM EMG signals between the groups indicate that the lower MVC torque in CF did not result from the direct impact of a CFTR defect on the sarcolemmal excitability; the differences more likely resulted from the less developed musculature in the patients with CF.

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Ildus I. Ahmetov, Olga L. Vinogradova and Alun G. Williams

The ability to perform aerobic or anaerobic exercise varies widely among individuals, partially depending on their muscle-fiber composition. Variability in the proportion of skeletal-muscle fiber types may also explain marked differences in aspects of certain chronic disease states including obesity, insulin resistance, and hypertension. In untrained individuals, the proportion of slow-twitch (Type I) fibers in the vastus lateralis muscle is typically around 50% (range 5–90%), and it is unusual for them to undergo conversion to fast-twitch fibers. It has been suggested that the genetic component for the observed variability in the proportion of Type I fibers in human muscles is on the order of 40–50%, indicating that muscle fiber-type composition is determined by both genotype and environment. This article briefly reviews current progress in the understanding of genetic determinism of fiber-type proportion in human skeletal muscle. Several polymorphisms of genes involved in the calcineurin–NFAT pathway, mitochondrial biogenesis, glucose and lipid metabolism, cytoskeletal function, hypoxia and angiogenesis, and circulatory homeostasis have been associated with fiber-type composition. As muscle is a major contributor to metabolism and physical strength and can readily adapt, it is not surprising that many of these gene variants have been associated with physical performance and athlete status, as well as metabolic and cardiovascular diseases. Genetic variants associated with fiber-type proportions have important implications for our understanding of muscle function in both health and disease.