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Stephen P. Sayers

Duchenne muscular dystrophy (DMD) is a disease affecting muscle fiber integrity in boys that leads to progressive weakness in skeletal muscle and premature death. Currently, there is no known cure for the disease. Different interventions have been explored to delay the progression of the disease and improve the quality of life for the DMD patient. Physical activity is one treatment that has generated controversy due to the increased mechanical stress placed on the muscle during contraction. This review explores the literature in animal models and human DMD patients and evaluates the known theoretical risks and benefits of increased physical activity in DMD patients.

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Donal Murray, Kevin C. Miller and Jeffrey E. Edwards

Clinical Scenario:

Although exercise-associated muscle cramps (EAMC) are common in ultradistance runners and athletes in general, their etiology remains unclear. EAMC are painful, sudden, involuntary contractions of skeletal muscle occurring during or after exercise and are recognized by visible bulging or knotting of the whole, or part of, a muscle. Many clinicians believe EAMC occur after an imbalance in electrolyte concentrations, specifically serum sodium concentration ([Na+]s) and serum potassium concentration ([K+]s). Studies that have established a link between EAMC occurrence and serum electrolyte concentrations after an athletic event are unhelpful.

Focused Clinical Question:

Are [Na+]s and [K+]s different in athletes who experience EAMC than noncrampers?

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Yujiang Xiang

Human carrying is simulated in this work by using a skeletal digital human model with 55 degrees of freedom. An optimization-based approach is used to predict the carrying motion with symmetric and asymmetric loads. In this process, the model predicts joint dynamics using optimization schemes and task-based physical constraints. The results indicate that the model can predict different carrying strategies during symmetric and asymmetric load-carrying tasks. The model can also indicate the risk factors for extreme loading situations. With such robust prediction capability, the model could be used for biomedical and ergonomic studies.

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Peter A. Farrell

Skeletal muscle proteins are constantly being synthesized and degraded, and the net balance between synthesis and degradation determines the resultant muscle mass. Biochemical pathways that control protein synthesis are complex, and the following must be considered: gene transcription, mRNA splicing, and transport to the cytoplasm; specific amino acyl-tRNA, messenger (mRNA), ribosomal (rRNA) availability; amino acid availability within the cell; the hormonal milieu; rates of mRNA translation; packaging in vesicles for some types of proteins; and post-translational processing such as glycation and phosphorylation/dephosphorylation. Each of these processes is responsive to the need for greater or lesser production of new proteins, and many states such as sepsis, uncontrolled diabetes, prolonged bed-rest, aging, chronic alcohol treatment, and starvation cause marked reductions in rates of skeletal muscle protein synthesis. In contrast, acute and chronic resistance exercise cause elevations in rates of muscle protein synthesis above rates found in nondiseased rested organisms, which are normally fed. Resistance exercise may be unique in this capacity. This chapter focuses on studies that have used exercise to elucidate mechanisms that explain elevations in rates of protein synthesis. Very few studies have investigated the effects of aging on these mechanisms; however, the literature that is available is reviewed.

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Dorina Ianc, Carmen Serbescu, Marius Bembea, Laurent Benhamou, Eric Lespessailles and Daniel Courteix

We investigated the effects of calcium supplementation and physical practice on the bone ultrasound properties and trabecular microarchitecture in children. 160 children aged 8−11 were randomly allocated to active or nonactive groups and to receive either a calcium-phosphate or a placebo powder for 6 months. Skeletal status was assessed using an ultrasound technique, which measures the speed of sound (Ad-SoS, m/s) at the phalanx. Bone microarchitecture was characterized by fractal analysis measured on calcaneus radiographs and the result expressed as the Hmean parameter, that has been shown to a good reliability of the bone texture quality. After 6 months, the calcium group had significantly gained Ad-SoS compared to the placebo group (P = 0.01) and Hmean increase was greater in the active than the nonactive group (P < 0.05). Exercise and calcium supplementation had a differential effect on the bone tissue, calcium being rather linked to a systemic effect whereas exercise has acted better onto the skeletal stressed site.

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João Valente-dos-Santos, Manuel J. Coelho-e-Silva, Filipe Simões, Antonio J. Figueiredo, Neiva Leite, Marije T. Elferink-Gemser, Robert M. Malina and Lauren Sherar

This study evaluates the contributions of age, growth, skeletal maturation, playing position and training to longitudinal changes in functional and skill performance in male youth soccer. Players were annually followed over 5 years (n = 83, 4.4 measurements per player). Composite scores for functional and skill domains were calculated to provide an overall estimate of performance. Players were also classified by maturity status and playing position at baseline. After testing for multicollinearity, two-level multilevel (longitudinal) regression models were obtained for functional and skill composite scores. The scores improved with age and training. Body mass was an additional predictor in both models [functional (late maturing): 13.48 + 1.05 × centered on chronological age (CA)—0.01 × centered CA2—0.19 × fat mass (FM) + 0.004 × annual volume training—1.04 × dribbling speed; skills (defenders): 7.62 + 0.62 × centered CA—0.06 × centered CA2 + 0.04 × fat-free mass—0.03 × FM + 0.005 × annual volume training—0.19 × repeated-sprint ability + 0.02 × aerobic endurance]. Skeletal maturity status was a significant predictor of functional capacities and playing position of skill performance. Sound accuracy of each multilevel model was demonstrated on an independent cross-sectional sample (n = 52).

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Erik A. Richter, Jørgen F.P. Wojtaszewski, Søren Kristiansen, Jens R. Daugaard, Jakob N. Nielsen, Wim Derave and Bente Kiens

In the present short review some factors affecting glucose utilization during exercise in skeletal muscle will be briefly described. Special focus will be put on the glucose transport step across the sarcolemma. Glucose transporters (GLUT4) are expressed at a surprisingly similar level in the different muscle fiber types in human skeletal muscle in contrast to findings in the rat. When working at the same absolute work load muscle glucose transport is decreased in trained compared with untrained muscle in part due to a decrease in GLUT4 translocation to the sarcolemma in trained muscle. However, when trained and untrained muscle are stressed severely by a workload taxing 100% of their peak oxygen uptake in a glycogen-depleted state, then glucose uptake is larger in trained than in untrained muscle and correlates with muscle GLUT4 content. Finally, the possible role of the AMP-activated protein kinase (AMPK) in regulating glucose uptake during exercise is discussed. It is indicated that at present no experiments definitively link activation of AMPK to activation of muscle glucose transport during exercise.

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Ronald L. Terjung, Ryszard Zarzeczny and H.T. Yang

Skeletal muscle mitochondrial capacity (mito), tissue blood flow (BF) capacity, and oxygen exchange capacity (e.g., DO2) appear to be well matched. The different skeletal muscle fiber types and muscle remodeled, due to inactivity >(e.g., related to aging or disease) or exercise training, exhibit widely differing aerobics capacities (V̇O2max). Yet, there are remarkably coordinated alterations in these 3 parameters in each of these conditions. With such a balance, there is likely shared control among these parameters in limiting (V̇O2max) of muscle, although this is a matter of considerable debate. The reduction in aerobic capacity in elderly can be improved by submaximal aerobic exercise training; this is related to increases in muscle mitochondria concentration and capillarity, but probably not BF capacity, as this is limited by central cardiovascular function. Thus, exercise-induced biochemical adaptations and angiogenesis occur in the elderly. The increase in muscle capillarity likely contributes to the increased oxygen exchange capacity, typical of endurance type training. The increase in [mito] appears essential to realize the increased in muscle V̇O2max with training and amplifies the rate-limiting influence of the muscle’s oxygen exchange capacity. Further, vascular remodeling induced by exercise in the elderly could be effective at improving flow capacity, if limited by peripheral obstruction. Thus, the limits to aerobic function specific to aged muscle appear most influenced by inactivity, whereas central cardiovascular changes impact whole body performance. Some may consider the aged myocyte as a small, inactive, normal myocyte in need of activity!

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Gillian K. Myburgh, Sean P. Cumming, Manuel Coelho E. Silva, Karl Cooke and Robert M. Malina

Purpose:

To evaluate relationships among skeletal maturity, body size, and functional capacities of elite junior tennis players.

Methods:

Participants were 88 elite British Junior tennis players (44 male; 44 female), 8–16 years of age (12.4 } 1.9 years). Skeletal age estimated maturty. Anthropometry, grip strength, countermovement jump, squat jump, forehand agility, backhand agility, Yo-Yo, 5-m, 10-m and 20-m sprints were measured. Comparative analysis for each sex was performed, relating advanced maturers (Male: 15; Female: 29) to a combination of on-time and late maturers (Male: 29; Female: 31). ANCOVAs were used to determine absolute differences between male and female players and between the 2 maturity subgroups, with chronological age as the covariate.

Results:

Advanced maturity afforded male players advantages in absolute measures of grip strength, speed, upper and lower body power but not in acceleration, agility or aerobic endurance. Male players were significantly taller than females in the U13-U16 age group. Advanced maturity in female players afforded advantages in absolute measures of grip strength, agility and overhead power, but not in backhand agility, aerobic endurance or squat jump power.

Conclusion:

It is important that talent identification protocols consider the maturity of youth athletes to more satisfactorily address athletic potential rather than transient physical capabilities.

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Olga N. Fedotovskaya, Leysan J. Mustafina, Daniil V. Popov, Olga L. Vinogradova and Ildus I. Ahmetov

Purpose:

In red skeletal muscle, monocarboxylate transporter 1 (MCT1) is required for lactate to enter the myocytes for oxidation. The A1470T polymorphism (rs1049434) in the MCT1 gene was shown to be associated with lactate transport rates in human skeletal muscles. The aim of the study was to compare genotype and allele frequencies of the MCT1 gene polymorphism in 323 Russian athletes and 467 nonathletic controls and to investigate the association of the MCT1 gene A1470T polymorphism with maximal oxygen consumption and maximal lactate concentration in rowers (n = 79).

Methods:

Genotyping for the A1470T MCT1 polymorphism was performed by PCR-RFLP method. Physiological measurements of 79 Russian rowers of national competitive standard were determined during an incremental test to exhaustion on a rowing ergometer.

Results:

Frequencies of the A allele (71.8% vs 62.5%, P < .0001) and AA genotype (59.8% vs 39.4%, P < .0001) were significantly higher in endurance-oriented athletes (n = 142) than in the control group. Mean blood lactate concentration was higher in male rowers with the T allele (AT+TT 10.26 ± 1.89 mmol/L, AA 8.75 ± 1.69 mmol/L, P = .005).

Conclusions:

MCT1 gene A1470T polymorphism is associated with endurance athlete status and blood lactate level after intensive exercise.