Medicine and sport science最新文献

Endurance training consists of a structured exercise programme that is sustained for a sufficient length of time with sufficient intensity and frequency to induce an improvement in aerobic fitness. Elite young athletes generally have higher peak oxygen uptakes (peak VO₂) than their untrained peers largely due to their greater maximal stroke volumes. Trained young athletes have faster VO₂ kinetic responses to step changes in exercise intensity but whether this is due to enhanced oxygen delivery or increased oxygen utilization by the muscles remains to be explored. Blood lactate accumulation in young athletes during submaximal exercise is lower than in untrained youth and this appears to be due to enhanced oxidative function in the active muscles. No well-designed, longitudinal endurance training studies of elite young athletes have been published. Even in the general paediatric population peak VO₂ is the only component of aerobic fitness on which there are sufficient data to examine dose-response effects of endurance training. The existence of a maturational threshold below which children are not trainable remains to be proven. The magnitude of training responses is independent of sex. Pre-training peak VO₂ has a moderate but significant inverse relationship with post-training peak VO₂ which suggests that elite young athletes are likely to experience smaller increases in peak VO₂ with further endurance training than untrained youth. Empirical evidence strongly indicates that both trained and untrained young people can benefit from endurance training but the relative intensity of exercise required for optimum benefits is higher than that recommended for adults.

Children and adults employ different thermoregulatory strategies, particularly in dealing with heat stress. Children rely more on 'dry' heat exchange, while evaporative heat loss is adults' foremost heat-dissipation venue. Several anatomical, physiological, and psychological factors can affect differential risk of thermal injury in the child vs. the adult athlete, in some situations. Children have greater surface-area- to-mass ratio, lower sweating rate, higher peripheral blood flow in the heat, and a greater extent of vasoconstriction in the cold. They can acclimatise to a similar extent but do so at a lower rate than adults. Differences in perceived exertion and thermal strain, cumulative experience, cognitive development, and decision-making capacity may negatively affect the young athlete's behaviour under competitive and other situations, possibly subjecting him/her to sub-par performance or to greater risk of thermal injury. However, except for very limited environmental conditions, children in general, and young athletes in particular, are physiologically as capable as adults to handle environmental challenges.

To date, much of the research concerning the performance of elite young athletes has focused on physical and physiological factors and how these relate to age and maturation. Little attention has been paid to other factors which might limit performance such as nutrition or environmental stressors. The paucity of research on the environmental effects on performance in young athletes is unsurprising given the need for experimental studies, the ethics of which would generally be untenable. As an outcome, there is a reliance on observational and case study data, e.g. observing the stressors which occur during jet lag and effects on sleep patterns, altitude and pollution. The effects of environmental factors have been predominantly researched from a health context in youngsters rather than a performance context. However, the evidence of those few empirical studies combined with coach and/or sports science support teams' experience have provided professionals with some guidelines. These applied guidelines include sleep patterns, jet lag, pollution and altitude research, to aid those preparing young athletes for training and competition in environments that present potential challenges to performance. The limitations of data extrapolated from adults are acknowledged and in all cases it is emphasised that recommendations and implementing practice should be based on data collected from young people.

Although in the past resistance and high-intensity exercise training among young children was the subject of numerous controversies, it is now well-documented that this training mode is a safe and effective means of developing maximal strength, maximal power output and athletic performance in youth, provided that exercises are performed with appropriate supervision and precautions. Muscular strength and power output values measured from vertical jump and Wingate anaerobic tests are higher in elite than in non-elite young athletes and normal children, and the specific training effects on maximal power output normalised for body size are clearly more distinct before puberty. At present, there is no scientific evidence to support the view that high-intensity and/or resistance training might hinder growth and maturation in young children. Pre-pubertal growth is not adversely affected by sport at a competitive level and anthropometric factors are of importance for choice of sport in children. However, coaches, teachers and parents should be aware that unsupervised high-intensity and resistance training programmes involving maximal loads or too frequently repeated resistance exercises increase the risk of injury. Resistance training alone is an effective additional means of developing athletic performance throughout planned youth sports training programmes. Strategies for enhancing the effectiveness and safety of youth resistance and high-intensity exercise training are discussed in this chapter.

Sport participation confers many varied benefits in children and adolescents, such as self-esteem, confidence, team play, fitness, agility and strength. Nevertheless, the age of initiation of intense training is decreasing and programmes which expose children to excessive amounts of exercise increase the risk of injury. We review sports injuries in young athletes and the long-term outcomes. Sports injuries can lead to disturbances in growth such as limb length discrepancy, caused by traumatised physeal growth induced by injury. Osgood-Schlatter lesion may also cause some sequelae such as painful ossicles in the distal patellar tendon. The apophysis can be fragmentised or separated, and this could be an adaptive change to the increased stress typical of overuse activities. These changes produce an osseous reaction even though they are not disabling. Participation in physical exercise at a young age should be encouraged, because of the health benefits, but decreasing the incidence and severity of sports injuries in young athletes is an important component of any athletic programme and may generate a long-term economic impact in health care costs. Active prevention measures are the main weapon to decrease the (re-)injury rate and to increase athletic performance.

Menstrual cycle irregularities are often observed among physically active women and athletes who participate in physical activity ranging from recreational to competitive exercise training. Further, such irregularities have been casually linked to an energy deficiency where caloric intake is inadequate for exercise energy expenditure resulting in a suppressive effect on growth and reproduction. Adaptations consistent with chronic energy deficiency, including reductions in resting energy expenditure and total triiodothyronine, have been observed in exercising women with functional hypothalamic amenorrhea (FHA). Gut peptides and adipocytokines also appear to be altered in exercising women with FHA and have been hypothesized to be involved in the etiology of FHA. Ghrelin concentrations are elevated in exercising women with FHA. Interestingly, while fasting ghrelin, an orexigenic peptide, is elevated in women with FHA, PYY, an orexigenic peptide, is paradoxically also elevated in women with anorexia nervosa and exercising women with FHA. Leptin, an adipocytokine, is also suppressed in FHA associated with exercise and anorexia. A critical leptin concentration threshold is suggested to be necessary for regular menses to occur. Ghrelin, PYY, and leptin all have the ability to cross the blood brain barrier and, in the hypothalamus, can modulate appetite and food intake, and are hypothesized to affect the hypothalamic-pituitary-ovarian axis. Future studies are needed to determine if ghrelin, PYY, or leptin play a direct role in the regulation of the hypothalamic-pituitary-ovarian axis, and if these signals can be altered by improving energy status secondary to increasing caloric intake and initiate the reversal of amenorrhea.

Adolescent female athletes are at increased risk for low bone mineral density (BMD) secondary to exercise-induced hypogonadism. Of particular concern is that the adolescent years are also a critical time for bone accrual, and deficits incurred during this period could lead to suboptimal peak bone mass acquisition and subsequent fracture risk in later life. Although weight-bearing exercise is typically associated with an increase in BMD, amenorrheic athletes have lower BMD than eumenorrheic athletes and nonathletic controls as a consequence of low energy availability and subsequent hypogonadism. It is important to recognize that critical interactions exist between net energy availability and the hypothalamo-pituitary-gonadal (H-P-G) axis that are key to the development of a hypogonadal state when energy intake cannot keep pace with expenditure. While the link between energy availability and gonadtotropin pulsatility patterns is well established, the actual metabolic signals that link the two are less clear. Decreased energy availability in athletes is associated with decreases in fat mass, and alterations in adipokines (such as leptin and adiponectin) and fat-regulated hormones (such as ghrelin and peptide YY). These hormones impact the H-P-G axis in animal models, and it is possible that in athletes alterations in fat-related hormones signal the state of energy availability to the hypothalamus and contribute to suppression of gonadotropin pulsatility, hypothalamic amenorrhea and consequent decreased BMD. A better understanding of pathways linking low energy availability with functional hypothalamic amenorrhea and low BMD is critical for the development of future therapeutic strategies addressing these issues in amenorrheic athletes.

Adipocytokines are signaling molecules released by adipose tissue with numerous functions, including regulation of metabolism, inflammatory process, and body mass. They are particularly interesting in youth, considering the rising prevalence of overweight/obesity and the linkage of this condition to inflammation. This chapter examines the relationship between body composition and select adipocytokines: leptin, adiponectin, IL-6, TNF-α, and resistin, in overweight, normal weight and anorexic youth. Leptin, which stimulates energy expenditure and promotes satiety, is highest in overweight youth, followed by normal weight and lastly anorexic youth. Adiponectin has similar functions to leptin but is negatively correlated with measures of body composition. Anorexic youth have the highest adiponectin per kg fat mass, followed by normal weight and overweight. Conversely, IL-6 is positively associated with body composition; however, research in anorexic youth is inconclusive. It has some pro-inflammatory effects and promotes glucose and fat use, therefore beneficial for maintenance of normal weight status. TNF-α is also a pro-inflammatory adipocytokine thought to be somewhat protective against cancer. TNF-α is highest in overweight, followed by normal weight and anorexic youth, similar to leptin. Finally, resistin is also involved in the pro-inflammatory response and the development of insulin resistance. However, far less research exists on this adipocytokine and its relation to body composition in overweight or anorexic youth is equivocal. In conclusion, several consistent relationships exist regarding adipocytokines and body composition; however, there is a need for additional research on these relationships in youth especially at extremes of adiposity such as overweight and anorexics.

Ghrelin and obestatin are two peptides associated with appetite control and the regulation of energy balance in adults. It is intuitive that they have an important role in growth and development during puberty. Therefore, it is acknowledged that these peptides, in addition to others, form part of the substrate underlying energy homeostasis which in turn will contribute to body weight regulation and could explain changes in energy balance during puberty. Both peptides originate from the stomach; hence, it is intuitive that they are involved in generating signals from tissue stores which influence food intake. This could be manifested via alterations in the drive to eat (i.e. hunger), eating behaviors and appetite regulation. Furthermore, there is some evidence that these peptides might also be associated with physical activity behaviors and metabolism. Anecdotally, children and adolescents experience behavioral and metabolic changes during growth and development which will be associated with physiological changes.

It is important to understand the factors that regulate the development of obesity during adolescence due to the increased risk of adult obesity, metabolic syndrome and the deleterious health effects of early puberty which may increase the risk of breast cancer later in life. Leptin, ghrelin, and adiponectin are peptides that affect energy homeostasis and insulin action. Similar to findings in adults, steady-state exercise does not change leptin concentrations and aerobic training without a change in body weight. A small amount of available data suggest that acute exercise does not increase circulating adiponectin concentrations in adolescents; however, it is very possible that more rigorous exercise protocols could acutely affect circulating adiponectin levels. Training studies indicate that shorter lengths of exercise training have a stronger effect on increases in adiponectin concentrations in male than female adolescents. It appears that if training is extended, increases in adiponectin levels will accompany improvements in insulin sensitivity. There are no studies of acute or chronic exercise on high-molecular weight adiponectin in adolescents and since this is thought to be the bioactive form of adiponectin, these studies are definitely needed. Investigations have demonstrated that exercise training increases total ghrelin levels in adolescents and that ghrelin is sensitive to reductions in body fat or increases in energy expenditure in this population. These findings are similar to those in adults. Moreover, there is evidence that luteinizing hormone is a predictor of total ghrelin levels in girls and suggests that ghrelin is a biomarker of energy imbalance across the menstrual cycle.