Aerobic fitness may be defined as the ability to deliver oxygen to the exercising muscles and to utilize it to generate energy during exercise. Aerobic fitness therefore depends upon the pulmonary, cardiovascular and haematological components of oxygen delivery and the oxidative mechanisms of the exercising muscle.
Maximal oxygen uptake (VO2max), the highest rate at which an individual can consume oxygen during exercise, is widely recognized as the best single measure of adults’ aerobic fitness. Maximal oxygen uptake conventionally implies the existence
of a VO2 plateau but this response is not typical of children and adolescents and it has gradually become more common to use the term peak VO2, the highest VO2 elicited during an exercise test to exhaustion, to describe young people’s aerobic
fitness.
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The work required to inflate and deflate the lung is governed by the equation: work = total intrapleural pressure × change in lung volume. For a given lung volume, it is therefore the internal pressures opposing inflation and deflation that dictate the efficiency of the lung.
Three main components contribute to intrapleural pressure other than the active contraction of the diaphragm and respiratory muscles: airway resistance, respiratory system compliance and elastic recoil. Each of these components change with growth, and have important implications with regard to ventilatory patterns in children.
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Insulin increases the uptake of glucose by the muscles, promotes the synthesis of glycogen, inhibits lipolysis and increases lipid synthesis; therefore it has been speculated that age-dependent changes in blood insulin concentration might influence metabolic characteristics.
Supporting data are, however, not convincing. In one of the most comprehensive studies, Wirth et al (1978) determined the blood insulin, FFA and glucose concentrations of prepubertal, pubertal and post pubertal boys and girls at rest and in the 15 min of an exercise test at 70% peak VO2 on a cycle ergometer. They observed that insulin levels increased during exercise in the prepubertal children, remained constant in pubertal individuals and decreased in the postpubertal groups. However, neither glucose nor FFA concentrations changed during exercise and differences between sexes or stages of maturity were not present.
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Oxidative metabolism is relatively slow to adapt to the demands of exercise and the time constant of the response to heavy exercise is about 25 s. The rate at which ATP can be resynthesized is aerobically much slower than that of anaerobic ATP resynthesis but oxidative metabolism can use carbohydrates, free fatty acids (FFAs) and even amino acids as substrates, although protein catabolism contributes less than 5% of energy provision during exercise.
Oxidative metabolism therefore has a much greater capacity for energy generation than anaerobic pathways and although it makes a relatively minor contribution during short-term high intensity exercise the contribution to ATP provision gradually increases with time and the oxidative contribution is dominant during exercise of longer than 90 s duration. Adults’ lipid stores are sufficiently large to fuel 30 marathons.
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Typically, during childhood and puberty, strength increases coincide with changes in fat-free mass (FFM). Moderate to strong correlations have been found for knee extension and flexion torque versus FFM in 8- to 13-year-old wrestlers (Housh et al 1996). However, further studies have reported age-related increases in torque per FFM for knee extensors and elbow extensors and flexors that could not be accounted for by changes in FFM.
The age effect for increases in strength independent of FFM may be attributable to an increase in muscle mass per unit of FFM or neural maturation which allows for a greater expression of strength. The proportion of FFM that is skeletal muscle has been suggested to increase with age. In addition, the proportion of muscle mass that is distributed at various sites is thought to vary and at birth approximately 40% of total muscle mass is located in the lower extremities, increasing to approximately 55% at sexual maturity in both boys and girls.
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