Learning from children: the emergence of pediatric exercise science.

WHICH UNIQUE INSIGHTS can be gained from the investigations of the physiology of the growing child? Are there pragmatic outcomes to research in children’s exercise science that bear specific utility to this age group or to their future life? What importance do exercise responses in youth bear to our understanding of the nature of human movement? The reviews appearing in this highlighted topic series have been selected with the goal of providing answers to these questions. Each contribution not only examines a current issue in the field but also highlights specific aspects of exercise research that are particular to children—the problems of identifying causality in growing subjects, surmounting obstacles created by ethical constraints, and translating maturational differences in physiological features to motor performance outcomes. As a prelude to these discussions, it is appropriate to introduce the reader to some general considerations regarding exercise science in youth. Other than the obvious factor of chronologic age, the feature most distinguishing children’s exercise responses from those of adults is change. From the moment of birth to becoming a grownup teenager, the human body is engaged in a continuous process of evolving physically, psychologically, socially, biochemically, physiologically—by any marker one wants to measure—from an undeveloped state of dependency to that of the complex mature adult. Name the exercise markers (mechanical efficiency, peak oxygen uptake, sprint and endurance performance, standing jump height, daily caloric expenditure), the myriad of determinants of such variables are in constant change over the course of this temporal journey. Some of these simply reflect increases in body dimension. Maximal cardiac output rises with growth, a reflection of increase in stroke volume, which in turn reflects augmentation of left ventricular size. Other variables contributing to the evolution of physiological responses to exercise are size independent, such as glycolytic capacity and other metabolic functions. Which are which? And how do they relate? What determines the rate of change in exercise physiological determinants as children grow? Those seeking to define causal relationships in responses to exercise in youth have, in effect, a moving target. Confounding the issue, the rate of biological maturation does not necessarily mirror that of chronological age—at any given age a group of children can be expected to exhibit significant developmental diversity. Pediatric exercise physiologists must, as a requisite of their trade, be constantly attentive to the variations created by growth and biological development in their investigations. They need to confront, as well, the question of how physiological variables, such as V̇O2max, should be best “normalized” for body size or level of biological development. The issue is clearly critical in assessing individual changes over time or performing group comparisons. Allometric scaling appears to be most appropriate in many situations, but when the more traditional “ratio standard” (i.e., per kg body mass) should be used instead remains problematic. The nature of this evolving physiology creates some unique ways of approaching exercise-related issues. For example, one can traditionally consider the etiology of obesity in thermodynamic terms—there must exist an imbalance of energy in and energy out, possibly biologic (an intrinsic “error” in energy regulation), possibly extrinsic (an effect of influences of our contemporary culture that facilitates food intake and discourages regular exercise). But in youth, the major factors that contribute to energy balance are all in the process of constant change. During the growing years, relative to body size, food intake steadily declines and basal metal metabolic rate falls, as does relative energy expenditure in the form of daily physical activity. A central regulator must be in charge of matching them up (or almost—something must be left over for growth). So, the process of becoming an obese 10 year old must relate to an imbalance in the relative rates of these changes, and one might consider biological determinants of obesity in children in respect to factors that are responsible for changes in these factors over time. Such an understanding might have therapeutic or preventive implications. By contrast, factors in adults affecting energy balance should, at least intrinsically, be more static over time. Investigating the physiological responses to exercise specific to children may prove enlightening in our understanding of basic physiological principles. For example, it is well-established that prepubertal children respond to a period of endurance training with very little improvement in peak V̇O2 compared with adults, and some studies report no change in maximal aerobic power at all. Why is this? It’s a difficult question to answer, given that the primary factors effecting changes in aerobic capacity with training are not entirely clear at any age. Until recently, spontaneous physical activity during childhood has been thought to be sufficient to elicit the phenotype potential for aerobic fitness. But, if one could identify the variables that explain the dampened aerobic trainability in children, such insights might be provided. The current burgeoning interest in this field is being driven, however, by more pragmatic matters. Relationships between Address for reprint requests and other correspondence: T. Rowland, Dept. of Pediatrics, Baystate Medical Center, Springfield, MA (e-mail: Thomas. [email protected]). J Appl Physiol 105: 322–324, 2008; doi:10.1152/japplphysiol.90624.2008.