This is an excerpt from Athlete's Clock, The by Thomas Rowland.
Malina: Numerous studies have documented this effect.Rosters of participants in youth sports, particularly hockey, are biased toward those with birth dates in the first portion of the year, as compared to the last. The usual explanation is that those who are chronologically older are also more likely to be more biologically mature—stronger, heavier, and taller—offering a competitive advantage. No surprises here. Yet it is intriguing that this same bias for early birth date is seen in highly successful young-adult athletes after puberty as well. Studies of competitors in professional baseball, ice hockey, soccer, and basketball have all indicated a skewing of birthdates toward the quarter of each sport year. Somehow, the early maturers, who are more talented early on, seem to have continued to dominate in their sport. How to explain this? I believe that the size and performance advantages of early maturing boys within their respective age groups attract the attention of coaches and others interested in the search for talent.
Rowland: Before leaving this point, it should be added that in addition to birth date, location of birth also seems to be important. If you want to be a star athlete, grow up in Austin, not New York City or Miller's Corners. Well, that's a bit of hyperbole, but the point is that studies indicate a disproportionate number of professional athletes grow up in cities with populations less than 500,000 but greater than 10,000. Psychosocial environment seems to have some importance in the pathway to sport success. For example, Jean Côté and coworkers reported that about half of the U.S. population lives in cities with more than 500,000 inhabitants, but these cities account for only 13% of National Hockey League players, 29% of professional basketball players, 15% of participants in major league baseball, and 13% of the players in the Professional Golf Association.
Malina: Agreed. Did they provide any explanation for these findings?
Rowland: Well, you can pick your favorite explanation among the many offered: better physical environment of the smaller cities with more unstructured play activities permitting experience in different sports, more local support for sport teams, less competitive milieu, greater chance for early success, and so on.
Malina: So, where did you grow up, Tom?
Rowland: Alma, Michigan.
Malina: That explains everything.
Rowland: Could be. So, remember, we have been talking here about the steady, normal improvements in motor performance that occur as children grow. No training, just normal development, governed by growth-promoting hormones and affected by genetic endowment and level of sexual development.
Malina: We should add a third component, behavioral development, to this normal progression, too. This is a cultural concept. In their social milieu, children develop cognitively, socially, emotionally, and morally—they are learning to behave within the constructs of our society. This also applies to motor development and sports, specifically societal expectations and rewards associated with proficiency or lack of proficiency. The demands of sports may influence, or even sometimes conflict, with this normal behavioral development.
All three of these trends—physical growth, biological maturation, and behavioral development—are in a state of constant change and interaction during childhood and adolescence, and the rates of change and interactions vary greatly from one child to the next. So when thinking about the effects of sport training on children, we must appreciate that adult-organized programs are superimposed on a constantly changing base.
Rowland: Let's turn now to the effect of athletic training on the natural progression of events. To what extent can athletic training accelerate the normal improvements in physical performance of children and adolescents as they grow (that is, beyond the stimuli provided by Mother Nature)? And, as a corollary, can a child's ultimate development into a performing athlete be enhanced by early sport training?
Just to be clear, by training, we mean a program of systematic instruction and practice of certain frequency, intensity, and duration. That improvement in sport skill should be achieved through training is based on the precept that regular exercise stresses body tissues, which, in response, compensate by improving function. That function is then translated into greater performance outcomes. Also, in sports involving complex neuromuscular skill, such as tennis, there is the concept that repeated practice grooves neuromuscular connections, providing a kind of muscle memory.
Malina: It should be readily apparent that when thinking about the applicability of these principles to youth, a number of considerations are important. For instance, are the children at a proper stage of readiness or preparedness to respond to such stimuli in terms of their personal stages of growth and development? Are there critical periods when children are optimally sensitive to improvements associated with instruction and practice (in the case of sport skills) and physical training (regarding biological dimensions and functions)? How do differences in coaching strategies and instructional techniques at different ages make a difference? Our understanding of these issues is very limited.
Rowland: Let's begin by considering some examples of physiological fitness. Like muscle strength. It wasn't that long ago that people felt that it was impossible, and maybe even dangerous, for children to try to improve their muscle strength with resistance training, like lifting weights, before puberty. It was thought that you had to have circulating testosterone to do this. But now we know this is false. A good number of studies have documented that children, both boys and girls, gain strength in properly designed programs, just as adults do. So, we should expect that weight training would help children improve performance in sporting events where strength is an issue, most directly in, say, wrestling or powerlifting, but also in football, swimming, and even baseball. The studies haven't been done yet to really prove this, but it makes sense.
Aerobic endurance performance, like distance running, is associated with V.O2max, the highest amount of oxygen utilized by the body in an exercise test on a cycle or treadmill. With a period of aerobic endurance training, children typically show a rise in V.O2max, on the average about 5 to 6%. Since that number is less than what is observed in adults (15-20%), some have suggested that this implies that prepubescent children might be less trainable than adults in aerobic endurance sports. But it is not clear if the dampened response of V.O2max to training in children can be translated into a similarly limited increase in aerobic endurance performance itself. That's because performance on, say, a 5K road race is dependent not only on V.O2max but also on factors like economy of running energy.