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Friday, September 19, 2008

When should a child specialise in sport?

"It's Allometry Dear Watson" (Sherlock Holmes)
(This is the updated version of an earlier article)

In all but a few sports, there is a very good reason why we should not, as coaches, have young people overly specialise before physical maturity: the reason is "allometry".


Allometric Scaling
The science of allometric scaling dictates that if an organism doubles in size, power increases only by a factor of 2/3rds. 

This is why an ant can lift several times its weight, whereas an elephant cannot lift its own weight. The larger the organism, the less its power to weight ratio. This is why an ant would collapse under its own weight if it were scaled to elephant size and why an elephant can't jump. The same maths applies to accelerating and decelerating a body: The more mass, disproportionally more energy is required to accelerate and decelerate it.

So a 50kg weight lifter can lift more per kg body weight that a 150kg lifter. 100m sprinters tend to be smaller than 200m sprinters because the smaller mass is more quickly accelerated, bestowing a slight advantager over a larger opponent over the shorter distance. This is why a small rugby player tends to be able out-manoever a larger opponent. 

Size is an advantage in sports that do not require lifting and lowering body weight. Therefore; a big rower or kayaker will have an advantage over a smaller opponent. The same applies to cycling - a big rider will have an advantage on a flat course; whereas the lighter rider will have it over a big boy on the hills. When running, a small runner has the advantage over a larger one in any event longer than about 200m. This is because each stride raises and lowers the body weight by several centimeters. Power to weight ratio outweighs total power. This is principally why the slightly built North African runners dominate world running.

Let's do the maths:
The maths are simple when working out power to weight ratios: Example: a 65 kg rider with a 4 liter O2 uptake = 61.5 ml O2/Kg 85 kg rider with a 5 liter O2 uptake = 58.8 ml O2/kg On the flat, the 85 kg rider will have an advantage over the 65 kg rider, whereas on a hilly course or one that requires constant accelerating and decelerating and changing of direction, the 65 kg rider will have the edge, other than on any long descents. We see these rules of physics being played out in races like the Tour de France the 60 kg riders dominate the mountain passes; whereas the bigger riders like Jan Ullrich tend to dominate the flat sections, including the time trials.

You are your Mother and your Father:
As a young person goes through puberty, body changes can be dramatic and one can not defy physical laws such as those to do with allomteric scaling. One can get an idea what that skinny little kid might grow into by looking at his/her parents and close relatives. This is why a young person that demonstrates talent should not over-specialise in one particluar discipline until well on the way to physical maturity - about 18-20 years. This gives them the option to switch to the event that their body best suits and to also find out what they really enjoy doing. Of course, there are always exceptions to the rules of physics, but these people are rare.

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