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Monday, October 02, 2006

"Its allometry dear Watson" - S.Holmes

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".

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.

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.

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.


Wayne said...

having done a lot of sprinting and studied it, i disagree regarding sprinters, if you check out the weights of sprinters 100 metre sprinters are every bit as heavy if not heavier than 200 metre sprinters, in anaerobic effort muscle bulk counts the energy is generated in teh muscle, the more muscle the more energy you can generate getting more acceleration, 200 metre runners tend to be lankeir and taller if anything, longer legs usually afford better top speed but they take longer to get gong, like carl lewis who took 70 metres to hit top speed. look at peter snell he was as mig as any track runner gets at over 80 kilos he was built like a sprinter and his bulk gave him the benefit f superior acceleration, if anything smaller size affords better top speed but NOT better acceleration unles you are short. I have bulky legs and I"ve never been out accelerated in a race by anyone smaller than me, but i have ben outpaced.

Gary Moller said...

A low centre of gravity and a smaller overall mass is an advantage when accelerating or changing direction. The taller, lankier runner will take longer to wind up but should have a higher terminal velocity. This is why 200m runners tend to be taller. Carl Lewis was a good example of this in action.

Wayne said...

you and me on the tarck gary, head to head he he he

Wayne said...

the advantage in hill climbing of the light person is more to do with teh fact that they have less weight to get up the hill and overcome gravity , it takes les enrgy for a lighter person to climb a hill on a big hill acceerlation is neither here nor there if you can't maintain a high average speed.
and regarding acclertation, , its an anaerobic thing, more muscle mass means more power output in anaerobic effort ben johnson won because he generated more muscle mas with steroids. and he was useless above the hundred metres because the superior acceleration his muscle mass gave him, inhibited his top speed in longer distances, less flexibility and stride length and harder to keep his mass moving at high speed for anything other than the shortest distance.