Why kids get tired just walking around

14 November 2010
Posted by Chris Smith.

As any parents, aunties or uncles know full well, kids get much more tired out simply walking around than adults do, and now scientists have taken a step closer to understanding why this is. It's not just that they run up and down more, or that we get more efficient at walking as we grow taller, but in fact it's all about having longer legs.

Publishing in the Journal of Experimental Biology, Peter Weyand from Southern Methodist University in the US led a team of researchers who investigated the metabolism and gait of a group of walking volunteers ranging from 5 to 32 years old who were between 3 and over 6 feet tall.

Baby carrierIt's already known that across all sorts of different animals, those with smaller bodies use more energy per unit of mass than larger animals. But this doesn't actually give us a full answer as to why little people use up more energy walking around - it's more than just a factor of their different metabolic rates.

Weyand's team monitored the energy used up by each volunteer as they walked on a treadmill at different speeds ranging from half to 1.9 meters per second, and discovered that smaller people use the same amount of energy per stride as taller folk - but with littler legs, they have to take more strides to cover the same ground, using up more energy in the process.

So essentially, if you scaled up a 3 foot, 5 year old to be the same size as a 6 foot adult, they would walk exactly the same way revealing that grown ups don't have a different, more economic way of walking they just benefit from having longer legs.

Based on their findings the research team have come up with an equation that calculates the energetic cost of walking, which will have lots of useful applications including for pedometers and treadmills. By inputting not just your weight, as many machines currently do, but also your height, we now have a much more accurate way of working out how many calories you'll burn over a certain distance.

So this is great - because it means next time I'm at the gym on a treadmill running next to an elegant tall athlete, I have a genuine scientific excuse for finding it more hard work, because just like toddlers, us vertically-challenged folk use more energy just getting around.

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