New equation calculates cost of walking for first time

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New equation calculates cost of walking for first time

http://www.eurekalert.org/pub_releases/2010-11/tcob-efw110410.php

Equation for how much energy we use when walking discovered

Any parent that takes their kid out for a walk knows that children tire more

quickly than adults, but why is that? Do kids and small adults walk differently

from taller people or do they tire faster for some other reason? Weyand

from Southern Methodist University, USA, is fascinated by the effect that body

size has on physiological function. 'This goes back to Max Kleiber's work on

resting metabolic rates for different sized animals. He found that the bigger

you are the slower each gram of tissue uses energy,' explains Weyand, who adds,

'It's interesting to know how and why metabolism is regulated that way.'

Intrigued by the question of why smaller people use more energy per kilogram

body mass than larger individuals when walking, Weyand teamed up with Maurice

Puyau and Butte, from the USDA/ARS Children's Nutrition Research Center at

Baylor College of Medicine, and undergraduate Bethany . Together they

decided to measure the metabolic rates of children and adults, ranging from 5 to

32 years old, weighing between 15.9kg and 88.7kg and ranging in height from

1.07m to 1.83m, to try to find out why larger people are more economical walkers

than smaller people. Weyand and his colleagues publish their discovery that

walkers of all heights use the same amount of energy per stride, making short

people less economical because they take more steps. They also derive a

fundamental equation to calculate exactly how much energy walkers use with

direct applications in all walks of life. The team publishes its discovery on 12

November 2010 in The Journal of Experimental Biology at

http://jeb.biologists.org/cgi/content/abstract/213/23/3972.

First Weyand and colleagues filmed male and female volunteers as they walked on

a treadmill at speeds ranging from a slow 0.4m/s up to 1.9m/s. Meanwhile, they

simultaneously measured the walkers' oxygen consumption and carbon dioxide

production rates to obtain their total metabolic rate. Next the team calculated

the amount of energy that each person used for walking by subtracting the basal

metabolic rate (energy required to maintain the body's basic metabolic

functions) from the total metabolic rate measured while walking. Finally, the

team compared the way each person walked, measuring the walkers' stride lengths,

stride durations and the proportion of each stride they spent in contact with

the ground (duty factor) to find out if large and small people walk differently.

Analysing the walkers' styles, the team found that all of them moved in exactly

the same way regardless of their height. Essentially, if you scaled a 5 year old

up to 2m, the giant child would walk in exactly the same way as a 2m tall adult.

So large people are not more economical because they walk differently from

smaller people.

Next the team calculated the metabolic cost of a stride as each walker moved at

their most economical pace and they discovered that walkers use the same amount

of energy per stride regardless of their height. So, big people do not become

more economical because they walk in a more economical style. Something else

must account for their increased economy.

Finally, the four scientists plotted the walkers' heights against their minimum

energy expenditure and they were amazed when they got a straight line with a

gradient of almost -1. The walkers' energy costs were inversely proportional to

their heights, with tall people walking more economically than short/smaller

people because they have longer strides and have to take fewer steps to cover

the same distance. So smaller people tire faster because each step costs the

same and they have to take more steps to cover the same distance or travel at

the same speed.

Based on this discovery the group derived an equation that can be used to

calculate the energetic cost of walking. 'The equation allows you to use your

height, weight and distance walked to determine how many calories you burn,'

says Weyand. The equation could also be built into popular pedometers to provide

users with a more realistic idea of how many calories they expend walking

throughout the day. Finally, the team is keen to extend the equation to

calculate metabolic costs at any speed. 'This has clinical applications, weight

balance applications and the military is interested too because metabolic rates

influence the physiological status of soldiers in the field,' explains Weyand.