Why do children tire more quickly than adults when out for a walk?

Like most people who have had to carry a tired child home after a long walk, Peter Weyland of Southern Methodist University asked himself that same question. Scientists had long recognized that smaller people use more energy per kilogram body mass than larger individuals when walking.

He wanted to know why.

The reason smaller people use more energy is not due to a different gait or a less efficient metabolic rate per stride. The key was something simpler: their height.

Smaller people tire faster because they take more steps to cover the same distance or travel the same steps as taller people. Their strides are shorter.

Weyland teamed up with three other researchers to study the issue. Their findings are published in the Journal of Experimental Biology.

To test the cost of walking, the team measured the metabolic rates of children and adults. Participants ranged from 5 to 32 years old, from 35 to 195 lbs and from 3'6 to a little over 6 feet tall.

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The volunteers were filmed walking on treadmills. Their oxygen consumption and carbon dioxide production were measured to gauge their metabolic rate.

The team compared their walks too -- measuring their strides, the way they walked, stride durations and the proportion of each stride spent in contact with the ground.

Results showed everyone moved in the exact same way, no matter if they were 4 feet or 6 feet tall. Analysis also found that everyone used the same amount of energy per stride, regardless of height. So, the energy discrepancy was not due to the style of walking.

Finally, the researchers plotted walkers' heights against their minimum energy expenditure. The results excited them. The walkers' energy costs were almost perfectly inversely proportional to their heights. Ergo, tall people walk more economically because they have longer strides and take fewer steps to cover the same distance.

The team produced an equation to calculate the energetic cost of walking that will factor in your height, weight and distance walked. They hope to incorporate it into some consumer pedometers to provide users with more realistic data tailored to walkers of all sizes.