When adults and kids go on a walk together, often the kids get tired faster. This is because smaller people use more energy per kilogram body mass than larger individuals.
Peter Weyand from Southern Methodist University and his team have discovered the equation for how much energy we use while walking.
They have linked 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," explained Weyand.
Intrigued by the question of why smaller people use more energy per kilogram body mass than larger individuals when walking, Weyand and colleagues decided to measure the metabolic rates of children and adults, to try to find out why larger people are more economical walkers than smaller people.
They found 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 derived a fundamental equation to calculate exactly how much energy walkers use with direct applications in all walks of life.
First Weyand and colleagues filmed male and female volunteers as they walked on a treadmill. 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 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.
They also discovered that walkers use the same amount of energy per stride regardless of their height.
Moreover, the researchers found that 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," said Weyand.
The findings were published in the Journal of Experimental Biology.