Scientists have come up with a new equation to determine how much energy people actually use while walking.

While previous work has conjured many ways to measure the energy cost of walking, the new equation is among the first to account for the impact of body size, taking into account individuals' height and weight.

The equation has many possible applications. It could be used to design pedometers that, in addition to distance walked, provide an estimate of calories burned, taking into account a person's body size. The military may also find the equation handy, possibly using it to calculate how much energy soldiers expend — and thus how many calories they will need — while carrying different loads, said study researcher Peter Weyand, of Southern Methodist University in Dallas.

The findings are published today (Nov. 12) in the Journal of Experimental Biology.

Why height and weight matter

Scientists knew that shorter people, including children, use up more energy per pound of their body mass when walking than taller people, but they didn't know why.

To find out, Weyand and his colleagues had 48 subjects, ages 5 to 32, walk on a treadmill at speeds varying from 0.9 miles per hour (0.4 meters per second) to 4.3 miles per hour (1.9 meters per second). The subjects ranged in weight from 35 to 195 pounds (15.9 to 88.7 kilograms), and in height from 3.5 to 6 feet (1.1 to 1.8 meters). The researchers measured how much oxygen the subjects used to calculate their metabolic rate. They also examined they way each subject walked, including their stride length and duration.

The results showed that everyone used about the same amount of energy for each stride they took. But because people with shorter legs take more steps to cover the same distance as people with longer legs, short people used more energy over a given distance.

In other words, tall people are more economical walkers because they can take bigger steps.

For instance, someone who is 5 feet tall and weighs 100 pounds burns 0.44 calories per pound of their body mass (49 calories total) to walk one mile, while someone who is 6 feet tall and weighs 190 pounds burns 0.37 calories per pound of their body mass (about 71 calories total)  to walk the same distance, Weyand said.

So although taller people are more economical walkers and burn fewer calories on a per pound basis, they do tend to burn more calories. This is because they are generally supporting a greater mass against gravity, which requires more energy, Weyand said.

The equation is based on the walking speed that was most economical for the subjects, it does not accurately estimate calories burned by people walking very fast or slow.

Kids and adults walk the same

The study is also the first to show that people, young and old, do not differ in their gait.

"Regardless of your height, weight and age, people, after they reach 5 years of age, all walk in the same way," Weyand said.

This finding was contrary to earlier hypotheses which proposed that younger people burn more calories when they walk because of some factor of development.

Weyand's study has implications for the study of human evolution. Some evolutionary biologists have proposed that humans started to walk upright because the posture is a more energy efficient way of walking. The new equation could provide better estimates of the energy costs of walking for early hominids, Weyand said, to see if the change in posture did indeed provide the supposed benefits.