The study examined how people budget their time as they travel on foot to reach a destination at a particular appointed time.
It found that when people have neither too much time nor too little time to reach their destination, they naturally switch back and forth between walking and running, which turns out to be the best strategy for saving energy.
The study is the first of its kind not conducted on treadmills, and it supports the notion that the human body has an innate sense of how to vary speed to optimise energy when we're on the move in our natural environment.
"We don't live our lives on a treadmill," said Manoj Srinivasan, co-author of the study and assistant professor of mechanical engineering at Ohio State University.
"Once we have a good enough understanding of how we naturally move to conserve energy, we can build a solid theory and apply it in practical ways - designing better shoes, or better prosthetics that let people walk using less energy," Srinivasan said in a statement.
In tests, 36 college students were asked to travel a distance a little longer than a football field, either on pavement outdoors or inside a school hallway. They were given a stopwatch, and told to arrive at their destination at a specific time - not before and not after, but right on time.
The students were free to set their own pace to achieve that goal, and were not told whether to walk or run. Sometimes the students were told to make the trip in two minutes, so they could walk the entire way at a leisurely two miles per hour if they chose.
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At the other extreme, they were allotted only 30 seconds, so that they had to run the entire way at a brisk pace of nine miles per hour to get there on time.
The study found the existence of a "transition region" between 4.5-6.7 miles per hour when the students tended to make the trip through a mix of walking and running.
Using data previously recorded by other researchers who measured the typical human energy costs for walking and running at various speeds, Long and Srinivasan calculated that dividing up the trip into spurts of walking and running saved energy.
"Students seemed to naturally break into a run or slow down to a walk to save energy while ensuring that they arrived at their destination on time," Srinivasan said.