Steudel's team tested both male and female runners at six different speeds on a treadmill while measuring their oxygen intake and carbon dioxide output. As expected, each runner had different levels of fitness and oxygen use but there were ideal speeds for each runner that required the least amount of energy.

Overall, the optimal speeds for the group were about 8.3 mph (about a 7:13 minutes per mile) for males and 6.5 mph (9:08 min/mile) for females.

The most interesting finding: At slower speeds, about 4.5 mph (13 min/mile), the metabolic efficiency was at its lowest. Steudel explains that at this speed, halfway between a walk and a jog, the runner's gait can be awkward and unnatural.

"What that means is that there is an optimal speed that will get you there the cheapest," Steudel says.