We've known for a long time that a lower rate of energy consumption at any given speed means an athlete can run farther. This is the idea behind all sorts of new training methods that measure caloric intake against mileage outtake and such, but what we haven't known is why.

Until now.

Melanie Scholz of the Free University of Amsterdam, Netherlands began to wonder if this measure of efficiency was actually a measure of stored energy—specifically of elastic energy stored in a runner's Achilles tendon.

The idea is pretty simple. When a foot lands it compresses the tendon, storing energy that gets released the next time the runner lifts that foot, thus helping to power the next stride. But what's really interesting here is that the amount of energy available for storage turns out to be directly related to the size of one's heel.

Scholz figured this out by mathematically modeling the effects of heel length (measured as the distance between the Achilles and the inside and outside ends of the ankle bone) on distance running prowess.