So far, the fastest anyone has run is about 27½ miles per hour, a speed reached (briefly) by sprinter Usain Bolt just after the midpoint of his world-record 100-meter dash in 2009.

This speed limit probably is not imposed by the strength of our bones and tendons. Rather, a 2010 study suggested that the limit comes from our bipedal stride, in particular how quickly we can rearrange our legs while still leaving time to push off from the ground.

Peter G. Weyand, a biomechanics researcher and physiologist at Southern Methodist University and one of the authors of the 2010 study, said that our running speed is limited because we are in the air for most of our stride. During the brief moments that our feet are touching the ground, we have to exert a lot of force.

“If I have to point to one mechanical limit for bipedal runners, from all the work that we’ve done, it’s the minimum period of foot ground contact,” he said. “A human who’s really fast, like Usain Bolt, is on the ground roughly 42 or 43 percent of the total stride time. But for a fast-running quadruped” — a cheetah, a horse — “it’s two-thirds of the stride time.”

Of course, humans are fully capable of running on four limbs without any magical help. A 2016 paper by Ryuta Kinugasa and Yoshiyuki Usami noted that the Guinness World Record for a human running 100 meters on all fours has improved from 18.58 seconds in 2008 (the first year the record was tracked) to 15.71 seconds in 2015. The researchers extrapolated from this rapid rate of improvement to make one of the stranger predictions published in a scientific paper: That by 2048, a person on all fours could go faster than a person running upright.

Read more:  How Fast Can a Human Run?