Written by Randall Munroe
Q: How fast is it physically possible for a human to run?
— Steve, Davis, California.
A: So far, the fastest anyone has run is about 27 1/2 mph, a speed reached (briefly) by sprinter Usain Bolt just after the midpoint of his world-record 100-meter dash in 2009.
This speed limit is probably 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 was 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% of the total stride time. But for a fast-running quadruped” — a cheetah, a horse — “it’s two-thirds of the stride time.”
During the brief period of ground contact, our legs must push us forward and push us upward to support our body weight. That’s a lot of force to exert in a short time — and it’s why humans can skate faster than they run, Weyand said: “On skates, you’re on the ground most of the time, like the quadrupeds, instead of being in the air.” Keeping your skates on the ground longer helps support your body during the glide phase, taking some of the load off the pushing leg.
I asked Weyand how he would redesign humans to run faster. I tossed out four options: longer legs, really wide hips, extra legs or extra knees.
“Adding more knees is probably the trickiest one,” he said. Extra knees might let you extend your legs to stay in contact with the ground longer. But if your feet get too far out from under your body, it’s hard to generate enough leverage to push down against the ground. “If you were designing robots or whatever, of the options you included, I think that’s probably the least likely,” he said. “That and the wider hips.”
Longer legs might help, he said; it is one reason ostriches run faster than us. But the best option would be more legs, so that we could have one or two of them on the ground most of the time, like a quadruped.
“The key thing is having more of the total stride time be grounded,” Weyand said. “It’s really that physical principle that’s all-important, from everything we’ve done — devices, quadrupeds, bipeds, artificial limbs. That comes through every time.”
So if you want to run really fast, you could try persuading a wizard to turn you into a centaur.
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.
That is a bold prediction, to put it mildly. But if you can’t find a wizard, you could always give running on all fours a try.