The Flash: Here's the biological limit to how fast humans can actually run

There is no firm consensus on how fast The Flash, the DC universe’s speediest superhero, can run. One story clocks him at 2,535 miles per hour (over 4,000km/h), for instance, which is faster than most fighter jets. While in Zack Snyder’s Justice League, he runs faster than the speed of light, 186,000 miles per second – so fast, in fact, that he violates the laws of physics and turns back time.

What is certain, however, is that for the likes of you and me, running at those speeds would be as unlikely as they would be perilous. Our faces would melt, our legs would break apart… I can barely do the ‘Couch to 5K’ plan as it is.

So how fast can human beings run? For that, we would need to look at the closest thing humanity has to The Flash: retired sprinter Usain Bolt.

In 2009, Bolt set a new world record by running the 100m sprint in 9.58 seconds – achieving a top speed of 27mph (43km/h) – at the World Athletics Championships in Berlin. It’s a record that is yet to be broken and, according to Mark Denny, a biology professor at Stanford University, perhaps never will be.

In 2008, a year before Bolt broke the world record, Denny published a paper analysing the top speeds of various athletics competitions – as well as those of greyhound and horse races – since the 1920s. And in many cases, he found the same pattern.

“Performances were clearly plateauing in all races,” he says, “and has plateaued in some of them.”

He found that horses, for example, reached their limit with Secretariat in 1973. “No horse has come close to the records he set,” says Denny. “They keep breeding horses to go faster and they just keep breaking down. The limits are real.”

Is there a limit to how fast humans can run?

As for humans, Denny has used his data to make a bold prediction: no human being will ever run the 100m sprint faster than 9.48 seconds, just 0.1 seconds under Bolt’s current record.

“There was a real sense of disappointment in response to the paper [with the prediction],” he says. “People didn’t like the idea of an Olympics where world records wouldn’t be broken. There was a chance that Bolt could have done it in 9.48 but then he aged out on that. I was rooting for him to break it.”

According to Denny, the reason for this proposed plateau is due to the basic biological limits of the human body. Four-legged animals like cheetahs – with their long legs, light body weight and flexible spine – are designed for short-term speed. Bipedal runners, meanwhile, are built for endurance.

“Muscles can only contract as fast as the actin and myosin [two key components of muscle fibre] can turn over,” he says. “You can get some advantage by making longer filaments in the muscle but then you can only make them so long.

"Usain Bolt is 196cm tall. You might get somebody who is 218cm and really well-proportioned. Yet even then, tendons and bones can only put up with so much. Even if you made the bones more robust, they would then be heavier, negating what you’re trying to do.”

The only way you could go beyond these biological limits, says Denny, is with genetic engineering. Although don’t go expecting us to be trotting around like centaurs any time soon. “It’s more likely to be used to gain more powerful muscles and longer legs,” says Denny.

“Or to tweak the design here and there: ‘let’s move this particular muscle to a different place’ and so on. That’s going to be really strange. Performance-enhancing drugs are scary enough but I hope I’m not around to see that.”