This episode of SciShow is sponsored by Blinkist.

Blinkist takes all the need-to-know info from thousands of nonfiction books and condenses them into just fifteen minutes of content. To learn more, click the link in the description. {♫Intro♫}.

In world-class running competitions, athletes just keep getting faster. Like, Usain Bolt — widely known as the world's fastest human — has clocked in at nearly forty-five kilometers per hour. And that kind of makes you wonder… how much faster can we get?

Is there a limit here, or will someone eventually leave that record in the dust? Well, no one can say for sure, but there is evidence a human speed limit should exist. And not for the reasons you might think.

There are a ton of variables involved in running, from how fast you swing your legs to the type of shoes you're wearing. But, generally, there's one thing that seems to matter most at the professional level: what happens when your feet are on the ground, during what's called the stance phase. The fastest sprinters are the ones who can push off the track with the most force.

So you might think that the stronger their legs are, the more force they'll get out of each step, and the faster they'll get. Pretty simple, right? Yeah, well… there's more to it than that.

As you go faster, your feet actually spend less time on the ground during each step. Like, when an Olympic-level sprinter is at top speed, their rubber meets the road for less than 1/10 of a second. So to run faster, they need to push harder in a vanishingly short window of time.

And that's where the trouble comes in. See, when a muscle cell gets the signal to contract, it doesn't generate force right away. It starts off kind of slack.

So, before it can do anything to help you move, it needs to get nice and taut. Then, it can start producing force. The problem is, that process can only happen so fast.

And while some people do have faster muscle fibers than others, overall, it takes significantly longer than one-tenth of a second for muscles produce their maximum force. This means that, during a race, an athlete can't use all of their strength. Their feet just aren't on the ground for long enough.

Sure, they can improve their technique, like by slamming their feet into the ground to get rid of slack faster. But even then, they'll reach a point where they can't improve — where their muscle fibers won't be able to provide more force during the tiny windows their feet are on the ground. When that happens, they've reached their speed limit and won't be able to go much faster.

Scientists think this limit exists because humans didn't evolve to be sprinting machines. Instead, our leg muscles are optimized for multiple tasks, like standing, walking, and running. That means they need to be strong as well as fast — and that strength comes at the expense of a little extra speed.

Right now, it's hard to say exactly what the maximum human running speed is, especially because some people have leg muscles that twitch a little faster than others. But one 2010 study looked into it, and they suggest we probably can't get much faster than about 50 km/h — even with genetic advantages, like extra speedy muscles. Someday, if we did want to go faster than that, we'd likely have to turn to genetic engineering or find a different gait that allowed our feet to stay on the ground for longer.

That's actually why animals like cheetahs and greyhounds are so fast, even though they have muscles with essentially the same limits as ours. When they run, their feet stay on the ground for longer, so they have more time to push off and generate force. Unfortunately, galloping like a cheetah… probably isn't an option for us, since humans evolved to run in a specific way.

But, hey. If that 2010 study is right, it looks like we haven't hit our maximum speed yet. So one way or another, it looks like someone could still beat Usain Bolt.

I'm no sprinter, but I can say that watching races is super exciting. Events like the hundred-meter dash cram all sorts of action into only a few seconds! And in a way, that's what the company Blinkist is trying to do with reading.

Blinkist is an app that takes all the best insights from nonfiction books and condenses them into something you can read or listen to in about fifteen minutes. For instance, they have a condensed version of Bill Bryson's A Short History of Nearly. Everything that you could probably read over your lunch break.

If you want to check it out or read one of Blinkist's thousands of other titles, you can click the link in the description. If you're one of the first 100 people to sign up there, you'll get free, unlimited access to the site for seven days and 25% off if you decide to get a full membership. You can also cancel any time during that trial period.

If you check them out, let us know what you think! {♫Outro♫}.