The science explaining how Usain Bolt became the fastest human in the world

Life as Laboratory
Life as Laboratory

Eight years ago, Usain Bolt made history in less than 10 seconds at the International Association of Athletics Federations World Championship in Berlin, Germany.

The Jamaican sprinter set the world record for the 100-meter dash, clocking in at 9.58 seconds. Since then, no one (not even Bolt himself) has been able to best that time. On Saturday, August 5, Bolt will once more run the 100-meter dash at the IAAF World Championship (assuming he makes it through the qualifying race on August 4). This will be his last race; Bolt is set to retire after this running season (there’s some speculation he may still race in the 2020 Olympics, although as of now Bolt has said he doesn’t want to).

There’s no such thing as a perfect human running machine. But Bolt comes close—thanks to a combination of having all the advantages of a natural-born sprinter and putting in the effort needed to minimize any of his disadvantages.

Broadly speaking, Bolt has the unique muscular build shared by most of the very best sprinters. All human muscles are made of a mix of slow- and fast-twitch fibers—as well as some that are undifferentiated, and will become slow- or fast-twitch depending on how we use them most often. Slow-twitch fibers are built for efficiency and use oxygen to generate energy from sugar. They’re most effective for activities sustained over a long period of time, like distance running. Fast-twitch muscle fibers are used to generate huge amounts of force, but they don’t use oxygen and as a result can’t carry us far. Training can help shape undifferentiated fibers into either slow- or fast-twitch, but for the most part the best runners were born with an imbalance of one or the other. Elite marathoners have way more slow-twitch fibers, and sprinters like Bolt have an abundance of fast-twitch ones.

The best sprinters also run with a different form than the rest of us. It’s not that they move their legs significantly faster; it’s that they hit the ground harder (paywall). Most of the force sprinters generate is directed straight into the ground for vertical movement; only about 5% is used to propel them forward, Peter Weyand, a physiologist studying human speed at Southern Methodist University in Texas, told Popular Science in 2013. The more force a sprinter can pack into the ground with a quick foot strike, the faster he or she goes.

In a 2010 study, Weyand’s lab conducted an experiment where subjects jogged, ran, and hopped on one foot on a treadmill. They found that the most force came from hopping, thanks to the leg’s longer airtime. The researchers then calculated that if a runner were to generate the maximum hopping force possible with each step, he or she’d be able to reach a speed of 19.3 meters per second (63.3 feet per second)—which would make for a 5.18 second 100-meter dash.

This is just a fun theoretical experiment; it’s impossible to actually sprint and jump straight up and down at the same time. But it appears Bolt generates a powerful punch to the track—maybe the most powerful ever.

Earlier this year at a conference Cologne, Germany, Andrew Udofa, a graduate student at Southern Methodist University currently working with Weyand, presented results of a study in progress analyzing Bolt’s strike force. Udofa slowed down and analyzed footage of Bolt running in the 100-meter dash in the 2011 World Athletic Championships in Monaco. Udofa estimated that Bolts strikes the track with more than 1,000 pounds of force on average.

Bolt’s stride is uneven: He generates about 1,080 pounds of force for an average right leg stride and 955 pounds on his left. That’s likely due to the runner’s reported scoliosis, which makes one of his legs slightly longer than the other, the New York Times reports (paywall). (Bolt has written about his scoliosis in his autobiography; mild scoliosis occurs in roughly 3% of the population doesn’t often need any kind of medical treatment.)

But it’s not Bolt’s uneven stride that gives him so much power; it’s his form. “In order to have greater forces, runners have a high knee lift, but then hit [the ground] very stiffly,” says Udofa. The stiffer they keep their limbs on impact, the more force makes it into the ground and isn’t lost to other parts of the body, like wobbly joints. (The same physics principles apply when seeking to achieve maximum force in other sports, too.) That’s why, Udofa explained, although distance runners may kick their legs behind them a little more, the best sprinters pick their knees almost straight up on each stride to try to generate maximum force when they bring their leg down again.

Interestingly, most physiologists would predict that Bolt’s height at 6 ft, 4 inches (193 centimeters) would put him at a disadvantage. “Bigger people are going be slower out of the blocks,” says Udofa, simply because it takes more force to get them going. It’s the same thinking why animals like T. Rex and elephants are slower than you might expect; it takes a lot of force to get them going, and their muscles can’t maintain the energy needed to get them up to speed and then keep that pace.

Based on our size relative to the rest of the animal kingdom, humans should actually be speedy. Unfortunately, we just weren’t built with speed in mind the way other animals, like cheetahs, were. An adult cheetah weighs around 160 lbs, about the same size as many professional human runners (Tyson Gay, the world’s second-fastest sprinter, for example, is 5 ft 10 inches, and weighs 165 lbs; Bolt’s height means he weighs more than most). But cheetahs have long, flexible spines that give them a massive stride length between their front and hind legs. We humans just don’t get the same range with two legs and our hips. Plus, cheetahs are way more aerodynamic when they run. They’re narrow with more horizontal bodies, whereas when we stand tall we get more resistance from the air.

Yet Bolt doesn’t seem to be slowed down by his height. He seems to get himself going at the same speed that smaller runners do, and his long legs carry him farther with each step. In an event like the 100-meter dash, Bolt probably takes four to five fewer steps than other runners, Udofa says.

And even though his legs are reportedly two different lengths, this asymmetry doesn’t seem to be a problem for Bolt. “A lot of people are going to have slightly asymmetries in terms of leg lengths,” says Udofa. “People will self-optimize to get what’s best for them,” in terms of getting the most use out of their muscles. In fact, research from earlier this year (paywall) proved we all run with a stride that most efficiently uses the energy we get from oxygen—although this doesn’t mean we’re naturally our fastest, as the Verge reports. Perfecting a stride technique with flawlessly raised knees and straight ankles takes years of training; Bolt seems to have combined all of that coaching with what works best for his body.

To top it all off, Bolt has one more trick up his sleeve. To those watching in the stadium or on TV, “it looks like Bolt kicks into another gear” at the end of a sprint, Udofa says. But actually, he’s just slowing down the least. Even the fastest sprinters mostly can’t hang on to their top speeds much farther beyond the 70-meter mark. Although they’re not slowing down in a noticeable way (at least to the untrained eye), they’re decelerating more than Bolt does. That’s what makes Bolt appear to glide so smoothly and easily ahead of the competition.

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