Sprinters who have taken on Usain Bolt in the 100-meter dash often describe a moment in the second half of the race when the world’s fastest-ever human just runs away from them.

One minute they are shoulder-to-shoulder with Bolt, believing that this will be the night the legend will be toppled. The next they are staring at his back, watching him raise his hands in triumph, sometimes many meters before he crosses the finish line.

Last week Bolt expressed his usual, unflappable confidence, even though a hamstring injury kept him from Jamaica’s track and field trials. Granted a medical exemption by the country’s athletics federation, he was named to the team even though he couldn’t qualify at the national trials.

“My chances are always the same: Great!” he said. “If everything goes smoothly the rest of the time and the training goes well, I’m going to be really confident going to the championship.”

Fans have grown familiar with his methods, too. Bolt, who is 6 feet, 5 inches tall, has won all but one Olympic and World Championship100-meter race since 2008. The lone exception is the 2011 World Championships, where a false start got him disqualified.

“Anybody can be beaten, but he is a crazy talent and something that we have never seen in the sport,” said Lance Brauman, who has coached many of the world’s top sprinters, including Tyson Gay, who won the 100 at the 2007 World Championships, right before Bolt’s era of dominance began. “You are hoping you have your best day and hope he doesn’t have his.”

Bolt seems to have another gear that no one else does. He accelerates, and no one can keep up. At least that’s what our eyes tell us is happening—but it’s not so.

Bolt is no different from every other incredibly fast man, hitting his top speed of about 27 miles per hour at about the 70-meter mark. From there, his speed drops, if only by a few hundredths of a second for each 10 meters, but in a race that is determined by whiskers, every fraction of a second is vital.

What this means is that Bolt isn’t kicking into another gear and running away from the field. Instead, he’s slowing down at a slower rate than anyone else.

So, beating him should be simple, right? Just don’t slow down. Of course it’s not that easy, and scientists are still figuring out why humans—and cheetahs and pronghorns and other fast mammals—slow down so quickly.

Bolt crossed the finish line to win the final of the men’s 4×100 meter relay for the Jamaican team at the 2015 IAAF World Championships in Beijing in August 2015.PHOTO: FRANCK FIFE/AFP/GETTY IMAGES

For decades, researchers have theorized that deceleration starts as energy stored in the muscles is used up. “All mammals engaged in intense exercise, be it a human marathoner, a cheetah trying to catch prey or the prey trying to avoid becoming a meal, rely on energy stored in the body, usually as glycogen,” said Karen Steudel, a professor of zoology at the University of Wisconsin. “Once this is depleted, the human or cheetah is basically out of gas.”

However, a 2012 study by Matthew Bundle of the University of Montana in Missoula and Peter Weyand at Southern Methodist University in Dallas, showed that the greatest decrease in muscular performance occurs within the first seconds of a sprint when runners are still accelerating, which would suggest that deceleration in a race as short as 100 meters may not be related to how sprinters metabolize glycogen.

“Muscle fatigue happens contraction by contraction,” Weyand said. He argues that the biological process that causes the fatigue is still a mystery. It also is very hard to measure, because it is difficult to examine what is happening to an incredibly fast person’s muscles when he can only run at full speed for roughly three seconds.

Still, the idea that muscle fatigue begins instantaneously and with each muscle contraction may say plenty about why Bolt is so hard to beat.

Bolt is significantly taller than the competition, and his ability to take quick steps, known as stride frequency, is about as good as anyone else’s. He can cover more ground with fewer steps, allowing him to complete 100 meters with just 41 strides, while his opponents average about 45.

If the muscles are becoming less powerful with each step, then by taking fewer steps, Bolt’s muscles are becoming less fatigued. That would explain why in the final 20 meters he essentially is slowing down slower than everyone else.

So, take fewer steps and you can beat Bolt, right? Well, no. Sprinting effectively means finding the right balance between stride length and stride frequency. Long strides that stretch too far beyond a sprinter’s center of gravity act like a jab to the chin. Each too-long stride breaksforward momentum. However, strides that are too short don’t cover enough ground, and human legs can only turn over so quickly.

“You’re always asking, ‘How can I get a little stronger, have a little more finesse, have a little more patience and run faster?’ ” said John Smith, considered by many to be the top sprint coach in the U.S.

Every sprinter says the key to winning is to pay attention only to what is happening in one’s own lane, because that is all that can be controlled. Bolt is trying for an unprecedented “triple-triple”—gold medals in the 100, 200 and 400-by-100 meter relay. If he pulls it off, he will surely be considered the greatest track and field athlete of all time.

Go ahead, try not paying attention to that.

Write to Matthew Futterman at matthew.futterman@wsj.com

Source: The Science Behind Sprinter Usain Bolt’s Speed – WSJ