### THE GIST

- A little tailwind, altitude and a faster start time could lower Usain Bolt's 100-meter world record from 9.58 to 9.45.

- No one can predict what the ultimate limit of human speed will be.

- Sprinting world records tend to stagnate for a decade before dropping again.

With his current world record of 9.58 seconds in the 100-meter dash and a top speed of more than 27 miles per hour, Jamaican sprinter Usain Bolt has already defied many expectations of how fast human legs can go.

Yet, without much effort, Bolt could run even faster, according to new calculations. With a few slight but still-legal boosts from tailwinds, altitude and a better reaction time at the start, argues Cambridge University mathematician John Barrow, Bolt could easily clock in at 9.45.

And while elite athletes will likely run even faster than that some day, no one can say for sure how fast people will eventually go -- or if we'll ever see a sprinter finally reach the limits of the human body.

"There will be an ultimate limit, but just because there's a limit mathematically, that doesn't mean you'll ever reach it," said Barrow, author of Mathletics: A Scientist Explains 100 Amazing Things About the World of Sports. "You can draw a curve that's always increasing, but never goes higher than the particular level where it's bounded."

Bolt surprised the running world when he broke the 100m record in the spring of 2008, partly because the top times had been stagnant for years. At 6 feet, 5 inches tall, Bolt also seemed too big to be a sprinter. By 2009, he had lowered the record from 9.74 to 9.58 -- a dramatic drop for such a short distance.

As speculation circulated about how fast Bolt might eventually go, Barrow started doing some basic calculations, focusing on three simple factors that are known to affect sprinting speed. He started with Bolt's notoriously slow reaction time to the starting gun.

Under official rules, runners are called on false starts if they leave the starting blocks less than 0.1 seconds after the signal sounds. The best starters are consistently off and running after about 0.12 seconds. If Bolt could get his sluggish start time of 0.165 -- the second slowest in the final heat at the Beijing Olympics -- down to 0.12 and still run at his top speed, Barrow said, that alone would lower his record to 9.55.

Bolt surprised the running world when he broke the 100m record in the spring of 2008, partly because the top times had been stagnant for years.Getty Images

With a maximum allowable tailwind of two meters (6.6 feet) per second on top of an improved start time, Barrow calculated with known relationships between wind, drag and running speed, the sprinter could lower his record to 9.5.

Finally, Barrow considered what would happen if Bolt ran at an altitude of 1,000 m (3,280 feet), the highest allowable elevation for running records to count. At that height, the density of air is low enough to reduce drag and facilitate another drop in speed. If he also started well and had a tailwind, altitude would give Bolt the ability to run a 9.47.

As for actual running technique, studies have shown that the most important factor driving sprinting performance is how hard runners can hit the ground in relation to their body weight, said Peter Weyand, a physiologist and biomechanist at Southern Methodist University in Dallas.

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The amount of time people spend in the air between foot strikes doesn't matter much, Weyend said. Neither does the speed with which they cycle their legs around. Instead, elite sprinters produce vertical forces that are as much as five times greater than their body weight. That propels them upwards like a spring, while momentum carries them foreword.

Scientists still don't know how the fastest runners generate ground forces as high as 1,000 pounds. And even though studies have connected certain body shapes and running styles with speed, it's always possible that everything will be different once people start running faster than they ever have before.

"We can figure out what the relationships are that allow people to run fast, what the important factors are and where the limits are from the standpoint of experience," Weyand said. "Once you move outside the range of data, you have no way of knowing if those relationships are going to break down. Any relationship you have within a given range doesn't necessarily hold at the extremes."

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Compared to distance running, very little is known about the detailed physiology of elite sprinting, added Michael Joyner, an exercise researcher at the Mayo Clinic in Rochester, Minn. What's almost certain, though, is that someone will eventually run faster than Usain Bolt.

In fact, at least two runners may have already unofficially beat Bolt's pace. In the 1964 Tokyo Olympics, for one, American sprinter Bob Hayes was clocked with a handheld stopwatch at 8.5 seconds in the final leg of the 4 x 100 relay. And last season, Bolt's teammate Yohan Blake ran the second fastest ever 200m with a time of 19.26 and a dismally slow reaction time at the start of 0.269. Taking all that into account, Barrow figured, Blake's 100m split would've been 9.495 -- faster than Bolt's current record.

Generally, times for the 100m tend to stagnate for five, 10 or 15 years before someone chips off another tenth or two-tenths of a second, Joyner said. He suspects that, a decade from now, the next top sprinter will lower the record to 9.4 or so. Beyond that, the future of sprinting is anyone's guess.

"Every time we say there's a limit, someone goes faster," Joyner said. "Who knows what that is?"