*This story appears in the July 12 issue of ESPN The Magazine and was adapted from a chapter of "The Perfection Point: Sport Science Predicts the Fastest Man, the Highest Jump and the Limits of Athletic Performance," by John Brenkus.*

Give even a moment'sthought to the limits of human potential and you have to wonder: How fast is it

possible for someone to move over flat ground using nothing but his own two legs?

There are dozens of different competitive running events, but the most fascinating is also the simplest and the shortest. It's the 100-meter sprint, and the world record-holder at this distance is almost certainly the fastest human alive. Why? Because if you just put the hammer down and give it everything you've got, somewhere around the 55-meter mark, you're going to hit the top speed of which you're capable. For the remaining 40 or so meters, you try to slow down as little as possible. But in a longer race, such as the 200-meters, you'd never hit that top speed. If you did, you'd run out of gas and finish dead last by an embarrassingly wide margin.

Still, for a race so simple, the 100 is surprisingly complex. It consists of four distinct phases:

1. Reacting to the gun;

2. Getting out of the blocks;

3. Accelerating to top speed;

4. Hanging on for dear life at the end.

To predict the fastest 100 that could ever be run, we'll use Usain Bolt's record-setting time of 9.69 seconds at the 2008 Beijing Olympics as our starting point, then break it down into these four phases. For each phase, we'll look at how much better Bolt could have done if the conditions had been more favorable. That will give us a new starting point. From there, we'll consider how much faster it will be possible for a future

sprinter to run, given physiological improvements.

The end result is stunning.

### PHASE 1: REACTING TO THE GUN

International rules require that each runner take off in reaction to the sound of the gun. If, instead, he guesses when the gun is going to be fired, kicks off based on that guess and happens to hit it just right, he's technically guilty of a false start. But how do we know what's in someone's mind?

We don't. What we do know is the shortest interval of time over which it's possible to hear the gun and push off the blocks. In international competition, that's assumed to be .1 of a second. React faster than that, and it's a false start.

In Beijing, Usain Bolt's reaction time of .165 of a second was downright pedestrian. Had he gotten off the line at the Olympic standard of .1 of a second, his final time could have been 9.62.

### PHASES 2 AND 3: GETTING OUT OF THE BLOCKS AND ACCELERATING

The force with which air impedes the motion of

a moving body is called aerodynamic drag, and it increases with acceleration. Unfortunately for runners, when you double the speed you

quadruple the drag. So anything that reduces a runner's drag will improve his finishing time.

One way to do that is to run with a tailwind. IAAF standards allow for a "following" wind of up to two meters per second, or about 4.4 mph. That's less than the breeze you feel coming out

of your car air conditioner on its lowest

setting. Because a sprinter is moving so fast, he's never going to feel a tailwind, but he will feel less headwind.

The impact is not trivial. Every meter per second of following wind knocks about .05 of a second off a 100-meter sprinter's time. When Bolt broke the world record in Beijing, there wasn't a breath of wind. Had there been the maximum allowable breeze, he might have chopped .1 of a second off his time. Factor in that adjustment and we're down to 9.52 seconds.

There is also an IAAF standard for the

maximum altitude of the track; the higher the track, the thinner the air, which means less

resistance. The standard is 1,000 meters, or 3,280 feet. The track at the Beijing Games was only 164 feet above sea level. Using standard corrections

for 100-meter times at various altitudes, we can

assume that, had Bolt been running on a track at the allowed maximum altitude, he would have taken another .06 of a second off his time. That gets us down to 9.46.

### PHASE 4: HANGING ON AT THE END

Cranking up to 25 mph from his start took Bolt about four seconds. Squeezing an additional 2 mph to reach his top speed of 27 mph took another two seconds. Since he was probably creating roughly the same amount of force during all of that

time, why did it take so long to accelerate from 25 to 27 mph?

There are a number of reasons. The first has to do with aerodynamic drag again. From 5 to 10 mph, the drag on Bolt's body quadrupled. Then it quadrupled again from 10 to 20 mph. At that speed, most of the force he was generating went toward overcoming air resistance, and some went to overcoming the friction of his feet hitting the track. There's not much power left to produce additional acceleration.

There are also limits to how fast muscles can move. A sprinter not only has to push off

powerfully with each leg, but he must also get that

leg forward quickly to prepare for the next step. Also, unlike an ostrich, the fastest animal on two legs, we don't have muscle-tendon units that act like springs to help propel the leg forward.

In fact, once a sprinter reaches the limit of how fast he can take individual steps, the best he can do if he has power in reserve is use it to increase stride length so he can cover more ground with each step. World-class sprinters typically take 43 or 44 steps over the course of 100 meters. Bolt needed only 41 in Beijing, covering well over eight feet with every step in the middle of the race.

Bolt hit his top speed of about 27 mph somewhere after the 50-meter mark, which is where most sprinters reach their top speed. About 10 or 20 meters later, they begin slowing down. While it's

difficult to tell exactly how fast sprinters are

moving at any point in the race, we know their splits -- the time they took to complete each 10-meter segment -- with great accuracy. Below you'll find the splits for six record-breaking 100-meter sprints. (Reaction time is included in the 0-10-meter segment.)

Two factors pop out. First, Bolt had the fastest 10-meter split in history, at .82 of a second. In addition, he had three of those in a row. In fact, for a 50-meter stretch starting at the 40-meter mark, Bolt never dropped below 26 mph.

Where would a future athlete go from there?

To find out, I consulted with Bassil Aish, a sports medicine doctor and one of the world's foremost authorities on the science of running. Aish has

calculated the ideal muscle mass and fiber ratio for a marathon runner. We asked him to do it for a sprinter. The explanation of how he conducted his analysis is technical and difficult to convert into a specific speed prediction, so I asked him to express his conclusions about the "ideal sprinter" via comparison to Usain Bolt.

"He's fairly close," Aish reported back, "but there's plenty of room for improvement."

How much room, exactly? "A sprinter who achieved ideal proportions," he says, "could

accelerate to a high enough speed and hold it for a long enough time to run the 100-meter distance 3.7% faster than Bolt did."

It's a dramatic conclusion, but it comes with a caveat. "We're talking about something that might happen 1,000 years from now," Aish warned. "At the rate humans are evolving, we're not the same species we were even 200 years ago, and we're definitely not going to be the same species 1,000 years from now."

We're still not quite ready to calculate the

perfection point in the 100, because we haven't yet dealt with the single most extraordinary aspect of Bolt's Olympic race: He practically quit racing with nearly 20 meters remaining. Even the most conservative experts agree that he could have gone .1 of a second faster if he'd kept the hammer down

instead of showboating. And, in fact, on Aug. 16, 2009, Bolt ran the 100 meters in Berlin with no slowing down and no showboating. His time was 9.58 seconds, an incomprehensible .11 of a second faster than his Olympic performance.

So now we're ready to do the final calculation.

We start with Bolt's Beijing Olympic time of

9.69 seconds. We assume he reacts at the start as quickly as allowed, and subtract .07 of a second. Next we subtract .1 of a second for the advantage he would gain with a legal tailwind. By moving the track to the allowable standard of 3,280 feet above sea level we can subtract another .06 of a second. Then we take off the .1 of a second his showboating added to his time.

We now know what Bolt's time would have been had all the conditions been ideal: 9.36 seconds.

All that's left is to factor in how much faster a future sprinter might run. If we apply Aish's calculation of a 3.7% improvement to our adjusted starting point of 9.36 seconds, we cut the record to 9.01 seconds.

At last, it seems that we're done. We've run out of reasons to think a 100-meter race could get any faster. There are simply no more sources of additional speed.

Except one.

Going from 9.01 seconds to sub-9 in the 100

requires an improvement of .02%. Where could it come from? Maybe from a muscle spasm, a midrace gust of wind that doesn't register on the measuring device or an equipment improvement. Maybe it will come from nothing more than population growth: Today, there are about 6.8 billion people in the world; in 50 years, there will be 10 billion, giving us a larger pool from which to draw runners. Maybe it will come from new technology: NASA spent $25.5 billion during the space race getting us to the moon, so surely $800 million in R&D over 800 years by Nike and Reebok can shave another .02 of a second off the 100.

I don't know where we'll find that last .02 of a second, but I do know this: Come that close to a number like nine seconds, and betting against it falling will be like betting against the sun rising in the morning. We can't say when it will happen, but it will happen, because our desire to overcome seemingly insurmountable obstacles is more

powerful than any of the factors we've considered. When a gauntlet is thrown down, we find a way to conquer it. Failure is inconceivable. It's in our

nature to find a way, as Roger Bannister, Sir

Edmund Hillary, Charles Lindbergh, Roger Maris and countless others have done.

The perfection point for the 100-meter sprint is 8.99 seconds.

Unless the species itself changes, it's the fastest a human will ever run.

*John Brenkus is the author of "Perfection Point" and the host of ESPN's "Sport Science."*