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.
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."
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?"