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MELISSA BLOCK, host:

When Jamaican sprinter Usain Bolt blasted through his own world record yesterday, he barely had time to savor that incredible feat before people were asking, how much lower can he go? Bolt slashed a full eleven-hundredths off his previous record time in the 100-meter finals at the World Track and Field Championships in Berlin. His new record: 9.58 seconds.

So, where could it stop? Is there a human speed limit? We're going to put that question to professor Peter Weyand, a physiologist at Southern Methodist University in Dallas. He specializes in the biomechanics of running.

Professor Weyand, welcome to the program.

Professor PETER WEYAND (Physiologist, Southern Methodist University): Thank you. It's nice to be here.

BLOCK: And what do you think? Can this record keep tumbling?

Prof. WEYAND: Oh, absolutely. I expect that it will, and I think there's a lot more to come than what we've seen so far.

BLOCK: Well, how far? I mean, Usain Bolt was asked about this right after the race. He said, well, maybe 9.4 seconds. Maybe that would be the limit, but he said you never know.

Prof. WEYAND: You never do. And I would say that he's being conservative. I think it could fall a lot more than that. You know, I would say in the next decade, it wouldn't surprise me if it goes below nine seconds.

BLOCK: Nine seconds?

Prof. WEYAND: Sure. It could go faster than that - and it probably will.

BLOCK: Well, what determines this physiologically? I mean, when you study the biomechanics of running and you've looked at these 100-meter sprinters, what do you see that's contributing to these faster and faster times?

Prof. WEYAND: Well, what they do is really remarkable because it comes down to how hard they can hit the ground in relation to how much they weigh, and how fast they can do it. So, for somebody of Usain Bolt's caliber, he will hit the ground with a peak force of about five times his body weight. So if he's just under 200 pounds, that's almost 1,000 pounds of force. And they do that in about a tenth of a second or less. So what they do is hit really hard, really fast.

BLOCK: And you're figuring that the more he runs, the better he'll get?

Prof. WEYAND: Yeah. I think the - when the ultimate - the question of the ultimate speed limit is posed, it really comes down to a limitation of how fast the muscles can turn on. And we know biologically, we're not anywhere close to that limit.

BLOCK: If you leave out anything you might chemically add to your body to get it to do things it wouldn't ordinarily do, what else can the human body do to improve on these incredible speeds that we're seeing?

Prof. WEYAND: Since the scientific understanding of speed is not terribly mature, we're seeing some training programs that have come along in the last five to 10 years that seem to be a lot more effective. And those training programs being implemented progressively more effectively by these elite athletes certainly has the potential to bring times down by another half a second or so.

BLOCK: Now, Usain Bolt is 6'5". He's taller than those other runners. Is that an advantage to him or not? I mean, it takes him longer to get out of the blocks, I guess, but he has this enormous stride.

Prof. WEYAND: It should be a disadvantage being that tall, and he is unusually tall. What sprinters do is a trade-off between starting and finishing. And when they start, it's a disadvantage to be taller and bigger because it takes longer to get the body going. But once they get up to speed, it's an advantage.

So if you look at athletes, how tall they are and how big they are by event specialty, the sprinters that run the short indoor dashes tend to be shorter bodies because most of that race is the accelerating or speeding up part, where it's an advantage to be short. But outdoors on the track, for example, if you go from 100 to 200 to 400 meters, the specialists get progressively taller as you go from 100 to 200, and then again from 200 to 400.

So, what's remarkable about him, he should be disadvantaged at the start, but he's not. He gets out just as fast or faster than his competitors, which is at odds in terms of muscle physiology and body size as we understand it currently.

BLOCK: What about the age factor for Usain Bolt here? He's 22. That's pretty young for a sprinter?

Prof. WEYAND: It is pretty young. Although in recent years, we've seen sprinters continue to compete well into their early 30s. So he's got a lot of time left.

BLOCK: He could do a lot.

Prof. WEYAND: Oh, yeah. Absolutely.

BLOCK: Professor Weyand, thanks very much.

Prof. WEYAND: Okay. Thank you, Melissa.

BLOCK: Professor Peter Weyand is a physiologist and biomechanist at Southern Methodist University in Dallas.

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