Posted by Dave on July 28, 2011 | 25 Comments
The speeds attained by world-class sprinters like Usain Bolt are simply unfathomable to me. During today’s workout, I ran at nearly top speed for a set of 400-meter repetitions: About a 6-minute-mile pace, or 10 miles per hour. Sure, I could probably hit 15 mph over 50 meters or so, but that would be about it. Bolt, on the other hand, attained a pace of .82 seconds per 10 meters during his world-record sprint: 27.3 miles per hour, or nearly twice as fast as I can run.
But this article, which has been making the rounds lately, suggests that despite Bolt’s prodigious speed, his legs are moving no faster than my own:
When Bolt established the current 100-meter world record in the 2009 world championships, running it in 9.58 seconds, he did so by moving his legs at virtually the same pace as his competitors. In fact, if you or I were to compete against Bolt, our legs would turn over at essentially the same rate as his.
How could that be? Don’t Bolt’s feet need to contact the ground during his run? And isn’t the ground moving past his feet at nearly twice the rate it does when I’m running at top speed?
The Yahoo Sports article I quote here mentions a 2000 study led by Peter Weyand, so I decided to look up the original study and see if Yahoo is getting it right. Indeed, the study’s title suggests that the Yahoo article is correct: “Faster top running speeds are achieved with greater ground forces not more rapid leg movements.” But it still didn’t make sense to me that the fastest sprinters move their legs no faster than ordinary runners, so I gave the article a close read. What did I find?
As you might expect, it’s a little more complicated than the article’s title suggests. To be fair, the Yahoo Sports article touches on the key issue: The length of time a world-class sprinter’s legs are on the ground: .08 seconds for a sprinter versus .12 seconds for an average runner. That 33 percent difference explains pretty much the entire difference between an Olympic athlete’s pace and an average runner’s pace. But it’s still not clear how Bolt’s legs could be moving the same speed as an ordinary runner. To understand what the researchers are talking about takes a bit more explanation.
Take a look at this figure from the original study:
The graph at the bottom shows the amount of force the runner’s foot applies to the ground with each step. It looks like the runner is spending about half the time in the air, but in fact each foot spends most of the time in the air, because the feet alternate. So when you look at the full range of the stride for each foot, in this case the foot is on the ground for about 180 milliseconds, but off the ground for about 450 milliseconds. Most of the time, your foot isn’t propelling you forward at all, it’s just getting ready for the next stride.
What Weyand’s team did in this study is recruit 33 runners of varying ability, from average to very fast. They set up each runner on a treadmill with a harness suspended from the ceiling to keep them from being launched off the back of the machine if they fell down. Runners held themselves over the belt using the handrails while it was brought up to speed, then lowered themselves down and ran, trying to match the speed of the belt. After they had successfully run at least eight steps, they stepped off the belt and the speed was increased. This was repeated until the belt was going so fast that the runners just couldn’t keep up. I’d love to see the video outtakes from this study, because I imagine there were a lot of stumbles and falls (though the researchers report that the harness setup was effective in preventing serious incidents).
For a typical runner, the results looked like this:
This graph shows the length and frequency of strides. As you might expect, the faster the runner went, the longer and more frequent the strides. Bolt takes about 41 strides to run 100 meters, while his competitors take more like 44. Fewer strides over the same distance means each stride is longer.
But the key to this research is found here:
These graphs compare all the runners, when running at top speed. In Graph B, we’re looking at the amount of time the runners’ feet are in contact with the ground with each step, and as you can see, the faster runners spent significantly less time contacting the ground. In Graph C, we’re looking at the time the runners spend airborne, with neither foot touching the ground. In this case, there’s no significant difference between the faster and slower runners. But if you add these two measures together, you do find a difference between faster and slower runners. The total time it takes to take a step is less for the fastest runners. What’s surprising is that this difference is fairly small—only about 16 percent, even though the fastest runners go about 80 percent faster than the slowest runners.
So while Usain Bolt doesn’t bring his foot forward any faster than I do, when his foot is on the ground, there’s no question that his leg is moving faster than my leg does. That means his stride rate is a little faster than mine, and when combined with the fact that his strides are much longer than mine, he can run a lot faster than I do.
So since Olympic sprinters spend most of each stride with the foot not in contact with the ground, it is true that most of the time their legs are not moving faster than ordinary runners. However, when it counts, they do indeed move much faster!