Track coaches have long claimed that the best sprinters are born, not made. Now, new research on the biomechanics of sprinting suggests that at least part of elite athletes’ impressive speed comes from the natural shape of their foot and ankle bones.
Using ultrasound imaging, researchers compared the feet of 12 top college sprinters with those of 12 mere mortals. Surprisingly, the athletes had particularly short heels and longer-than-average toes — features that actually put them at a mechanical disadvantage when running.
“What we found is that sprinters actually had less mechanical advantage than the non-sprinter subjects that we tested,” said biomechanics researcher Stephen Piazza of Penn State University, co-author of the study published Friday in the Journal of Experimental Biology. “This was surprising to us because we expected that sprinters needed all the help they could get.”
Piazza and his co-author, kinesiology graduate student Sabrina Lee, launched their study after they happened to measure the Achilles’ tendon of a former NFL wide receiver, and were shocked by how little leverage his tendon provided.
“If you think of your foot as being kind of like a wheelbarrow,” Piazza said, “when you grab the handles of the wheelbarrow and pull up, you’re doing what the Achilles tendon does. The longer those handles are, the easier it is going to be to lift up the load. If you had really short handles, you would have poor mechanical advantage.”
Similarly, having a short “lever arm” on your Achilles tendon makes it harder to pull your foot off the ground — which is why the researchers were surprised to find short heels on a professional sprinter. But further research proved the football player wasn’t an aberration: On average, top sprinters had heels that were 25 percent shorter than their non-athlete counterparts, as well as significantly longer toes.
To understand the paradox, the researchers set up a computer model of a sprinter’s push-off. The simulation revealed that despite providing a mechanical disadvantage, the short lever arm of a sprinter’s heel actually produced more force than the longer lever arm of a non-sprinter.
“It turns out that there’s a trade-off that we think is going on,” Piazza said. “The larger the lever arm of the Achilles tendon, the more the tendon has to travel up when you point your toes. What that means is that the calf muscles have to shorten more rapidly, and muscle that is shortening more rapidly can’t generate much force.”
In other words, sprinters sacrifice the mechanical advantage of a long lever for the benefit of a stronger push-off. Since quick acceleration over a short distance is the key to winning a short race, Piazza says the trade-off makes sense for sprinters. “He has to be able to generate a lot of force, but he also needs that leverage,” he said. “It turns out that by giving up some leverage, you actually gain more in terms of force generation and get a net benefit.”
According to the computer simulation, having long toes also makes sprinters speedier, by extending the time that a runner’s foot makes contact with the ground. “Early in the race, the only way you have to speed up is through interaction with the ground,” Piazza said. “If you want to speed up quickly, you need to have some meaningful interaction with the ground.”
But like short heels, long toes come with a cost. Earlier this year, a group of anthropologists reported that long toes are less energetically economical for long-distance running. Led by evolution researcher Campbell Rolian of the University of Calgary, the group found that modern humans have much shorter toes than their early hominid ancestors, suggesting that the need for endurance probably superseded the need for speed and acceleration in our ancient relatives.
“The two studies are actually nicely complementary, and show that long toes provide more power for propulsion, but that this comes at a cost of greater muscle effort,” Rolian wrote in an e-mail to Wired.com. “So there may be an optimal length at which you can get both a capacity to push off and some muscle economy.”
Hat tip: @geekstats
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