Apparently short heels and long toes are a surprising recipe for speed — if you don’t understand mechanical advantage:
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.
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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.”
Have these researchers ever driven a stick-shift? High-gear isn’t faster off the line.