Longer toes and a unique ankle structure provide sprinters with the burst of acceleration that separates them from other runners, according to biomechanists.
"At the start of a sprint the only way a runner can speed up is through the reaction force that results from the action of leg muscles pushing on the ground," said Stephen Piazza, associate professor of kinesiology, Penn State. "Long toes provide sprinters the advantage of maintaining maximum contact with the ground just a little bit longer than other runners."
Piazza and his colleague Sabrina S. M. Lee, former Penn State graduate student now a post-doctoral fellow at Simon Fraser University, Vancouver, Canada, studied the muscle architecture of the foot and ankle to look at the differences between sprinters and non-sprinters.
They matched 12 collegiate sprinters with 12 non-athletes of the same height. They measured the distance between the heel and the end of the toes and used ultrasound imaging to measure the sliding of the Achilles tendon during ankle motion, from which the leverage of the tendon can be calculated.
"What we found was that the lever arms (distance between the tendon and center of rotation of the ankle) were significantly shorter -- about 25 percent shorter -- in sprinters," said Piazza, whose findings appeared recently in the Journal of Experimental Biology. "This difference might be explained by a tradeoff between leverage and muscle force-generating capacity."
Because the lever arms are shorter, the muscles shorten less for the same joint rotation. If muscles shorten less, they shorten more slowly, which helps them to produce greater force that more than compensates for the reduced leverage.
While there is little published work on foot shapes and sprinting, previous work on animals suggests that ostriches, greyhounds and cheetahs have feet built for sprinting.
To understand the kind of human foot that would produce a similar sprinting advantage, the researchers developed a simple computer model that could analyze the physiological data they had collected earlier.
"We wanted to see how much acceleration we could get out of the model when we changed the tendon lever arm and the length of the toes," said Piazza. "What we found is that when the Achilles tendon lever arm is the shortest and the toes are longest, we get the greatest acceleration."
Piazza cites other recent research suggesting that shorter toes in modern humans could be an evolutionary adaptation for efficient distance running.
"Maybe our ancestors with longer toes were better sprinters. Or maybe longer toes were selected for at a time when navigating in trees was more important and our toes became shorter as endurance running became more important for our survival," he added.
The Penn State researcher cautions that while the study could be a piece of the puzzle in determining who could potentially be a good sprinter, other physiological components such as body type, cardiovascular physiology and muscle fiber types should also be taken into account.
It is also unclear whether sprinting ability is congenital or whether training can influence the shape of bones in the foot.
"It is not too far-fetched to think that training can help accentuate the shape of the bone," said Piazza. "But if sprinters' skeletal characteristics were shown to be immutable, it would support the coaches' adage that sprinters are born and not made."
The National Science Foundation funded this work.
- Lee, S. S. M. and Piazza, S. J. Built for speed: musculoskeletal structure and sprinting ability. Journal of Experimental Biology, 2009; 212 (22): 3700 DOI: 10.1242/jeb.031096
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