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Some scientists say we are natural-born runners, that our body has evolved to run. Now, the leading proponent of the so-called human runner school concludes that we do it more efficiently without shoes.

NPRs Christopher Joyce has this story on barefoot running.

CHRISTOPHER JOYCE: Anthropologist Dan Lieberman says human ancestors needed to run well - away from big animals and after small, tasty ones, for example. He based that view on fossil bones.

Lately, though, hes been studying runners, living ones. It started at a lecture he gave before the Boston Marathon. A barefoot runner - someone who runs long distances without shoes - peppered the professor with questions he couldnt answer. So Lieberman took him to his lab at Harvard University. He had him run over a flat, metal plate that measures the collisional force of a footfall. Lieberman says runners generate a lot of collisional force.

Professor DAN LIEBERMAN (Anthropologist): Most runners, when they land and they heel strike - they land on their heel - they generate this sudden impulse, this sharp spike of force. So its like somebody hitting you on the heel with a hammer about one-and-a-half to three times your body weight.

JOYCE: Lieberman was surprised by the extremely low force readings made by the barefoot runner.

Prof. LIEBERMAN: He ran across the force plate and he didnt have it. And I thought, gee, thats really amazing and it kind of makes sense because that spike of force hurts, and I wonder if other barefoot runners do that.

JOYCE: So Lieberman tested several groups of runners: Kenyans whod been walking and running barefoot all their lives, and Americans who grew up walking and running in shoes, and some who switched from shoes to running barefoot. He found that runners in shoes usually land heel-first. Barefoot runners land farther forward, either on the ball of their foot or somewhere in the middle of the foot, and then the heel comes down. That spreads the impact force across the foot, ankle and lower leg muscles. There' s less sudden vertical force shooting up the leg that way.

Also, people who switched from shoes to barefoot running eventually, without prompting, adopted the barefoot style. Lieberman, who runs marathons himself, says the reason is simple.

Prof. LIEBERMAN: Its pain avoidance. Its very easy to do. And your body naturally tells you what to do.

JOYCE: Running shoes dampen the shock of a heel-first landing. So, Lieberman says, thats probably why shoed people run that way. But is that the most efficient way to run? Lieberman thinks not.

Prof. LIEBERMAN: Turns out that the way in which barefoot runners run seems to store up more energy.

JOYCE: To understand how that works, I talked with anthropologist Brian Richmond at George Washington University. He points out that the human foot has an arch with ligaments inside that stretch and contract with every footfall.

Professor BRIAN RICHMOND (Anthropologist, George Washington University): It allows the arch of the foot and the calf muscles to act as a better spring and to store up energy, and then give it back in the beginning of the next step.

JOYCE: Think of a compressed mattress spring pushed down and then released. Richmond agrees with Lieberman that the front-first landing of barefoot running probably capitalizes on that spring mechanism more than heel-first landing - it gets more spring out of the spring.

Richmond, in fact, has discovered fossilized footprints dating back a million and a half years. Those human ancestors who left them had an arch. They were walking when they left the prints. But Richmond now suspects that when they ran, they landed front-first.

Prof. RICHMOND: It looks like this is how our ancestors have been running for a million years or more. Its only been in the last 10,000 years weve had any kind of shoes, really.

JOYCE: Lieberman published his findings in the journal Nature. He received research funding from a company that makes minimal shoes, which mimic barefoot conditions. He adds that he received no personal income from the company. He also says hes not taking sides over which style of running is better or safer.

Prof. LIEBERMAN: I think we have to be really careful about what we do and we dont know. We have not done any injury studies, so this is not an injury study.

JOYCE: That, he says, is next.

Christopher Joyce, NPR News.

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