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Washington - Harvard biologist and runner Daniel Lieberman had a simple question: "How did people run without shoes?"
The answer he got is: Much better.
At least running barefoot seems better for the feet, producing far less impact stress compared to feet shod in fancy, expensive running shoes, according to a study by Lieberman in Thursday's issue of the journal Nature.
The study concludes that people seem to be born to run barefoot.
The research was funded in part by a company that makes minimalist running shoes that try to mimic barefoot running. But Lieberman, who disclosed the grant, said the company had no say in the design of the study and didn't influence the outcome.
People who grew up running barefoot - such as boys in Kenya's Rift Valley province, which is known for endurance running champs - tend to land mostly on the front or middle of the foot when they touch ground. And when these runners do use shoes, they continue to run in that way.
People who have always worn cushioned running shoes usually hit the ground heel first.
The difference in the way the foot strikes the ground is important. Lieberman's study examined the physical stresses on feet with different types of running and found that people with running shoes strike the ground with the mass of the entire leg, nearly 7% of the body. That is more than three times the weight of impact for barefoot running.
"It's really about how you hit the ground," said Lieberman, who specialises in human evolutionary biology. "When you hit the ground, some of your body comes to a dead stop."
For runners in cushioned shoes, "it is literally like someone hitting you on the heel with a hammer," Lieberman said. But, he said that "the way in which barefoot runners run is more or less collision free."
But runners should be cautious about ditching their shoes or using new ones that mimic barefoot running, Lieberman said. If you change the way you run quickly "you have a high probability of injuring yourself," he said. In general, changes either in running shoes or distance should be no more than 10% a week, he said.
The study does not look at injuries from running and does not conclude that shod runners are injured more often. That specific research should be looked at next, he said.
Lieberman has looked at the evolution of long-distance running; 2 million years ago our pre-human ancestors used that approach to wear out prey during prolonged hunts. He found that the 1970s invention of the modern running shoe changed our strides. And it was not necessarily for the best.
The study turned Lieberman into a barefoot-running convert, weather permitting.
"We did not evolve to run barefoot in New England in the winter," he said, referring to the northeast United States where winters can be severe. Yet, he said hard surfaces, glass, nails and concrete are not a real problem for barefoot running. Acorns are.
Dr. Pietro Tonino, chief of sports medicine at the Loyola University Health System in Chicago, was not part of Lieberman's study but said it makes sense because of what he sees every day.
"When you look at runners, the most common thing they have is, in most cases, heel injuries," Tonino said. The No. 1 foot injury that Tonino sees is plantar fasciitis, a painful irritation and swelling of the bottom of the heel.
Tonino said cushioned running shoes work against evolution which developed the foot properly for endurance running.
"Your body is basically just very passive in the running activity compared to probably what it was designed to do," Tonino said.
Tonino does not recommend barefoot running for Americans who have gotten used to modern running shoes, but said for people who do not have foot injuries, less constrictive shoes might be a good idea.
For his part, Lieberman said barefoot running "is a movement that ain't going away."
