"People who don't wear shoes when they run have an astonishingly different stride," said Daniel Lieberman, Harvard University researcher, in a statement.  "By landing on the middle or front of the foot, barefoot runners have almost no  impact collision, much less than most shod runners generate when they heel-strike."

Specifically, researchers analyzed the foot strikes and running strides of runners in the United States and Kenya, using high-speed cameras, scales and 3D motion analysis.  Each participant ran at least 12.4 miles per week, with comparisons of runners who always wore shoes and those who did not.

They looked for the amount of impact barefoot runners generated by landing heel-first and forefoot-first, while runners with shoes had heel-first impact measured.

Barefoot runners are less likely to be injured because they land on their forefoot and mid-foot, as opposed to runners with shoes more likely to land on their heels.  The increased impact caused increased torque on the hip and knees, with a higher likelihood of tibial stress fractures and plantar fasciitis.

"So it's like someone hitting you on the heel with a hammer, about 1 1/2 to 3 times your body weight," Lieberman noted when runners land and heel-strike.

Running barefoot may have worked during testing, but obviously isn't feasible in most places in the United States.  The sidewalks and streets are littered with shards of glass, trash and other debris that could potentially cause injuries to runners.  Some tracks have signs up telling runners to wear footwear while on the track.   

Vibram fivefinger shoes (VFFs) have increased in popularity over the past 12 months, with supports noting how much more comfortable, natural the VFFs feel when compared to regular running shoes.