Barefoot, five-finger, super-minimal, zero-drop. Whatever joggers embrace as the approach-du-jour for improving form, most of these trends stem from one physiological principal: people who grow up running sans footwear—the way our ancestors did for hundreds of thousands of years—run by landing on their fore- or mid-foot.

A new study finds, however, that not all habitually barefoot runners today actually run that way. The research was published online January 9 in PLoS ONE.

One of the ideas behind barefoot and minimal-shoe running has been that by cutting out our super-cushy sneakers, we will land more gently on the mid- or fore-foot, thereby avoiding a jarringly hard heel strike. This rear-foot landing, along with extra force allowed by the cushion of modern running shoes, many have argued, might be to blame for a host of running-related injuries that tend to plague these fitness chasers.

But the initial research into barefoot runners focused on a particular group of unshod runners—the Kalenjin, of Kenya—who tended to land on their forefeet. The new research investigates running styles in another Kenyan group that also runs shoeless. "The Daasanach people grow up without shoes and continue to spend most of their lives barefoot," Kevin Hatala, a graduate research in Hominid Paleobiology at George Washington University and co-author on the new paper, said in a prepared statement.

Nevertheless, these people, it seems, land farther back on the foot when they run.

"We were surprised to see that the majority of Daasanach people ran by landing on their heels first," Hatala noted. "This contradicts the hypothesis that a forefoot strike characterizes the 'typical' running gait of habitually barefoot people."

The researchers recruited 38 Daasanach adult volunteers (half of whom were female) and had them run down a hard-packed sand track that had a pressure-sensor pad placed midway down the length. Each runner ran across the pad at least three times at a comfortable, self-chosen "endurance" pace and at least three more times at a faster "sprint" pace.

At the endurance pace (three meters per second), 72 percent of landings measured were rear-foot strikes and just 4 percent of landings were on the forefoot. Only once runners accelerated to more than six meters per second did a majority of runners switch to mid-foot strikes. Few (14 percent) landed on their forefoot—even at this quicker pace.

The striking—so to speak—difference between the two groups might be explained, at least in part, by a host of factors. As Hatala and his colleagues point out in their study, the Kalenjin runners from some of the original barefoot running research were athletes and all put in more than 20 kilometers or running per week—more than did the Daasanach in the new study. So a fore- or mid-foot strike might have been necessary to protect the body from the impact of so much running.

Additionally, much of the Kalenjin running was done on hard surfaces, against which they would want to guard for hard landings—perhaps by focusing on the softer-landing forefoot.

And finally, in the original studies, the Kalenjin forefoot strikers also ran at a much faster pace (about five to six meters per second) than did the Daasanach when they tended to land on the rear of their feet (three meters per second). More recent observation, however, has found that the Kalenjin tend to keep up their forefoot striking even at slower paces.

The findings don't necessarily mean that a goal of mid- or forefoot striking for recreational (shod or unshod) runners is misplaced—or that those looking for a more minimal or "natural" running form should opt for a rear-foot strike instead. The research simply shows that there appear to be more than one style of running for people who have grown up running without shoes.

"The challenge ahead is to identify the most important factors that influence how barefoot people run and the healthiest style for today's runners," Kevin Richmond, an anthropologist at George Washington and study co-author, said in a prepared statement.

The new findings might also help researchers who study the evolution of the human gait, in characterizing preferred running styles for activities regarded as important for our ancestors, such as persistence hunting or effective scavenging. "It is not clear which experimental sample, if either, represents a better 'model' for the distances and frequencies of running in early humans," the authors noted in their paper.

In the meantime, the discovery that it might be possible to run for years with a heel-strike—even shoeless—adds another bump in the road to any quest for finding a one-size-fits-all running style.