Jodi Hilton for The New York TimesThe women then returned to the lab, where researchers analyzed their form, foot-strike patterns and the force at which they hit the ground under three different running conditions - with regular running shoes, barefoot and while wearing the Vibram FiveFingers.

The researchers found that half of the women who switched to barefoot running or minimalist sports shoes failed to adjust their form, resulting in more wear and tear on their bodies, not less.

The study showed that when the women were wearing traditional running shoes, they all used a rear-foot strike, meaning they landed predominantly on their heels. But when the women switched to barefoot running or the Vibram FiveFingers, only half of them adjusted their form, as recommended, to a forefoot strike pattern, which entails landing mainly on the ball of the foot. The other half of the women kept the same form whether running barefoot, in Vibrams or in their cushy running shoes - landing first on their heels as they propelled themselves along.

Women who used the correct form experienced lower-impact forces on the foot while running barefoot or in Vibrams. But among the women who didn't change their form and continued to land on their heels, the impact forces created by barefoot and Vibram running were nearly twice as high as in regular athletic shoes.

"People who run, they've run in shoes for so long, landing on their heels, that some of them are going to continue to do that," said John P. Porcari, professor of exercise and sports science. "When you land on your feet, the force gets transmitted up the kinetic chain - to feet, to ankles, to knees, to hips, to back. That's why runners can have injuries from their toenails to their belly button."

Cedric Bryant, the chief science officer for the American Council on Exercise, said the study shows how important it is for new barefoot and Vibram runners to pay attention to form and slowly transition out of traditional shoes.

"Take the slow and steady approach," he said. "Rather than going out and trying to run your typical distances at your typical speeds, give yourself ample time to adapt and adjust to this new style."

The council recommends that runners who want to switch to barefoot or minimalist shoes begin with brisk walking. Once they start running, new barefoot and Vibram runners need to shorten their stride and focus on landing on the forefoot as opposed to the heel. Someone who has been running four or five times a week should try the minimalist shoes for only a portion of one of their runs until they adjust to the change, Dr. Bryant said.

"The key thing our study seems to suggest is that it's really important you take some time to really adjust your running form or running style," said Dr. Bryant.

In addition, the study found that all the runners bent less at the knee while running barefoot or in Vibrams, a change in form associated with fewer injuries. However, compared with barefoot runners, shod runners and those in Vibrams showed more pronation, which is the natural side-to-side movement of the foot during running. Excessive pronation is associated with more injuries.

While the research shows that runners who are able to change their form may benefit from going barefoot or wearing Vibrams, longtime runners who are doing fine may think twice about making the switch.

"If you're not injured, I wouldn't change anything," said Dr. Porcari. "If you're constantly getting injured, you may want to try these things. Maybe, biomechanically, shoes just aren't for you."