A woman stretches out her legs as she sits in grass. She wears barefoot running shoes.It’s the latest thing among avid runners: “minimalist” shoes that approach the way humans first ran—barefoot.

But a new study suggests that runners over the age of 30 who transition from traditional running shoes to minimalist shoes should do so cautiously to avoid injury.

They ”probably need to do it much more slowly, over a longer time period,” said study lead author Dr. Scott Mullen, a researcher at the University of Kansas Hospital Sports Medicine and Performance Center in Kansas City.

He was slated to present the findings this week at the annual meeting of the American Academy of Orthopaedic Surgeons in Las Vegas.

As Mullen explained, with traditional running shoes the heel strikes the ground first. But the focus in the lighter minimalist shoe is to have the forefoot strike first.

In a conventional running shoe, your heel is higher than your forefoot. But a minimalist shoe is flatter, with less drop from the heel to toe.

Complicating things is the fact that the human foot appears to adapt to shoe changes differently, depending on a person’s age. For example, Mullen cited a recent study that found that teens appear to adapt easily to minimalist shoes—changing quickly from a heel-strike to a forefoot-strike pattern when transitioning to the barefoot-like shoes.

His team switched the focus to older runners. Mullen’s group tested 26 runners—all over 30 with at least 10 years of running experience—and found they were not as adept at transitioning to the new forefoot strike pattern favored by the minimalist shoe.

Mullen’s team tested the older runners on the treadmill at various speeds, both in their conventional running shoes and barefoot.

“They ran barefoot because the minimalist shoes are an attempt to mimic barefoot running in a more protected fashion,” Mullen explained.

Runners were tested at running speeds of 6, 7 and 8 miles per hour for women and 7, 8 and 9 mph for men. As they ran, a camera captured their feet’s strike pattern.

“In adolescents, the amount of heel strike decreased with increasing speed,” Mullen said. Not so in adults. “There was persistence of the heel strike in the adults, regardless of speed,” he found. In fact, “43 percent of the adults continued to heel strike barefoot at the highest speed, compared to about 12 percent of the adolescents.”

That means that older runners who want to transition to the lighter shoes may need more time for their feet to adjust.

With a minimalist shoe, landing on the forefoot allows the foot to naturally dissipate the forces of the foot strike better, Mullen explained. But when people don’t change their foot strike from the heel to the forefoot, this can boost their risk of injuries such as stress fractures, he said.

Some of these injuries may only mean a little time taken off from running, but others could be so serious that they require surgery, he said.

One expert who reviewed the study said the findings make sense.

Dr. Cathy Fieseler is president of the American Medical Athletic Association. She said that “adolescent runners are new to the sport and can more easily alter foot strike patterns. That makes sense: anyone new to an activity can modify movement patterns more easily than someone who has been performing the same activity for a long period of time.”

But the study did have its limitations, added Fieseler, who is also a primary care sports medicine physician at Trinity Mother Frances Hospitals and Clinics in Tyler, Texas. She theorized that some runners in the study may have been slower to change their foot strike because they ran on conventional roads, not on a treadmill as was used in the research.

However, Fieseler agreed that a slower transition to the minimalist shoes is probably best. In her clinic, she said she’s already treated people who tried to transition too quickly and gotten stress fractures and tendon injuries as a result.

Older runners who are thinking of changing the type of shoe they wear might also consider a gait analysis to help runners decide which shoe is best for them, Mullen advised. This test is offered at many running stores, in physical therapy departments or in some hospital sports medicine departments.

If your shoe is comfortable and seems to work well, it might be better to continue wearing it, Mullen said. If you do switch to a lighter barefoot type shoe, experts suggest reducing your mileage, alternating running with walking, and running on level surface. Increase your miles gradually, using the ”10 percent rule” so you up your mileage only 10 percent each week from the week before.

Even before training in the minimalist shoes, wear them to do errands or around the house to get used to them, Mullen said. “You probably can’t go too slowly” in the transition process, he said.

Experts note that findings presented at medical meetings are typically considered preliminary until published in a peer-reviewed journal.