People began running way before there were any shoes, but that was a long time ago. It can be very difficult for a human foot that's worn shoes all its life to adjust to running without them.

That's why runners interested in switching to minimalist running shoes should make the transition very slowly to avoid injury. It may even take a whole year to adjust, according to a Brigham Young University study.

In the study, 10 weeks was not enough.

Minimalist running shoes are a compromise between running barefoot and traditional running shoes. They're thinner and more flexible than regular running shoes. They also offer less cushioning and shock absorption than shoes with thicker soles.

Ten of the 19 runners switching over to the five-finger shoes showed significant edema, including two runners who had stress fractures.

One type of minimalist shoe, called a barefoot shoe, has a sole that is only a few millimeters thick and a zero heel drop — the heel of the foot is no higher than the toes when wearing the shoe. Traditional, heavily cushioned running shoes have a 10-12 millimeter heel drop, or about half an inch.

The Brigham Young study looked at the effect of switching to five-finger running shoes, a barefoot model that has separate slots for each individual toe. It included 36 experienced runners who had been running between 15 and 30 miles a week in traditional running shoes. Seventeen continued to run in their usual shoes while 19 transitioned to running in five-finger shoes over 10 weeks.

The transitioning was done along lines suggested by the shoe manufacturer: one short (1-2 mile) run in the new shoes the first week and an additional short run each week, so runners ran at least three miles in the new shoes by the end of Week 3. The runners were then told to add mileage at the rate they felt comfortable with for the remaining seven weeks.

Runners' feet were scanned for inflammation and possible stress injury to foot bones using magnetic resonance imaging (MRI). The images helped researchers measure bone marrow edema or swelling.

Ten of the 19 runners switching over to the five-finger shoes showed significant edema, including two runners who had stress fractures. Only one member of the control group had significant edema.

Also, almost all of the runners in the minimalist shoe group were running fewer miles at the end of the 10 weeks than they had been at the start. Researchers suspect that this is because their feet hurt.

Rather than taking their results as an indictment of minimalist or barefoot shoes, the study's authors think it shows how difficult the transition to them can be. Even the substitution of a single mile in the barefoot shoes in the first week is possibly too much. More research will need to be done.

Both barefoot running and running in minimalist shoes encourage landing on the front or middle of the foot. Traditional running shoes encourage landing on the heel. When the foot landing zone changes, the forces that the foot and the entire body feel change as well. Scientists are just beginning to understand how these changes affect individual body parts.

Among non-scientists, there are passionate adherents for both heel strikes and for forward strikes, for traditional shoes and for running barefoot.

Only some of the runners in the Brigham Young study who switched shoes developed serious foot problems. Why some did and others didn't isn’t yet known. The researchers are analyzing additional data from the study to see if any personal characteristics make runners more prone to foot injury or possibly protect against such injuries when switching to lighter footwear.

The study was published online in Medicine & Science in Sports & Exercise and will also appear in a future print issue of the journal.