Summer is when the rubber hits the road. We're talking about flip-flops, the ubiquitous sandals that consist of little more than a sole held onto the foot by a Y-shaped thong. Flip-flops used to be relegated to the beach or the locker room, but in the last few years they seem to have become the footwear of choice for a whole generation. Back in 2005 some members of the Northwestern University national champion women's lacrosse team drew flack for wearing flip-flops when they met with President Bush at the White House.

Questions of etiquette aside, flip-flops may not be the best choice for health reasons. In a study presented last week at the annual meeting of the American College of Sports Medicine, researchers at Auburn University found that flip-flops actually alter the way wearers walk. That change in gait can cause persistent foot and ankle pain—the kinds of problems usually associated with a fondness for Manolos and Jimmy Choos.

Justin Shroyer, a doctoral student in biomechanics at Auburn, and Dr. Wendi Weimar, the director of Auburn's biomechanics laboratory, were tossing around ideas for a research project when they hit upon a subject that seemed ripe for exploration. "We're biomechanists," explains Shroyer. "We can't go anywhere without analyzing the way someone walks." They noticed that when students came back from summer vacation they often complained of pain in their feet, ankles and lower legs. The same students were also likely to be flip-flop fans (as is Shroyer, as a matter of fact). Could there be a connection?

To find out, Shroyer solicited 39 college-age female and male volunteers. The participants wore thong-style, flat-soled flip-flops and then regular athletic shoes while walking on a platform that measured the force they exerted when their feet struck the ground. Shroyer also filmed them as they walked so he could study differences in the movements of their hips, legs, knees, ankles, feet and toes.

After digitizing all these images and analyzing the data, Shroyer came up with some disturbing conclusions for those of us who treasure the freedom of flip-flops. He found that flip-flop wearers take shorter steps. The result is more stress on the body because you have to move more to go the same distance as people wearing other kinds of shoes. That could mean a higher risk of muscle and joint pain in the legs.

Toes are another problem area. "When you wear flip-flops, you kind of scrunch your toes to keep the flip-flop on your foot," Shroyer says. That constant pressure often adds up to throbbing and tenderness in the toes. "The body is an amazing machine," Shroyer explains. "When you do one thing, other things turn off and on. By engaging the muscles that scrunch your toes, you are turning off the muscles that would bring your toes up." That also means that you can't lift your foot up as much when you walk—hence the characteristic flip-flop shuffle.