A flip-flop is a flat, backless, usually rubber sandal held loosely on the sole by a Y-shaped strap that passes between the first and second toes and around either side of the foot. They do appear to be the preferred summer footwear, particularly among young people. And you're right - they aren't the best kind of foot protection for daily or prolonged wear. A study presented at the 2008 meeting of the American College of Sports Medicine (ACSM) concluded that wearing flip-flops repeatedly can lead to leg or foot pain, probably because they can alter an individual's stride and do not provide arch support. Researchers compared flip-flops and sneakers in 39 men and women aged 19 to 25 to evaluate the angles and force with which the feet hit the ground. They found a statistically significant decrease in force with flip flops and concluded that this difference may change normal gait in those who wear them. This may explain why some people who wear flip-flops experience lower leg pain.
Justin F. Shroyer, a graduate student at Auburn University in Alabama who was the lead author of the study, cautioned that flip-flops are not good for prolonged use or for walking long distances; instead, they should be reserved for casual use and worn only for short periods of time. What's more, if you do wear them, replace them every three or four months, Shroyer said. (Do the same with running shoes.) If you notice any leg pain and still want to wear flip-flops, he advises looking for a pair with more support. While they're likely to be more costly, flip-flops with support may help you avoid disturbances of gait that might lead to pain.
By the way, another study presented at the ACSM meeting found that wearing high heels when you walk down stairs can cause an unstable gait that might result in actual injury to the leg or foot.
Andrew Weil, M.D.