By compiling the forces that the foot is exerting on the ground, they found a statistically significant decrease in the vertical (straight-down) force in flip-flop wearers. This decreased force may explain anecdotal evidence flip-flop wearers alter their normal gait and may shed light as to why some experience lower leg pain.
“Flip-flops are not designed for prolonged use or for walking long distances,” says Justin F. Shroyer, lead author on the study. “They lack the support that a walking or running shoe provides. Flip-flops should only be worn casually and for shorter periods of time.”
Shroyer also noted:
• Some flip-flops on the market may provide more support, but are usually more expensive. For someone with lower-leg or foot problems, Shroyer recommends they limit their usage and choose a flip-flop with more support.
• Flip-flops should be replaced often. Despite claims that they are the most comfortable when they are “broken in,” they should be replaced every few months just as a runner would with running shoes.
Flip-flops aren't the only shoes causing our feet to ache. Read High-Heel Stats for the numbers on heel wearers in pain.