While the foot is bare in a flip-flop, it will not have normal function during gait cycle.  In order to keep the flip flop from falling off during gait, one must flex or “scrunch” toes to keep the sandal in position.  While flip-flops are fine for most people, chronic use of flip-flops can create problems with toe deformity and create foot pain.  So if you have significant foot pain and constantly wear flip-flops, you might consider not using them for a while just to make sure they are not causing the problem.  This happened to one of my clients once—when she stopped wearing flip-flops exclusively, all the foot pain went away, and she was able to return to her normal exercise routine.