Cause: During a vigorous workout, ill-fitting bottoms that are too big or too small often migrate to...somewhere in the middle.
Solution: First, make sure that your bottoms provide ample coverage and have four-way stretch so that they move when you do. You've probably discovered that cotton, which can feel so loose and liberating when you put it on, tends to bunch and sag when it becomes wet with sweat—so instead, look for breathable wicking material like Coolmax or DriFIT. Patagonia's Active Hipster briefs are form-fitting without being form-squeezing and have chafe-free leg openings that won't creep up. And the Asics ASX bikini brief has a smooth, supportive fit that prevents cheek creep with a knit weave that feels soft against the skin.