Chafing is skin irritation caused by repeated friction, usually from fabric rubbing against skin or skin rubbing against skin, common at the thighs, underarms, and nipples during long runs. It's prevented with lubricant or anti-chafe balm and well-fitted, moisture-wicking clothing, and it gets worse the longer you keep running without addressing it.
Repetitive motion combined with sweat and friction irritates the skin over time, especially on long runs or in humid conditions. Loose or rough-seamed clothing tends to make it worse.
Common spots are the inner thighs, underarms, nipples, and anywhere clothing seams or waistbands rub repeatedly. Longer distances and hotter, sweatier conditions increase the risk everywhere friction occurs.
Apply lubricant or an anti-chafe balm to friction-prone areas before the race, and wear clothing you've already tested on long runs. Race day is not the time to try new gear.
Clean the area gently, let it dry, and apply a soothing, fragrance-free moisturiser or barrier ointment. Avoid re-irritating the area with tight clothing until it heals.