Friction, or rubbing, caused by tight clothing and repetitive pressure (such as backpack straps rubbing against shoulders) can lead to a type of acne known as ‘acne mechanica’. This type of acne can occur anywhere on the body, although it is most common on areas that are more frequently subjected to friction, such as the back, shoulders, buttocks and forehead (when tight hats are worn). Tight fabrics, sweat and heat (caused by excessive rubbing) can block hair follicles and create a warm, moist environment for bacteria to multiply in. This combination of blocked hair follicles and multiplying bacteria can lead to acne forming.
Acne mechanica is often referred to as sports acne, as it is common amongst athletes due to the repetitive heat, sweat and friction that their skin is exposed to. It is also common amongst students and soldiers as well.