Chafing

There are two types of chafing: skin-on-skin and fabric-on-skin. 

Skin-on-skin chafing is where your thighs or underarms rub together. Fabric-on-skin is when your shirt, sports bra, or bottoms rub the skin and aggravate it. 

Chafing is caused by several different factors, including: Extra body weight, loose-fitting clothes, non-breathable fabric, hot or humid weather, sensitive skin, large muscles. Although minor chafing may not seem like an issue, it can turn out to be pretty painful and can hinder your overall performance, even forcing you to stop running in some situations. A little lubrication prevents the friction that leads to chafing.