There are many runners who are not aware of this or refuse to act on this fact. Running shorts that have built-in underwear are to be used without underwear. The liner inside already provides the support that you need and by wearing an extra pair of underwear you are risking chafing. Furthermore, liners are created from a fabric that has great moisture management properties. This means that by wearing an unsuitable type of underwear you are simply not using these properties. Also, the underwear liner is built to prevent bacterial infections and keep you dry and comfortable.