[header] How to stop your legs from itching when you run [title] Change your detergent or fabric softener. [step] Chemicals in the detergent or fabric softener you use may be irritating your skin. Even if you haven't had any problems before, when your skin is warm and you're sweating, you may have an increased sensitivity. 
[substeps] Switch to detergents and fabric softeners designed for sensitive skin, or that contain no dyes or perfumes. Typically you can find these wherever you normally shop for laundry supplies at the same price as the detergent and fabric softener you usually buy.