This is Frictional hypermelanosis, it is an uncommon condition. The clinical finding is characterised by asymptomatic, diffuse, brownish patches located mainly in the skin above bony prominence's. Dry skin will aggravate the condition and make it look more patchy. Consult a Dermatologist for the same.
Health Tips
keep the skin well hydrated with a lot of Medicated moisturisers. avoid Dettol or Savlon application