Yes this is one thing that is very interesting to me as well
I was watching an episode of the simpsons where they going skiing and marge gets a broken leg from the clock that falls off the wall. After they take the cast off her leg was full of coarse dark hair and the Doctor made a joke and said to order 1000 cc's of leg wax or something like that
I have also heard from some friends who had a cast on that their hair gets coarser and darker after it gets taken off
I wonder if people who wear alot of hats and beanies have the same effect....