Here is an interesting article giving some reasons as to why Clay gives Andy's knees some problems
http://www.sportinglife.com/tennis/news/story_get.cgi?STORY_NAME=tennis/09/05/13/TENNIS_Murray_Nightlead.htmlMurray admits, though, that playing on clay does tend to exacerbate the long-standing problem he has with his knee.
"The thing is that with my knee, because of the thing I was born with, the bipartite patella, I can't go past 90 degrees and that's not good when my knee goes over my toe," he explained.
"If I'm standing up and I fall forward and I hit whatever it was, the name board, that caused my knee to go forward and that gives me some sharp pains.
"It's a little bit sore. On clay it's worse than on other courts because there is more sliding and what not.
"You get much lower to hit the balls and you're having to hold in wide positions especially when you slide out to your forehand and stuff, so I'm going to have a bit more pain on clay than on other courts."
