Can somebody explain to me (in words of one syllable - why clay is so tough on the body? Also and this isn't a dig against Nadal (for a change), why doesn't it effect his knee injury?).
It's tougher because it's slower. You aim to hit the ball hard and, to achieve the same speed of shot on clay as you do on other surfaces, you need to apply a lot more physical action to your shots - more rotation, more muscle etc.
You also hit a lot more shots - the average rally on clay is probably about ten shots, as compared to about five on other surfaces, so you have to work harder to win each point. Of course, some rallies on clay can be 40+ shots.