Originally Posted by Clarky21
Clay is not hard on the body at all. It's the best surface to protect the joints on. Hardcourts are joint killers and is the surface that is the hardest on the body.
Hard courts result in more shock as the surface is harder and there is more twisting on the knees and ankles as you don't slide when changing direction.
But, clay court matches tend to turn into long grind fest which is very tough on the body too. Many greats have said that the French is the most challenging to win from a physical perspective as the points and matches last a long time with many more strokes played. Grass is the best surface for the body. Soft surface, quick points that reward shot making instead of grinding groundstroke play.
If Nadal's knees are not near 100%, even clay will be very tough on his knees.