Originally Posted by Mahboob Khan
Federal and Nadal's forehands: Watch them carefully. Yes, their hitting arms are straight at contact but bent in the backswing and follow-through.
I think straight arm at contact is not good for every one. It might cause you pain.
Actually, the arm extension is nearly completed before the forward swing for Federer, if not completed... Nadal clearly extends his arm later during swing than Federer does, however.
As for your comment regarding pain, I do not see the point. What would make it more prone to injuries? Hitting can't cause the arm to hyper-extend and, since in both types of hitting position you're making contact with some sort of wrist extension, I doubt there is any more risk for that same issue at the wrist either.
Besides, for consistency, some people find it easier to set up with at more extreme position at the elbow. As stated above, it's one less part that risk moving.