..it could have something to do with the fact that they play so much before their bodies are fully developed??
I don't know, but someone like Jelena is (most probably) still growing, and if she's - as a child basically - exerting the kind of stress on her muscles that could be one reason. Same with the problems Clijsters has.
That's one theory (might be completely wrong though!).
I hope the mri is just precautionary, to make sure for certain that it's nothing to serious.
"Mentally prepare? Why? When you plan something it never works. It's simple: get in, drive car, see what happens."