I've been stretching for ages when I first caught wind of 10 P. I'm a noob, but I can hit sorcerer (yay for flexibility), and never ONCE had knee problems doing RG, or any RG techniques..Idk what those people are doing that they're injuring themselves, you just have to follow each step exactly. I think maybe they just torque the hell out of their knee without ever stretching a single day of their lives when going for RG, that's the only thing I can think of when it comes to knee injuries from RG.