So I started doing these for the first time last week, and I've been having lots of soreness behind my knees. Until now, I had been doing leg curls, hyperextensions, and things like that for hamstrings. I was always told to avoid stiff leg deadlifts because they're dangerous. But now, after practicing my form, watching countless hours of youtube instructional videos and whatnot, I started doing them but now I'm really sore right behind my knees. Not muscle soreness, but inflexibility soreness, if that makes any sense. Do any of you do any stretching or anything that loosens up that area? Thanks