As I'm apt to over-do things, I'd like some guidance in distinguishing acceptable muscle soreness from more serious discomfort. As an example, I wish to lengthen my hamstrings by using the "wall" drill. It hurts well before my back and arms are parallel to the ground. The pain and discomfort are shared between my hamstrings and my lower back. There's nothing at all comfortable about that procedure. As I don't have a mirror in a location that would facilitate guarding against spinal curvature, I just proceed until it hurts somewhere. I wish to "make all deliberato haste" in this method, but I realize that there is much to be undone. Perhaps I could "make do" with just some informed reassurance.