Hey!
I'm having some trouble with my reverse crunch... I think I'm doing something wrong. I've noticed that there are numerous version out there, and I've tried 3 of them allready. The typical straight leg lift keeps sort of popping my left knee on each rep (and it seems to be working my quads more than my lower abs). Another version cracks my back on every rep and a third feels like I'm not actually doing anything! Anyway I was hoping you folks might have some pointers. I'm open to sugestions for other lower ab excercises too.

Ps. I have a pilates ball, but thats about it.