Use these two exercises as a progression from the ab wheel. The videos don't do them justice. You've got to try them to understand it.
Keep these in a low to moderate rep range, multiple sets of 5-8 reps. Going high rep on them can make the low back kick in. But you've been warned: This will leave even veteran lifters shaking like a leaf on a tree.
Not everyone was meant to squat deep, but a simple test will tell you how deep you can safely go. And if "butt wink" is a problem, there are easy ways to fix it.
Prior to the early 90's, hardly anyone used scientific references to support their notions – not in real life and certainly not in the magazines. You just said what you believed and most of the time no one questioned you.