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.
If you're a bodybuilder and you've spent your weightlifting life doing sets of 8,10, 12, etc., doing singles will absolutely blow you up. A terrific idea and a terrific program that'll raise you head and shoulders above the competition.