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.
When I first approached corrective and performance exercise specialist Paul Chek about writing for Testosterone, his first concern was whether I'd choose to soft-peddle his article. In other words, he was worried that I'd try to "dumb down" his material.