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.
The farmer's walk is as old school as it gets – just grab a pair of heavy weights and walk! But with just a few technical tweaks, you can take the farmer's walk to the next level of safety and effectiveness.