The BOSU allows you to isolate and fully engage your core while challenging your stability. This single-leg bicycle crunch can be performed while alternating legs on each rep, or by

working one side and then the other. Your obliques will thank you for this move!

You may perform the plank with oblique crunch while resting on your forearms or with both arms fully-extended. You fire up your entire core while in this yoga-inspired position, but

your obliques will get a great burn here as well. Your entire body will increase upper and lower body strength with regular practice.

If you really want to challenge your abs, remove all stabilizing surfaces and try hanging exercises. The hanging leg raises shown in this video, for example, target lower abs and obliques.

You must develop upper body strength and shoulder stability before attempting any type of hanging exercise. But when you do, you will have endless options to try!

Shemar Moore demonstrates hanging crunches using arm straps in this video. He finishes each set off with a twist--several, actually. Check it out and then give it a try!

Shemar demonstrates straight leg raises with rotations. This exercise engages his entire core while raising his heart rate. Just press play.