Let's face it, crunches are overrated. When it comes to working your abs, there are plenty of exercises that work your abdominals and back together. When the front and back of your torso work together, it's a win-win situation for the core. This variation of the quadruped (named such since you're on all fours — get it?) also targets the obliques. Do 15 reps on each side to make a set. Aim to do at least two sets, but three is even better. You can even add a light dumbbell to increase the difficulty.