Since this series is all about getting stronger and more in-tune with your body, it makes sense that our version of an ab workout is less about looking like a fitness model and more about getting functional from your legs to your stomach — and getting a rock-hard core at the same time. All you need is a sturdy box, a pull-up bar, and a medicine ball. But don't be fooled: While the gear is simple, these moves will challenge every muscle in your body.

When you're done with this lesson, check out Getting Fit From Scratch: Arm Day, Upgraded.