6. Hyperextensions on Stability Ball: I'm sure you're wondering why this is on the list. Well, when working a muscle group, it's also important to strengthen and work the opposing muscle. If you want to strengthen your abs, it's necessary to strengthen your lower back as well. I personally have a weak lower back, so I feel that this is a doable and necessary exercise so I don't hurt myself while doing the other exercises. When doing hyperextensions, you can place your arms where they are comfortable as long as they don't get in the way of your movement. To make this exercise more challenging you can hold a weighted plate.