A Brief Anatomy Lesson

There are four main muscle groups that need to be trained for a strong and healthy core.

1) Rectus Abdominis
This muscle is your "six-pack" muscle and runs down the front of your stomach. Their primary function is to FLEX the abs. For example, bringing the chest toward the hips i.e. crunches, or bringing the thighs toward the chest i.e. leg raises.

2) Obliques
These muscles run down the side of your waist. They are activated through ROTATION (e.g. broomstick twist) and LATERAL FLEXION (e.g. bending from side to side).

3) Transversalis Abdominis
This muscle group lies underneath your Rectus Abdominis, and is responsible for sucking your tummy, a.k.a. compression (e.g. when you see a pretty lady walking toward you).

4) Erector Spinae
These are the muscles of the lower back, which help extend the abdominal region a.k.a. hyperextension.

5) * Serratus/ Intercoastals
Although not considered part of the "core", a set of well-developed serratus add a finished-look to any set of abs. They are the fan-shaped muscles that lie just above the rib cage, and they make an average physique look incredible. Exercises that train the serratus include cable (rope) crunches, and dumbbell pullovers.