When you're talking six-pack you're really talking about more muscle groups than one. The abdominals are made up of four distinct muscle groups: the Transverse Abdominis (also called core), the External Abdominal Obliques, the Internal Abdominal Obliques, the Rectus Abdominis (which also happen to be handily divided into upper and lower abdominals). The Master Pack workout takes care of them all.