A strong core is important for efficient movement patterns and good musculoskeletal health. But having good core control does not mean you have a six pack, or even that you want a six pack. Despite all the hype every year about getting ready for swimsuit season, the muscles that give a defined six pack are superficial, and are not an indication of good core control or strength.
The core includes the muscles all around your trunk, including the transverse abdominis (deep core), internal and external obliques (side of the trunk), which you might expect; but also includes multifidus (deep back muscles), erector spinae (vertical back muscles), the diaphragm, the gluteus maximus (butt), pelvic floor muscles, and the trapezius (top of the back).
Weak core muscles can have a trickle down effect, causing lower back pain, knee pain, or IT band issues. Without your core muscles working together muscles imbalances will occur and dysfunctional movement patterns may result. So don’t focus on getting those superficial six pack abs, focus on strengthening the whole core, so you can move better, feel better, and do more of the activities you want to do.
For more information on the importance of the core, check out this article.