Mainstream fitness see's the core muscles as these different sets of muscles that aught to be isolated....do this for the lower abs, this for the upper abs, hypertension for the lower back and a machine for the sides and other things in between. They want you to believe that in order to get a good set of looking Abs, you need to break them down into a million pieces like a jigsaw puzzle. Hate to break it to you but unless you have the patience of a monk or into bodybuilding, you'll hate working so much for very little results. The truth is, the Core Muscles are meant to work as a unit and utilized as a whole and not a bunch of little things meshed together. Train your abs using full body movements and isometric holds. The more you learn and physically do that, you'll come to realize how much bullshit mainstream fitness has fed you.
When it comes down to it, Isometrics are one of the most powerful entities in the entire realm of exercise and health. In order to even move our bodies, our Core needs to stabilize and be tightened in order to do our best forms of movement whether it be weights, bodyweight, gymnastics, bridging or anything else. Isometrics teach us a form of Muscle Control that helps us fight off injury and protects the lower back from harm. Although you are flexing the muscles themselves, the key to staying strong and durable is how you breathe. To get the most out of your Core Training specifically is to flex every muscle in the body but not to the point of bursting a vein but enough to where you feel it and being in those moments of controlling the body. Isometrics unlock plateaus that you didn't think were possible and the strongest muscles are our legs and Core.
It isn't the greatest thing in the world to have six-pack abs, but yet it sure as hell isn't a bad thing either. There are however two different types of muscles when it comes to being truly strong and just looking like it; its Counterfeit Muscles & Conditioned Muscles. Counterfeit is when you have nothing but show and you can actually get hurt fairly easily and you're left with bundles of muscles that are crap without function; conditioned is when you are functional, strong and mobile from as many angles as possible, move forward/backward, side to side, diagonal and anywhere else possible and you're less prone to injury and can withstand some hard hitting damage. If you want real six-pack abs, condition them with laser-like intentions and not showing off just because they look like they're carved out of granite.
The biggest aspect of having strong and powerful Core Muscles is that they protect you instead of working against you. They keep harm away for the organs and bone structure. Having protective muscles helps build power in the tendons and ligaments in the rest of the body. Want to have a healthy life full of vigor and vitality; strengthen the body to help protect you. You have only this life in the world, don't waste it on useless training methods, drugs and crappy food. Do the best to your abilities and become as strong as possible having the strongest Core you can possibly imagine.