For the last decade or so, core exercises and core training have been the big buzz phrases in the fitness and exercise world. Fitness professionals and fitness enthusiasts alike throw the phrases core exercises and core training around pretty regularly, but few of these people understand what the "core" is (and, truth be told, this is subject to individual interpretation) or what the purpose of core exercises actually are.
My interpretation of the core is any muscle lying between the bottom of your rib cage and the front of your hips (your hip flexors) on the front side of the body, and any muscle lying between the middle of your back and the bottom of your butt cheeks on the back of the body. Basically, all the major muscles of your trunk and pelvis. People often talk about the core as being just your abdominals and lower back muscles, but it also includes your glutes (butt muscles), your hip adductors, abductors, and rotators and your spinal erectors, quadratus lumborum, etc.
The purpose of core exercises and core training is to increase stability in the trunk and pelvis, and also to allow the muscles of these regions to better resist movement. Ab crunches, side bends, back extensions, twists, etc. are not "pure" core exercises. Yes, they "work" many of the muscles contained in the core, but they do not train stability or anti-movement qualities optimally. Furthermore, repeated flexion, extension, and rotation of the lumbar spine (crunches, back extensions, twists, etc) opens up the door for low back (and even shoulder) problems and dysfunction.
With all of the above being said, I really don't think one needs to over complicate core exercises and core training. If we want to stay specific to what core training is all about-stabilization and resistance of movement-and avoid injury and dysfunction, there are really only a handful of movements and variations of these movements that need to done. All of these movements will get your "core" plenty strong and "hard" for any athletic or aesthetic purpose. Here they are...
1. Pillar Bridge: work up to a point where you can perform this exercise with the correct body alignment for 3 minutes. Once you can do this, you can make this exercise more demanding by placing additional weight on your back (sandbags of various weights work well and are comfortable, or you can wear a weighted vest) 2. Side Pillar Bridge: work up to a point where you can perform this exercise for up to 2 minutes on each side from the floor. Once you can do this, you can make things more challenging by wearing a weighted vest, or elevating your feet onto a bench or box. 3. Barbell or Ab Wheel Rollouts: work up to a point where you can perform 20 full range rollouts from your knees with the proper body alignment. You can make this exercise more difficult by doing this from your feet...you may need to start by just lowering yourself, going to your knees, climbing back up and starting again...it takes great strength to come back in from your feet, at least initially. 4. Palloff Press: increase the resistance or narrow your feet to make this harder...just don't compromise body alignment (no leaning or rotating inward or outward) 5. Isometric Palloff Press: same guidelines for the normal palloff press above.
Here is a playlist of all the exercises:
The Isometric Palloff Press is not shown in the playlist...all you need to do is hold the end position (arms extended) of the regular palloff press in a fixed position for time.
In my opinion, these 5 core exercises are really all you need for ab focused core training. Drop the crunch and sit-up variations (or at least cut back on them)...your spine will thank you. If you round out your "core" program with exercises which emphasize the glutes and hips more-deadlift variations, lunge variations, glute bridges, pull throughs, etc-you'll have all the core work you really need.
Finally, remember, without a proper fat loss nutrition regimen, no amount of core training or core exercises are going to "make your abs show" and give you the "6 or 8 pack" everyone is so fond of. People constantly seek out new abdominal exercises in hopes of finding that magic bullet core exercise which is finally going to give them "ripped abs". This exercise doesn't exist, so keep it simple and just implement the 5 core exercises featured and discussed above and focus on improving the strength, function and health of these muscles. Use your diet and overall level of energy burning activity to make your abs "show through".