The core is not meant for spinal flexion. The core is meant to protect the spine during movement.
So why are you doing the "crunch master 1000", or "100 sit-ups to a six pack"???
The core is made up 4 major muscles, in this order:
Rectus abdominis (six pack)
The deepest layer and the most important, is the transverse abdominis. It wraps from the front all the way back to your spine. A large portion of low back pain comes from a weak transverse abdominis, causing the spine to not be fully supported. Put it this way, when Aaron Rodgers "puts on the belt" and or does the discount double check, he is actually visually explaining where the transverse abdominis is...
The cure for a weak core is 4 For The Core
"The abdomen, lower spine, and pelvis comprise the trunk (core) of the body. This area must be stable so the limbs have a fixed base from which to create powerful movements. The abdominal and back muscles form a supportive ring around the spine. Soldiers are only as strong as their weakest link; so all these muscles must be trained in a manner that mimics their function." -ArmyPRT.com
How to ACTIVATE your transverse abdominis: Lay on the ground with your knees bent and both feet on the ground. Actively press your lower back into the ground. Actively pull your rib cage down. Actively draw your naval (belly button) into your spine. Got all that? Great! Now flex your core hard as if you were bracing to be punched in the gut... Now that you have that down, you must be able to do that on these 4 moves:
1) The Bent Leg Raise: Lay on the ground (activate your transverse abdominis) with your feet in the air, knees slightly bent, head 1 inch off the ground with your chin slightly tucked. Put your arms at a 45 degree angle from your sides and or slightly under your low back to feel that you are pressing your low back down. Extend your legs away from you, keeping your knees slightly bent, and hold for 30-60 seconds. If you cannot maintain your transverse abdominis activation, keeping your low back on the ground, then bring your knees back over your chest and rest for 3-5sec and go right back out.
2) Glute Ham Bridge March: Lay on the ground (activate your transverse abdominis) with your knees bent and both feet on the ground. Arms at 45 degrees from your sides, head on the ground. Pick your toes up and drive your heals in the ground. Slowly extend at the knee one leg at a time, in a marching manner for 30-60 seconds. And yes, keep your transverse abdominis activated. You may rest your hips for 3-5 seconds on the ground at a time.
3) Side Plank: You will do 30-60 seconds on each side. With the left side, put your left elbow under your left shoulder and actively pull your shoulder away from your ear, don't relax your shoulder as if you were at the beach. Right foot in front of left foot, or cross it on top. Put your right hand across your navel. You should have a straight line from your heal to your shoulder, and your head in a neutral position. Pick your hips up and hold for 30-60 seconds, resting for 3-5 seconds if unable to keep your transverse abdominis activated.
4) The Quadraplex or "Bird Dog": The Quadraplex or "Bird Dog": On your hands and knees. Hands undershoulders, knees under hips. Extend opposite arm and leg, keeping your toepointed straight down. Head in a neutral position with your transverse abdominis activated! Hold for 30-60 seconds, rest for 3-5 seconds when needed.
These four movements will work on core stabilization without overworking your iliopsoas muscle. Any "sit-up" that you do it strengthening your iliopsoas, not your core. If you are looking togain a six pack, look no further than your own diet. The "six pack" is made in the kitchen. A strong, stable core that will promote good back healthfor a lifetime is made with 4 for the core. Check out this video to understand the iliopsoas muscle and it's function in the sit-up: