Here is more info on why you should avoid ab crunches and sit-up exercises.
One other reason I avoid traditional ab exercises like crunches is because the worst thing you can possibly do for your low-back is to get up first thing in the morning and start doing sit-ups and crunches. When you sleep, fluid accumulates into your intervertebral discs making them larger than normal, and the ab flexion motion in sit-ups and crunches can compress these discs and by doing so, you greatly increase the chances of herniating a disc.
I learned this information from Dr. Stuart McGill, an expert in lower back health and performance. He sees a lot of disc herniations from spinal flexion that occurs when you round your lower back. You want to avoid rounding your lower back in all exercises, but that is the exact motion that occurs in crunches and sit-ups.
So skip those exercises.
The first thing for beginners to do to start improving their abdominal endurance and lower back health is to use exercises like the plank and side plank. Those aren’t very exciting exercises, but they’re critical in helping you avoid lower back trouble.
The plank exercise is done when you’re supporting your body weight on your elbows and feet. Your body is in a straight line hovering above the ground, and you maintain that position, breathing normally and bracing your abs. You are going to hold that position, eventually working up to holding it for a full two minutes. Once you can do that, you’ve built excellent abdominal endurance that will protect your lower back.
For back health and performance, abdominal muscle endurance is more important than the ability to do hundreds and hundreds of crunches. Even with the total body ab exercises, I want you to focus on keeping your abs braced—and when I say “braced,” I mean as if someone was going to punch you in the stomach.
Dr. McGill also recommends you totally avoid any type of machine crunch found in most gyms; they just add to the dangers of spinal flexion and increase the chance for injury. Advanced bodybuilders have to weigh the risks and benefits of building ab muscle and putting themselves through movements that aren’t necessarily the safest for the lower back. But that’s up to them, and the results they’re after aren’t really the same as Turbulence Training.
More importantly, everyone basically has “abs,” so the most important factor in being able to see your six-pack is to lose the belly fat that covers them up. You don’t need to risk your lower back health doing hundreds of crunches and sit-ups to get a six-pack.