Dr. Stuart McGill, professor of spine biomechanics and one of the world's foremost researchers into matters of core and back health recently published an article aimed at personal trainers that breaks down the hows and whys of core training and back health.
This week I'm using this topic for my last educational forum at Equinox, because it never ceases to amaze me the number of trainers I see having clients do set after set of crunches when I know damn well those trainers know better than that.
Yes I know people may demand "cut abs" and still buy into the broscience regarding core work. But we as fitness professionals are supposed to educate our clients, not acquiesce to something we know is counterproductive.
Back to Dr. McGill's article, he states that repeated spine flexion (crunches) are commonly believed to be a good way to train the abs. However the rectus abdominis and abdominal wall do not function optimally to bend the torso, but rather to brace the spine and transfer power from the hips to the upper torso. Or as he puts it a "elastic storage and recovery device."
In other words your six pack is used to stabilize and stiffen the spine, not flex it.
Further, Dr. McGill says that our lumbar discs can only take so many reps of flexion before injury and pain happen, so you'd better save them for tying your shoes rather than endless reps of crunches that do literally nothing for developing a strong, healthy core.
As to why some people can tolerate crunches and some can't blame your parents. We all know people that are naturally lean or strong, or those that have done crunches for 20 years with no problem. Those are the lucky few. Why keep rolling the dice and wasting your time?