This past weekend I attended a workshop on training pre and post Natal women. An informative time, if you have a chance to see Annette Lang speak or teach go see her.
One of the topics was core training (yeah yeah I know alot of trainers are rebelling against the "core" label), and the importance of core stability for pregnant women. Naturally as a woman's stomach gets bigger doing movements such as crunches are not only uncomfortable, but may create problems (stomach herniation anyone?).
By the way this goes for any guy with a big stomach too. Carrying alot of internal fat and doing crunches can lead to your guts popping out. Not a good look.
During this I asked a knowledgeable colleague if he still had anyone do crunches anymore, to which he replied no.
I can't remember the last time I had a client do a crunch.
Annette referenced Dr. Stuart McGill, probably the foremost expert on back issues as related to exercise and injury. Dr. McGill doesn't like the crunch either, noting undue stress on the low back, neck, and basically how crunches neglect important anterior core muscles better addressed with other movements.
And I should probably bring up the fact here that doing a million crunches will not get you a six pack, or help anyone "tone" their midsection in any way whatsoever. I'm now confident that nobody will ever ask about crunches again. Right.
So now that we've dispensed with that let's move on to productive core movements.
Woodchop variations, planks, renegade rows, plankouts, Pallof press are all exercises that use not only the rectus abdominus (six pack muscles) but also deeper core muscles that are important in stabilizing the back.
Chances are anyone with low back pain is not as strong as they should be in this area. The function of core muscles such as the Transverse Abdominus is to protect the lumbar vertebrae by not letting them flex. Or to put it another way we need to stabilize the low back, not flex it as happens when you do a crunch.
My own low back situation has gotten alot better since I've focused on stabilization movement patterns, along with getting my hips more mobile.
I've seen Gray Cook, a noted physical therapist, speak a number of times now. He addresses this issue as well as anyone I've ever seen so check it out.