In fact, most crunches work the lower
back as hard as they do the abs
, and all that twisting can be a strain. Not that anyone knows how to best work the abs, though, as far as I can tell. For six pack abs, quite frankly, most who have them don't get them through exercise -- it's a sure sign of steroid use, since the belly should have some fat. You'll never see agricultural or hunter-gatherer cultures with flat stomachs. Other than this, the above post is right on.