Check out this video from Mark Young. He shows exactly what can and does happen with countless reps of traditional ab training (spine flexion). I found it on another website and thought it was too good to pass up. It may be a little graphic but it is just a pig spine.
I’ve said it several times that I don’t do traditional ab / core training, as in crunches, situps, russian twists, etc. with our athletes. We train the lumbar spine for stability and anti-rotational strength. To do this we use ISO ab variations (planks), ISO hip raises variations activating the glutes and spinal erectors, pushup variations that move the arms and legs creating torque within the torso, and a countless array of other exercises based on stabilizing the spine, while mobilizing joints around it. If you don’t know my thoughts on the low back you can find some of them in the previous post Quick Notes on the Low Back.
Crunches and such just are worth the risk in my opinion. Not only does spinal flexion create disc problems, but when you look at the big picture the torso doesn’t function like that in sports. I believe it should be trained like it is utilized in sport and in life, for rock hard stability and to resist movement.