Are crunches (exercise) bad for spine? & what about planks. I heard they are also bad for in a certain way. What are good ab exercises?

Situps. Setups were condemned after bad experiments. They now are great abdominal exercises shown to be perfectly safe. Be sure to setup to each side to get the obliques . After all, I never heard of a patient blowing a disc on passing a constipated stool. Also, a six pack refers to your abdominal wall not what's in it.