This classic exercise is bad for your spine and does not work anything but the 8-pack. You should avoid it because it might lead to a hunchback. Remember, Quasimodo was

only considered beautiful on the inside. 

All these do is put necessary pressure on your back. It could lead to irreparable spine damage. Bad, right? Yes. 

If you're doing these, then you're mostly wasting your time. While not SUPER dangerous, they are only working out the side of your body. You want to focus on exercises that work out

more than one muscle group.

The twisting motion of this popular exercise puts unnecessary strain on your back. The rotating screws up all sorts of disks.

Bike crunches are surprisingly cardio intensive and they work out a wide range of ab muscles.

If it has ever appeared in an infomercial then don't try it. It's useless. Shocking, right?

These are awkward and they don't provide much benefit. A lot of the time you will be working your arms more than your abs. 

Once again, these have little to do with your abs. It's more of a total body exercise. Which isn't bad, per say, but there are better total body movements out

there. 

Total core strength is all about balance and anything involving a stability ball will get you there. Crunches on their own just don't help (or hurt) much.

These don't focus on more than one muscle group and they could lead to back pain. Big no-no, right?

These won't put a strain on any no-no zones and they will work your entire core because you're focused on balance. Yay!