I don't like crunches (which makes me a heretic in Muay Thai circles).
Here's an excerpt from
Alwayn Cosgrove's Blog that I found the other month on the topic . . .
And when it comes to abdominal workouts, I urge you to discard
crunches, and hack them from your program because they are unessential.
In fact, the truth about abdominal crunches is that they are useless, ineffective, and even damaging to your spine.
The
latter fact is covered in great detail by Dr. Stuart McGill in his
book, "Ultimate Low Back Fitness & Performance". His research will
shock you (and the scare the bejesus out of you) if you are one of
those gym-rats who spends 30 minutes on abdominal crunching type
movements.
I found out crunches are useless the hard way,
spending ten minutes per day doing crunches to no avail while in my 3rd
year of University. They didn't do anything for my abs that my regular
workouts of squatting, deadlifting, hockey, and soccer couldn't do.