" In the past, before the physiology of abdominal training was well understood, bodybuilders used to do a lot of “conventional” abdominal exercises such as Sit-Ups and Leg Raises. Unfortunately, those are not primary abdominal exercises but instead work the iliopsoas muscles—the hip flexors. "

― Arnold Schwarzenegger , The New Encyclopedia of Modern Bodybuilding: The Bible of Bodybuilding, Fully Updated and Revised