Pavel Tastouille (SP?) published an ab exercise I like very much. You need a partner or a special apparatus to do it. It was developed by someone with a doctorate and lots of credentials out of Europe. The technique is to get into a crunch position and have someone or something hold on to your calves. Now clench your glutes and press your legs into your partner's hands while executing a crunch. What's happening is this: When you push your legs down you activate your hamstrings and glutes. These are the antagonist muscles to the deep hip flexors that you activate during a situp. Because the activation of an antagonist disables the agonist, the deep hip flexors relax when you push your legs down. This leaves the rectus abdominis to do the work, and boy does it work. This version of a crunch/situp is truly the isolation exercise for the abs. I've done cable crunches, incline situps, weighted situps, etc. This is the king. It has a specific name that I cannot recall. Google "Pavel" and "situp" or "crunch" and you'll likely find it.