when trying to get maximum definition in a muscle. one must really hurt that particular muscle with the build up of lactic acid, when you train your shoulders you feel pain...likewise when training abs you have to feel pain, incredible pain, the best exercise for this is crunches (for me anyway). no excuses, if you want good abs you have to hurt them, there are no shortcuts...forget machines, resort to crunches, try 5 sets of 80 2 min rest between each set twice a week...your abs will hurt...meaning that you have worn down muscle fibres thus causing growth to occur and your abs to eventually get the way you want them...there arent shortcuts, just hit your abs hard.