
There are a ridiculous number of crazy abdominal exercises with an attempt to increase the definition or (dare I say...) "tone" of abdominals. Most of these exercises are done at the expense of "shredding your back/spine" (remember...that thing in your back that many of your core muscles attach to?) through repetitive flexion.
One thing to consider that the abdominal muscles (or core muscles as a group) are composed mostly of slow twitch muscle (as are most postural muscles). This means they are great for endurance, but not great for producing power or speed. So training with repetitive fast movements is more to a detriment than a benefit. Slow twitch muscle does not respond as well to increased loading, so adding weight to abdominal exercises will marginally increase hypertrophy of slow twitch muscle.
