Originally posted by: sgrinavi
The abdominals are a single muscle and act as one, yes, you should vary your workout, but none of them will target the top or bottom of that muscle. THe resaon you are only seeing a four pack is one of two reasons.
Rectus abdominis is a single muscle, I will not argue that. However, I propose a single test. Do crunches to the point of failure, as many as that takes you, weighted, whatever. Now, do leg raises (again, weighted if they need to be). You should be able to perform the leg raise despite the fact that your abdomin is too weak. Where are you feeling the burn? It is possible that your lower back is helping out in the exercise some, but most likely you will feel a very deep burn in your lower abs.
So how do we explain this if it is a single muscle? I suppose this could be considered range of motion in the abdomin. Anyone familiar with range of motion knows that doing any sort of exercise improperly can cause an ineffective workout even in an isolated exercise. Consistent poor range of motion in a bicep workout will create a non-ideal visual result for that muscle.
There are exercises that will better target and give a more thorough workout of the entire rectus abdominis. The standard crunch is not necessarily the best choice because it isolates out your legs. Something like a bicycle crunch is a much more involved workout since it brings that lower half back into the workout as well as the obliques.