The Range Of Motion
A lot of people are doing a lot of ab exercises but they never target the abdomen. The reason why is because they are just hitting their hip flexors.
Let’s think about how the abdomen works
It opens and closes your stomach region. And in a lot of ab exercises like bringing your feet up, you’ll work your flex muscles if you don’t open your abs at the full range. When doing this exercise in the wrong way, you will feel your abs working, but it’s just a little bit to stabilize yourself.
To really target your abs and to build great abs you wanna make sure that in any movement that you do, it opens up and then it closes. A good way to compare is a bicep curl, when you fully extend your arm (the muscle opens) and then bring it towards your shoulder (the muscle closes).
To build great abs you need to do the exact same way.