Have you ever felt your hips when you perform a crunch? Typically, people who are performing a crunch incorrectly will feel their hip flexors activating throughout the movement, and the residual burn afterward. But if you feel your hip flexors, are you really working your core?
The purpose of your Rectus Adomninis (6-Pack, everyone has one; shown in red) is to provide flexion, i.e. bending, of your body's trunk (torso) and lumbar spine. When you perform flexion through your Rectus Adominis, you'll notice that the opposite reaction is a rounding of the mid/lower back. Therefore, when doing any type of crunching motion, I focus on bringing my ribcage down to my pelvis and making sure I am rounding through the mid/lower back and not bending at my hips. You'll notice that the range of motion is much smaller.
The other thing to note is that you can only work muscles by how they attach at their insertion points. Because the Rectus Admoninis attaches in two spots (pubic crest & xiphoid process), any time you perform a crunch you target the ENTIRE muscle group. Flexion of your trunk happens either by bringing your ribs down to a stable pelvic position (crunch) or by bringing your pelvis up to a stable ribcage position (reverse crunch/leg raise). It is not possible to target your "lower abs" or your "upper abs." Sure, you may feel the exercise in one area compared to another, but you are working the entire Rectus Abdominis.
Next time you do some ab work, pay attention. What's working? Hips or abs?