Finally Feel Your Abs
Most people never fully contract their rectus abdominis. Instead, they're using their hip flexors during ab flexion and crunches.
If you can do 30 crunches, your form is off and your abs probably aren't working very hard. Those hip flexors are taking over and doing most of the work.
Don't believe it? Try manipulating your muscles to see how strong your abs really are. The goal is to deactivate the hip flexors. We can do that by activating the hamstrings. (This is similar to the Janda sit-up popularized by Pavel.)
Tie a medium or heavy band around something secure. Then set up for any crunch exercise but have the band pull on the back of your ankles. The band should have adequate tension and you should feel slight hamstring activation. This will ensure your hip flexors turn off.
Now do as many reps as you can focusing on flexing your lumbar spine and squeezing your abs as hard as possible. Aim to get as high as possible without neck involvement, only abs. As you lower yourself continue to squeeze the abs until you reach the bottom of the rep.
If you do this correctly, you won't be able to do too many reps because your abs are lifting your bodyweight on their own. Here are three variations you can try:
1. Hamstring Activated Crunch
This one is the most basic with the easiest set up. It has the least range of motion, but you'll get plenty of ab stimulation.
2. Hamstring Activated Swiss Ball Crunch
This one is more advanced and allows for greater range of motion, plus it throws in some stability work.
3. Hamstring Activated Weighted Crunch
If you want to really challenge yourself, add a plate or a dumbbell. This is a great way to get a lot of ab stimulation in a short amount of time.
After your first set, you'll probably realize your abs aren't as strong as you thought. Do any of these variations for 1-3 sets per workout. After a few weeks, add 1-2 more sets or use a stronger band to further deactivate the hip flexors and improve the mind-muscle connection in the abs.