Crunches are not "functional" (how often do we fold ourselves in half in the real world?) so they don't really train the core muscles much. Just rectus abdominus "six pack" muscle. Planks (and Pilates, and Yoga, and dance) are more functional, kick in your transversus abdominus, and get your lumbar and pelvic stabilizers and slow twitch postural muscles all warm and happy.

At least, that's how I rationalize my distaste for crunches!

When I want to really make myself tired I do bridges with my feet on a physioball. Straight leg, bent leg, and curling-toward-you. I can only do 5 or 10 of each. I aspire to 20 of each. My abs and hamstrings cry out for mercy.

And I aspire to the next step in the plank progression: side planks where you lift and lower the top leg. I can't do that yet at all.