What's your usual working weight? If your back is going out just from doing a warm-up maybe you shouldn't be doing that exercise until you heal more.
As for ab exercises... can someone describe or point me towards a link which describes hanging pikes or discusses them in more detail? I did a search but didn't find anything other than passing references.
On a related note, my favorite ab exercise is one I created myself.
Place a swiss ball (yeah yeah, I know) right next to a flat bench. Sit on the ball with your back facing the bench and lie backwards so your shoulders are lying on the bench -- much like a DB pullover except your lower back is resting on the ball, your hands are gripping the edge of the bench overhead to stabilize, and your ass/legs hanging over the edge.
Bring your knees as close to your chest as you can without flexing your abdominals to maximize your leverage advantage, and then -- without ANY hip movement at all -- curl your hips and lower back up off the ball, then as you relax let your spine drape over the ball again fully before doing your next rep.
This exercise is just great and a welcome change from my usual DB-weighted swiss ball crunches (and of course, squats, deadlift, etc).