update: just worked abs yesterday. yeah, it's the damn hanging straight-leg raises. i find myself bending my lower back in an effort to tighten my abdominals and help raise my legs. i decided to do my leg raises on a machine like this one:
http://www.e-weightloss-fitness.com/abdominal_exercises/weightloss_diet_1307.JPG
(No... that's not me. I'm not too keen on wind pants anyway)
using this machine keeps my lower back straight, allowing me to exclusively use the abdominal muscles to raise the legs. much easier on my back. i'm guessing that because my lats are flexed in the hanging position, i really can't bend my upper back to crunch my abs together, so all that bending takes place right where the lats do (lower-mid back). just my guess.
give it a shot and let us know.