i go to the gym as often as i can...been slacking lately, but anyways. i used to exersize my abs directly, but i got to a point where it just started to hurt around my site, so i've stopped exersizing them directly. (ie situps, curls) but keep in mind i was using quite a lot of weight/resistance.
if you kept the weight to a minimum it wouldn't be a problem at all....BUT, then what's the point, right? which brings me to: exercizing them indirectly...when you walk, or lift or do anything really, you're exersizing your abs. i think indirect is just as effective as low resistance direct, and you don't waste your time on the direct exersizes. so everytime you use a machine or weights at the gym for your arms/back/legs, you're using your abs too! you won't get a toned 6pack, but the strength will be there, i lift up and carry 100-120lb bags of apples & grain on my shoulders pretty often without issue.
for more strength in your back, try some reverse situps. your gym should have a machine where you can select the resistance. or try that machine where you sit and pull down on a bar, assisted pullups, (arms wide) dips etc. those are all good indirect ab workouts too. something else you could try, stand vertical, then bring your chest forward until it's perpendicular to your legs keeping your head inline with your back. your arms are now parallel to your legs. then back up to a standing position. add some dumbells for some resistance.
anyway i hope this makes some kind of sense lol
Dec 2006: Proctocolectomy/permanent ileostomy
Since 1991: Indeterminate crohn's/ulserative colitis