Need a safe exercise for stomach muscles

At 2 months post op I started back exercising.

I started out slow and graduated into 40 minutes of walking and 20 minutes on the elipse. I noticed that the muscles in my lower abdomin felt a little twingy toward the end of my workout.

I decided to start doing a few crunches for my tummy. I didn't feel pain, just those same lower abdomin twinges.

After a week of working out my back went out on me. I couldn't do anything without a lot of back pain. I went to 3 Doctors and they did nothing for me except give me pain pill and tell me to rest.

Well, my back pain is gone, but my weak stomach muscles are still here. I don't want to hurt my back again, but I really need to get back exercising.

Any ideas?

I feel good after doing the following "light" stomach workouts:

In bed (at night or in the morning):
Lying on your back, suck your stomach muscles in ... hold and breathe. Do this several times. Isometric exercise!

While walking, do the same thing. I am walking 30 mins. to 60 mins. (not every day). I suck my stomach in while walking and hold and breathe. I do it as often as I can think to do it, without doing it the whole walk. This results in my standing very straight - think "stomach in, chest out!" I find this helps strengthen the muscles. I won't attemp crunches yet (I am in my 7th week post op).

I've experienced the bad back pain (from back muscles compensating for the weak stomach muscles) and I notice it happens (both times) on days when I do a lot of standing or even sitting at the computer and carrying things (even though they may be light) and do not rest between these activities - either for a good clip at night (on the bed or sofa with my heating pad on stomach, so nice!) or for short rests in a recliner.

This morning my back was a little stiff - so I did a little gentle stretching (brining my legs up, holding backs of thighs with hands) to get some kinks out.

I highly recommend the balance ball for draping your body over it and getting good stretches for the core while being supported - try that. My fiance claims it cured his back pain (he does stomach crunches while sitting on the ball).

Good luck.