Getting your heart rate up is pretty much the definition of exercise intensity. I don't want to second guess your doctor, but it is possible that he wasn't meaning intensity in this sense of the word. He may have been meaning just exercise that is easier on your body (especially your back).
Next time you talk to your doctor, ask him exactly what he meant, and perhaps what sort of exercises you can and can't do safely. The impact of running is an obvious no-no, and it sounds like the twisting and the like involved in Zumba is also causing you problems? A couple of possibilities you could ask him about:
* Adding some hills to your walking route will increase the heart rate (and the calories burned) of your walking, without changing the motion (and your back seems OK with walking at the moment?)
* The stationary bike/spinning might be another option that allows vigorous exercise without too much back movement.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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