Hi surreal,
I've been told, and beleive that using a steadicam correctly, is an excellent way to keep your back in shape.
I too have heard of people claiming back injuries after operating for 2 or 3 years. One guy I know is trying to sell his, because he can't stay "in it" for more than 3 or 4 minutes, and complains that he can't get a steady shot because of the strain. He's has his rig for 4 years. And he's in shape...more so than I.
I'm 5'7", 140lbs, and I can stay in my rig for much longer than that. Sure, it's tiring, but not painful.
I also have met several operators that have been operating for 10, 12, 20 years, and are doing fine.
I think having a proper operating posture helps prevent the back injuries you might hear about.
just my general thoughts
If you are still concerned about it, arrange to spend a day with an operator that has gone to a workshop, or has been operating "pain free" for several years. Odds are they have, and can teach you the correct posture.