[poletopole]

I wonder what property makes that true of a yoga ball? I almost always find myself leaning forward on my desk as if I'm riding a bike. Perhaps it absorbs the stress more than a conventional chair or maybe it's because it's omnidirectional in terms of possible postures? I have one of those Ikea mechanical adjustable height desks (forget what it's called) and wouldn't trade it for the world. My boss used to work standing up, but neither did he program for 12+ hours a day.

[Shatnerz]

I would guess it's because it requires more active stabilization of the back and core and as a result muscle tone and posture either stays the same or slightly improves.

I also find that I'm and moving around more often or changing positions more frequently since switching to an exercise ball. I'm guessing several short stints sitting are better than 4-8 hour sessions.

[moistly]

It takes a fair effort to slouch on a yoga ball. It’s unstable.

There are a number of wobbly chair designs that force one to sit correctly, and that strengthen core muscles. Well worth looking into them, IMO.