a "kneeling chair" (http://en.wikipedia.org/wiki/Kneeling_chair) is a similar solution that keeps your spine straight but doesnt put stress on your legs. i have a friend who has one and he loves it
I helped my dad start an office furniture store back in the 90's, so I learned a bit about ergonomics by osmosis.
The central problem with kneeling chairs is that one of the most important ergonomic principles is that you have a natural (and necessary) propensity to shift positions every few hours. Kneeling chairs don't allow you a range of positions the way that conventional chairs, exercise balls, standing, etc. do. The ergonomics people I knew always went out of their way to say kneeling chairs were actually a bad idea.
Having said that, whatever works for you, works for you. But for people considering a kneeling chair, you want to be careful not to just try it in the store for 10 minutes, then buy it.
I had a kneeling chair once. They are great if you have a regular chair to alternate with every few hours, but kneeling for an entire day gets hard on the knees. The main benefit for me was to switch between my kneeling chair and my regular one every 3-4 hours.
That was the point. You have to build up the small muscles that support your spine. Until that has happened, your lower back will hurt ... but that is a good sign. Once the muscles have developed, you can sit for hours and your posture is dramatically improved. That is what happened for me.
But now I'm thinking standing or walking is the way to go. There is deep evidence that walking while working improves performance ... and is more fun.
The central problem with kneeling chairs is that one of the most important ergonomic principles is that you have a natural (and necessary) propensity to shift positions every few hours. Kneeling chairs don't allow you a range of positions the way that conventional chairs, exercise balls, standing, etc. do. The ergonomics people I knew always went out of their way to say kneeling chairs were actually a bad idea.
Having said that, whatever works for you, works for you. But for people considering a kneeling chair, you want to be careful not to just try it in the store for 10 minutes, then buy it.