This article focuses on the physiology of sitting. It mentions challenges for meditators with steps to strengthen sitting ability.

http://www.zafu.net/whatswrong.html

I began sitting on an exercise ball at my office job about 5 years ago. I felt tired at the end of the workday because of the extra effort of holding my torso upright without support. That lasted for a week. I don’t notice it at all now.

Using an exercise ball for a chair ($13 at Target), I move more frequently. I found a stable position that works well too: I sit with my tailbone at top center, knees wide, heels tucked into the ball — so my hips are higher than my knees.

I add air once or twice a year to keep it firm.