I have had a Balan's Chair for decades, the kind without a back support. It came highly recommended by my orthopedic MD because I have scoliosis in the small of my back and had to be able to sit upright while sewing. I absolutely love it because it's so comfortable for me, keeps my back straight, and keeps me from "hunching over" my sewing machine. It's been a Godsend! Here's a link so you can see what I'm talking about. Mine has just the seat rest and knee rest, no back and no rolling casters.
http://www.bizchair.com/kneelingchai...4-ece2f0b7394e