There is a company called SpinaliS that makes these crazy spring-based ergo chairs for ‘active sitting’. I tried them in the showroom near my work in Warsaw and rhey seemed to cause some relief in spine muscle fatigue. I’ll be doing a 5-day trial with a leased chair some time soon to see if it works as a full-time chair. A bit expensive tho. Company itself seems to be from Ljubljana, (of kors, and so on and so on).