I used to love https://en.wikipedia.org/wiki/Kneeling_chair as a kid.
e.g. https://www.varierfurniture.com/en_gb/explore-our-chairs/kne...
Opsvik was partly responsible for the wave of interest in ergonomic seating in the 1980s. He eventually wrote a book, Rethinking Sitting [2], about the topic.
[1] https://varierchairs.com/product/variable-balans/
[2] https://www.amazon.com/Rethinking-Sitting-Peter-Opsvik/dp/03...
Another product that was moved over to the Balans brand was the Move chair by Per Øie [1], which is an actual saddle chair.
[1] https://www.archiproducts.com/it/prodotti/varier-furniture/s...
They look awkward to sit on, you actually have to try them to realise you naturally adopt an upright (not slumped) sitting position.
I also just found some plans, might make one so it's adjustable my way: http://www.craftsmanspace.com/free-projects/kneeling-chair-p...
From what I understand (and years of use) they merely place a small amount of weight on the front of your shins, which is at most merely uncomfortable for some people. (Most of your weight should be supported by your bottom.)
For an average human body, I have a hard time imagining what biomechanics could lead to a knee issue from it? A quick Google search didn't show anything.