That's the Balans [1] (the original model of which is now marketed as Variable Balans), by Norwegian designer Peter Opsvik.

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...