In the realm of active seating, Italian industrial designer Giorgio Gurioli designed this experimental Bikechair:
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3D printed in ABS, the chair provides one leg while yours are meant to take up the slack.
This begs the question: How does the chair stay standing when unused?
The answer:
It’s not intended for production, of course. The Bikechair was created as an exercise (promotion?) in conjunction with Delta Wasp, a manufacturer of industrial 3D printers. If the yoga-ball-as-chair thing took off, this is how a 3D printing powerhouse would attempt to capitalize on it.
Source: core77