DIY

https://makezine.com/article/craft/homebrew-the-elliptigo-glide-bike/

When my friend Bryan Pate asked if I could build him an elliptical trainer he could ride outdoors like a bike, as a substitute for running, I was surprised he couldn’t buy it. Once I was convinced that it didn’t exist commercially, I set about designing one. After extensive 3D modeling, finite element analysis, and many design iterations, the ElliptiGO was born.

Here's a DIY example: https://www.instructables.com/wooden-kneeling-chair/

Anyone tried them or some other alternative chair like those exercise ball chairs?

Ball chair DIY example: https://www.instructables.com/How-to-Make-a-PVC-Ball-Chair/