Try the Wigli, a Dutch stool which, well, wiggles. It has a three-legged milking-stool design, and the round seat wobbles. This offers support for your office-bloated body, but you have to steady yourself with those rarely-used muscles in your lower back and abdomen, which strengthens your core and stops you getting back pain. It’s kind of the grown-up version of sitting on a ginat rubber ball.
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-joel