This site talks about how to pick the proper size of exercise ball and how to use one as a chair:
Tutorial: How to use an exercise ball as a chair
I like a non-rolling exercise ball:
Gold’s Gym 65 cm Stay Ball for heights 5’3′ to 5’10”
Gold’s Gym 75 cm Stay Ball for heights 5’10” and above
Use at your own risk and begin by having someone “spot” you, so you don’t bonk your head.
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October 18, 2017 Barnes Disclaimer