When you sit on top of a stability ball (a.k.a., exercise, fit, Swiss, Physio ball) your thighs should be parallel to the floor and your knees should be at a 90 degree angle. This piece of equipment is used to help improve your balance, flexibility and coordination. But before you jump aboard any exercise ball be sure you are using one that is best suited for you.

Size Chart
Height	Ball Size
5'-5'7	55 cm
5'8-6'2	65 cm
6'3-6'9	75 cm
Over 6'10	85 cm

Good luck and be careful when using this versatile exercise tool.