I've been seeing a lot of people buying and using the wrong size stability ball.
If you are using it for exercises where you lie prone (on your stomach) or supine (on your back) on the ball, or when you are sitting on the ball you want to make sure that the ball is large enough to help you stay in correct alignment. The way to measure this is to sit on the ball. Your knees should make a 90 degree angle. The size of the ball will probably be between 55 cm and 70 cm.
