"Fitballs" are available in more durable and burst-resistant versions and have a firm tactile experience. Recommended for the "hyperactive" or whenever durability is paramount. They will support over 600 lb. (272 kg) of weight, and they are latex free!
Selecting the correct ball size is easy. Just measure the length of the user's arm from the armpit to the finger tips. The arm measurement will be the target diameter of the ball you should select. Since ball diameters will not match arm measurements exactly, a tolerance of plus or minus 5" (13 cm) arm length is a good fit-range for ball use. The larger balls are offered for activities that may not require independent arm or foot contact with the floor.