For Aviva's Ball Chair (shown in main photo above; PLU 11883) can help take pressure off the spine, release muscle tension, correct posture, improve balance and strengthen the torso. This ball, which can be inflated up to ~52 cm (10.4 inches) in diameter (it fills using a needle valve on a pump (not included), and comes with a long stopper).
Apart from use with these chairs, these balls are also suitable as exercise/yoga balls.
Various Ball Chair Wheel Sets
The wheels pictured at top of the photo at right are standard, free-rolling wheels (PLU 11294). The wheels pictured bottom right are manually locking (PLU 17519): depending on how the small tab is positioned, it either enables sliding/movement while seated, or it locks the wheels in place to keep the chair stationary (two locked wheels suffice to hold the chair in place).
The auto-locking wheels (not pictured; PLU 11246) are the ones normally sold with the chair; they are weight-activated, to prevent rolling when you are seated, and the locks release when you get up, so the chair can be repositioned.
Ball Chair Height Extension Adjusters
Set of 4 chair leg height extenders recommended for taller users or those sitting at a slightly higher desk surface. These 5 cm extensions (pictured right; PLU 10459) are for use with the Aviva Ball Chair and also fit the BalanceBall Chair from Gaiam.