A stability ball chair is a type of desk chair that uses a stability or exercise ball as the seat cushion. Some people simply use large exercise balls as a desk chair, as opposed to a designated stability ball chair; however, an actual exercise ball chair will typically feature a stability ball mounted on casters, similar to any other rolling desk chair, with a support bar for the lower back. There are arguments for and against the use of these chairs.
A stability ball chair is often considered to be an ergonomic chair because it keeps the spine in a more natural posture, helping to prevent slumping, and because it forces the abdominal muscles to engage to remain sitting upright. Using a stability ball chair requires constant, small corrections in the muscles of the abdominals, legs, and lower back, to stay balanced on the ball. Many of these muscles are rarely used, particularly by people who spend all day sitting at a desk, and the stability ball helps to correct this.
Of course, some people simply use a stability ball as a desk chair, rather than a designated stability ball chair. This can be perfectly fine if the height is correct; it is generally recommended that the thighs should slope downward, and not be raised above the hips. The arms should be relatively parallel to the keyboard, without needing to tilt the wrists up. A chair with a support bar may provide a some additional support to the lower back as well, which does not exist on a plain stability ball.
For people with lower back or hip problems, some doctors advise against using a stability ball chair, stating that it puts extra pressure on the lower back. Unless one has any specific health issues, it is usually a matter of personal preference whether or not one chooses to use a stability ball as a chair. It is also entirely possible to save some money and use the same ball that is used for exercising as a chair. It is best to begin slowly, and gradually work up to using the ball all day.
For instance, one might first begin by using the stability ball chair for only half an hour or a few hours at a time. Once that becomes comfortable, gradually add time each day until the ball is being used for the whole day. Only continue to use the ball if it is comfortable; if not, there are a number of other ergonomic desk chair options that may be preferable.