Promotes healthy, upright posture with the added benefits of core strength, increased relaxation, decreased stress and muscle tension. The Evolution Ball Chair has a height adjustable base and comes with a 55cm ball, which may be inflated or deflated to achieve the most comfortable position for the user. The moving base provides the user with the mobility and efficiency of an office chair, while experiencing the core strength benefits of a stability ball. The most economical and versatile ball chair on the market.