Challenge the deepest stabilizing muscles in your body while exercising on the unstable base of this 55 cm (22") Stability Ball™. Designed to improve balance and strength as well as tone your body, this Stability Ball can also be used with the ergonomic Halo® Trainer Plus. When used in conjunction with this evolutionary device core workouts are maximized and controlled instability dampens exercise intimidation. Can be used for all fitness levels. Tested up to 800 pounds.