Great for partner or individual core work or atlas stone style exercises and many other functional exercises. The Hydropower Ball can take your strength gains to a whole new level. Easily change stability level and resistance of your activity by adding or removing water. Available in two sizes. (33lbs.) water; Medium: Blue, 40 cm. (16") dia., holds 25 kgs. (55lbs.) water; Large: Red, 50 cm. (20") dia., holds 40 kgs. (88lbs.) water. Green is no longer available.