Fitter First DuraBall PRO Exercise Ball Blue (65cm) World's Strongest Exercise Ball Chair. Strongest and safest burst resistant exercise ball. Its perfect for larger individuals and is also suitable as a ball chair when underinflated. Burst Resistant up to 1,100 lb and Static Load Rating of 4,400 lb. Available in 45 cm,55 cm,65 cm,75 cm,85 cm sizes with diffrent colors.