These new Burst-Resistant FitBalls are much firmer. They are
weight tested to 600 pounds (270 kg) and resists tearing if punctured.
These FitBalls offer more firmness for users who want a solid
support while using them in balance stability and core training
exercise and rehabilitation routines.
Although FitBall Stability Balls
are burst-resistant, they are not recommended for use during
heavy weight training.
Also see Ball Pumps