A gymball has to offer a fine balance between support and instability, if the ball is too rigid or too soft then the effects on your core stabilising muscle will be compromised. The Reebok gymballs all perform to these optimal levels if you are simply using the ball to sit on at a desk or performing dynamic exercises. All Reebok gymballs are anti- burst; now that does not mean you can stab it or kick it around. No, what it actually means is if you were unfortunate to get a puncture in the ball whilst you were using it instead of going ‘pop’ like a balloon it deflates slowly to avoid the risk of injury.