Featuring a 65 cm diameter, the Onyx Black PTP CoreBall is recommended for individuals from 165 cm (5 ft 4) up to 185 cm (almost 6 ft 1). Back pain often stems from poor posture, muscle imbalances and weak core engagement. Add this essential piece of equipment to your home gym to build a strong core, foundation to any functional movements. We have included a range of exercises carefully designed for you to learn just how to do so. You can also improve your posture by simply using your CoreBall as an alternative to your office chair.