CoreDisks With Squellet, Bean, PVC Balls or Foam Filler offer users different degrees of stability for positioning. You can also make them more unstable for development of core strength. The only tool required to change the possible uses is a standard athletic pump. Less air results in more stability, more air means less stability. When uninflated each type of CoreDisk takes on the sensory property of the filling. By adding air the disks shift from the original concave shape to more of a therapy ball shape, and from the sensory feel of pellets or balls to the feel and response of an air-filled ball. Measures 12″ diameter (30.6cm).