The GoFit 13 In. Core Balance Disk with Training Manual is an ideal way to add a new level of difficulty to your workout. The soft disk can be utilized in pilates yoga or regular workouts to work your core muscles and increase balance and stability. Difficulty may be increased by adding air or decreased by releasing air. It comes with a training manual to optimize your knowledge of the product and exercises. * Before beginning any weight loss or exercise program you should consult with your physician.