Finding balance means the core is engaged and getting stronger. The flat side of the Donut Ball works the core and provides stability while sitting, making it perfect for work or the classroom. The rounded side is for active exercise. The unique design allows users to add additional resistance and adjust their traditional workouts by including a Donut Ball. For instance, placing one between the legs makes sit-ups harder to complete. This ball is red, has a 30" diameter and is recommended for users 67"-73" tall. The instability of the ball is the key as users constantly need to engage stabilizing muscles, working to improve coordination and balance. The size and shape make these perfect to use at home, in the gym, at the beach or park, anywhere you like to work out. Kneeling and balancing while using the Donut Balls freshens workouts and adds additional benefits as different muscle groups get worked. Donut Balls are available in 5 color-coded sizes for adults and children. Choose the right donut based on the expected user's height, just like an exercise ball. The balls range in diameter from 18" through 34". CanDo Donut Balls are non-slip and Latex-free. Their durable PVC material supports up to 300 lbs. Please note, inflation pump is not included.