What’s the deal with those big bouncy balls taking up space around the gym? Stability balls — also called exercise balls, balance balls, Swiss balls, or fitness balls — are more than just fun to sit and bounce on; they’re a great way to improve strength, cardio endurance, and balance. To get the most from a bouncy fitness routine, make sure to choose the correct size stability ball. Most balls come in three diameters based on the user’s height, though some come in four or five.