Exercise balls are useful, in a gym or therapeutic setting, for strengthening your core muscles. The inherent instability of the round, prone-to-rolling exercise ball forces core muscles to constantly stabilize your body while you're on the ball. Even though big-name trainers like Bob Harper from "The Biggest Loser" endorse the use of exercise balls in place of desk chairs, some still oppose the idea.
Simply rolling your exercise ball over a fallen thumbtack might cause it to suddenly deflate. If you’re sitting on it when this happens, a serious injury might result. Help cut the risk of this happening by selecting an exercise ball that is clearly labeled as either puncture- or burst-resistant, or with the letters SDS (slow deflate system).