Whether you call it an exercise ball, a fitness ball, a yoga ball or a balance ball, we have all seen it at the gym. People exercise and stretch on it. But, how many people do you know use an exercise ball as an office chair? An exercise ball office chair not only helps your posture, but it has the capability of strengthening your core muscles, all while sitting. Thanks to some help from the Internet, I was able to build my own exercise ball office chair for around $25.