Exercise bikes have been around for some time. They also provide a good cardio workout as well, while limiting impact and injury to knees and hips. Stepping height can be adjusted to accommodate different body sizes and fitness levels, however, it can be used to perform a very effective ab exercise. The Roman Chair is a somewhat taller machine with two stationary, parallel arms and a straight, padded back. There is nothing to sit on. In conclusion... The bodily changes and improvement of physical abilities you seek are not determined by the equipment you need. Crunches can also be done on an exercise ball and many feel it is more effective than standard crunches. You sit on the ball until you are lying with your back flat on the mat. Relax your abdominal muscles and bend your knees slightly. Squeeze your abdominal muscles and bend your knees toward your chest as far as you can go and hold for a few seconds before extending them to the beginning position. This exercise was number two in a San Diego