The general rule for aerobic stepping is not to let your knee bend more than 90 degrees. So, a shorter person should use a shorter step. Note: Using a step for muscle toning is a little different. Some workouts do have you bending your knees at more than a 90 degree angle (it's a good way to target your buttocks). But that's only if you have no joint or knee problems. Of course, if it hurts - stop!