This scale was created to help people gauge their level of exertion and to use it in exercise testing when heart rates are unreliable or unable to be obtained. You may have seen a copy of the category-ratio scale in your local gym or favorite fitness magazine. This is most widely used in fitness classes and the like to help participants figure out how hard they should be working. As mentioned in the article about exercise and pregnancy, the RPE scale is a great tool for gauging exertion level as heart rate has not been shown to be the best indicator. Here are the two scales.