The original scale introduced by Gunnar Borg rated exertion on a scale of 6-20. This is especially used in borg rpe scale 1 10 pdf diagnosis of breathlessness and dyspnea, chest pain, angina and musculo-skeletal pain.
The CR-10 scale is best suited when there is an overriding sensation arising either from a specific area of the body, for example, muscle pain, ache or fatigue in the quadriceps or from pulmonary responses. The sensitivity and reproducibility of the results are broadly very similar, although the Borg may outperform the Likert scale in some cases. The seemingly odd range of 6-20 is to follow the general heart rate of a healthy young adult by multiplying by 10. For instance, a perceived exertion of 12 would be expected to coincide with a heart rate of roughly 120 beats per minute.
6 means “no exertion at all” and 20 means “maximal exertion. When a measurement is taken, a number is chosen from the following scale by an individual that best describes their level of exertion during physical activity. Psychophysical bases of perceived exertion”. Perceived exertion as an indicator of somatic stress”.