The Borg RPE scale is a numerical scale that ranges from 6 to 20, where 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 perceived level of exertion during physical activity. The scale was constructed to roughly correlate to 10% of heart rate in a healthy 20-year-old. This explains why the rating starts at 6, which would roughly correspond to a resting heart rate at about 60 per minute. In older individuals, the correlation becomes higher than 10% at the high-end of the scale, as
maximum heart rate declines with age. ==Borg RPE scale and the external environment==