The Borg Scale of Perceived Exertion (RPE) is a numerical tool, ranging from 6 (No Exertion) to 20 (Maximal Exertion), used by coaches and athletes to subjectively gauge the intensity of their warm-up and training sessions. This scale is cleverly linked to heart rate, as multiplying the RPE score by 10 gives a decent estimate of the user's actual heart rate, with the target for a proper warm-up being 13 ("Somewhat Hard") to reach the target heart rate zone.