Rate perceived exertion and fatigue.
The Borg scale runs from 6 at complete rest to 20 at maximal effort and was designed so the rating roughly tracks heart rate when multiplied by ten. Perceived exertion alone misses accumulated fatigue, so this combines it with current soreness and last night's sleep into a single index: exertion carries the most weight, soreness next, and sleep shortfall against an eight-hour reference last. Readiness is the complement of that index.
Normalised components
Exertion load = (RPE - 6) / 14 x 100; soreness load = (soreness - 1) / 9 x 100; sleep debt = (8 - sleep hours) / 8 x 100
Fatigue index
Fatigue = 0.45 x exertion load + 0.30 x soreness load + 0.25 x sleep debt
Educational estimate only. Not medical advice — discuss results with a qualified clinician before changing treatment, medication or exercise.
Borg anchored it so the rating multiplied by ten approximates heart rate in beats per minute, which puts complete rest near 60 and maximum near 200.
As a rough guide, above 70 suggests replacing intensity with easy aerobic work or rest. Below 35 usually means you can absorb a hard session.