A self-reported score of how hard exercise feels.
Rating of Perceived Exertion is a subjective scale (classically Borg 6–20, or a modern 0–10 CR10 scale) on which a person rates how hard an effort feels. It is a cheap, validated way to gauge intensity and, when multiplied by session duration, to estimate training load (session-RPE). Because it integrates many physiological signals, it often tracks internal load well.