RPE & Heart Rate
topic
Aerobic exercise intensity is measured by rate of perceived exertion (RPE, the subjective 6–20 scale of Borg or the simplified 1–10 scale), heart rate as a percentage of maximum (HRmax), heart rate reserve (Karvonen formula), power output (watts, most precise in cycling), pace (minutes per kilometer/mile), and blood lactate concentration (most physiologically accurate for zone demarcation). Each measure has different accessibility and precision, with heart rate being the most practical real-time metric despite its confounding by temperature, hydration, caffeine, and emotional state.
Role
Intensity measurement transforms exercise from vague effort into quantifiable, progressible stimulus — the difference between 'I went for a run' and 'I spent 35 minutes at zone 2 intensity producing the specific mitochondrial adaptation I was targeting.' Most recreational exercisers have no objective intensity monitoring and rely entirely on subjective effort perception, which is notoriously inaccurate: people consistently overestimate their exercise intensity in early exercise, underestimate it when fatigued, and have no means of identifying the specific intensity zones that produce the specific physiological adaptations they are pursuing.