Training zones divide exercise intensity into bands based on percentage of maximum heart rate, or more precisely, percentage of heart rate reserve — the range between resting and maximum heart rate. Different zones emphasise different physiological adaptations: light zones build aerobic base and recovery capacity, hard zones build anaerobic capacity and speed.
Why heart rate reserve beats a simple percentage of max
A simpler method just takes a percentage of maximum heart rate directly (60% of max, say). The Karvonen method used here instead works from heart ratereserve — the gap between resting and maximum — which better accounts for individual fitness level. A very fit person with a low resting heart rate of 45 bpm and an unfit person with a resting rate of 80 bpm, both with the same calculated max heart rate, get meaningfully different target zones under Karvonen, whereas a simple percentage-of-max method would give them identical targets despite very different baseline fitness.
The formula’s real limitation
The 220-minus-age estimate for maximum heart rate is a population average from decades-old research, and individual variation around it is substantial — a standard deviation of roughly 10-12 beats per minute is typical, meaning a meaningful fraction of people have a true max heart rate 10+ bpm away from what this formula predicts. Fitness professionals and researchers use several alternative formulas that claim modest improvements, but none eliminate this individual variation. A supervised maximal exercise test remains the only way to know your actual maximum heart rate with confidence.
What each zone is generally used for
- Zone 1-2 (50-70%). Recovery and aerobic base building — sustainable for long durations, builds the cardiovascular foundation other training sits on top of.
- Zone 3 (70-80%). Moderate, sustained effort — the zone many people default to without meaning to, sometimes called the “grey zone” because it is harder than base training but not targeted enough to build peak fitness efficiently.
- Zone 4 (80-90%). Hard efforts that build lactate threshold and race-pace capacity.
- Zone 5 (90-100%). Maximum effort, sustainable only briefly — builds top-end speed and anaerobic capacity.
A note on relying on heart rate alone
Heart rate responds to factors beyond exercise intensity — heat, dehydration, caffeine, illness, sleep debt and stress can all shift it independent of effort level. Heart rate zones are a useful guide, not an infallible one, and anyone starting a new exercise program, especially with any underlying health condition, should get guidance from a doctor before using target heart rate zones to structure training.