Heart-rate zone training: a complete guide
Heart-rate zones are the cardiovascular equivalent of weights on a barbell — they let you prescribe and reproduce exercise intensity precisely, rather than just "going for a run". For endurance athletes, polarised training based on heart-rate zones is one of the most consistently effective ways to build aerobic capacity, raise lactate threshold and improve race-day performance. For general health and fat loss, zone-aware training prevents the common mistake of running every session at the same medium intensity — slow enough to be tiring, fast enough to never recover.
The five-zone model
The standard model divides exercise intensity into five zones based on percentage of maximum heart rate (or heart-rate reserve when using Karvonen). Each zone targets a specific physiological adaptation. Zone 1 (50–60%) is recovery — very easy effort that flushes metabolic waste, supports mitochondrial maintenance and adds aerobic volume without accumulating fatigue. Zone 2 (60–70%) is the aerobic base zone, where most endurance training should happen. Sustained zone 2 work increases mitochondrial density, fat oxidation capacity and capillarisation — the foundational adaptations that determine long-term endurance.
Zone 3 (70–80%) is the tempo zone — comfortably hard, sustainable for 30–60 minutes by trained athletes. It improves lactate clearance and is useful in moderation but is widely overused; spending too much time here creates fatigue without optimal adaptation. Zone 4 (80–90%) is threshold work, hovering near the lactate threshold for 15–40 minutes. This zone raises the speed at which you can sustain aerobic work — the single most important variable for race-pace performance. Zone 5 (90–100%) is VO2 max work, performed as intervals of 30 seconds to 5 minutes near maximal effort. It develops maximum oxygen uptake, the ceiling on endurance performance.
Polarised training: the 80/20 rule
Research on elite endurance athletes consistently shows a polarised distribution: roughly 80% of training time in zones 1–2 (easy aerobic) and 20% in zones 4–5 (hard intervals), with relatively little time in zone 3 (the "grey zone"). This 80/20 model is one of the most robust findings in endurance science, validated across runners, cyclists, rowers, cross-country skiers and triathletes from amateur to Olympic level.
The trap most amateur endurance athletes fall into is the opposite distribution — most sessions in zone 3, rarely truly easy and rarely truly hard. Zone-3 training is moderately fatiguing but produces sub-optimal adaptations: not slow enough to maximise aerobic base development, not hard enough to drive top-end fitness. Switching from "moderate all the time" to polarised training is one of the fastest performance unlocks for recreational runners and cyclists.
Karvonen vs simple % of max
Two methods exist for calculating zone boundaries. Simple % of max takes your maximum HR and multiplies by the zone percentage — Zone 2 lower bound = max HR × 0.60. This is easy but ignores individual fitness. A 50-year-old with a max HR of 170 and a resting HR of 80 sits at a very different intensity at 60% of max than a 50-year-old with a max HR of 170 and a resting HR of 50.
The Karvonen heart-rate reserve method fixes this. It calculates heart-rate reserve (HRR) as max HR − resting HR, then applies percentages: Zone 2 lower bound = resting HR + 0.60 × (max HR − resting HR). This normalises for fitness, producing zones that better reflect actual relative effort. Karvonen requires you to know your resting HR — measure it first thing in the morning before standing up, ideally with a chest strap, averaged over several days for accuracy.
Estimating max heart rate
The classic formula 220 − age (the Haskell equation) is widely cited but systematically under-predicts max HR in adults over 40 and over-predicts in those under 25. The more accurate Tanaka equation (208 − 0.7 × age) was developed from a meta-analysis of 351 studies and produces estimates within roughly ±10 bpm of true max for most people. The Gulati equation (206 − 0.88 × age) is calibrated specifically for women and tends to outperform sex-blind formulas in female populations.
Predicted max HR has a typical error of ±10 bpm because individual variation is genuinely large. If you train seriously, get your actual max from a graded exercise test or a hard field test — a 3-minute all-out hill or bike effort after thorough warm-up usually surfaces a number within 2–3 bpm of true max. Using your real max is meaningfully better than any formula prediction.
Using zones for fat loss
The "fat-burning zone" marketing is technically true but misleading. The body burns the highest percentage of fat at lower intensities (zones 1–2), but burns more total calories and more total fat at higher intensities because total energy demand is higher. For fat loss, the dominant variable is total energy burned — and total energy burned depends primarily on duration × intensity. A 30-minute zone 4 session burns more total fat than a 30-minute zone 2 session, even though a smaller percentage of those calories came from fat.
The practical implication: for fat loss, follow the polarised model. Build aerobic base with longer zone 2 sessions (60–90 minutes) and pepper in 2–3 high-intensity sessions per week in zones 4–5. The combination produces more fat loss per hour of training than steady-state moderate cardio alone — and it builds the cardiovascular capacity to handle harder work over time.