Heart rate is a proxy for how hard your body is working. It's imperfect — heat, caffeine, sleep, stress and dehydration all shift it — but it's continuous, cheap to measure, and far more honest than perceived effort on a day when you feel good.
Zones divide the range between rest and maximum into five bands. Each drives a different adaptation, and training in the wrong one is the most common reason people plateau.
What each zone is for
| Zone | What it does | % HRR |
|---|---|---|
| 1 — Recovery | Blood flow without stress. Warm-ups, cool-downs, active recovery days. | 50–60 |
| 2 — Aerobic base | Mitochondrial density, capillary growth, fat oxidation. The foundation. | 60–70 |
| 3 — Tempo | Aerobic capacity under moderate load. Useful, easy to overuse. | 70–80 |
| 4 — Threshold | Raises the point where lactate accumulates faster than you clear it. | 80–90 |
| 5 — VO₂ max | Peak oxygen uptake. Short intervals only; costly to recover from. | 90–100 |
The talk test
You don't strictly need a monitor. Zone 2 is where you can speak in full sentences but wouldn't choose to. Zone 3 is short sentences. Zone 4 is a few words. Zone 5 is none. This maps onto measured zones surprisingly well, and it needs no batteries.
The polarised pattern
Analyses of how elite endurance athletes actually distribute their training keep finding the same shape: roughly 80 per cent of sessions easy, in zones 1 and 2, and the remaining 20 per cent genuinely hard, in zones 4 and 5. Zone 3 gets comparatively little.
Recreational athletes tend to invert this. Left to instinct, most people drift into zone 3 for nearly everything — hard enough to accumulate fatigue, not easy enough to build a base, not hard enough to drive peak adaptation. Sports scientists call it the moderate-intensity rut, and escaping it usually means deliberately slowing your easy days down further than feels natural.
If your easy runs feel a bit hard, they're not easy runs. Zone 2 pace is slow enough to be mildly embarrassing at first. It gets faster over months as aerobic fitness improves — that improvement is the whole point.
How accurate is a formula?
Not very, individually. The classic 220 − age has a standard deviation of about 10 to 12 beats per minute, so roughly a third of people sit more than 10 bpm from their predicted max, and it systematically overestimates in the young and underestimates in older adults. Newer equations — Tanaka's 208 − (0.7 × age), and 211 − (0.64 × age) from the large Norwegian HUNT study — fit observed data better, but none escapes the underlying spread.
Two things improve on this. A field test — a thorough warm-up followed by a genuinely maximal three-minute effort on a hill or a bike, with the peak reading taken from the last seconds — gets you close to your true maximum. And over time, your own data tells you: the highest number you ever see in a hard race is a better anchor than any equation.
Before maximal testing: if you're over 40, have been inactive, or have any cardiovascular history, family history of heart disease, chest pain on exertion or unexplained breathlessness, get cleared by a doctor first. Maximal effort testing is not the place to find out about an undiagnosed condition.
Common questions
Why is my heart rate higher some days at the same pace?
Cardiac drift from heat and dehydration, poor sleep, illness, caffeine, and accumulated training fatigue all raise it. A resting pulse 5 or more beats above your normal on waking is a reasonable prompt to keep the day easy.
Are wrist-based monitors accurate enough?
For steady work in zones 1 to 3, generally yes. For intervals they lag and can lock onto cadence instead of pulse. A chest strap remains substantially more reliable for anything with sharp changes in intensity.
Is the fat-burning zone real?
The proportion of energy coming from fat is higher at low intensity, which is true and where the label comes from. But total energy expenditure is lower too, and body composition responds to total energy balance over weeks rather than to the fuel mix during any one session. Train zone 2 for the aerobic adaptations, not for the fuel label.