What heart rate zones are and why they matter
Heart rate zones are ranges of your heart rate (HR), each of which stresses the body differently and develops a different quality — from recovery all the way to maximum oxygen uptake. Training by heart rate is a way to dose intensity objectively instead of by feel.
Without zones, most amateurs fall into one trap: they make easy sessions too hard and hard sessions not hard enough. Everything ends up cooking in a medium intensity that tires you out but develops little. Properly calculated zones fix exactly this.
Max heart rate: why 220 minus age is crude
Maximum heart rate (HRmax) is the highest HR your heart can reach under all-out effort. The classic 220 − age formula is convenient, but its error reaches ±10–12 beats per minute. Two people of the same age can have true maximums that differ by 20–30 beats.
Why:
- HRmax is highly individual and barely depends on fitness;
- the formula is an average over a large sample, not a measurement of you;
- an error in HRmax propagates into every single zone.
It is far more reliable to learn your maximum from real data — for example, from peak values at the end of a hard interval or test where you genuinely emptied the tank.
Resting heart rate and heart-rate reserve (HRR)
Resting heart rate (HRrest) is your HR at complete rest, best measured in the morning right after waking. It is lower in trained people: the heart pumps more blood per beat. Wearables (Garmin, Oura, Whoop, Fitbit) compute it for you from overnight data.
Heart-rate reserve (HRR) is the gap between maximum and rest:
HRR = HRmax − HRrest
HRR is exactly what makes zones personal: two people with the same HRmax but different resting HR will get different zone boundaries.
The Karvonen formula: how to calculate zones
The Karvonen formula computes a target heart rate not as a percentage of the maximum, but as a percentage of the reserve added back to the resting rate:
Target HR = Intensity × (HRmax − HRrest) + HRrest
Where intensity is the fraction of the reserve (for example, 0.7 = 70%).
Example. HRmax = 190, HRrest = 50, so HRR = 140. The 70% boundary:
- 0.7 × 140 + 50 = 98 + 50 = 148 bpm
For comparison, the naive method (70% of 190 = 133) gives a very different number. Karvonen is more accurate because it accounts for your resting heart.
LTHR — an even better anchor
If your LTHR (lactate threshold heart rate) is known, zones are best built from it. The threshold is the intensity your body can hold for a long time before fatigue spikes, and it correlates with real performance far better than an estimated maximum. LTHR is found from a 20–30 minute tempo test.
So the priority of the calculation is: LTHR → Karvonen (HRR) → % of HRmax — from the most reliable anchor to the crudest.
The five heart rate zones
| Zone | % of HRR | What it develops | How it feels |
|---|---|---|---|
| Z1 Recovery | 50–60% | Recovery, blood flow | Very easy, you can sing |
| Z2 Aerobic base | 60–70% | Endurance, fat use, the "engine" | Easy, full sentences |
| Z3 Tempo | 70–80% | Aerobic power | Moderately hard, hard to talk |
| Z4 Threshold | 80–90% | Lactate threshold, speed | Hard, single words only |
| Z5 VO2max | 90–100% | Maximum oxygen uptake | All-out, no talking |
- Z1–Z2 is the foundation. This is where the aerobic engine is built and mitochondria and capillaries multiply.
- Z3 is deceptive: it feels like work but develops less than you think.
- Z4 is threshold work that raises your sustainable pace ceiling.
- Z5 is short, sharp intervals for maximum heart and lung power.
The 80/20 rule and the grey-zone mistake
Studies of endurance athletes converge on one thing: roughly 80% of training time should be easy (Z1–Z2) and only ~20% hard (Z4–Z5).
The amateur's core mistake is the grey zone (Z3): the session runs too fast to be easy and too slow to be developing. Fatigue accumulates without matching gains. Signs you are stuck in the grey zone:
- easy days end with you out of breath;
- hard intervals never reach a true maximum;
- progress stalls while fatigue piles up.
The cure is simple: make easy truly easy and hard truly hard.
How Sport AI Link computes your zones
Sport AI Link does not plug in a generic 220 minus age. The bot takes your real data from your wearable — resting HR and maximum from overnight and workout records — and computes personal zones by the priority LTHR → Karvonen → % of HRmax. As soon as a more reliable anchor appears (for example, an LTHR estimate), the zones refine automatically.
The numbers are computed by deterministic code, while the coaching read — to the point and without hand-holding — is voiced by AI: where your intensity landed, whether you are stuck in the grey zone, and what to do next.
Want to know your real heart rate zones instead of a one-size-fits-all average? Connect your device to Sport AI Link in Telegram and get a personal calculation from your own data.