Heart Rate Zones
Dividing training intensity into numbered bands, typically three to seven, based on percentage of max or threshold heart rate, each with a different physiological purpose.
Heart rate zones divide the range between resting and maximum heart rate into numbered bands, each corresponding to a different training intensity and a different physiological demand. The idea predates any single wearable brand: coaches have used heart rate to estimate effort for decades, well before GPS watches made it something everyone could see mid-session.
How zones get set
Most models set zone boundaries as a percentage of either maximum heart rate or threshold heart rate (the heart rate at your functional threshold), with threshold-based models generally considered more precise since two athletes sharing a max heart rate can have very different fitness. A three-zone model (easy, moderate, hard) is simple and rough. Five-zone models, like Joe Friel's, and seven-zone models, like British Cycling's, split the same range into finer bands to separate, for example, easy aerobic work from tempo work that feels similarly comfortable but trains something different.
What each general band is roughly used for
Across most models, the pattern is broadly similar even when the exact cutoffs differ:
- Lowest zones (often labeled 1-2): easy, purely aerobic effort, sustainable for hours, used for building aerobic base volume without much accumulated fatigue.
- Middle zones (often labeled 3): a "moderate" intensity that's harder than easy but not clearly hard, sometimes called the gray zone because it's easy to drift into without training either quality well.
- Upper-middle zones (often labeled 4): threshold and tempo work, sustained efforts near the edge of what's holdable for an extended period.
- Highest zones (often labeled 5+): short, hard efforts above threshold, used for VO2 max and anaerobic capacity work.
The exact number of zones and their labels vary by system, but the underlying shape, mostly easy work, a smaller amount of threshold work, and a small amount of very hard work, shows up across most well-supported training approaches.
Why Zone 2 isn't a synonym for the whole system
Zone 2 has become a popular shorthand in endurance training discussion, to the point where it sometimes gets used loosely to mean "easy training" in general. It's actually one specific band within this broader system: the low, purely aerobic zone near the bottom of most models, not the zone system itself. Heart rate zones are the whole framework, dividing the full intensity range into bands from easy to maximal. Zone 2 is one piece of that framework, the one most models place just above the very easiest recovery effort and below the point where breathing shifts and easy conversation gets harder to hold.
The practical catch
Heart rate zones set from an estimated max (like the old "220 minus age" formula) can be meaningfully off for a given individual, sometimes enough to shift what should be an easy day into the next zone up without the athlete noticing. Zones set from an actual tested or estimated threshold tend to be more reliable. Either way, RPE is a useful sanity check against the numbers, since heart rate itself can drift with heat, fatigue, caffeine, and hydration independent of actual effort.