Heart-rate zones are five effort bands, each defined as a slice of your maximum heart rate, and each one trains your body differently. Knowing which zone you’re in turns a vague workout into a deliberate dose — and the research is clear that mixing the extremes works better than grinding the middle.
The common model runs from Z1 to Z5 as a share of your maximum heart rate. Z1 (roughly 50–60% of max) is active recovery — easy enough to hold a conversation. Z2 (60–70%) is the endurance base, building aerobic capacity and fat-burning efficiency. Z3 (70–80%) is the “tempo” middle. Z4 (80–90%) pushes your lactate threshold, the effort you can just barely sustain. Z5 (90–100%) is the maximal band, trainable only in short, hard intervals.
The low zones and the high zones do different jobs. Z2 builds the slow, durable engine: more mitochondria, more capillaries, a heart that pumps more per beat. Z4 and Z5 sharpen the top end — your ability to clear lactate and hold a hard pace. Z3, the popular “comfortably hard” middle, is where many people accidentally spend most of their time: too hard to recover from, too easy to drive peak adaptation. It has a place, but it shouldn’t be the default.
Every zone is anchored to your max HR, so the estimate matters. The famous 220 − age formula is a rough population average and can be off by 10–20 beats for any one person. A field test (a hard, sustained effort to genuine exhaustion) or a lab test gives a far more personal number. If you only have the formula, treat your zone edges as fuzzy bands, not hard lines — and adjust once your real numbers show up in training.
Endurance research keeps pointing to a polarised distribution: roughly 80% of training time easy (Z1–Z2) and about 20% hard (Z4–Z5), with little time deliberately parked in Z3. The easy volume builds the base without piling on fatigue; the hard, short doses drive the sharp adaptations. The trap is the “grey zone” — chronic Z3 that feels productive but leaves you tired without being either restful or truly stimulating.
Open any workout in Vitra to see your time in each of the five zones, plus average and max heart rate and pace — reconstructed on demand from your Oura data, never stored on a server. A time in HR zones summary across your recent workouts shows whether your mix is genuinely polarised or quietly drifting into the grey zone, all computed locally on your machine.
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