Zone 1 (50–60% max HR): active recovery. Zone 2 (60–70%): aerobic base, fat burning. Zone 3 (70–80%): aerobic tempo. Zone 4 (80–90%): lactate threshold. Zone 5 (90–100%): maximal sprint effort.

It estimates population averages with a standard deviation of ±10–12 bpm, meaning your true max could be 20+ bpm higher or lower. A supervised all-out effort test or lab test gives a far more personal result.

Zone 2 (60–70%) burns the highest proportion of fat as fuel. However, Zone 4–5 work burns more total calories per minute. Both play a role: Zone 2 for base building, Zone 4–5 for caloric output.

It helps significantly. A chest strap (e.g. Polar H10) is more accurate than wrist-based optical sensors. Alternatively, the "talk test" works for Zone 2: you should be able to hold a conversation but not sing.