Heart-Rate Zones, Explained Without the Jargon
Z2 builds your engine, Z5 sharpens its edge. A practical guide to training where it matters.
Five zones, two that matter most
There are five training zones, but for non-elite athletes the leverage sits in two of them: Z2 — comfortable, conversational pace — and Z4/Z5 — hard intervals at or above lactate threshold. The middle, "moderately hard" zone is the famous junk-miles trap.
Why Z2 is non-negotiable
Z2 builds mitochondrial density and aerobic base — the engine that powers everything else. The norm in elite endurance training is 70–80% of total training time in Z2. Most amateurs reverse this and wonder why they plateau.
Use Karvonen, not 220 − age
Anchor zones to your heart-rate reserve (max − resting). The Karvonen formula respects how fit you already are. Our heart-rate calculator does both.
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