Why zones in the first place
Heart rate is a workable proxy for metabolic effort during steady aerobic exercise. The 5-zone framework — popularised by Sally Edwards in the 1990s and adopted by British Cycling and most modern endurance coaches — anchors training prescription to physiology rather than perceived effort or pace alone. Pace drifts with terrain, weather, fatigue and altitude; HR is more stable.
The five zones
- Z1 (50–60%) — recovery rides, warm-up, cool-down.
- Z2 (60–70%) — fundamental endurance, the largest portion of an endurance plan. Builds mitochondrial density and fat oxidation.
- Z3 (70–80%) — tempo, “comfortably hard” pace. Builds aerobic capacity and lactate clearance. Spend less time here than the watch suggests.
- Z4 (80–90%) — threshold work. 20–60 minute intervals at lactate threshold. Key for race performance.
- Z5 (90–100%) — VO2max intervals. Short, sharp efforts (30 seconds to 5 minutes). Highest training stress.
Two methods, when to use each
%HRmax is what most fitness watches default to. It is simple and works for general fitness. Karvonen heart-rate reserve uses the difference between resting and max HR — for trained athletes with resting HRs in the 40s, Karvonen zones land 10–15 bpm higher than the equivalent %HRmax bands and better track metabolic effort. For prescribed endurance training, Karvonen is the more useful default.
When zones break down
Heart-rate-based prescription assumes a stable response curve. It breaks down with cardiac drift in heat or dehydration, altitude adaptation, beta-blockers and other heart-rate-modifying medication, atrial fibrillation, pregnancy, and acute illness. In those cases, prescribe by RPE (rate of perceived exertion) or power, not HR.