Heart rate zones are bands of effort expressed as a percentage of your maximum heart rate, or — when you supply a resting rate — of your heart rate reserve, which is the gap between resting and maximum. The reserve method, published by Karvonen and colleagues in 1957, produces more personal ranges because it accounts for how efficient your heart already is at rest.
How to measure resting heart rate properly
Take it in the morning, before caffeine and before getting out of bed if possible, after lying or sitting still for at least five minutes. Count the pulse at your wrist for 30 seconds and double it, or read it from a chest strap. Do it on several mornings and use the average. Illness, alcohol the night before, dehydration, poor sleep and stress all raise it — which is exactly why an unexplained jump of five to ten beats above your own baseline is a useful early signal that you are getting sick or under-recovered.
Why zone 2 gets so much attention
Zone 2 — comfortably conversational, breathing noticeably but able to speak in full sentences — is where the largest share of aerobic adaptation happens per unit of fatigue. It builds capillary density and mitochondrial function without generating the recovery cost of hard intervals, so you can do a lot of it. The practical guideline behind it is unglamorous: at least 150 minutes a week of moderate activity, or 75 minutes of vigorous, plus two sessions of strength work.
What changes your numbers
Maximum heart rate falls with age and is essentially not trainable. Resting heart rate, by contrast, responds well to training — a well-conditioned adult may sit in the 50s where they once sat in the 70s. Heat, altitude, dehydration, caffeine, stress and illness all push heart rate up at a given workload, a phenomenon called cardiac drift, which is why the same pace can sit in a different zone on a hot afternoon than it did in the cool morning.