Target Heart Rate (Karvonen Method)

Martti Karvonen's 1957 refinement anchors training zones to heart rate reserve, the span between your resting and maximum rates, rather than to the maximum alone. The reasoning is straightforward: two people with the same maximum but resting rates of 45 and 75 are not doing the same work at 140 beats per minute, and a percentage of the reserve says so while a percentage of the maximum does not.

Because it builds on a maximum heart rate, everything uncertain about that number flows into this one. Take the resting rate properly, lying still before getting up, since a rate measured after coffee and a flight of stairs quietly shifts the entire zone. And treat the answer as a rough guide rather than a prescription: perceived effort and the ability to hold a conversation are honest signals that no formula can replace, and anyone with a heart condition or on rate-altering medication should get their zones from a clinician.