Maximal Heart Rate and Training Zones

Heart rate zones are percentages of the athlete's maximal heart rate. Every number on the watch during a cardio session, from the warm-up walk to the last interval, comes from that one maximum. If the maximum is wrong, every zone is wrong in the same direction: an estimate that is too low turns an easy jog into an alarm, and one that is too high lets an "easy" session drift into hard work. This document says how the maximum is estimated, what a heart rate recorded in training can and cannot tell, and what to do when the athlete's age is not known.

Estimating the maximum from age

Maximal heart rate falls with age, by about seven beats per decade. The equation that best describes this in healthy adults is 208 − 0.7 × age. It comes from a meta-analysis of 351 studies with more than 18,000 subjects, and a laboratory study of 514 adults that measured the maximum directly gave almost the same line. The relation did not differ between men and women, nor between sedentary and trained people. A study that followed the same adults through repeated maximal tests over 25 years found almost the same slope.

The older rule, 220 − age, has no comparable evidence behind it. It overestimates the maximum of young adults and underestimates that of older adults: at 30 years the two rules agree within a few beats (187 and 190), at 60 they differ by about six (166 and 160).

Any age equation is a population average. In a study of 762 sedentary adults who were tested to exhaustion, the 208 − 0.7 × age equation had no systematic error, but an individual's true maximum fell within about ±11 bpm of the estimate in two cases out of three, and within about ±22 bpm in 95% of cases. The estimate is a good starting point and a poor final answer.

What a heart rate recorded in training says

The highest heart rate recorded in a session is never above the athlete's true maximum, unless the sensor is wrong. It is a lower bound: the maximum is at least that high. It is not the maximum itself, because most sessions never come close to an all-out effort. A walker whose highest recorded value is 105 bpm has a maximum far above 105; using 105 as the maximum would put zone 1 at 53–63 bpm, below the resting heart rate of many adults.

So a recorded peak can raise the estimate from age, but never lower it.

Recorded peaks also carry sensor error. Chest straps measure the electrical activity of the heart and agree almost perfectly with an ECG. Wrist sensors measure blood flow optically: in a comparison across treadmill, bike and elliptical, their error ranged from about ±15 to ±34 bpm depending on the activity, and was worst at high intensity with arm movement. A known failure in running is "cadence lock", where the wrist sensor reads the rhythm of the arm swing, about 160–180 per minute, as the heart rate. A single high value is therefore weak evidence.

The rule this app applies

1. With the athlete's age, the estimate is 208 − 0.7 × age, rounded.
2. A recorded peak raises it only when it is credible:
   • it is the highest value reached in at least two cardio sessions of the last 90 days, so a single spike does not count;
   • it is no more than 22 bpm above the estimate from age. Above that, outside the range where 95% of real maxima fall, the value is treated as sensor error and ignored.
3. The maximum is the higher of the estimate and the credible peak.
4. Without the athlete's age, there is no estimate to correct, and a peak alone only says the maximum is at least that high. The zones are then used by name only, with no beats per minute: "zone 2" instead of "111–130 bpm". Apple Watch applies its own zones, computed from the age in the Health app. Once the age is known, the numbers come back.

Points 2 and 4 are working assumptions of this app, built on the evidence above rather than taken from a study that tested them directly.

The five zones

The zones are bands of 10% of the maximum, the same model most sports watches use by default:

Zone	% of maximum	Name	Used for
1	50–60%	Recovery	Warm-up, cool-down, walking recoveries
2	60–70%	Aerobic	Easy and long sessions, the base of the week
3	70–80%	Tempo	Steady tempo work
4	80–90%	Threshold	Threshold repeats, strides
5	90–100%	VO2max	Short hard intervals

These bands sit inside the general classification of exercise intensity used in exercise guidelines, where moderate intensity is roughly 64–76% of the maximum and vigorous intensity roughly 77–95%. In this app, swimming sessions carry no heart rate target.