Field note
Heart rate zones are five effort bands that split every heartbeat of a workout so you can see the shape of the session, not just one flat average.
Two runs can share an average and be nothing alike. Time in each band keeps the distribution the average throws away.
A beat of 150 means one thing for a sprinter and another for someone walking uphill. Zones usually sit on your working range: the span between resting rate and maximum. That span is often called heart-rate reserve.
The common method traces to Karvonen, Kentala, and Mustala (1957): take maximum, subtract resting, and place each boundary at a share of that reserve. Apple Fitness uses a related reserve model. Zone 2 does not start at a fixed number for everyone. A fit heart and a tired one can be working equally hard at very different beats per minute.
The American College of Sports Medicine also classifies intensity as a percentage of maximal heart rate. Reserve and percent-of-max are cousins, not rivals. Both still depend on the two numbers underneath.
| Idea | Honest read |
|---|---|
| Heart-rate reserve | Max minus resting; zones as shares of that room |
| Percent of max | Simpler; ignores resting rate |
| Threshold-based zones | Need a hard-effort test, not a formula alone |
| Session average | Flattens easy jogging mixed with hard intervals |
| Minutes in each band | Keeps the shape of the workout |
Most time in Zone 3 tells you more than an average of 145. A run that bounces between easy jogging and hard intervals can post the same average as a flat tempo run, even though the two sessions ask different things of your body.
Endurance coaches care about where the minutes land. Seiler (2010) reviewed intensity and duration distribution in endurance athletes and described a common pattern of roughly four fifths easy and about one fifth hard. That is an observed pattern in trained athletes, not a quota you must hit. It explains why distribution matters. It does not hand you a weekly prescription.
The easy majority is the same idea behind What is Zone 2 training?: conversational work you can repeat. Zones are the ledger that shows whether that majority actually happened.
Zone 2 is sometimes called the fat-burn zone. That label describes the fuel mix the body tends to lean on at that effort. It is not a promise about weight or calories burned. Treat it as the sustainable aerobic band, and leave the fat-burn name as a footnote about fuel, not a goal.
When labs measure where a runner's easy effort actually ends, the answer lands across a wide spread. A 2025 pooled review summarized in Running Writings put the first threshold anywhere from about 69 to 94 percent of maximum heart rate across seven studies and 140 runners, and the harder threshold anywhere from 80 to 98 percent across nine studies and 160 runners. One in ten sit outside those spreads even so.
Friel's TrainingPeaks guide sets running zones from lactate-threshold heart rate. Threshold scales say little about where easy effort begins unless you also measure that floor. ACSM percent-of-max tables are another common map. Offering one method with no alternative claims more certainty than the literature supports.
A field shortcut many coaches use: run hard for about thirty minutes and take the average beat over the last twenty. That is close enough for setting threshold-based zones. It is still a field estimate, not a lab gas-exchange test.
Maximum is ideally the highest beat you actually recorded in hard work. Where nothing has been recorded, population shortcuts fill the gap. Robergs and Landwehr (2002) traced the history of the familiar "220 minus age" equation and showed how weakly it fits individuals. True maximums scatter widely around that formula. Every boundary can drift when the max is a population average dressed up as yours.
Resting rate should be a calm personal usual, not one rough night. One bad sleep should not move five boundaries.
Zones are also only as honest as the sensor. Gillinov and colleagues (2017) and Pasadyn and colleagues (2019) found variable accuracy in wearable heart-rate monitors during aerobic exercise, especially when intensity jumps. Optical wrists smooth and trail on short sprints, so a brief all-out bout may show less top-zone time than really happened. A chest strap is usually more honest for intervals. That sensor limit pairs with what What does all-day heart rate tell me? says about busy-wrist samples.
Zones do not diagnose fitness or disease. They do not prove you burned fat. Changing how you draw the lines rebins the same heartbeats, so history can look different without the workout itself changing. The honest limit, said once: zone charts are a map drawn from resting and maximum estimates plus an optical or chest sensor, not a lab threshold test unless you actually took one.
Read the minutes in each band. Treat the average as a headline that hid the plot.
I write these because I build Aera, an iPhone app that reads Apple Health against your own baseline and puts the research and the error bars underneath every number it shows you, including this one. You do not need it to use anything above.