Heart rate zones are bands of beats per minute that divide exercise intensity into steps, usually three or five, each defined as a share of your maximum heart rate, your heart rate reserve, or the heart rate at a measured metabolic threshold. They let you label each minute of a workout as easy, moderate or hard and count how much of your training falls in each band.
Every zone system uses the same arithmetic, and the systems still disagree. "Zone 2" on your watch, in a lactate lab and in a training study can describe three different intensities for the same person. The bpm boundaries matter less than knowing which anchor produced them and how far that anchor sits from your own physiology.
01The four zone models
Percent of max heart rate
The simplest model multiplies an estimated maximum heart rate by fixed percentages. Polar's five zones run 50 to 60, 60 to 70, 70 to 80, 80 to 90 and 90 to 100 percent of max, and Polar suggests 220 minus age as the simplest way to estimate the max (Polar, n.d.). Titan and most consumer platforms start here because the only input is one number.
The model is only as good as that number. Tanaka and colleagues pooled 351 studies and 18,712 subjects and found max heart rate follows 208 − 0.7 × age, with 220 minus age underestimating it in older adults (Tanaka, 2001). A longitudinal analysis of 908 maximal tests in 132 people over 25 years produced 207 − 0.7 × age (Gellish, 2007). The HUNT Fitness Study measured 3,320 healthy adults, fitted 211 − 0.64 × age, and reported a standard error of 10.8 bpm around that line (Nes, 2013). At age 60 the three formulas give 160, 166 and 173 bpm. Any of them can miss you by 10 bpm or more, and every boundary moves with the error. The max heart rate entry covers the formulas and field tests in detail, and how to calculate heart rate zones works through the arithmetic for each model.
Percent of heart rate reserve
Karvonen's method subtracts resting heart rate from max heart rate before applying the percentage, then adds it back (Karvonen, 1957).
Target HR = resting HR + fraction × (max HR − resting HR)
The logic is physiological. In 63 adults on a cycle ergometer, percent of heart rate reserve matched percent of oxygen uptake reserve almost exactly, with a slope of 1.00 and an intercept of −0.1. Against percent of VO2 max, the same data gave an intercept of −11.6 and a slope of 1.12 (Swain and Leutholtz, 1997). A treadmill study of 50 adults found the same pattern (Swain, 1998). Reserve-based zones account for two people with the same max and different resting rates, and they move when your resting rate falls with training.
Threshold-anchored zones
Threshold models build the bands around the heart rate at a measured metabolic transition, usually the lactate threshold or a field estimate of it. Andrew Coggan's levels, published by TrainingPeaks, put Endurance at 69 to 83 percent of threshold heart rate and Lactate Threshold at 95 to 105 percent (Coggan, 2026). Threshold heart rate comes from a lab test or a sustained time trial, so these zones cost more effort to set and track your physiology more closely than any percentage of max.
Three physiological zones
Most intensity distribution research uses three zones separated by two measured markers. Zone 1 sits below the first lactate or ventilatory threshold, zone 2 between the first and second thresholds, and zone 3 above the second, the region of the anaerobic threshold and maximal lactate steady state. Seiler's review of elite endurance athletes found about 80 percent of sessions performed at low intensity, around 2 mmol/L blood lactate, with the rest built around hard intervals near 90 percent of VO2 max (Seiler, 2010). That finding is the basis of polarized training.
Five-zone scales can be mapped onto the three. Tønnessen and colleagues published the scale Norwegian champion skiers trained by, averaged from the self-reported cut-offs of 29 elite skiers (Tønnessen, 2014).
| Zone | Typical heart rate, % of max | Typical lactate, mmol/L | Three-zone model |
|---|---|---|---|
| 1 | 54 to 73 | under 1.2 | Below LT1 |
| 2 | 74 to 83 | 1.3 to 2.0 | Below LT1 |
| 3 | 84 to 88 | 2.1 to 3.6 | LT1 to LT2 |
| 4 | 89 to 93 | 3.7 to 5.7 | Above LT2 |
| 5 | above 94 | above 5.8 | Above LT2 |
The authors state that individual adjustments are required. The percentages describe typical elite skiers.
02Why the models disagree
Thresholds sit at different percentages in different people
Iannetta and colleagues tested 100 adults and located each person's lactate threshold and maximal lactate steady state. The lactate threshold fell anywhere from 60 to 90 percent of max heart rate, and maximal lactate steady state from 75 to 97 percent. Women reached both at a higher share of max than men (Iannetta, 2020). A fixed percentage of max therefore lands in a different metabolic domain for different people. At 80 percent of max, one person in that range is still below the lactate threshold and another is already above maximal lactate steady state.
Heart rate reserve narrows part of that gap by accounting for resting rate. Mann and colleagues' review notes that exercise at a fixed percentage of reserve produces a similar between-person spread in blood lactate to exercise at a fixed percentage of max (Mann, 2013). Jamnick and colleagues reviewed the methods and found that fixed percentages of maximal anchors have little merit for producing domain-specific responses, while the first lactate and ventilatory thresholds have reasonable support for marking the boundary between moderate and heavy exercise (Jamnick, 2020). Threshold-anchored zones fit best, with the cost that thresholds need testing and retesting.
One runner, five versions of zone 2
Take a 40-year-old runner with a measured max of 184 bpm, a resting heart rate of 52 bpm, an LT1 at 138 bpm and a threshold heart rate (LT2) of 165 bpm. These values are illustrative.
| Method | Inputs used | "Zone 2" in bpm |
|---|---|---|
| Polar default, 60 to 70% of max | Max 184 | 110 to 129 |
| Coggan Endurance, 69 to 83% of LTHR | LTHR 165 | 114 to 137 |
| 60 to 70% of heart rate reserve | Max 184, resting 52 | 131 to 144 |
| Titan default, 74 to 80% of max | Max 184 | 135 to 147 |
| Three-zone research model | LT1 138, LT2 165 | 138 to 165 |
The first four bands are all presented as easy aerobic work. The top of Titan's default Zone 2 sits 9 bpm above this runner's LT1, so part of a Titan Zone 2 run falls in the research model's zone 2, the moderate band that polarized programs keep small. For a runner whose LT1 sits at 150 bpm, the same Titan zone would sit entirely below threshold. Zone 2 covers the competing definitions of that one band.
Heart rate drifts and lags
Heart rate is a response to work, and it moves on its own clock. During prolonged exercise, stroke volume falls progressively after 10 to 20 minutes, and Coyle and González-Alonso attribute this cardiovascular drift mainly to heart rate rising as body temperature climbs (Coyle and González-Alonso, 2001). A steady pace that starts in Zone 2 can finish in Zone 3 without any change in effort at the muscle. Heart rate also trails changes in workload. Coggan notes that average heart rate may never reach the target range in short VO2 max efforts because of the slow heart rate response and the ceiling set by max heart rate (Coggan, 2026). For short intervals, pace or power is the better target, and heart rate zones describe long steady work more accurately than short repeats.
03How Apple Watch sets heart rate zones
Apple describes its Heart Rate Zones as "a percentage of your maximum heart rate" that are "automatically calculated and personalized using your health data," presented as five segments from easier to harder. The zones are calculated only if your date of birth is entered in the Health app (Apple, n.d.). Apple does not publish the formula. You can switch to Manual under Settings > Workout > Heart Rate Zones on the watch, or in the Watch app on iPhone, and set lower and upper limits for zones 2, 3 and 4.
During a cardio workout, the Heart Rate Zone view in the Workout app shows your current zone and time in it, and the Fitness app on iPhone graphs estimated time in each zone after the session (Apple, n.d.). Titan calculates its own zones from the heart rate samples in Apple Health, so the zone shown on your wrist and the zone Titan assigns to the same minute can differ. Neither is wrong. They use different anchors.
04How accurate wrist heart rate is during exercise
Apple Watch measures heart rate with photoplethysmography. Green LEDs flash hundreds of times per second, and photodiodes read how much light the blood in your wrist absorbs with each beat. The optical sensor covers 30 to 210 bpm (Apple, 2026a). Apple lists what degrades the reading. Low skin perfusion, for example in the cold, some tattoos, and irregular movements such as tennis or boxing all make readings less reliable, while rhythmic movement like running or cycling gives better results. Apple recommends a snug fit on top of the wrist during workouts and suggests a Bluetooth chest strap when the optical sensor cannot get a consistent reading (Apple, 2026b).
The research agrees with that guidance.
- Treadmill, 50 adults. Apple Watch agreed with ECG at a concordance of 0.91, and 95 percent of its readings fell within −27 and +29 bpm of the ECG. Accuracy was best at rest and fell with exercise (Wang, 2017).
- Treadmill, bike and elliptical, 50 adults. Apple Watch reached a concordance of 0.92 across all conditions, second only to the chest strap at 0.996. No wrist device was accurate on the elliptical with moving arm levers (Gillinov, 2017).
- Sitting, walking, running and cycling, 60 adults. Apple Watch had the lowest heart rate error of seven devices, and six devices kept median error below 5 percent during cycling. Error was higher during walking and in participants who were male, had a higher body mass index or had darker skin (Shcherbina, 2017).
- Meta-analysis of 44 studies. Wrist devices averaged a bias of −0.51 bpm on the treadmill, −4.55 bpm during cycling and −7.26 bpm during resistance training. In resistance training the error grew by about 3 bpm for every 10 bpm rise in heart rate (Zhang, 2020).
- Skin tone. Bent and colleagues found no significant difference in accuracy across skin tones, and found absolute error during activity about 30 percent higher than at rest (Bent, 2020). A systematic review of 10 studies found four that reported lower accuracy in darker skin, four that found no effect and two with mixed results (Koerber, 2023).
For zone work, the pattern is clear. Wrist readings are close enough to sort steady running and cycling into zones. They are least reliable where heart rate changes fastest, in intervals, lifting, and sports with irregular arm movement. A Zone 4 interval that reads as Zone 3 may reflect the wrist sensor trailing a fast change in heart rate, where Bent and colleagues found devices differed most. If you do structured intervals or strength circuits and care about the minutes, pair a chest strap with your watch.
05How Titan sets your zones
Titan uses five zones built from percentages of max heart rate. It does not use resting heart rate, so Titan's zones follow the percent-of-max model. Each zone starts at a set percentage and runs to 1 percent below the next, and you can move each start in 1 percent steps. Tap your avatar on Today to open You, open Your Training, and under Heart Rate tap Heart Rate Zones. The heart rate zones help article covers each control.
| Zone | Name | Default range | Training load focus group |
|---|---|---|---|
| 1 | Very Light | 53% to 73% | Low aerobic |
| 2 | Light | 74% to 80% | Low aerobic |
| 3 | Moderate | 81% to 86% | High aerobic |
| 4 | Hard | 87% to 92% | Anaerobic |
| 5 | Maximum | 93% to 100% | Anaerobic |
Titan's defaults sit close to the Norwegian scale above, which places the Low aerobic and High aerobic boundary a few percent below a typical elite skier's LT1.
With Use Custom Max Heart Rate off, Titan compares the single highest heart rate sample in Apple Health over the last 30 days with 220 minus your age, and uses the higher number. The age estimate works as a floor, so a month without hard efforts does not pull your zones down. If you know your max from a race finish or repeated all-out efforts, turn on Use Custom Max Heart Rate and enter it. The value ranges from 80 to 240 bpm.
Every heart rate lands in one of the five zones. Readings below the Zone 1 start count as Zone 1, and anything above the top of Zone 4 counts as Zone 5. Titan keeps the resolved max and thresholds for up to seven days and recalculates immediately when you edit a zone or change the custom max. When the saved thresholds change, a Heart Rate Zones Updated card on Today shows the old and new upper limit for each zone. With no custom max, no heart rate in the last 30 days and no date of birth, Titan falls back to fixed thresholds, with Zone 1 ending at 132 bpm and Zone 5 starting at 168 bpm.
06How Titan uses zone time
Zone minutes feed three parts of the app.
- Daily training load and Exertion. Each workout minute is multiplied by its zone number, so a Zone 1 minute counts 1 and a Zone 5 minute counts 5. Thirty minutes in Zone 2 scores 60 and the same thirty minutes in Zone 4 scores 120. Only heart rate recorded inside a workout counts (training load, Exertion score).
- Training load focus. Trends sorts zone-weighted load into Low aerobic (Zones 1 and 2), High aerobic (Zone 3) and Anaerobic (Zones 4 and 5), each with its load and share of the total, which reads as a three-zone distribution when your boundaries sit near your thresholds.
- Time in zone. The Heart Rate Zones card on Today shows the day's minutes in each zone during workouts. The Cardio distribution chart in Trends shows weekly minutes in Zones 2 through 5 and leaves out Zone 1.
Because load weights each zone by its number, an error in your zone boundaries becomes an error in training load. A max set 10 bpm too high can move minutes down a zone and understates your load, and a max set too low does the reverse. The TRIMP entry covers other ways to turn heart rate into training load.
07Setting zones to your own thresholds
The best zones are the ones anchored to your thresholds. If you have a lab test, or field estimates from a drift test and a time trial, convert each threshold to a percentage of your max and move Titan's boundaries to match. For the runner above, LT1 at 138 bpm is 75 percent of 184 and LT2 at 165 bpm is 90 percent. Lower Zone 2's start to about 64 percent, then set Zone 3 to start at 76 percent and Zone 4 at 90 percent. Zone 2 now ends at LT1 and Zone 4 begins at LT2. Low aerobic, High aerobic and Anaerobic then follow that runner's physiology.
Retest after a block of base training or a clear change in fitness. A rising LT1 heart rate, or a faster pace at the same heart rate, shows the aerobic base improving. Heat and long durations raise heart rate at a given pace through cardiovascular drift, so compare sessions run in similar conditions and at similar points in a workout.
08Further reading
The endurance training guide covers the polarized and pyramidal trials, the field tests for LT1 and LT2, and how to check your measured distribution against your plan in Titan. The performance physiology guide traces the physiology behind thresholds, zone models, cardiac drift and training load.
09References
- Apple (n.d.). View Heart Rate Zones on Apple Watch. Apple Watch User Guide. https://support.apple.com/guide/watch/view-heart-rate-zones-apd897dccddf/watchos
- Apple (2026a). Monitor your heart rate with Apple Watch. Apple Support. https://support.apple.com/en-us/120277
- Apple (2026b). Get the most accurate measurements using your Apple Watch. Apple Support. https://support.apple.com/en-us/105002
- Bent B et al. (2020). Investigating sources of inaccuracy in wearable optical heart rate sensors. npj Digital Medicine 3:18. https://doi.org/10.1038/s41746-020-0226-6
- Coggan A (2026). Power training levels. TrainingPeaks. https://www.trainingpeaks.com/blog/power-training-levels/
- Coyle EF, González-Alonso J (2001). Cardiovascular drift during prolonged exercise, new perspectives. Exercise and Sport Sciences Reviews 29(2):88-92. https://doi.org/10.1097/00003677-200104000-00009
- Gellish RL et al. (2007). Longitudinal modeling of the relationship between age and maximal heart rate. Medicine and Science in Sports and Exercise 39(5):822-829. https://doi.org/10.1097/mss.0b013e31803349c6
- Gillinov S et al. (2017). Variable accuracy of wearable heart rate monitors during aerobic exercise. Medicine and Science in Sports and Exercise 49(8):1697-1703. https://doi.org/10.1249/MSS.0000000000001284
- Iannetta D et al. (2020). A critical evaluation of current methods for exercise prescription in women and men. Medicine and Science in Sports and Exercise 52(2):466-473. https://doi.org/10.1249/MSS.0000000000002147
- Jamnick NA et al. (2020). An examination and critique of current methods to determine exercise intensity. Sports Medicine 50(10):1729-1756. https://doi.org/10.1007/s40279-020-01322-8
- Karvonen MJ, Kentala E, Mustala O (1957). The effects of training on heart rate, a longitudinal study. Annales Medicinae Experimentalis et Biologiae Fenniae 35(3):307-315.
- Koerber D et al. (2023). Accuracy of heart rate measurement with wrist-worn wearable devices in various skin tones, a systematic review. Journal of Racial and Ethnic Health Disparities 10(6):2676-2684. https://doi.org/10.1007/s40615-022-01446-9
- Mann T, Lamberts RP, Lambert MI (2013). Methods of prescribing relative exercise intensity, physiological and practical considerations. Sports Medicine 43(7):613-625. https://doi.org/10.1007/s40279-013-0045-x
- Nes BM et al. (2013). Age-predicted maximal heart rate in healthy subjects, the HUNT fitness study. Scandinavian Journal of Medicine and Science in Sports 23(6):697-704. https://doi.org/10.1111/j.1600-0838.2012.01445.x
- Polar (n.d.). Heart rate zones, the basics of heart rate training. Polar. https://www.polar.com/en/guide/heart-rate-zones
- Seiler S (2010). What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance 5(3):276-291. https://doi.org/10.1123/ijspp.5.3.276
- Shcherbina A et al. (2017). Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort. Journal of Personalized Medicine 7(2):3. https://doi.org/10.3390/jpm7020003
- Swain DP, Leutholtz BC (1997). Heart rate reserve is equivalent to %VO2 reserve, not to %VO2max. Medicine and Science in Sports and Exercise 29(3):410-414. https://doi.org/10.1097/00005768-199703000-00018
- Swain DP et al. (1998). Relationship between % heart rate reserve and % VO2 reserve in treadmill exercise. Medicine and Science in Sports and Exercise 30(2):318-321. https://doi.org/10.1097/00005768-199802000-00022
- Tanaka H, Monahan KD, Seals DR (2001). Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology 37(1):153-156. https://doi.org/10.1016/S0735-1097(00)01054-8
- Tønnessen E et al. (2014). The road to gold, training and peaking characteristics in the year prior to a gold medal endurance performance. PLOS ONE 9(7):e101796. https://doi.org/10.1371/journal.pone.0101796
- Wang R et al. (2017). Accuracy of wrist-worn heart rate monitors. JAMA Cardiology 2(1):104-106. https://doi.org/10.1001/jamacardio.2016.3340
- Zhang Y et al. (2020). Validity of wrist-worn photoplethysmography devices to measure heart rate, a systematic review and meta-analysis. Journal of Sports Sciences 38(17):2021-2034. https://doi.org/10.1080/02640414.2020.1767348
