Key takeaways
Three-zone models are built on two measured thresholds, while five-zone watch models split the same range by percentage of max heart rate
Fixed percentages of max heart rate or VO2max put different people in different physiological domains, so threshold-based zones are more precise
Heart rate drifts upward over long or hot sessions, so easy days are better capped by pace, power or breathing after the first hour
Your watch shows five heart-rate zones, the research papers talk about three, a cycling coach talks about power zones and a running plan gives paces. They are all trying to describe the same thing: how hard your body is working relative to its own limits. This guide explains how the models line up, which signal to trust when, and why heart rate drifts upward on easy runs. For the basics of the five watch zones, start with VO2max heart rate zones.
3 zone vs 5 zone training models
The three-zone model used in most endurance research is anchored on two physiological breakpoints: the first lactate or ventilatory threshold (LT1/VT1) and the second (LT2/VT2, close to critical speed or power). Zone 1 sits below the first threshold, zone 2 between them and zone 3 above the second. Seiler's review of elite athletes training 10 to 13 times a week found about 80% of sessions at low intensity (around 2 mM blood lactate or less) and about 20% dominated by hard work such as intervals near 90% of VO2max — the basis of 80/20 polarised training.
The five-zone watch model splits the range by fixed percentages of max heart rate. It is convenient, but its borders do not move with your thresholds.
| Three-zone (research) | Physiological domain | Approx. five-zone watch zone | Typical sessions |
|---|---|---|---|
| Zone 1: below LT1/VT1 | Moderate | Zones 1–2 (some of 3) | Easy runs, long runs, recovery |
| Zone 2: between LT1 and LT2 | Heavy | Zones 3–4 | Tempo, marathon pace, sweet spot |
| Zone 3: above LT2 / critical speed | Severe | Zones 4–5 | VO2max intervals, 3–5 km race pace |
The mapping is approximate. A well-trained runner's first threshold might fall at 80% of max heart rate, which is watch zone 4; a beginner's might be close to 70%.
Intensity domains: why percentages of max mislead
A 2020 review by Jamnick and colleagues in Sports Medicine judged intensity methods by whether they reliably place people in the moderate, heavy or severe domain. Their conclusion: prescribing a fixed percentage of VO2max, max heart rate or maximum work rate has little merit for producing distinct, domain-specific responses. LT1, the gas exchange threshold and VT1 had some support for separating moderate from heavy, while critical power and critical speed had the best support for separating heavy from severe.
An earlier review by Mann, Lamberts and Lambert reached a similar view: at the same percentage of VO2max or max heart rate, some people sit above a metabolic threshold and others below it. Threshold-based zones should give a more uniform stimulus, though they need lab or field testing and thresholds vary more day to day than max heart rate. If you only have a max heart rate, the heart rate zone calculator is a reasonable start; a lab test is better (VO2max tests explained).
Heart rate zones vs power zones vs pace
Each signal answers a different question:
| Signal | Measures | Strengths | Weaknesses |
|---|---|---|---|
| Heart rate | Internal load | Reflects heat, fatigue, illness; good cap for easy days | Lags 1–2 min; drifts; wrist sensors can misread |
| Power | External work (cycling, some running pods) | Instant, repeatable, good for intervals | Ignores how you feel that day |
| Pace | Speed over ground | Simple, matches race goals | Distorted by hills, wind, trails, heat |
| RPE / breathing | Perceived effort | Free, integrates everything | Subjective, needs practice |
A practical combination: set interval targets by pace or power, cap easy days by heart rate or breathing, and check both against each other. If you race on the road, the VO2max running calculator turns a recent race time into training paces. For how thresholds and FTP relate to VO2max, see VO2max vs lactate threshold and FTP.
Heart rate drift: what cardiac drift is
Run at the same pace for 90 minutes and your heart rate usually creeps up even though nothing else changes. This is cardiovascular drift. Coyle and González-Alonso described it as a progressive fall in stroke volume after 10 to 20 minutes of exercise, and argued that it is driven mainly by the rising heart rate itself rather than by blood moving to the skin as body temperature climbs.
The size of the drift depends on conditions:
- In a temperate room, nine recreationally active women exercising at 60% of VO2max showed heart rate rising 5.4% and stroke volume falling 11% between minutes 15 and 45 (Wingo et al., 2012).
- In 30 °C heat, trained men who cycled for 120 minutes without drinking lost 3.7% of body mass, and their stroke volume and VO2peak fell by 13.8% and 8.7%. Drinking during the ride largely prevented both (Ganio et al., 2006).
The practical upshot: on long or hot sessions, a fixed heart-rate cap forces you to slow down progressively. That is fine for easy days, but on a long run with race-pace sections, judge the effort by pace and breathing after the first hour. Coaches also use the drift between the first and second half of a steady run, at the same pace, as a rough marker of aerobic fitness. See heat training and VO2max for how heat acclimation reduces this strain.
Why is my heart rate high when running easy?
If your heart rate shoots into zone 3 or 4 on what should be an easy run, work through the usual suspects:
- Your easy pace is too fast. The most common cause. Slow down until you can talk in full sentences.
- Heat, humidity or altitude. All raise heart rate at the same pace.
- Dehydration or a long day on your feet. Lower blood volume means a higher heart rate.
- Poor sleep, stress, alcohol or a coming cold. A raised morning resting heart rate is a clue (resting heart rate, HRV and recovery).
- Sensor error. Wrist optical sensors can lock onto cadence, especially in the first minutes or in the cold. A chest strap settles it (how accurate is your watch's VO2max).
- You are new to running. Early on, the aerobic system lags behind your legs. Walk breaks are fine; the gap narrows within weeks (zone 2 training and VO2max).
If heart rate is unusually high at your normal easy pace for several days in a row, treat it as a fatigue signal and take an easy day or two.
Frequently asked questions
Is 3 zone or 5 zone training better?
Neither is better in itself. A three-zone model maps directly onto your two thresholds and suits planning an 80/20 week, while five zones give finer labels for workouts. What matters is that the boundaries come from your own physiology.
Should I train by heart rate or pace?
Use pace or power to set the effort of intervals, because heart rate lags by one to two minutes, and use heart rate to keep easy days easy. On hills, in heat or on trails, heart rate or effort beats pace.
What is cardiac drift?
Cardiac drift is the slow rise in heart rate during a long steady session at the same pace or power. Stroke volume falls as you heat up and lose fluid, so heart rate climbs to keep cardiac output up.
Why is my heart rate high when running easy?
Common reasons are an easy pace that is too fast, heat and humidity, dehydration, poor sleep, illness, caffeine or a wrist sensor reading cadence instead of pulse. If it stays high for days at the same pace, back off.
Sources
- Seiler S. What is best practice for training intensity and duration distribution in endurance athletes? Int J Sports Physiol Perform, 2010. PubMed
- Jamnick NA, Pettitt RW, Granata C, Pyne DB, Bishop DJ. An examination and critique of current methods to determine exercise intensity. Sports Med, 2020. PubMed
- Mann T, Lamberts RP, Lambert MI. Methods of prescribing relative exercise intensity: physiological and practical considerations. Sports Med, 2013. PubMed
- Coyle EF, González-Alonso J. Cardiovascular drift during prolonged exercise: new perspectives. Exerc Sport Sci Rev, 2001. PubMed
- Wingo JE, Salaga LJ, Newlin MK, Cureton KJ. Cardiovascular drift and VO2max during cycling and walking in a temperate environment. Aviat Space Environ Med, 2012. PubMed
- Ganio MS, Wingo JE, Carroll CE, Thomas MK, Cureton KJ. Fluid ingestion attenuates the decline in VO2peak associated with cardiovascular drift. Med Sci Sports Exerc, 2006. PubMed