Zone 2 is a steady endurance intensity just below your first lactate threshold, where blood lactate stays near 2 mmol/L or lower, you can still speak in full sentences, and fat supplies a large share of your energy. The same label also names two other intensities, a fixed band of max heart rate on most watches and the band between the two thresholds in training research, so the first question to ask about any zone 2 advice is which definition it uses.
The label matters because it sets how hard your easy days are. Ride at a watch's zone 2 when your own first threshold sits lower, and your "easy" hours become moderate work that leaves you tired for the sessions that count. Set it too low and you walk through workouts that could have built more fitness.
01Three definitions share one name
A band of max heart rate on a five-zone watch scale
Consumer devices divide max heart rate into five bands. Polar's zone 2, labeled Light, runs from 60 to 70 percent of max (Polar, n.d.). Apple Watch shows five zones that Apple describes as "a percentage of your maximum heart rate," calculated from your Health data and available only if your date of birth is in the Health app (Apple, n.d.). Titan's default Zone 2, also labeled Light, runs from 74 to 80 percent of max. None of these bands comes from your own lactate curve.
San-Millán's lactate and fat-oxidation definition
The zone 2 of podcasts and longevity books comes from Iñigo San-Millán. He places it at a blood lactate concentration of roughly 1.5 to 2.0 mmol/L, an effort that "feels sustainable for hours but clearly more demanding than a casual walk," and describes it as the highest intensity at which that mitochondrial efficiency can be sustained for hours (San-Millán, 2026). The physiological basis is his 2018 study with George Brooks, which measured blood lactate and fat oxidation during graded cycling in professional cyclists, moderately active adults and people with metabolic syndrome. Fat oxidation fell as lactate rose in every group, with correlations of r = -0.97, -0.98 and -0.92, and r = -0.76 across all data points (San-Millán and Brooks, 2018). In this definition, zone 2 is the top of the easy range, the last intensity before lactate starts to climb.
Zone 2 in the three-zone research model
Most training intensity research uses three zones bounded by two thresholds. The first lactate threshold (LT1) is where blood lactate first rises above resting levels. The second is the highest intensity at which lactate production and clearance still balance, the maximal lactate steady state (Faude, 2009). Zone 1 sits below LT1, zone 2 sits between the two thresholds, and zone 3 sits above the second. Seiler's review of elite endurance athletes put about 80 percent of their sessions at low intensity, near 2 mmol/L of blood lactate or below (Seiler, 2010). In this model, research zone 2 is the comfortably hard tempo band that polarized training keeps small. San-Millán's zone 2 is the top slice of research zone 1.
Lining the three up
Tønnessen and colleagues published the five-zone scale used by Olympic and world champion cross-country skiers, with typical heart rate and lactate values averaged from the self-reported cut-offs of 29 elite skiers and a note that "individual adjustments are required" (Tønnessen, 2014). It links the percent-of-max bands to lactate.
| Five-zone scale (Tønnessen, 2014) | Typical HR, % max | Typical lactate | Three-zone research model | Titan default |
|---|---|---|---|---|
| Zone 1 | 54 to 73% | under 1.2 mmol/L | Zone 1, below LT1 | Zone 1, 53 to 73% |
| Zone 2 | 74 to 83% | 1.3 to 2.0 mmol/L | Zone 1, below LT1 | Zone 2, 74 to 80% |
| Zone 3 | 84 to 88% | 2.1 to 3.6 mmol/L | Zone 2, LT1 to LT2 | Zone 3, 81 to 86% |
| Zone 4 | 89 to 93% | 3.7 to 5.7 mmol/L | Zone 3, above LT2 | Zone 4, 87 to 92% |
| Zone 5 | above 94% | above 5.8 mmol/L | Zone 3, above LT2 | Zone 5, 93 to 100% |
On this scale, San-Millán's 1.5 to 2.0 mmol/L sits in the skiers' zone 2 and in Titan's default Zone 2. Polar's 60 to 70 percent sits in the skiers' zone 1. A research paper's zone 2 corresponds to Titan's Zone 3. Every one of these correspondences assumes your thresholds fall near the population average, and many people's do not.
02What happens in your body at zone 2
Below LT1 you are in what physiologists call the moderate intensity domain. Oxygen uptake rises to a steady value, lactate stays near resting levels, and phosphocreatine and glycogen break down slowly. Storoschuk and colleagues note that the upper edge of this domain ranges from about 24 to 80 percent of VO2 max depending on fitness, which means zone 2 can be walking at a normal pace for a sedentary adult and cycling at about 300 W for an endurance athlete (Storoschuk, 2025).
Fat oxidation peaks within this range, and the peak is broad. In 18 moderately trained cyclists, the intensity of maximal fat oxidation (Fatmax) averaged 64 percent of VO2 max and 74 percent of max heart rate, and fat oxidation stayed within 10 percent of its peak from 55 to 72 percent of VO2 max (Achten, 2002). In 300 healthy men and women, Fatmax averaged 48 percent of VO2 max, equivalent to 61.5 percent of max heart rate, and came later in women than in men. Sex, VO2 max and self-reported activity explained only 12 percent of the differences between people (Venables, 2005). A trained cyclist's fat-burning peak can land in Titan's default Zone 2 while a less trained adult's lands in Zone 1.
Heart rate at a fixed zone 2 pace does not stay fixed. After 10 to 20 minutes of steady exercise, heart rate climbs while stroke volume falls, a pattern called cardiovascular drift. Coyle and González-Alonso argue that the rising heart rate is the main cause of the falling stroke volume (Coyle and González-Alonso, 2001). This drift is the basis of one of the field tests below.
03How to find your zone 2
The four common methods trade cost against precision. All of them locate LT1 or a proxy for it, and zone 2 then becomes the band just below that point.
| Method | What it locates | What you need | Main error source |
|---|---|---|---|
| Talk test | The last stage of comfortable speech | Nothing | Subjective, varies with fatigue and mood |
| Blood lactate | LT1 or a fixed 2 mmol/L point | Lab or portable analyzer, step test | 25 competing threshold definitions |
| Percent of max HR | A population-average band | A known max heart rate | LT1 ranges from 60 to 90% of max |
| Heart rate drift | Aerobic threshold heart rate | 40 to 60 minutes of steady effort | Terrain, heat and pacing changes |
Talk test
Quinn and Coons had 15 adults read a passage aloud during graded treadmill exercise and rate whether they could speak comfortably. At the last comfortable stage they were at 64 percent of VO2 max, 82 percent of max heart rate and an RPE of 12, and those values matched their measured lactate threshold more closely than their ventilatory threshold (Quinn and Coons, 2011). The practical rule is to find the pace where reciting a paragraph starts to feel uncertain and back off slightly. The talk test costs nothing and does not depend on knowing your max heart rate.
Blood lactate
A graded test with finger-prick lactate at the end of each stage gives the most direct answer, provided you know which threshold definition the tester used. Faude and colleagues catalogued 25 lactate threshold concepts, grouped into fixed concentrations such as 2 mmol/L, the first rise above baseline, and methods that estimate maximal lactate steady state (Faude, 2009). A fixed 2 mmol/L point and a first-rise point can land at different heart rates for the same person.
Heart rate as a percentage of max
Percent-of-max zones are easy to set up and poor at finding LT1. Iannetta and colleagues tested 100 women and men and found that LT1 occurred anywhere from 60 to 90 percent of max heart rate, and from 45 to 74 percent of VO2 max. Their conclusion was that fixed-percentage methods "conform poorly to exercise intensity domains" (Iannetta, 2020). A 74 to 80 percent band sits below LT1 for some people and above it for others. The error compounds if your max heart rate itself is an age-formula estimate.
Heart rate drift
Uphill Athlete's drift test estimates your aerobic threshold heart rate, close to LT1, from cardiovascular drift. Warm up for 10 to 15 minutes, then hold a steady effort for 40 to 60 minutes on flat ground or a treadmill without speeding up or slowing down. Divide the average heart rate of the second half by the first half and subtract one. Drift of 3.5 to 5 percent means your starting heart rate was your aerobic threshold. Under 3.5 percent means you should retest 5 bpm higher, and over 5 percent means you started above it (Uphill Athlete, n.d.). Trails and hills break the test, because heart rate follows every change in grade.
Drift also tracks durability over long sessions. In 82,303 recreational marathoners, the ratio of heart rate to speed rose by 16 percent on average between the 5 to 10 km and 35 to 40 km segments, and the runners with the least drift finished faster (Smyth, 2022).
04How accurate your watch is in zone 2
Steady zone 2 work is the easiest case for a wrist sensor. Gillinov and colleagues compared four wrist monitors with ECG in 50 adults. Apple Watch had the best agreement of the wrist devices across all conditions (concordance 0.92), agreed acceptably during cycling and treadmill running, and was the only accurate device on an elliptical without arm levers. No device was accurate on an elliptical with arm levers (Gillinov, 2017). Apple notes that rhythmic movements such as running and cycling read better than tennis or boxing, that cold and some tattoos can interfere with the sensor, and that the watch should sit snugly. Apple Watch can pair with a Bluetooth chest strap when readings are inconsistent (Apple, n.d.).
A zone boundary in the wrong place usually costs more than sensor error. With LT1 anywhere from 60 to 90 percent of max, a boundary set from a formula can miss by 20 or more beats.
05What the evidence says about mitochondria and fat burning
Where the case for zone 2 comes from
The popular argument runs from observation to prescription. Elite endurance athletes train mostly at low intensity (Seiler, 2010), and their lactate and fat oxidation curves differ sharply from those of people with metabolic syndrome (San-Millán and Brooks, 2018). The inference is that zone 2 builds mitochondria and fat-burning capacity, and that going harder would spoil it.
What controlled studies show
Storoschuk and colleagues reviewed this argument in 2025 and concluded that "current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity" (Storoschuk, 2025). They list two problems with the inference. Elite athletes also do large amounts of high-intensity work, so their mitochondria cannot be credited to easy training alone. Their volumes, often above 20 hours per week, are also far beyond the roughly 150 minutes per week that public health guidelines target.
The trials they gathered are mixed. Four weeks of zone 2 training did not raise citrate synthase activity, a marker of mitochondrial content, or mitochondrial respiration in recreationally active men. Five months of training with 86 percent of volume in zone 2 did not raise citrate synthase or succinate dehydrogenase in elite athletes. Twelve weeks of zone 2 cycling three times a week did speed phosphocreatine recovery, another mitochondrial marker, in healthy men and in men with type 2 diabetes. Duration seems to matter within a session. One study reported increased expression of PGC-1α, a gene that drives mitochondrial growth, after 60 and 90 minutes of zone 2 but not after 30 (all as summarized in Storoschuk, 2025).
Direct comparisons favor harder work for the same total work. MacInnis and colleagues trained one leg of each of 10 men with intervals and the other with continuous cycling at 50 percent of peak power, matched for total work. After six sessions, citrate synthase activity was 10.2 against 8.4 mmol per kg protein per minute in favor of the interval leg, with greater mitochondrial respiration as well (MacInnis, 2017). Granata and colleagues' review of training studies found that total training volume tracks changes in mitochondrial content, while relative intensity tracks changes in mitochondrial respiratory function (Granata, 2018). If volume drives content, zone 2 earns a place as the intensity that lets you pile up hours. Storoschuk and colleagues read the same analysis more strictly, as showing that training below about 60 percent of maximal work rate is not expected to raise mitochondrial content or respiratory capacity.
Fat oxidation
Zone 2 training does raise fat oxidation in untrained people. One year of confirmed zone 2 training increased Fatmax and maximal fat oxidation in previously sedentary adults, and 12 weeks at 40 percent of VO2 max increased fat oxidation in men with obesity. Training at higher intensities produces similar gains, and meta-analyses comparing interval training with moderate continuous training find no difference or small advantages for intervals in adults with overweight or obesity (as summarized in Storoschuk, 2025). Some individual studies favor zone 2 and others favor higher intensities, and the reviewers found no convincing evidence that zone 2 produces larger gains.
Where the evidence is thin
Storoschuk's review found few studies that prescribed zone 2 as it is commonly defined, with lactate checked to stay below 2 mmol/L. Most of the evidence on either side comes from studies at intensities that only approximate zone 2. The absence of proof for zone 2's superiority partly reflects the absence of trials designed to test it. The case for long easy sessions in endurance athletes rests on different ground, the ability to train more hours without the fatigue that intensity carries, and the review does not dispute that.
06How much zone 2 you need
The answer depends on your total training time.
Public health guidelines call for 150 to 300 minutes of moderate-intensity activity per week, or 75 to 150 minutes of vigorous activity, or a combination (Bull, 2020). Depending on your fitness, zone 2 can fall anywhere from light to vigorous on the guideline scale, and for unfit people it may not reach moderate intensity at all. Storoschuk's review notes that the ACSM guidelines treat 150 minutes of moderate work as possibly insufficient to raise cardiorespiratory fitness, and it argues that people training at guideline volumes should prioritize higher intensities (Storoschuk, 2025). If you have three hours a week or less, spending all of it in zone 2 likely gives up VO2 max gains that one or two harder sessions would produce.
At higher volumes, easy training carries most of the load. Tønnessen's Olympic and world champions trained about 800 hours in their best year and did about 90 percent of their endurance time below LT1. By their own five-zone scale, 86 percent of that time was in zone 1 and only 5.3 percent in zone 2, at most about 1.3 hours a week in any training phase (Tønnessen, 2014). Even the athletes whose habits inspired the zone 2 trend spent most easy hours below San-Millán's range.
Seiler's elite athletes, training 10 to 13 times a week, made about 20 percent of their sessions hard, which works out to two or three a week (Seiler, 2010). A practical reading of the evidence is to keep the hard sessions you can recover from and make most of the remaining time easy, at or below the top of your calibrated zone 2. Sessions of 60 to 90 minutes are more likely to produce mitochondrial signaling than 30 minutes at the same intensity, based on the limited data above. The endurance training guide covers distribution targets in detail, and heart rate zones covers zone systems in general.
07How Titan measures time in zone 2
Titan sorts heart rate into five zones set as percentages of your max heart rate. The defaults start at 53, 74, 81, 87 and 93 percent, so Zone 2, labeled Light, runs from 74 to 80 percent. Titan rounds each boundary to the nearest whole bpm. With Use Custom Max Heart Rate off, Titan compares the highest heart rate in Apple Health over the last 30 days with 220 minus your age and uses the higher number. You can set your own max instead, from 80 to 240 bpm. The heart rate zones help article documents the details.
For each workout, Titan reads the heart rate samples recorded during the session. If the samples linked to the workout cover less than 70 percent of its duration, Titan also reads samples between the workout's start and end times. The Heart Rate Zones card on Today shows your workout minutes in each zone for the day, and each workout's detail screen shows the same breakdown for that session.
Zone 2 minutes feed three other numbers. Each workout minute is weighted by its zone number, so 30 minutes in Zone 2 adds 60 to your daily training load and 30 minutes in Zone 4 adds 120. Training load focus in Trends groups Zones 1 and 2 as Low aerobic, Zone 3 as High aerobic, and Zones 4 and 5 as Anaerobic. The Cardio distribution chart in Trends plots weekly minutes in Zones 2 through 5 and leaves out Zone 1.
A worked example
A runner with a 185 bpm max gets a default Zone 2 of 136 to 148 bpm. A drift test on a treadmill puts their aerobic threshold at 132 bpm, 71 percent of max. With the defaults, every minute between 136 and 148 bpm counts as Zone 2 while sitting above their LT1, which is research zone 2.
To fix it, tap your avatar on Today to open You, open Your Training, and under Heart Rate tap Heart Rate Zones. Each zone's start moves in 1 percent steps and must stay above the zone before it, so lower Zone 2's start first, to about 64 percent. Then set Zone 3 to start at 72 percent. Zone 2 now runs from 118 to 131 bpm and ends one beat below the measured threshold. Easy runs capped at 131 bpm count as Low aerobic, and anything above shows up as High aerobic in Training load focus. Retest every six to eight weeks of base training. A rising aerobic threshold heart rate, or a faster pace at the same heart rate, shows the base is building.
08References
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- MacInnis MJ et al. (2017). Superior mitochondrial adaptations in human skeletal muscle after interval compared to continuous single-leg cycling matched for total work. Journal of Physiology 595(9):2955-2968. https://doi.org/10.1113/JP272570
- Polar (n.d.). Heart rate zones. Polar. https://www.polar.com/en/guide/heart-rate-zones
- Quinn TJ, Coons BA (2011). The Talk Test and its relationship with the ventilatory and lactate thresholds. Journal of Sports Sciences 29(11):1175-1182. https://doi.org/10.1080/02640414.2011.585165
- San-Millán I (2026). Zone 2 is a metabolic equilibrium. Substack. https://inigosanmillan.substack.com/p/zone-2-is-a-metabolic-equilibrium
- San-Millán I, Brooks GA (2018). Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals. Sports Medicine 48(2):467-479. https://doi.org/10.1007/s40279-017-0751-x
- 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
- Smyth B et al. (2022). Decoupling of internal and external workload during a marathon, an analysis of durability in 82,303 recreational runners. Sports Medicine 52(9):2283-2295. https://doi.org/10.1007/s40279-022-01680-5
- Storoschuk KL et al. (2025). Much ado about Zone 2, a narrative review assessing the efficacy of Zone 2 training for improving mitochondrial capacity and cardiorespiratory fitness in the general population. Sports Medicine 55(7):1611-1624. https://doi.org/10.1007/s40279-025-02261-y
- 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
- Uphill Athlete (n.d.). Understanding the heart rate drift test, a practical guide for endurance athletes. Uphill Athlete. https://uphillathlete.com/aerobic-training/heart-rate-drift/
- Venables MC et al. (2005). Determinants of fat oxidation during exercise in healthy men and women, a cross-sectional study. Journal of Applied Physiology 98(1):160-167. https://doi.org/10.1152/japplphysiol.00662.2003
