The Rise and Reassessment of Zone 2

Between 2019 and 2023, "Zone 2" went from a coaching term to a cultural one. It was on podcasts, in bestselling books, on the wrists of people who had never heard of a lactate threshold. Then, in 2025, the physiologists started pushing back. This piece traces the whole arc: where the idea came from, why it spread so far beyond its evidence, what the trials and meta-analyses actually show, and — the question that matters — who it is genuinely for.

A disclosure first. We build a Zone 2 app, so we have an interest in the answer. We've tried to let the literature lead, including where it doesn't flatter us.

1Origins: how an observation became a prescription

1.1 The descriptive tradition

The modern Zone 2 story starts with Stephen Seiler. In 2006, Seiler and Kjerland quantified the training of eleven nationally competitive junior cross-country skiers and found a strikingly consistent pattern: roughly 75% of sessions were performed clearly below the first ventilatory threshold, 15–20% contained substantial work above the second, and very little sat between the two^[1]. Seiler's 2010 review extended the observation across rowers, cyclists, runners and skiers: athletes training ten to thirteen times a week converge on a distribution in which about 80% of sessions are low-intensity and about 20% are dominated by hard work^[2].

Two features of this work are routinely lost in retelling. First, it is descriptive: it reports what successful athletes do, and Seiler was explicit that convergence across sports and nations suggests the pattern is a good solution to a constraint problem — how to accumulate enormous volumes of training "at an acceptable level of stress"^[2]. Second, the 80% refers to sessions, in athletes performing enormous numbers of them. The pattern is a property of high-volume training. Nothing in the original work suggested that a person training four hours a week should copy the ratio.

1.2 The mechanistic gloss

The physiological narrative that turned "easy volume" into "Zone 2 as a specific stimulus" came largely from Iñigo San-Millán, working with George Brooks. Their 2018 paper measured blood lactate and substrate oxidation during graded exercise in three groups — professional cyclists, moderately active individuals, and people with metabolic syndrome — and found that the professionals oxidised far more fat and accumulated far less lactate at any given intensity^[3]. The interpretation offered was that this reflected superior mitochondrial function, and that training at the intensity where fat oxidation is maximal and lactate begins to rise — Zone 2 — is what builds it.

That is a coherent hypothesis. It is not what the study tested. The design was cross-sectional: a comparison of different people at one point in time. It cannot distinguish "Zone 2 training produced these mitochondria" from "twenty years of 25-hour weeks produced these mitochondria" from "people with these mitochondria become professional cyclists". The paper is valuable as a diagnostic method; as evidence for a training prescription it is silent.

1.3 The longevity turn

Two things then happened at about the same time. In 2018, Mandsager and colleagues published data on 122,007 patients undergoing treadmill testing at the Cleveland Clinic, showing that cardiorespiratory fitness was inversely associated with all-cause mortality with no apparent upper limit^[4]. VO₂max became the longevity number. And in December 2019, Peter Attia — physician, longevity commentator, and host of one of the most influential health podcasts in the English-speaking world — released an episode with San-Millán titled "Zone 2 Training and Metabolic Health", followed by a second in 2022 and by Attia's 2023 bestseller Outlive^[5].

The synthesis that reached the public was clean and compelling: VO₂max predicts how long you live; mitochondria determine VO₂max; Zone 2 builds mitochondria; therefore Zone 2 is the exercise for longevity. Add that San-Millán was, at the time, the personal physiologist to Tadej Pogačar during his 2020 and 2021 Tour de France wins, and the idea arrived with the strongest possible halo.

2What Zone 2 actually is

Part of the difficulty in evaluating Zone 2 is that it has never had a single definition. A 2025 expert consensus in the International Journal of Sports Physiology and Performance, drawing on fourteen applied sport scientists and coaches, settled on: the intensity immediately below the first lactate or ventilatory threshold (LT1/VT1), which may be delivered as continuous, variable or interval sessions^[6]. That is the definition used throughout this piece, and the one our app targets.

In practice, however, people locate it by very different proxies — a percentage of maximum heart rate, a fixed lactate value, the "talk test", the point of maximal fat oxidation. Meixner and colleagues tested fifty cyclists with the common markers side by side and found coefficients of variation between individuals of 6–29% depending on which was used. VT1 and Fatmax agreed well with each other; fixed percentages of HRmax and fixed lactate values did not^[7]. The practical implication is that much of the "Zone 2" done in the world is not below LT1. Some of it is comfortably above. When a study — or a rider — reports doing Zone 2, it is worth asking which one.

A further ambiguity is that the term collides across zone systems. In Seiler's three-zone model, "Zone 2" is the region between the thresholds — the moderate-intensity band that polarized training deliberately avoids. In the five-zone systems used by most cycling software, "Zone 2" sits below the first threshold. A polarized coach saying "avoid zone 2" and a Zone 2 advocate saying "do 80% zone 2" are frequently describing the same week. It resolves a surprising share of online argument.

3The mechanistic claims, tested

3.1 Mitochondria

The central promise was mitochondrial. The best evidence on it is now very large. Mølmen, Almquist and Skattebo (2024) pooled 353 studies and 5,973 participants in a meta-regression of exercise-induced mitochondrial and capillary growth^[8]. After adjusting for frequency, duration and baseline fitness, mitochondrial content rose by 23 ± 5% with endurance training, 27 ± 5% with high-intensity intervals, and 27 ± 7% with sprint intervals — differences that were not statistically significant. Training frequency mattered: six sessions a week beat four beat two. And per hour of training, sprint intervals were about 3.9 times as efficient as endurance training at increasing mitochondrial content; high-intensity intervals about 1.7 times.

Three things follow. The claim that Zone 2 does nothing for mitochondria is false: a 23% increase is a large, real adaptation, and a 2026 meta-analysis restricted to moderate continuous training confirmed significant gains in mitochondrial volume density, though it rated the underlying evidence as low quality^[10]. The claim that Zone 2 is the optimal or unique mitochondrial stimulus is also false: it is matched by intervals per programme and beaten by them per hour. And there is one adaptation for which low intensity is genuinely superior — capillary density rose 13% with endurance training, 7% with intervals, and not significantly with sprints^[8]. Capillarisation governs oxygen delivery and substrate exchange at the muscle; it is plausibly part of why the long, easy hours matter for durability.

An earlier review by Granata and colleagues adds a useful distinction: training volume appears to drive mitochondrial content, while intensity drives mitochondrial respiratory function — how well each mitochondrion works, not just how many there are^[9]. On that reading the two intensities are complementary rather than competing, which is precisely what the polarized model has always implied.

This is the context for the 2025 narrative review by Storoschuk, Moran-MacDonald, Gibala and Gurd — "Much Ado About Zone 2" — which concluded that current evidence does not support Zone 2 as the optimal intensity for improving mitochondrial or fat-oxidative capacity, and that higher intensities should be prioritised, particularly at low training volumes^[11]. Read alongside Mølmen, that conclusion is correct in its scope. Its scope is the general population and the outcomes are mitochondrial capacity and cardiorespiratory fitness. It is a rebuttal of the longevity synthesis, and it lands.

3.2 VO₂max

Here the evidence is old, consistent and unfavourable to Zone 2. Milanović, Sporiš and Weston's 2015 meta-analysis of thirteen randomised trials found HIIT improved VO₂max by 4.9 mL·kg⁻¹·min⁻¹ against 1.9 for moderate continuous training^[12]. Burgomaster and colleagues showed in 2008 that sprint intervals produced muscle oxidative adaptations comparable to traditional endurance training with a fraction of the time commitment^[13]. If VO₂max is the goal and time is the constraint, low intensity is the wrong tool. There is no serious dispute about this in the literature; the dispute is about whether VO₂max is the right goal.

3.3 Fat oxidation

The fat-burning claim has better support than sceptics allow, but not of the kind its advocates suggest. Training at the intensity of maximal fat oxidation ("Fatmax", which coincides closely with VT1^[7]) does produce meaningful changes in obese adults: across eleven randomised trials, body mass fell by 4.3 kg, fat mass by 4.0 kg, and VO₂max rose by 3.0 mL·kg⁻¹·min⁻¹^[14]. But head-to-head comparisons with higher intensities in that literature were few and inconsistent, and the pooled analysis could not rank them. The honest position is that Fatmax training works for body composition and fitness in that population; whether it works better than the alternatives is unproven. And, as before, the dramatic fat-oxidation figures of professional cyclists are cross-sectional and cannot be attributed to a training intensity.

3.4 Recovery and the autonomic nervous system

The strongest evidence for low intensity is not about what it builds. It is about what it doesn't cost. Seiler, Haugen and Kuffel (2007) measured heart-rate variability recovery in trained and highly trained runners after sessions at different intensities and durations. After 60 or even 120 minutes below the first ventilatory threshold, HRV returned to baseline within five to ten minutes. After sessions at or above VT1, recovery took around 30 minutes in the highly trained and 90 minutes or more in the trained — and, notably, going harder still did not delay it further^[15]. The authors proposed that VT1 acts as a near-binary switch for autonomic stress.

This is the physiological basis of the polarized model, and it reframes the whole question. Low intensity is not valuable because it is a potent stimulus. It is valuable because it is the only intensity at which training load can be added without a recovery cost, which makes it the only lever left once the two or three hard sessions a body can absorb each week have been used. Everything that follows about "who benefits" turns on this.

3.5 Durability

The outcome that VO₂max-centred reviews cannot see is durability, defined by Maunder and colleagues (2021) as the time of onset and magnitude of deterioration in physiological characteristics during prolonged exercise^[16] — put simply, how much you fall apart three hours in. Matomäki and colleagues (2023) trained 35 recreationally active adults for ten weeks with either low-intensity work (about 6.8 h/week) or high-intensity work (about 1.6 h/week), then tested them over three hours. Durability improved similarly in both groups. VO₂max improved only with high intensity — and the change in VO₂max did not correlate with the change in durability^[17].

Two conclusions, pulling in opposite directions, both correct. First: durability is a distinct adaptation, invisible to a ramp test, and the kind of outcome for which an easy-volume programme is designed. Second: in untrained people, the high-intensity group got the same durability gain in a quarter of the time. The population matters enormously here; we return to it below.

4What the intensity-distribution trials show

If the mechanistic case for Zone 2 as a stimulus is weak, the case for polarized distributions as a programme is stronger — with important caveats about who it applies to.

Esteve-Lanao and colleagues (2007) randomised twelve sub-elite runners to five months at either an 80/12/8 or a 67/25/8 distribution. The group that trained easier improved their 10.4 km time by roughly half a minute more^[18]. Small, but in the expected direction. Stöggl and Sperlich (2014) randomised 48 well-trained endurance athletes to nine weeks of polarized, high-volume, threshold or HIIT-dominated training. Polarized produced an 11.7% rise in VO₂peak and a 17.4% rise in time to exhaustion, significantly more than the others. The finding that matters for this piece is what didn't work: the high-volume group, at 83% low intensity with almost no hard work, produced no significant improvement at all. Neither did threshold^[19]. In trained athletes, easy volume without intensity did nothing.

Filipas and colleagues (2022) added periodisation: over sixteen weeks in well-trained runners, a pyramidal block followed by a polarized block outperformed either model held constant, with pyramidal training improving lactate threshold and polarized training slightly favouring VO₂peak^[20]. The distribution is a phase, not a religion.

Then the correction. Rosenblat and colleagues (2025) performed a network meta-analysis of individual participant data from thirteen trials and 348 athletes — half recreational, half competitive — and found no statistically significant difference between polarized, pyramidal or other distributions for VO₂max or time-trial performance. Exploratory subgroups hinted that competitive athletes responded better to polarized and recreational athletes to pyramidal, but neither reached significance^[21]. Cove and colleagues (2025) pooled 41 studies and 797 trained cyclists and concluded that increasing training volume may not produce notable gains in VO₂max or time-trial performance in the well-trained^[22].

A 2025 review by Sun and colleagues supplies the observation that ties this together. Athletes training around 350 hours a year — about seven a week — tend to display threshold-like distributions. Above roughly 750 hours a year, polarized and pyramidal patterns predominate; low intensity makes up 75–91% of preparation-phase training in elite endurance sport^[23]. Nobody legislates this. It emerges from how much training a body can absorb. At fourteen hours a week, most of it has to be easy. At five, it doesn't — and the trial evidence suggests it shouldn't be.

5Who benefits: a population-by-population reading

The literature does not support a single verdict on Zone 2, because it does not describe a single population. Read by group, it is reasonably clear.

Population	What the evidence says	Zone 2's role
High-volume endurance athletes (10 h+/week)	Polarized/pyramidal patterns emerge above ~750 h/yr[23]; easy volume without intensity does nothing[19]; autonomic cost of intensity is the binding constraint[15]	Structural. Most of the week, by necessity. Enables the 2–3 hard sessions that drive adaptation; builds capillaries and durability
Time-crunched recreational athletes (< 6–7 h/week)	Intensity is 1.7–3.9× more efficient per hour[8]; threshold-like distributions emerge at ~350 h/yr[23]; recreational athletes trend toward pyramidal[21]	Supporting. Two hard sessions first. One long easy ride if there's room; otherwise easy days are recovery, not the main event
Long-event athletes (sportives, audax, ultra)	Durability is a distinct adaptation, uncorrelated with VO₂max change[17]; capillarisation favours low intensity[8]	Event-specific. The long easy ride is the closest thing to race preparation. Prioritise duration over frequency
Sedentary adults seeking health	Any modality builds mitochondria; frequency matters more than mode[8]; HIIT better for VO₂max[12] and insulin resistance[24]; Fatmax training effective for body composition[14]; supervised adherence similar for both[26]	Entry point, not ceiling. Excellent for building the habit and the base. Add intensity once established; don't stop there
Older adults	Generation 100: 1,567 people, five years; no mortality difference overall; HIIT vs moderate HR 0.51 (95% CI 0.25–1.02); moderate group no better than controls[25]	Insufficient alone. The assumption that moderate is the safe default for age is not supported; intensity appears at least as important
Injury-prone or returning from injury	Evidence on intensity and injury is conflicting; volume and frequency are also risk factors; speed work trends toward more injury[27]	Plausible, unproven. Lower tissue stress per hour is a reasonable argument, not a demonstrated one

Three of these deserve expansion.

5.1 The time-crunched

This is where the 2025 critique is most correct and where a Zone 2 app has least to offer. A rider with four hours a week who spends them all below LT1 is choosing the least time-efficient stimulus available, in the one population for whom time efficiency is the whole problem. The Mølmen efficiency data, the Milanović VO₂max data, the Stöggl finding that easy volume alone did nothing in trained athletes, and the Sun observation that low-volume athletes naturally drift to threshold-heavy distributions all point the same way^{[8][12][19][23]}. The first two hard sessions in a small week are worth more than the first two easy ones. Zone 2 earns a place — as the recovery glue between hard days, and as one long ride if the goal is a long event — but it is not the main event, and any Zone 2 advocate who says otherwise is selling.

5.2 The sedentary and the longevity-minded

This is the population the longevity synthesis was aimed at, and it is the population for whom the synthesis fails. For a previously inactive adult, any exercise produces large mitochondrial gains, and frequency matters more than mode^[8]. For VO₂max — the actual longevity metric — intensity is decisively better^[12]. For insulin resistance, HIIT outperformed continuous training in a meta-analysis of fifty studies^[24]. The concern that hard exercise is unsustainable is not well supported either: in supervised programmes, compliance was 89% for HIIT and 93% for moderate training, adherence 63% and 68% — a modest difference, not a chasm^[26].

None of this makes Zone 2 a bad place to start. It is accessible, low-risk, pleasant, and it builds the base that makes harder work tolerable. What the evidence does not support is Zone 2 as a destination — the idea that a few easy hours a week is the optimal longevity dose. If VO₂max is the target, the target requires intensity.

5.3 Older adults

The Generation 100 trial is the only long-term randomised test of exercise intensity and mortality, and it deserves more attention than it gets. 1,567 Norwegians with a mean age of 73 were randomised to five years of twice-weekly HIIT at ~90% of peak heart rate, moderate continuous training at ~70%, or national activity guidelines. Overall mortality did not differ between the exercise groups and controls. But the moderate group did no better than controls (hazard ratio 1.24, 95% CI 0.73–2.10), while HIIT versus moderate showed a hazard ratio of 0.51 (0.25–1.02) — a halving of risk that just missed significance^[25]. This is one trial, underpowered for mortality, and it should not be over-read. But it is direct evidence against the common assumption that moderate intensity is the safe and sufficient default in later life.

6Anatomy of a hype cycle

Why did an idea with this evidence base become this large? Because each link in the chain was individually reasonable, and the chain was never tested as a whole.

• A descriptive finding was read as prescriptive. Seiler documented what athletes doing 750+ hours a year do. The finding was applied to people doing 150.
• A cross-sectional study was read as causal. Professional cyclists oxidise more fat. The intensity they train at was credited; the twenty years and twenty-five-hour weeks were not.
• A metric was fused to a method that isn't the best way to move it. VO₂max predicts mortality; Zone 2 is among the least efficient ways to raise VO₂max. The synthesis survived because few people checked both halves.
• The definition was elastic enough to absorb any result. With 6–29% between-marker variability, "Zone 2" could mean anything from recovery pace to tempo. Successes were attributed to it; failures to doing it wrong.
• It was pleasant, and it flattered. A method that says go easier is easier to adopt than one that says go harder, and it carries an implicit compliment: the pros ride slow because they're smart, and now so are you.

The correction was inevitable, and it has arrived from the expected direction — one of the authors of the 2025 review is Martin Gibala, whose laboratory produced much of the foundational interval-training evidence^[11][13]. That is not a conflict of interest; it is a school of thought answering another. The risk now is that the pendulum swings past the evidence in the other direction, and "Zone 2 is oversold" becomes "Zone 2 is useless". The Mølmen data alone should prevent that.

7A defensible position

What follows is our reading, offered as a synthesis rather than a finding.

Zone 2 is a real stimulus, not a special one. It increases mitochondrial content by roughly a quarter, builds capillaries better than any other intensity, and does both with an autonomic cost measured in minutes rather than hours^[8][15]. It is not the optimal per-hour stimulus for mitochondria, VO₂max or metabolic health, and the popular case that it was should be retired^[11][12].

Its value is structural, and it scales with hours. The polarized distribution is what a large training volume looks like when arranged sustainably; it is not a ratio to impose on a small one. Below roughly six or seven hours a week, intensity should be protected first and easy riding fills the gaps. Above ten, easy riding is most of the week because nothing else can be^[19][21][23].

Its unique outcome is durability, and its unique population is the long-event athlete. If your event lasts hours, the long easy ride is your specific preparation, and a ramp test will not tell you whether it's working^[16][17].

It only works if it is actually Zone 2. Fixed heart-rate percentages misplace the threshold for a large minority of people^[7]. Easy riding done fifteen watts too hard accumulates the fatigue of tempo with the stimulus of neither — the moderate-intensity band both camps agree should be avoided. Holding intensity honestly below LT1 is not a detail; it is the entire mechanism. It is also, for what it's worth, the entire reason our app exists.

Go hard two or three times a week. Ride everything else genuinely easy. Do more of the easy riding as your hours allow, and less as they don't. That advice predates the hype, survived it, and survives the correction.

8Limitations of this review

This is a narrative review by a non-academic with a commercial interest, not a systematic one; we searched for the strongest evidence on each side rather than exhaustively. The trial literature on intensity distribution is small, short (mostly 6–16 weeks), and overwhelmingly male. "Zone 2" is defined inconsistently across studies, which means some of what is reported as low-intensity training is not. Outcome measures are dominated by VO₂max and time-trial performance, which under-represent durability, injury, adherence and health outcomes. And the two most important recent analyses — Mølmen's meta-regression and Rosenblat's network meta-analysis — are themselves subject to the quality of the studies they pool, which both sets of authors describe as limited.

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