Zone 2 has become the most talked-about intensity in endurance training and one of the most confused. The term means at least three different things, and the versions disagree by enough to be the difference between a recovery jog and a hard tempo run. That is worth untangling before you build a training week on it.
Three definitions, and they do not match
The research model. Almost every study of training intensity uses three zones, split by two thresholds: below the first lactate or ventilatory threshold, between the two, and above the second. This is the model behind every “80/20” statistic you have read.
Your watch. Garmin, Polar and most consumer devices use five zones by percentage of maximum heart rate, where Zone 2 is typically 60–70% of maximum.
The podcast version. Iñigo San Millán, whose name is attached to most of the current interest, defines it metabolically — roughly 1.5 to 2.0 mmol/L blood lactate, which sits at about the first threshold.
Here is the size of the disagreement
A 2025 study measured thresholds in 165 recreational runners. The first lactate threshold occurred at 83.6 ± 4.2% of maximum heart rate in women and 78.9 ± 4.4% in men.
So for a typical recreational runner:
- Your watch’s Zone 2 (60–70%) sits roughly 9 to 14 percentage points below your first threshold.
- San Millán’s Zone 2 lands in your watch’s Zone 4.
Someone diligently holding 65% of maximum because a screen says “Zone 2” is training considerably easier than the physiologist whose work they are following. Neither is wrong — they are different intensities with the same label. And no, five-zone “Zone 2” is not the same as three-zone “Zone 1”; it covers roughly the bottom half of it.
A study of 50 cyclists compared eight competing definitions of Zone 2 and found variation between them of 6% to 29%, with one definition producing about 25% lower power output than another. The disagreement is measurable, not theoretical.
Is low intensity actually special?
Here the honest answer is less exciting than the enthusiasm suggests. A 2025 review in Sports Medicine examined exactly this question and concluded: “current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity.”
The study most often cited as the foundation for Zone 2 training is worth knowing about. It compared 22 professional cyclists, 20 moderately active men and 10 men with metabolic syndrome — 52 people, all male, all cycling — in a single exercise test each. It has no training intervention at all. It shows that fitter people burn more fat at a given workload, which describes trained people rather than demonstrating what training made them that way.
The defensible case for easy running is simpler and still good: it lets you accumulate volume without accumulating fatigue. That is not magic. It is arithmetic, and it is why the elite distribution looks the way it does.
What runners actually do
World-class distance runners perform at least 80% of running volume at low intensity across the training year, on 160–220 km a week. That part of the story holds up.
We previously used the next finding to argue against the “grey zone” idea — that recreational runners cluster at a moderate intensity, training too hard on easy days. Re-reading the study, it supports that idea rather than refuting it, so here is the correction. Across 151,813 marathons by 119,452 runners the distribution was pyramidal, but the split was 49.0% easy, 35.3% moderate and 15.7% hard — better than a third of all training sat in exactly the moderate band the grey-zone critique warns about, and the polarisation index stayed near 1.25 at every ability level. Even the fastest men reached only about 67% easy running. What faster runners did differently is the useful part: moderate and hard training stayed roughly flat across ability levels while the proportion of easy running rose as marathon times got faster.
Is polarised training proven best?
No — and this is where the evidence thins out badly.
The most-quoted meta-analysis found a moderate effect favouring polarised over threshold-heavy training. It pooled three studies, all rated 4 or 5 out of 10 for quality. A larger 2024 meta-analysis of 17 studies found a small VO2max advantage that vanished in interventions lasting 12 weeks or longer, and no advantage at all on time-trial performance.
Most decisively, a 2025 network meta-analysis pooling individual data from 348 athletes — with Stephen Seiler, who originated the polarised model, as a co-author — found no significant difference between polarised and any other distribution, including pyramidal. What every dataset agrees on is that total volume, not its distribution, tracks with performance.
The measurement problem
All of this assumes you know your maximum heart rate. You almost certainly do not.
“220 minus age” was never derived properly. The definitive meta-analysis of 351 studies found individual standard deviations of 7 to 11 beats per minute around any age-based estimate, and noted the 220 figure was arrived at essentially arbitrarily. A study of 4,375 endurance athletes found age formulas underestimated actual maximum by 5–6 beats on average, with limits of agreement spanning roughly 28 beats. A 2025 comparison of seven equations concluded none “demonstrated high individual level accuracy”.
An error of ten beats on your maximum is wider than the zone you are trying to hit.
And the Maffetone “180 minus age” method has never been validated. Searching the published literature turns up no empirical validation study. The only peer-reviewed paper on it is by Maffetone himself, classified as “Hypothesis and Theory”, in which the authors state that the relationship “must be verified scientifically” and that the formula is “not a replacement for laboratory testing”. That is a fair description of a proposal. It is not evidence.
So what should you actually do?
- Stop using 220 minus age. If you use a formula at all, 208 − 0.7 × age is better derived — but treat any resulting zone as a wide band, not a line.
- Anchor to a recent race, not a formula. Estimating your maximum instead of measuring it increased threshold estimation error from 6.6 to 8.4 beats in the 165-runner study. Pace off something you actually ran.
- Use the talk test as a ceiling. Full sentences, comfortably, the whole way. Its failure mode — running slightly too easy — is the harmless one. Be aware it has been validated mostly in cardiac rehabilitation on bicycles rather than in runners, with correlations of 0.37 to 0.60. It is a guide, not a measurement.
- Cross-check with effort. In 2,560 people tested to exhaustion, the first lactate threshold sat at an average of 10.8 on the 6–20 scale (±1.8) and the second at 13.6 — so genuinely easy running is “light”, roughly 10 to 12, not the 13 to 14 most people drift into.
- Expect your real first threshold near 79–84% of maximum, not 60–70%. If you are holding 65% because a watch said so, that is a fine recovery run. It is not what the elite data is describing.
The single most defensible conclusion in this entire literature: run easy most of the time so you can run more, and do not spend much energy on the decimal places.
References: Nuuttila O-P, et al. Eur J Appl Physiol, 2025. doi:10.1007/s00421-025-05748-8 · Storoschuk KL, et al. Much Ado About Zone 2. Sports Med, 2025. doi:10.1007/s40279-025-02261-y · Rosenblat MA, et al. Sports Med, 2025. doi:10.1007/s40279-024-02149-3 · Silva Oliveira P, et al. Sports Med, 2024. doi:10.1007/s40279-024-02034-z · Tanaka H, et al. J Am Coll Cardiol, 2001 · Scherr J, et al. Associations between Borg’s rating of perceived exertion and physiological measures of exercise intensity. Eur J Appl Physiol, 2013;113(1):147–55. · Muniz-Pumares D, et al. The training intensity distribution of marathon runners across performance levels. Sports Med, 2025;55(4):1023–35. · Maffetone P, Laursen PB. Front Physiol, 2020. doi:10.3389/fphys.2020.00296