The easy running everyone suddenly has a name for
Few training ideas have spread as fast as Zone 2. It turns up in podcasts, longevity talks and training plans, usually described as a precise intensity that builds mitochondria, burns fat and lays the foundation for everything else. Runners have done easy, conversational running for as long as the sport has kept training logs, and the new label has brought with it a sense that the exact boundary matters a great deal.
A quick note on scale before the detail. For the vast majority of runners, the biggest improvement comes from running more, consistently, over months and years. Most of that running will be easy, and whether a given run sits a few beats inside or outside a Zone 2 band is a second-order question. Precision becomes a useful refinement once the regular running is in place.
Key takeaways
“Zone 2” means different things in different systems. In the three-zone model used in much of the endurance research, zone 2 is the band between the two thresholds. In most five-zone heart-rate models, Zone 2 is easy aerobic running that sits inside the three-zone model’s zone 1.
Descriptive studies of well-trained endurance athletes find that most of their training is done at low intensity, around or below a blood lactate of about 2 mmol·L⁻¹.
A 2018 study by San-Millán and Brooks found that professional endurance athletes oxidised more fat and had lower blood lactate than less-fit groups during exercise, and that the two measures were tightly and inversely correlated.
That study compared groups at one point in time. It describes what trained metabolism looks like and does not test whether training at one exact intensity produced it.
The strongest evidence supports a large volume of easy running with some harder work alongside it. Claims that one narrow zone is uniquely effective go beyond what the research shows.
Two models, one name
Much of the confusion starts with the numbering.
The endurance research literature mostly uses a three-zone model anchored to two physiological markers. Zone 1 sits at or below the first lactate or ventilatory threshold, where blood lactate begins to rise above resting levels, often around 2 mmol·L⁻¹. Zone 2 runs between the first and second thresholds, up to roughly 4 mmol·L⁻¹. Zone 3 is everything above that. In this model, zone 2 is the “middle” intensity of tempo running and threshold work, the band polarized training tries to keep small. We covered how elite distance runners divide their training across these three zones in polarized vs pyramidal training for marathoners.
The Zone 2 of popular training culture comes from five-zone models, usually set by heart rate. There, Zone 1 is very easy recovery running and Zone 2 is steady, comfortable aerobic running. In three-zone terms, it sits in the upper part of zone 1, below the first threshold. The version popularised in recent years is commonly described in metabolic terms: the highest intensity a runner can hold while blood lactate stays low and fat supplies a large share of the energy.
So a sports scientist talking about “zone 2” from a research paper and a runner talking about “Zone 2” from a watch may mean two quite different efforts. For most runners reading about Zone 2 today, the five-zone meaning applies: easy running near the top of the easy range.
What the research supports
Most training by well-trained athletes is easy
Stephen Seiler’s 2010 review in the International Journal of Sports Physiology and Performance pulled together descriptive studies of nationally and internationally competitive endurance athletes training 10 to 13 times a week. Across those studies, about 80% of training sessions were done at low intensity, around a blood lactate of 2 mmol·L⁻¹, with about 20% dominated by high-intensity work such as intervals at roughly 90% of VO2max. Seiler described athletes as self-organising towards a high-volume approach, with hard sessions placed carefully throughout the training cycle. He also found no convincing evidence that adding more high-intensity training gives already well-trained athletes long-term gains.
This is the solid core of the Zone 2 idea. The athletes in these studies run a lot, and most of it is easy. The review speaks about low-intensity training as a broad band, and it says little about a narrow sub-band within it.
Fat oxidation and lactate move together
The study most often cited for the metabolic version of Zone 2 is San-Millán and Brooks, published in Sports Medicine in 2018. The authors used indirect calorimetry and blood lactate measurements during exercise in three groups: professional endurance athletes, moderately active people and people with metabolic syndrome.
The professional athletes had significantly higher fat oxidation and significantly lower blood lactate than both other groups. Within each group, fat oxidation and lactate were strongly and inversely correlated (r = −0.97 in the athletes, −0.98 in the moderately active group and −0.92 in the metabolic syndrome group). The authors concluded that measuring lactate and fat oxidation together during exercise gives an indirect way to assess metabolic flexibility and oxidative capacity across people with very different metabolic profiles.
That is a useful finding about measurement and about what a highly trained metabolism looks like.
Where the hype runs ahead
The step from that study to “train in Zone 2 to get the same metabolism” is an inference. The design compared different groups at one point in time. It did not assign anyone to a training programme, so it cannot say which training, at which intensity, produced the athletes’ profile. Their years of high training volume, their genetics and their selection into professional sport are all part of the picture.
The intervention evidence is thinner than the confidence around Zone 2 suggests. In a nine-week trial from 2014, Stöggl and Sperlich randomised 48 well-trained endurance athletes to polarized, threshold, high-intensity or high-volume training. The high-volume group, which trained mostly at low intensity, showed little further change in most performance measures over that period, while the polarized group, which combined a lot of easy work with some hard work, improved most. Nine weeks is short and these were already well-trained athletes, so the trial leaves the question open, though it fits Seiler’s description of easy volume and hard sessions working together.
A few other points deserve caution:
Heart-rate zones move. Heart rate drifts upwards during long runs, in heat and with fatigue, so a fixed heart-rate band can mean different efforts on different days.
Formula zones are rough. Zones set from age-based maximum heart rate formulas or generic percentages can be well off for an individual. Lab-measured thresholds are more accurate, and few runners have them.
The boundary is fuzzy. The first threshold is a transition, so the difference between the top of Zone 2 and the bottom of Zone 3 on a given day is small.
Generic zone tables and weekly prescriptions in hours of Zone 2 apply the same numbers to everyone. kaizen is hyper-personalised: it accounts for the overall intensity of the running you actually do, from each run’s pace and elevation, to set your training target for the week. An easy run that drifted a little above a heart-rate band still counts for what it was.
How to use Zone 2 sensibly
What the evidence supports is fairly plain. Most of your running should feel easy enough to hold a conversation and to repeat day after day. Harder sessions have a place alongside that easy running, as they did in the training of the athletes Seiler reviewed. Heart rate is a helpful guardrail for runners who tend to drift into moderate effort on every easy run, and it works best read alongside how the run feels.
The published literature does not give a precise number of Zone 2 hours a recreational runner needs each week, and plans that set one are making a generic judgment. kaizen sets your weekly target from the intensity of the running you’ve completed and recalculates as you run, so a week of mostly easy miles and a week with a harder session each add up in proportion to what they were. You choose how to fill the target, whether that’s easy runs, a long run or a club session.
Sources and how sure we can be
The figures from Seiler 2010 and San-Millán and Brooks 2018 come from the published abstracts. The Stöggl and Sperlich results come from that paper’s abstract. We have not read every full text, and the abstract of San-Millán and Brooks does not report group sizes, so we have not given them.
The three-zone definitions are standard in the research literature. Five-zone models vary between systems, and the description of popular Zone 2 here is a general summary of how the term is commonly used, without a single defining paper behind it. The statements about heart-rate drift and formula-based zones are general physiology without a specific study cited here. Our reading that the metabolic case for a narrow Zone 2 band runs ahead of the evidence is an interpretation of cross-sectional data and short trials. Longer trials in recreational runners would make the picture clearer.
Easy running, week after week
Zone 2 is a new name for a very old part of distance training, and the research behind easy running is solid. The precise boundaries are the least important part of it. What moves nearly every runner forward is a lot of comfortable running repeated over months, with some harder work mixed in, and zone precision sits on top of that as a refinement.
See your easy miles count in kaizen
If you’re putting in the easy running and want to see it add up, kaizen calculates your fitness from your recent running and past races, then sets the weekly training target your goal requires. Every run counts by its pace and elevation, and the target recalculates as the week goes, so you can see how steady, consistent running builds over time. You decide which runs fill it. Open kaizen on iOS or Android and see your fitness.
