Zone 2 Is Not a Heart Rate

Why every Zone 2 number you have been given is different, why that is completely fine, and how to find yours.

When it comes to Zone 2, many people are confused. With many viewpoints and perspectives (often contradictory), the confusion is understandable. 

You have seen Zone 2 defined as 75% of your maximal heart rate. You have seen the user-friendly 180 minus age formula. You have seen a range as wide as 70 to 85% of maximal heart rate. Then you read that Zone 2 should feel easy, and somewhere else that it should feel moderately hard. Quite reasonably, you ask: which one is right, and why are the numbers so far apart?

The short answer is that all of them ‘can’ be right, and none of them is technically Zone 2. That sounds like a contradiction, so let’s start at the beginning, because once the underlying concept is explained, the confusion disappears for good.

Zone 2 Is a Metabolic State, Not a Heart Rate

Here is the single most important thing to understand.

Zone 2 is not a heart rate. It is a metabolic state. Heart rate is simply one of several ways we try to see that state from the outside, and it happens to be one of the least precise.

Physiologically, Zone 2 is the intensity sitting just below your first lactate or ventilatory threshold, what the scientific literature calls LT1 or VT1. Below that point your body is in what world expert Dr Inigo San Millan describes as a metabolic equilibrium. You are producing lactate, but you are clearing it just as fast as you make it. Blood lactate sits at roughly 1 to 2 mmol per litre and stays there. Your breathing is up, but you can still hold a conversation. You could keep this going for hours.

San Millan makes the point that lactate here is not a waste product or a sign that you have run out of oxygen. It is a fuel. At this intensity your muscles are making lactate, shuttling it around the body, and burning it in the mitochondria as fast as it appears. Zone 2 is the highest intensity at which that whole system stays balanced. Lactate testing does not create the Zone, it simply makes the equilibrium visible.

So why do the heart rate numbers vary so much? Because heart rate is a proxy for that metabolic state, not the state itself. Two people with identical maximal heart rates can reach their LT1 at very different heart rates, depending on fitness, fibre type and training history. The recent expert consensus paper from Sitko and colleagues, which pooled the views of 14 applied sport scientists and professional coaches, placed the Zone 2 heart rate at roughly 70 to 80% of maximal heart rate.  That is a wide window, and it is wide for a good reason: the physiology it is trying to capture does not sit at the same heart rate in everyone.

When we use 75% of maximal heart rate, or the 180 minus age guide, we are not claiming that these are Zone 2. This is a sensible starting estimate, a place to begin while we confirm the real thing.

The Zones Are a Map, Not a Set of Walls

Here is the most important point, and it clears up most of the remaining confusion. Zone boundaries are not walls. Your physiology does not flip from one mode to another the instant you cross a line drawn on a fresh lab test. The Sitko panel stated that they do not expect the adaptations at the top of zone 1 to be meaningfully different from those in the middle of zone 2, or from those at the bottom of zone 3. It is a continuum. The zones are a map, and like any map they simplify a landscape that is actually smooth.

This is exactly why chasing a single perfect heart rate number is the wrong goal. You are not trying to hit a magic value. You are trying to spend time in a band of intensity where a particular set of adaptations is switched on. Get into the neighbourhood and stay there, and the work gets done.

What Zone 2 Builds

So, what are those physiological adaptations?

At the cellular level, San Millan frames Zone 2 as the highest intensity at which three systems stay synchronised: your redox balance, meaning your ability to keep regenerating the cofactor that glycolysis depends on, your mitochondrial capacity to oxidise fuel, and the lactate shuttle that carries lactate from where it is made to where it is burned. Train repeatedly at this intensity and you build the machinery that runs all three: more mitochondria, denser internal structure, and more of the transporters and enzymes that shuttle and oxidise lactate. You also become better at using fat for fuel, which spares your limited glycogen stores.

The consensus paper lists a similar set of expected adaptations: increased muscle capillarisation, more mitochondrial enzymes in your type I (slow twitch) fibres, better metabolic efficiency, and modest gains in critical power and VO2max. One adaptation is worth highlighting on its own. The expert panel calls it the compression of LT1 toward LT2. As you train, your first threshold shifts upward, so you can move faster and hold more power before lactate starts to climb. In other words, your easy pace gets faster over time. That is what aerobic progress looks like.

There is also a durability benefit that any endurance athlete will recognise and appreciate. A well-developed aerobic base means you fatigue less over long efforts, and you hold more power at the same heart rate and perceived effort late in a session. That is what we mean when we say someone is aerobically fit.

The mitochondrial evidence is also worth exploring. A large meta-regression by Molmen and colleagues, pooling data from almost 6000 participants, found that endurance training, high-intensity interval training and sprint interval training all increased mitochondrial content to a similar degree once you account for how much training was actually done. Per hour of exercise the higher intensities were more efficient, but Zone 2 clearly does the job, and it does it with a fraction of the fatigue cost. That low fatigue cost is precisely what lets you accumulate the volume the adaptations require.

We Cannot Replace Zone 2 With Intervals

The most common objection to Zone 2 is this: If intensity is the potent stimulus, why bother with the easy and moderate work, why not just do the intervals? Here is the problem.

Intervals are the stimulus, but our aerobic base is what lets us perform them and recover from them. The bigger that base, the better every part of that job gets.

The higher our aerobic base, the more power or pace we can hold in each interval, the less we fade across the set, and the faster we recover in the short rest between efforts.

The recovery between hard efforts is an aerobic job: we rebuild our phosphocreatine stores and clear the lactate and hydrogen ions using oxygen, so the size of our aerobic system sets how much we can recover before the next rep.

Tomlin and Wenger showed this directly, with aerobically fitter

people resynthesising phosphocreatine faster, clearing lactate better, and holding their power output across repeated bouts.

There is a fuel side to it as well. When Hetlelid, Laursen and Seiler put well-trained and recreationally trained runners through the same hard session at matched perceived effort and blood lactate, the fitter runners oxidised nearly three times more fat, and the researchers attributed their better performance to exactly that, because burning more fat spares our limited glycogen and helps us hold quality deeper into the interval set.

Put those three together, better reps, less drop-off, and quicker recovery, and we have described durability. It is also why the best endurance athletes in the world spend roughly 80% of their training easy and keep only a small slice hard (Seiler and Kjerland).

In one well-cited comparison, Stöggl and Sperlich found that this polarised split improved key endurance markers more than a threshold-heavy or intervals-only approach. The base is what lets these athletes do the hard work

at true quality and back it up again a few days later. Skip it, and we feel the cost straight away.

Every hard session starts from a lower ceiling, we fade sooner within the set, we recover more slowly between reps and between sessions, and the risk of overreaching and injury increases. Going straight to the intervals does not save us time. It quietly caps how much good intensity we can actually do.

How to Find Your Zone 2

Now the practical question: how do you actually find yours? Think of it as a hierarchy, running from most precise to most convenient.

1.  Lactate testing. The gold standard. A graded test with finger-prick lactate samples lets us see your LT1 directly. Zone 2 sits just underneath it, where lactate holds at around 1 to 2 mmol per litre. This is exactly what San Millan means when he says lactate makes the equilibrium visible.

2.  Breathing and the talk test. If you do not have access to lactate testing, your breathing is a very good guide. At LT1 your breathing rate steps up noticeably. Zone 2 is the intensity just below that point, where you can still speak in full sentences but would find singing a strain. If you have to break your words up to catch a breath, you have gone too hard.

3.  Perceived effort. On the 6 to 20 Borg scale, zone 2 sits at roughly 10 to 12: comfortable and sustainable, clearly working, but a long way from stressed.

4.  Heart rate as a starting proxy. This is where 75% of maximal heart rate, or 180 minus age, becomes such a user friendly tool. Use it to get into the right neighborhood, then confirm with your breathing and perceived effort, and ideally with lactate when you can.

One important caveat on heart rate. Over a long session your heart rate drifts upward even when your pace and effort have not changed. This is cardiac drift, driven by accumulating fatigue, dehydration and rising body temperature. The consensus panel's advice, which we follow, is to prioritize keeping your breathing and perceived effort in the Zone 2 range, even if that means easing off the pace as the session wears on. Do not chase the heart rate number upward. Let the pace come down instead.

Zone 2 is the foundation, not the ceiling. It builds the aerobic engine that lets you tolerate and benefit from hard work, but it does not replace that work. High-intensity intervals deliver stimuli that Zone 2 simply cannot, and a complete program needs both. So, the question was never Zone 2 or intervals? It is Zone 2 and intervals, in the right proportions, in the right order.

Summary

So why are all the Zone 2 numbers different? Because each one is a proxy for something that isn't a number at all. Zone 2 is a metabolic state — the equilibrium just below your first threshold. Heart rate formulas point you in the right direction. Breathing and perceived effort narrow it further. Lactate testing confirms it.

The practical takeaway: aim for the zone, keep it conversational, account for drift, and prioritize time spent there. That's where the adaptations happen.

References:

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Lock M, Yousef I, McFadden B, Mansoor H, Townsend N. Cardiorespiratory fitness and performance

adaptations to high-intensity interval training: are there differences between men and women? A

systematic review with meta-analyses. Sports Med. 2024;54:127-167.

Mølmen KS, Almquist NW, Skattebo Ø. Effects of exercise training on mitochondrial and capillary growth in

human skeletal muscle: a systematic review and meta-regression. Sports Med. 2025;55:115-144.

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