Stop Arguing About Zone 2

The Argument of Intensity or Volume is Dumb. Let's Stop

If you’ve been in the exercise game long enough, you know everything follows a hype cycle. It gains in popularity, everyone is talking about it, and then suddenly the tables turn, and everyone is screaming, “X is overrated.”

Why does this occur? Because people initially want to jump on the new thing, but then as it becomes saturated, it no longer sets you apart to jump on the thing, so you go the other direction. You gain notoriety pointing out that the hyped thing isn’t the bees knees. It occurs with training, diet, you name it.

It occurred in the 2000-2010ss with HIIT and Tabata training. And now it’s occurring with ‘zone 2’ training.

It’s one of the most frustrating phenomenon in all of exercise land because it confuses the lay person, and most of the time the answer is somewhere in the middle. So if you ground yourself in that middle, you just watch people scream intensity or volume, as they whizz past you going way too far in one direction or the other in order to make a name for themselves.

Which brings us back to Zone 2. What is it? A fancy word for what we used to call normal easy runs. Your typical base building speed. Not slogging around so you’re recovering, but not going so fast that you’re getting closer to your tempo or threshold pace. It was largely popularized by folks like Peter Attia as the “best” way to train in the 2020s. But it’s origin came much much earlier, as we’ll get to shortly.

Recently, there’s been a push back. There’s even been a research review called “Much Ado About Zone 2” that’s made the rounds that it’s not the be all end all.

So…let’s break it all down. Is Zone 2 the greatest or a fraud, or somewhere in between? First, a brief history…

The Volume-Intensity Debate

For over 120 years, endurance training has swung back and forth between folks emphasizing intensity and those saying volume. At first, it was BIG swings. Coaches and athletes doing lots of walking versus those doing all short intervals. Over time, the swings have become more nuanced. Instead of arguing over either/or it’s the best combination of each. Everyone agrees that to run your best 10k you need a decent volume of easy running, now we just quibble over the details, whether that’s 70mpw or 100mpw, whether it’s tons of threshold or just enough, etc.

But in research land, we didn’t make as much progress. While, in coaching we left the extreme debates behind by about the 1960s. In research world, the idea of whether we should do mostly HIIT or mostly Zone 2 is still an argument.

Why? A few reasons. First in coaching land, performance is sacrosanct. You tell really quickly whether a model of training works or not based on real world results. So if something comes along and shows far superior results, it gets adopted and sticks around. That direct feedback mechanism is what helps training evolve relatively quickly.

In the lab, we don’t utilize this nice neat mechanism. Instead, we often measure surrogate markers or mechanisms. Instead of judging based on multiple performances over years, we measure things like VO2max, LT, Running economy, or maybe some enzymes or more invasive shifts to see what’s going on in the body. These are important. It helps us understand WHY training does what it does. But every level down we go, we get a little more disconnected from the thing that matters and tells us the most (holistic performance gives a sense of how the entire system functions together). So if performance is at the top, then the next level down is more direct contributors like VO2max or running economy, and the next level down is more indirect mechanistic stuff (muscle fiber, enyzmes, mitochondria, Red blood cells, etc.).

Second, in the performance arena we get to see long-term results. How training impacts an entire team for years on end with performance data. In the lab, we’re often looking at 6-10 week snapshots. That short term effect can confuse folks on what actually works. (Think of it like this. If I measured performance after a 6 week based period or a 6 week intense sharpening period, performance would improve more after 6 weeks of sharpening. But does that mean the intense work is better then the base? Of course not. If I tried to sharpen for 24 weeks, I’d be training like it was 1940, and my performance would eventually stagnate or I’d burn out…)

So what? While in the real world, we saw a natural evolution of training towards a mixture that includes lots of easy, a solid amount of moderate, and some intense, with shifting depending on the time of year…In the lab we saw a never ending debate over HIIT vs. Zone 2, or any iteration.

In the 2000s, Stephen Seiler tried to help out by doing research based on what he was seeing in the real world. This is where the 80/20 principle (80% easy) came about. Now it’s important that this was a real of thumb, not an exact prescription. But Seiler’s work was important, because it dragged us out of the HIIT/VO2max focused training of the 1990s and said, “wait, people actually doing this at the highest level don’t follow our advice, maybe we should ask why.”

Which brings us to today…

The Zone 2 Hype Cycle

While Seiler was observing elite endurance athletes putting in lots of relatively slow training, this wasn’t new. It’s the innovation that Lydiard and others brought to the forefront all the way back in the 1960s. It took us away from the days of Zatopek and lots of 400m repeats into the modern era where we first accumulated lots of easy to moderate training. So this isn’t new, it’s long established in the real world.

Seiler and others yanked us out of the VO2max/intensity doldrums and science being science, they couldn’t settle for the easy intstructions to just run easy enough where you can talk, added a “zone” around it. One that if we’re honest is pretty arbitrary. No, it’s not tied to precise adaptations where if you train a little too slow or too fast you miss out. It’s mostly tied to arbitrary physiological markers that allow us to say there’s a meaningful difference. Most of that is overblown. In truth, it just means: mostly easy.

But…people who aren’t coaches like Peter Attia took this zone classification to the absurd end, telling us his zone was exactly between 1.7 to 1.9 mmol of lactate.

And that’s where we went wrong. We not only over-indexed on a false precision, but folks started claiming it did everything. You started to hear a buzz word bingo of what zone 2 did best: mitochondria biogenesis, capilirization, and so on. We went wild for mechanisms to explain WHY folks needed easy running.

Why? Because of sciencyness. As someone in the actual arena explaining to a new person, it doesn’t convince many people that we’ve known this for 60 years because some former milkman experimented with training, his runners dominated, and then everyone else tried a similar approach and it was adopted. It sounds a lot better to use fancy terms. Everyone knows mitochondria are the powerhouse of the cell. Who wouldn’t want more of that?!

So…Is Zone 2 Out?

So what we ended up with is a kind of meme version of the original real world classification of easy running. A false precision, over-indexing on mechanisms, and with the nuance of timing or periodization of training mattering a lot.

And that’s where the current research backlash comes in. It’s critiquing the meme easy running, not the actual thing. Before we dive into what the research got wrong, let’s start with what it got right.

1. There is no magic zone 2 switch. Nothing magically shifts from 1.8 to 2.0 mmol/l or what have you.

2. The mechanistic work on mitochondria development varies so much, that no we can’t say zone 2 is the be all end all for mitochondria biogenesis.

3. If all your doing is easy training, no you won’t max out aerobic abilities.

4. Yes, Attia and others over sold the mechanisms and the precision.

All of that is true and accurate. The hype cycle took us from Lydiard and Seiler’s observation to “do lots of easy running” and gave us some bastardized version of running between 1.8-2.1 mmol/l will solve all your problems…

As I mentioned in the history section, no one in the real world debates whether or not we need easy running or even intense training. We accept you need all of it. It’s not even up for debate. But…where the research review got wrong is they ignored what Lydiard and 100+ years of coaches figured out that many scientists still can’t seem to understand… we need to leave the false dichotomy of easy versus intense running behind. The devil is in the details, not one over the other. They are synergetic, not oppositional.

People like Attia and even more so Inigo San Milan never argued for either/or. They argued for building a foundation and topping it off with other intensities. It’s the same argument Seiler made. He didn’t say elite endurance athletes weren’t doing intense work, just way less than the research was suggesting at the time.

Let’s walk through some mistakes.

1. Zone 2 is horribly defined everywhere…all the time.

Part of the problem in this research and others is that zone 2 is kind of nebulous. In the review, they included research that looked at everything from what I’d consider a fast walk to easy running in some cases. And then waved away studies that were broader or may have included some zone 3 or what have you. This creates problems, because of course there’s a minimum intensity (that varies by person) that will give us lots of aerobic adaptations. A well-trained runner isn’t getting a boost from lots of walking, no matter how long he’s going for. And on the other side, almost no one trains exclusively in ‘zone 2’ so it eliminates any observational studies that just follow base building or regular endurance training.

I’m not entirely faulting the researchers her, it’s why I hate zone 2 as a concept.

When a kind of nebulous concept that was really developed as a kind of rule of thumb for coaches (the 5 zone training model), is rigidly applied, it doesn’t work. First, because few studies rigidly look at zone 2 training, because it’s really hard to define and control. Secondly, because of this, it allows you to pick and choose what’s included as zone 2.

2. It’s mechanism focused

One of the problems is that it’s mechanism focused, not performance. Which isn’t inherently bad, but when you’re considering mechanisms, you have to understand the nuance we discussed previously. For example, one study they pointed to found that 5 months of easy training didn’t improve citrate synthase activity ( a marker of mitochondria changes)….Well, that was in elite endurance athletes! Who almost certainly had topped off their easy volume based adaptations from years of lots of easy. As Canova pointed out, once the aerobic house is big enough, it’s no longer creating new rooms, you’re just maintaining the ones you have and making sure it doesn’t shrink, while you start to decorate the interior.

We can see this throughout the paper. A large share of the mitochondrial argument goes like this:

1. Zone 2 produces smaller changes in AMP, ADP, lactate or phosphocreatine.

2. It therefore activates AMPK or CaMKII less.

3. PGC-1α expression is sometimes smaller.

4. Therefore, chronic mitochondrial adaptation is probably smaller or absent.

Acute signaling is not chronic adaptation. A transient signalling pathway activation is highly variable. For instance, A 2026 meta-analysis of moderate training found significant increases in mitochondrial volume density and VO₂max while finding no consistent changes in PGC-1α.

As my grad school advisor warned decades ago, “be careful falling in love with pathways. Chances are there are dozens we don’t know about or what they do. You’ve got to get to functional adaptation. That’s the king.”

When we over-index on signalling pathways, we miss the forest for the trees.

A similar mistake can be seen throughout the review where they emphasis intensity causes more changes in a variety of markers (pH, AMP/ADP, AMPK, lactate, etc.) to show it’s superiority. But in reality these are just acute markers of an applied stressor. The body does not award adaptation points according to how miserable one session feels or how much stress it causes.

The name of the game is absorbing and adapting. Which again, is why long-term performance measures are your best marker for adaptation.

3. It’s own research counters it’s claim.

The paper's own anchor citations contradict it. Its single biggest source (Mølmen, Almquist & Skattebo), found mitochondrial content rises equally with continuous, interval, and sprint training (23/27/27%, no significant difference). They waved it away on a technicality saying its definition of endurance training was "too broad" to know whether Zone 2 specifically benefits. The other paper (Granata 2018), they use to argue against low intensity, actually concludes mitochondrial content tracks volume. The authors focus on that they found intensity is important to mitochondria function, but left out the other half of the finding…

Even if all we did was care about mitochondria (hint, we don’t), most of the research shows that yes, both volume and intensity produce mitochondria increases, sometimes to the same degree. But volume of training is generally what it tracks.

And, much of this research is hard to do or track. Why? Because for a clear comparison you need long-term training studies. Volume takes longer to work. It’s why Lydiard told people essentially, spend months building your base, and then once you add intensity you can be 6 weeks from near peak fitness. Intensity works fast. Volume takes time. If your studies aren’t long-term, it misses the differentiator.

We can see this in the researchers own claims. They cite Inglis 2024 to show "moderate intensity did not increase V̇O₂max.” It’s a single 6-week study, which itself is contradicted by Mølmen's pooled finding that continuous training raises V̇O₂max similarly to HIT.

This is the problem when we cherry pick data without considering time horizons, mechanisms, performance, etc.

4. It asks the wrong question.

“Which intensity is optimal?” is bad question.

Optimal for what? Aerobic ability, performance, longevity, mitochondria density, VO2max, time constraints, and on and on.

There is no optimal training intensity. All matter.

High intensity generally performs well when you have limited time. If I told you that you have 6 weeks to run fast, cranking out intervals may be your best bet. If I told you that you only had 30min to train, sure something more intense might win.

This time component is often touted as “more efficient" for the modern person. But it neglects a few items. First, with intense work, you have to warm-up. For an easy run, you don’t. Second, after an intense workout, you cool down or at least have to recover. After an easy run, you walk right into the shower. So much of the ‘time saving” is a mirage from not including the warm-up, cool-down, and recovery time.

The 80/20 Debate

I’m sure the researchers were well intended. But this review misses the mark. And the reason I’m calling it out is because it’s making the rounds on social media and podcasts, and does damage to what actually freaking works in the real world.

The review was narrowly focused, mostly on mechanisms instead of functional outcomes. And it created a kind of strawman of intensity vs. volume and then trying to declare intensity the winner based on “time efficiency.”

We can see that most in the argument they make against Seiler’s work. They argue that the “lots of easy” is based on folks training 15-20 hours per week. But for lower volumes, we shouldn’t apply that.

The problem with that is a few fold. First, Seiler included runners in his analysis. Unlike cycling where you can accumulate a crazy high number of hours, in running you are limited by pounding. Even if you are running 100 miles per week that’s ~11 hours per week for sub-elite competitor. Much less than the cyclist. But if we consider most college, high school, and so forth runners don’t run 100mpw all the time, the lots of easy still holds. Most good high school kids may run 60mpw, that’s 7 hours per week. Lower level high school kids running 30-40mpw, (3.5-5hours) still do lots of easy, and not all intervals.

Meaning, that the lots of easy still holds. Sure, it might not be precisely 80/20. But even for relatively low volumes that we’d see in amateurs, the idea that most of your training is easy holds. It’s why if we compare HS to pro runners, the number of hard workouts per week doesn’t vary much. It’s not like HS kids are doing 2 hard workouts and pros are doing 6 a week. No, everyone is doing 2-3 hard workouts a week, or some mixture with more moderate that equals that out.

Now, at some point, if you are only training 3 days a week, will you need more intensity? Absolutely. You can only pack in so much volume in 2-3 days a week, so you have to compensate somewhere.

But even then, the answer isn’t lots of HIIT. It’s to build your base with more moderate intensity or tempo work, as we see with folks who’ve modified the norwegian double method for recreational runners. (Or what Jim Peters showed us 70 years ago.).

And it ignores the point of easy running. One of the reason it works more consistently, is its easier to build up to and handle a decent volume of easy running, then hammering intervals 4-5 days a week. One is demanding and exhausting. The other eventually becomes an easy stroll where you talk the whole time with your friends. Which side note, why volume matched research (which is used in this review sometimes) is not appropriate here. If we match 10mpw of 400s vs 10mpw of easy running, of course the intense work comes out on top. The whole point of easy running is that you can accumulate more of it safely!

So no, the lots of easy isn’t a mirage of high volume training. It’s something that HS, college, and pro runners have known for half a century. Whether you’re running 4 hours or 12 hours per week, you need mostly easy running to build up that aerobic base.

The False Debate

The real reason this irks me is that it sets us back. It puts us arguing, once again, over a false dichotomy: volume over intensity. When that problem was solved many many years ago.

You need both, in the right dose. What’s the right dose? It will vary for individual and event. If you’re training for an 800m, you need more intensity than volume, for example.

But for anything middle-distance and up, or for general aerobic development, no one is arguing that we need only one or the other. That’s a debate that literally occurred between 1930 and 1950. We’ve moved on.

Now, we acknowledge that lots of easy builds the foundation. It allows you to safely accumulate volume to set the stage and build all of those wonderful adaptations, that then are enhanced once we add some moderate and then more intense work on top of that. How much? Depends on your aims. But underlying almost all of it is: lots of easy, add in some moderate as you need to push adaptations, then top of with some intense.

It’s not that HIIT is bad. We need it. But the HIIT evangelicals are stuck in the 1930s. Even Igloi, who did intervals all day every day, realized that he needed to build a foundation with easy intervals. Not HIIT.

Lastly, I want you to imagine a scenario. You have a high school cross-country coach who gets new freshman in. These kids are low volume, they haven’t trained before. What would you say if that coach prescribed them 4 days a week of 200s and 400 intense repeats? You’d think that coach sucked. That he was a football coach who didn’t understand cross-country. And those kids might improve a lot initially (clean slate phenomenon) but soon would be burned out, injured, or quit because they were untrained folks with no background hammering intervals.

Why do we think that kind of advice is good for adults? It’s not. Even more for novices or amateurs, you need to build a foundation. It’s something every high school or amateur coach understands.

And before someone goes, “you’re talking running performance, we’re talking zone 2 or HIIT for health…” the point is that performance captures the underlying items that correlate with health better than singular physiological markers. I’ve shown this before. And the data on sedentary folks and longevity backs me up.

When you actually dig into the longevity data, the biggest wins don't come from getting people to sprint. They come from getting people who do nothing to do something, and then to do a lot of that something. The accelerometer studies are about as clear as it gets. Folks racking up around five hours a day of light activity see somewhere in the neighborhood of almost a 30% drop in all-cause mortality. The whole public health game is volume of sustainable movement.

And the review pivots to fitness to make its case. The argument goes something like, “VO2max is the strongest predictor of mortality, zone 2 might not budge it much, so prioritize intensity.” But even there, on the battlefield they chose, the easy stuff holds up. Mølmen’s pooled data shows continuous training nudges VO2max about as much as the hard intervals do. And a history of training tells us that if you want to increase your Vo2max the most, you do lots of easy and top it off with intense. So even if we drag the goalposts all the way over to VO2max, it’s the same answer. Turns out we don’t need to make grandma do Tabata after all.

I hate to be a call-out guy, but can we just stop? Can we stop ignoring a century of training history with some hand waving and valuing a 6 week training study over the natural experiment of performance data? It’s annoying. And I’d argue it’s bad science. You’re intentionally ignoring good quality data.

We need lots of easy. What lots is will vary. What easy is will ebb and flow, for some really slow, others moderately slow. But you can’t escape that fact.

It hasn’t been up for debate for over 60 years. High intensity and volume work aren’t opponents. They are compliments. Can we get with the times?

-Steve