#379 - AMA #79: A guide to cardiorespiratory training at any fitness level to improve healthspan, lifespan, and long-term independence

I'm your host, Peter Atia.

“At the end of this short episode, I'll explain how you can access the AMA episodes in full,”

along with a ton of other membership benefits we've created. Or you can learn more now by going to Peter AtiaMD.com/subscribe. So without further delay, here's today's sneak peek of the Ask Me Anything episode. Welcome to Ask Me Anything AMA episode 79. In today's AMA we take a comprehensive look at Cardiovascular Fitness, one of the most common

topics we get questions about, and of course an area that is central to not only lifespan, but also healthspin. Now, like we've done a lot of content on this topic, but we wanted to put together an episode

that brought it all together in one place, and as always to include any of my updated thoughts,

something which, of course, I'm constantly updating my thinking on topics and hopefully sharpening it. So the goal of this episode is to provide a practical guide that allows you to structure your training in a way that meaningfully impacts your health, your functional capacity, and maintains independence as you age. We'll discuss why Cardiovascular Fitness is one of the strongest modifier predictors of healthspin and lifespan, what zone two training actually represents, and why it's different from

higher intensity work, and there seems to be still some confusion about this. How to think about exercise volume, intensity, and the practicality of training in different zones, and using different approaches, how to measure and track improvements in zone two specifically, VO2 Max targets age adjusted goals, and planning for the marginal decade, structuring and effective work out routine around your zone two goals and your VO2 Max goals.

How to balance zone two with higher intensity work across varying weekly training volumes.

“Again, this is a very important part of the discussion, because as I alluded to,”

there are going to be some of you listening to this, that shouldn't be doing zone two at all. And my hope is that by the end of this podcast, you've been able to identify yourselves. Adjusting training for beginners, metabolically unhealthy individuals, long-time trainees, older adults, etc., specific considerations for women specifically, including some misconceptions, around zone two or other forms of cardio training, common training mistakes, and how to avoid

platos, burnout, and mis-targeted intensities. Practical strategies for sustainability, progression, and long-term adaptation. If you're a subscriber and you want to watch the full video of those podcasts, you can find it on the show notes page, and if you're not a subscriber, you can watch a sneak peek of the video on our YouTube page. So, without further delay, I hope you enjoy AMA 79.

“Beer. Welcome to another AMA. How you doing? I'm doing very well. Thank you for having me back.”

Always welcome to have you. I see every time we do these, you bring something more and more

to each recording. What have I brought this time? I don't know. I'm not even aware. Nothing you can think of. Nothing jumps out. No new additions to the body. No, but given that we've now introduced carveouts at the end of these, it's made me that much more aware of how much I am the perfect target of YouTube and Instagram ads, because I could create an entire podcast called the carveout, where I just talk about the things that I buy when they're served up to me as ads

that I end up liking. Have you bought one thing today? Not today, but I did get something really awesome two days ago. It arrived two days ago. Anything you'd like to share with a group. Heck, no, I'm saving it for a carveout. Maybe next month. I put these things to the test. Let me tell you, I am a serious tester of product. So, the thing that I got so far, I've already tested it once. It was insanely good. I need a few more reps with it, and if I'm still digging it in a month,

it might make it to the carveout list. That's great. It's exciting because I don't think people realize what you're talking about could be a legitimate thing that has actually been official to health and longevity. And it could be the dumbest $20 gadget that has ever existed. And we have no way of knowing which one it is when it comes to that spectrum. Well, I'll give you a hint. It was served up on a YouTube ad, so it's definitely not the former, but I will say it's also not the

latter. Even people hang in. There's a lot of daylight between those two. Yes, you just tend to live on the spectrum, so you kind of go one or the other. That's true. Don't think you do a moderation as moderation, which turns out is the same with engaging in YouTube and Instagram ads. You like to go all in. So with that said, what we're covering today, one topic, cardio respiratory, fitness, and simpler terms for people, zone two, VO2 Max. This is a topic we have talked about over the years

about by far the most, partly because of the interest in it, and also I think because of how

open you are on how it is the biggest and strongest modifiable predictor of both health span and

“life span, meaning it's the biggest impact that someone can do something about it. So that's why”

we decided to kind of dedicate this AMA, gather all the questions, and try to make it a one-stop shop for everything relating to how to measure, track, improve, zone two, VO2 Max through training. We'll cover how this relates to people who have a lot of time to work out. People who have a little time to work out will look at how it relates to people who are just starting training. People who have been training for a long time, older adults. If anything

changes for women in particular and more, we'll also look at if your opinion has evolved around some recent debates and discussions around zone two, lactate, how to balance, volume and intensity, with the goal of not having your best exercise month ever, and then stopping, but more so long turn. So with all that said, anything else you want to add before we get started? Yes, this was an idea that when the team pitched it to me, my initial response was, I don't think

this is worth it. We've already generated plenty of content on this. It certainly would be within

“the top five things that I talk about, and the team, I think, was able to get me convinced,”

and I believe rightly so by saying, yeah, Peter, that's kind of the point is, if someone were to try to go out there and aggregate everything you've said on this topic, it would be a full-time job, and I think somebody even shared with me how many hours and hours of content it would be, and it was triple digit hours. And they said, that's great for the person with an encyclopedic memory who is a lifelong devotee who doesn't have a job, but most people aren't going to fit

into that category, and it would be really helpful to have a practical guide, not just a theoretical guide to this. That kind of won me over, and so I guess I would just say kudos to the team for convincing me that this was the way to do it. I'm really happy with the way they've crafted a story around this. Let's dive in. Before we do a quick question, do you think if the team started to put their arguments and forms of videos that we ran as Instagrammer YouTube ads, you'd be more willing

“to listen? If you could be good enough, but you have to catch me within the first 10 seconds of the ad,”

or I'm skipping it. I don't know that that's a skill set that exists on our team. We don't practice that skill of catching you in the first 10 seconds. No, we practice more of, we will get you at the end of two hours after explaining your rigor. So with that said, I think what would be helpful to start is looking at real quick, why is cardiovascular fitness a central pillar and not only your approach to lifespan, how long you live, but health span? Yeah, so again, if you've been listening to

me talk about this for years, you can literally go to your podcast player and hit forward for a couple of minutes. You don't need to hear this, but I do want to spend at least a minute on this idea

that cardiovascular fitness is one of the most important and modifiable. It's very important that

we're talking about modifiable predictors of both how long you're going to live and how well you're going to live. So if you look at all the predictors of all cause mortality, which remember, that's the holy grail metric of longevity, cardiovascular respiratory fitness outperforms every other variable we can measure. This includes blood pressure. This includes cholesterol. This includes BMI smoking. It even includes age, which just blows my mind. So cardiovascular fitness ERF represents how

efficiently your heart and lungs and blood vessels and muscles can work together to deliver and utilize oxygen. So the more efficient that system is, the more physiologic reserve your body has and if this reserve that allows you to tolerate stress, this stress can come in the form of an infection, surgery, or just frankly the day-to-day demands of living. This has been most typically and most repeatedly measured using a test called VO2 Max. For me, talk about this, of course,

and it's become a very popular thing that people talk about. It's the maximum rate at which the body can utilize oxygen, tested, of course, during maximal efforts, which require exercise. So this number is expressed in milliliters of oxygen per kilogram of body weight per minute, but it can be estimated using something called metz or metabolic equivalence where one met is equal to 3.5 milliliters per kilogram per minute of oxygen uptake or utilization. So I would say that the reason that VO2 Max

has become such a popular way to do this is because it is a standardized test that doesn't

mean it's always done correctly and we've got plenty of examples of how this can be done incorrectly,

but for the most part a well-trained technician can do this consistently and that makes it easy

“to study and that's why in the literature you're going to see so much discussion where it comes”

down to metz or VO2 Max the two can be used interchangeably and you won't for example see that when it comes to zone two. So we're going to talk a lot about that today, but I just want to point out zone two is a much more difficult area to navigate because it's not a maximal effort. It's in

between effort. VO2 Max is a maximal effort so when you tell somebody to basically floor it until

they're going to kill over that's actually much easier to achieve. Now to put some context around the importance of VO2 Max in mortality again because it's been studied if you're in the bottom quartile or quintile so bottom 20 to 25% of the population perspective VO2 Max you've got a four to five fold higher risk of mortality all cause mortality in any given year than those in the top three percent two to three percent. That's a pretty big jump but keep in mind even tiny little jumps

say moving from the second quartile to the third quartile will still have easily a 50 to 75%

“improvement in all cause mortality. So why is this such a powerful relationship and I think it comes”

down to not just what the number represents which is everything I've talked about vis-a-vis oxygen delivery

in utilization. I've said this before but I think it bears repeated measures like VO2 Max just like strength they're actually integrators of work done so if a person has a VO2 Max that is low and their aspiration is to have a very high VO2 Max they can but it will take potentially years and countless hours of work done and that work will be done at the level of their cardiovascular system their pulmonary system their hematologic system muscular system metabolic system and all of

those things will have to work and work and work for hundreds of hours to get a desired outcome. And if you think about that that's much more frankly impressive from a physiologic perspective

than taking a pill that lowers your cholesterol. It's not to say that taking a pill that lowers your

cholesterol doesn't improve outcomes but it's not going to come close to improving outcomes as much as this does on average. There are edge cases there are some individuals with familial hyperclesteralemia where that pill that lowers their cholesterol will have an outsized benefit but by and large this is why things that improve cardiovascular fitness or strength tend to have such an impact on mortality. Beyond mortality can you also talk about the

health span benefits so what you refer to as not only how long it can help you live but how well

“it can help you live. Yeah I think the argument here is just as strong of course the data are not”

quite as objective because health span is not as objective so what I might aspire to be able to do that would define good health span for me might not be the same as you Nick and is not going to be the same as every person that is listening to us right now. But what we do know and I think we'll show at least one figure to that effect today is that VO2 max declines quite predictably with age at about 10% per decade but the oxygen cost of doing things doesn't change so whether it be climbing

stairs or lifting something up or chasing your kids around or playing a sport those things don't change. So if you have a declining capacity to deliver and utilize oxygen in the presence of constant demand at some point those curves cross and what that effectively means is you start losing the ability to do these things again we'll talk about this in much more detail when we get there but as I think a figure can represent better than what I'm saying necessarily our objective

is to be able to maintain optionality around being physical for as long as possible and that is 10 amount to having as high a VO2 max as possible in addition to being as strong as possible. When talking about cardiovascular respiratory fitness in the past to kind of help people understand it in a simpler way you've often talked about the base and peak model can you just walk through a little bit more about that framework and how different exercise and intensities can contribute

to each component of that. I talk about this cardiovascular respiratory fitness triangle and I can't take credit for this at all it was one of my cycling coaches that came up with this so the idea was that you picture a triangle with a base and a peak and the base is what we think of as your capacity to do sustained sub maximal effort over a long period of time so think of something you

sustain for 5 to 10 minutes obviously there are so many gradations here your functional threshold

power which is what you could obtain for an hour is obviously smaller number than the peak and a shorter number than the base. So anyway the goal here if you're trying to maximize your total aerobic capacity is to maximize the area of this cardiovascular respiratory triangle and of course to do that you want to have the widest base and the highest peak possible and these require different forms of training so if you just trained at one intensity level the whole time you would increase both

of these things. I want to be clear on that point because it creates so much confusion. If you only park yourself at one level of training you would through enough volume increase both of these but that's not the way to maximize the problem and it's certainly not the most time efficient

“way to do it nor is it necessarily the best way to do it in fact it's almost assuredly not given”

the fact that no high level athlete trains that way. The base is ideally built through adaptations that help you utilize oxygen more efficiently to convert fuel but mostly fat into ATP. What this is really geared towards is improving mitochondrial density and efficiency and optimizing fat oxidation and lactate utilization. Conversely the peak which again is I've built to max represents the ceiling for oxygen delivery primarily but utilization and it's driven

by how well this system can deliver oxygen to the mitochondria. That's primarily the bottleneck. It's how much oxygen can you deliver to mitochondria versus the base which is how much can you utilize substrate efficiently. So when it comes to delivering oxygen to the mitochondria there are really four big drivers. There's the diffusion of oxygen from the lungs into the

“blood. There's cardiac output so that's heart rate and stroke volume. Then there's the oxygen”

carrying capacity of the blood namely hemoglobin and then there's the muscle's ability to extract this but as I said a moment ago it's the cardiac output that is the main driver here and it is the one we are most sensitive to in reduction. So again what drives cardiac output primarily is stroke volume how much blood comes out of the heart with each pump and heart rate and of course when you're at a VO2 max effort you're getting to maximum heart rate. So somewhere

between 70 and 85% of the variability in VO2 max is accounted for just by this one variable. In the show notes we'll include a whole bunch more detail on this if anybody kind of wants to nerd out on this stuff. I love this stuff but I don't want to spend anymore time on it right here. So as I kind of alluded to it's very tempting to and I want to apologize if I've ever created the impression or oversimplified this and it's possible that I have. Zone 2 is what you do

exclusively to build your base and high intensity workouts is the only thing you do to build your peak. As I said these systems work together and if all you did was zone 2 you would absolutely get a wider base you would also raise your peak. Similarly if you did higher intensity training you would increase your peak but you would also widen your base a little bit. The key as we'll get into in the nuance is what is the optimized way to utilize time around different volume and

intensity requirements. So how much total work can you do? How much cardiovascular fitness training can you do? That's probably the single biggest determinant but that involves a min max problem which

“is a big part of what we're going to talk about today. So that's how I think about the triangle.”

When talking about zone 2 you've often talked about fan oxidation, mitochondria, lactate. Oftentimes I think these terms can be a little confusing for people and so I think it's

always helpful to kind of re-look at them and explain it. So do you mind just spending a few minutes

walking through the cellular mechanisms that are involved in cardiovascular respiratory fitness just so everyone is kind of familiar with the terms you may or may not use throughout here? Yeah so it's really funny because I've noticed some amazing memes on Instagram where you basically have people that are making fun of anybody that uses the word mitochondria. So somehow because I don't really pay attention to the wellness influencer health space. Apparently the word mitochondria is

now just one of those buzzwords that you should throw around as much as possible. And so if you're playing sort of wellness influencer bingo you're going to get a lot of points for mitochondria.

Can't remember some of the other awesome words that are just basically pathonomonic for

got biome like they've got all the buzzwords. Protein, another one. Protein, I'm sure. You've set me up now

to trigger a bingo card. But I guess you're right. You can't have this discussion without doing this. So hopefully I'm going to get an exemption for my use of the word mitochondria here. At the foundation of your cardiovascular respiratory system are these organelles called mitochondria

“and of course all of you who took a high school class in biology will remember that they're”

referred to as sort of the little power units of the cell. And the majority of our ATP is produced by them. And again ATP is the currency for energy just because I can't resist giving one more level of detail. The way ATP work is they donate ATP has three phosphates. They donate one of those phosphates. And it's that liberation of energy that comes from that chemical bond that creates energy. The mitochondria can generate ATP from either fatty acids or pyruvate

pyruvate is a intermediary breakdown product of glucose via a process called glycolysis. And both of these processes are constantly occurring. It's just the question is what's the balance in which they're occurring and of course are these both equal know they're not? Each process has a trade-off. The trade-off would simply be stated this way. If you are optimizing for efficiency, and you don't care as much about the speed with which you can deliver ATP, you want to take that

more aerobic pathway, meaning utilizing oxygen and shutling the breakdown product of fatty acid or glucose, either in the form of pyruvate or acetylchoe into the mitochondria, to use an oxidative pathway to generate lots of ATP per units of carbon that go in. The problem with that is as the

“demand for ATP accelerates, you have to make a trade-off. You have to make a sacrifice. The body says,”

"I'm sorry, I can't do this anymore. I have to go down this quicker path using glycolysis where

I turn glucose into pyruvate ultimately until lactate. I don't get nearly as many ATP for it,

but I can deliver much more ATP to the muscle." Now, I can't do this indefinitely. There's a whole problem associated with that which we'll talk about, but that's effectively at the high level the trade-off. Another way to think about this is through the lens of the fibers that are involved. Again, these are terms we've used on the podcast before, but the goal of this podcast is to kind of tie this all together. At lower intensities, you have these type one or slow twitch

muscle fibers. Again, I think the term slow twitch, it does to some extent reflect the speed with which they twitch, but I think a more important way to think about them is they're slow to fatigue and they're more endurance-based fibers. Again, at lower intensities, they're the ones that are doing all the work. Very rich in mitochondria, deep red, they excel at oxidizing fat, and they're very, very efficient. As the intensity increases, we have to start recruiting

more of the type two fibers. These are fast twitch fibers, which again, are more contractile in their force, but they're also fast to fatigue. They have less mitochondria, and they're going to recruit and rely more heavily on glycolysis. That's happening outside the mitochondria. So, initially, lactate, which again, kind of gets a bit of a bad wrap, but again, we've done

“an entire podcast on this, and we'll link to the podcasts on this topic, but the most important”

of these is definitely the one with George Brooks. Initially, the lactate gets recycled locally, so it's shuddled to neighboring type one fibers. It gets generated in the type two fiber, gets shuddled to a type one fiber, gets converted back into pyruvate, and then the pyruvate goes into the mitochondria to produce more ATP. That's called the lactate shuddled. But again, these things are constrained by demand, and therefore, as output increases, and demand increases,

lactate production in the type two fibers begins to exceed the capacity for what can be done locally in the mitochondria adjacent. And at that point, lactate spills into the bloodstream. So, if you were measuring lactate in the bloodstream, with a continuous lactate monitor, which, by the way, these things are easily and prototype, and there's some that are probably above the markets. This is something you can appreciate in the future. You might start out an exercise

session where your lactate is resting at 0.5 mm, everything I just described up until this point would not increase that, even though locally lactate levels are rising. But once it starts filling the bloodstream, now you actually have to rely on other tissues in the body. The heart, other muscles that are not being utilized at this point in time, they have to start clearing it using lactate as fuel, as we've even learned from George Brooks. The brain will do this as well.

The liver also can convert that lactate back into glucose via gluconeogenesis, and this basically

allows the body to maintain certain levels of lactate at a new baseline that is above the original

healthy individual, and someone who's metabolically flexible, meaning they can go back and forth between utilizing glucose and fatty acids, this falls at about 2 mm of lactate. That is what we refer to as zone 2. Again, if some of those conditions aren't met, if you're not a metabolically

flexible person, using that first threshold of lactate at 2 mm is not going to happen. There are

people who walk around at rest with a lactate level above 2. But the point here is you can maintain your now at a new steady state where, if you remember, the first steady state is where the local tissues are able to offset lactate production at the rate that it's being produced, consumption and production are equally locally. Then you have this second level, which we refer to as the first lactate threshold, because it's the first one we're measuring in the plasma,

and that's where now the systemic tissues are able to balance it. But now we get to a third level of lactate, which is really called the second lactate threshold, and that's at higher and higher levels, and at this point, once the body gets above that level. And this level varies quite a bit by individual. Maybe if we have time I'll go into how you can measure that. I talked about this at

length in the first podcast with Olaf Alexander Boo, but we can come back to that. But anyway,

these higher levels about put glycolitic lactate production in the working muscles completely surpasses the body's ability to clear it. At this point, blood lactate starts to rise much more sharply. It's accompanied by hydrogen ion, because the lactate is negatively charged. The hydrogen is positively charged, so they're balanced kind of one to one. You have this acidity that occurs. It turns out that it's the hydrogen ion and not the lactate that is effectively poisoning the

muscle. It actually prevents the actin in myusin filaments in the muscle from being able to relax. Again, for most people, that second lactate threshold or really third one depending on how you're

“counting them occurs somewhere between four and five millimole of lactate. That's how much more”

variable number. Okay, so I'm going to stop there. There's a lot we could talk about there, but hopefully that kind of sets the groundwork. Very much so. I think maybe we're clicking on to zone two before we get further in a little bit of a different way, because that seems like there's been a lot of discussion lately on whether it has unique benefits, whether it's just better to focus on higher intensity work only. So how do you think about this question? I think it comes

down to context. I think there's a lot of confusion around this, so hopefully I'll do my best to dispel that. There's ideas out there challenging the idea that zone two, special or magical, or there's anything that's good about it, and there are certainly people who would assert that high intensity work produces the same or even greater adaptations. And I think, honestly, in the framework that some people are proposing that it is true. So let's now think through this

for the proponents of high intensity exercise. People who say, "Don't waste your time doing zone two," the shorter the amount of exercise time that a person has, the more true that is. Because remember, something I said a few minutes ago, which is, if you really want to maximize

“the area of your triangle, nothing beats volume. Now, walking won't do it. So you have to get to”

zone two, this first place where you have some adaptation, but the more time you spend there,

the better. And so if we're going to talk about a professional athlete or even a recreational athlete, if you're going to talk about 10 to 15 years ago the way I train, where by some miracle, I still was managing to spend 14 to 16 hours a week on a bike, then we can get into the nuance of how that time should be divided. So now let's turn this over to someone who's going to adhere to the general guidelines. So the general guidelines says you should exercise ideally 150 minutes

per week. So that's two and a half hours per week. And I'm sure the guidelines would be happy if you did more, but that's what we're trying to get people to. Most people are not exercising two and a half hours per week. And truthfully, if that's all you can adhere to, then zone two is not going to be an efficient use of your time because it doesn't provide a sufficient enough training

“stimulus to drive the adaptations to make the triangle bigger. And remember that 150 minutes is total”

exercise. Well, part of that's going to have to be some resistance training. So even if you said, I'm going to carve out an hour for 230 minute resistance training workouts in a week. Then you've got an hour and a half for cardio. Truthfully, I would say then all of that time should be done at high intensity. You should probably have two forty five minute high intensity workouts. But when I talk about training, then maybe I should be, but I'm generally not talking to that population. When I'm talking

only got two and a half hours this week to exercise, we're going to craft your program around that. But I'm talking to a person who is really thinking about how to optimize and achieve their best results over both lifespan and health span over decades. And if that's the case, then you're going to need more volume than 150 minutes a week. And then that means you're going to have to utilize different levels of intensity. So because zone two is this point at which lactate rises to the

level where it's now in the bloodstream. So local tissues can't clear it, but your body is able to

clear it. You're stressing this system. This is the first place where you're now really stressing

the system enough to recruit more glycolytic fibers. But what's nice about this is the intensity is low enough that you can keep going for a long enough period of time. And this is why endurance athletes who are training for 15 to 20 hours a week are indeed spending basically 80% of their time in this zone. Because the intensity is low enough that they can do it for so long and yet they are still getting a training adaptation. So if you are training in zone two while you're not

getting as much adaptation as you're getting at zone five, you're still applying a strong enough training stimulus to activate both fuel systems, right? So you're not maximal, but you're near maximal for fat oxidation. You have some glycolysis, you have lactate shuttle, but you don't have the wear and tear of the acidity and the fatigue that comes when lactate production completely overwhelms clearance systemically. So again, you can pack in volume of training in a way that

you can't with very high intensity. There are other benefits to zone two, by the way, if you're an athlete, which is it comes with the benefits of improved movement efficiency. I discussed this on a podcast as well. So I guess I hope that clarifies kind of the context around one versus the other. In looking in terms of spending more time than the bare minimum, when it comes to exercise,

“how does then exercise intensity play a role into the relationship of volume and sustainability?”

Once you're not constrained by that 150 minutes per week, and honestly, that's my hope. I hope is that everybody listening to us right now, even though once in a while they might be constrained by that, but that they can find more time to exercise. The limiting factors start to become fatigue and recoverability, and even to some extent adherence. And I think that especially as you get older fatigue and recoverability become real limiting factors. So higher intensity

work out, work out in zone five, very important, and should always be a part of your training,

you can't do that much of it once you get into your 40s and 50s. When you're in your 20s and even into your 30s, you can still hammer these workouts. But I can't do those workouts three or four times a week anymore, and I don't think most people listening right now can either. So if you're going to be able to devote more time to your training, you're going to have to be able to do so at a lower physiologic cost. And again, volume drives adaptation. That's the thing to

“remember. It's volume above all else that's driving adaptation provided that volume is at least”

at zone two, where you start to undergo all those changes we discussed. So basically, there's a cost of doing high intensity work. And by the way, part of that is adherence based. It's more painful. It's more fatiguing, and it's harder to sustain. So one of the things I tell patients who are bored when they're doing zone two is look, use it as an opportunity to get really it caught up on your favorite podcast or your favorite audiobook or something like that. Something

that frankly is a little bit harder to do during a high intensity workout where you're probably

not able to concentrate about it. So taking it back to basically the critics of zone two,

they're correct in a narrow sense in that per unit time high intensity training delivers more physiologic adaptation, but they're kind of wrong in the way that it matters. It's not that zone two is magical. It's that it's practical. And it becomes more and more valuable as your volume increases. So in short, I think zone two is the cornerstone that lets you do enough work, enough volume,

“safely, and consistently so that you get the adaptations you need to be an athlete for life.”

Moving on now to look at how someone can measure zone two and be out to max understand what they are. When is it useful? How do you go about measuring it? And let's start with zone two just because we were

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