Welcome to the Empirical Cycling Podcast. I'm your host, Kolie Moore, joined as always by Kyle Helson. And I want to thank everybody for listening and ask you guys to please subscribe to the podcast if you have not yet already, and also share the podcast with your friends. And also, feel free to share the podcast online when somebody's got a question and you feel we've explained it really well here. You can do that. And the only reason I say that is because you guys are doing it already, and it tickles me every time I see that. And if you'd like to go to iTunes and give us a rating, you can do that too. And also remember that we are an ad-free podcast. So if you'd like to donate and support the show, you can do so at empiricalcycling.com slash donate. We have the show notes on the website with all of these studies that we reference in this episode. And so if you have any coaching or consultation inquiries or questions and comments, you can send an email to empiricalcycling at gmail.com. We also have merch at empiricalcyclingpodcast.threadless.com. So, Kyle, we're continuing our series on VO2 Max today. So what do you hear most commonly about the limiters of VO2max? Oh, I think that's a good question. I think a long time ago there was this sort of generally accepted anecdote, if you will, that VO2max was this like predictor of athletic endurance performance and also that it wasn't Trainable, and that if you were born, and then you grew up, and you had some VO2 max, and whatever that was, as soon as you got it measured, that determined and predicted your entire endurance athletic outcome, and there's nothing you could do about it, you couldn't change it, and that was it. And so I remember hearing people say, like, oh, like... Long time ago, like athletes wouldn't want to get their VO2 max measured because they didn't want to have that like mental block in their mind of, oh, I'll never be better than this. And obviously today we know that's not true, but you hear lots of things about VO2 max and it being a good predictor of both fitness and potential when you talk about VO2 max in relation to FTP. But in terms of limiters of VO2 max, people think like, oh, they like to talk about like, oh, like lung volume or, you know, how good your cardiovascular system is or your heart or all of these other different elements because, you know, you kind of think like, okay, if your body's going to be breathing in this oxygen, like something must be capping that and You know, I think at first inspection, you think like, oh, well, you breathe in through your lungs and all this stuff. And that's kind of like the first barrier. So maybe there's, you know, things like that. So today we're going to get into understanding the physiological components of VO2max. And so when we get to talk about training it, that everybody's on the same page who has been listening in so far. So first we need to talk about something called the Fick equation, which is one of those things that gets trotted out a little bit. now and then by actual experts and dilettantes alike, obviously to varying degrees of understanding. So what is this equation? Actually, there's a couple because Adolf Fick was a smart guy who obviously lived in the 1800s since we don't see that name a lot anymore for some reason. So we're going to see two of Fick's equations today. The most well-known is just called the Fick equation and it regards VO2. the volume of oxygen that we intake during existence or exercise specifically. So the equation looks at the constituent parts of oxygen uptake, distribution, and utilization. So it looks like this. VO2 equals arterial venous O2 difference times cardiac output. So VO2 max would just be, so let's break down cardiac output into its two constituent parts. So VO2 max would be max Stroke Volume, Times Max Heart Rate, Times Arterial Venous O2 Difference. And so, arterial venous O2 Difference, or AVO2 Difference, as we're going to refer to it, is basically utilization of oxygen by our muscles. So when we use the Fick Equation, as we're going to actually see later, we can actually Calculate one of the values of the Fick equation if we have all of the others. So if we have VO2 max and we have, you know, stroke volume and we have heart rate, we can figure out a VO2 difference, not a problem. Before we get too much deeper into that, the Fick equation presents interesting questions on training philosophy. Most importantly, are all parts of the Fick equation equally important? And by that, I mean, do they limit VO2 max equally? So will increasing cardiac output have as much impact on VO2 max as increasing a VO2 difference or possibly increasing heart rate? Not that that's easy to do. So most, if not all, I actually have not tallied them because, you know, who would want to take that kind of time, of the training media that I see only take a cursory glance at the Fick equation and I think... In this cursory glance, it's because it's so cursory, they will all say yes, and that all parts contribute equally. And so we're going to look into that today. The Fick equation being a relatively simple formula sort of leads people on a cursory inspection to be like, oh, all of these parts are equally important. Because since it's just... Multiplying two different or four different things depending on how you break it down. That's it. It's just four, you know, components and there's no weird fractions or exponents or logarithms or whatever. So you're just like, oh, it's just multiplication. There's no weird like calculus operators. Exactly. Yeah. Yeah. It's deceptively simple. So today we're going to look very specifically at the utilization part of this, the AVO2 difference. And then in the next episode or two, we're going to look at the rest of the components of the Fick equation and then we're going to get to some training suggestions when we tie all this together. So when we start going through this, we're going to want to keep this question in our heads. So if we train... Our muscles, as well as possible, to be packed to the brim with mitochondria and enzymes, and we've got a ton of capillaries in our muscles, so we can really distribute blood to our muscle fibers, will our VO2 max go up? In other words, if it does, then we should be able to measure that as an increase in the AVO2 difference. We should be able to measure it as an increase in utilization. And that's going to affect our VO2 max. So the basics first, why do we care about utilization? So if you didn't use any oxygen, your VO2 would be zero and your hemoglobin and all of your other globins would just be fully saturated all the time. You need to remove oxygen from the blood in order to have quote unquote space to put it when you inhale. which is the open binding sites on hemoglobin. So hemoglobin being the protein that carries oxygen through the blood obviously. So how is utilization measured? How is AVO2 difference measured? So there are a few ways to measure and report values. So where we measure O2 concentration and how it's reported matters because most physiologists and textbooks and studies, et cetera, Just consider a VO2 difference as quote-unquote mixed blood, which means we're measuring, well, arterial blood, obviously it's not mixed, it's just coming straight from the lungs and the heart, then it's getting distributed to tissues, so there's nothing to really mix. So a lot of the time, they'll measure it in like your left brachial artery, a big artery in your arm. It's easy to get to, and it's coming right fresh from the heart, but with veins, You know, the blood gets distributed to tissues, then it comes back to the heart once it's been deoxygenated a little bit. And that means, you know, this is the mixing of the blood is when we get it back to our, into our main vein that goes back to our heart, you know, it comes from everywhere. So sometimes when you actually see AVO2 difference written, the V has a bar on top of it, V bar. And this is just like you learned in high school stats class. This means it's the average. So it's the average venous blood, since it's the average blood from all of the tissues. And the other thing that we should be careful of here, obviously, is the way that it's reported, because there's a bunch of different ways to report oxygen in the blood, from like saturation to volume percentage and other things. So for some reason, the medical field loves deciliters or 100 milliliters. So they use volume percentage, which is milliliters of O2 per 100 milliliter of whole blood. So for a deciliter, if we have 18 milliliters of oxygen to start with and we get down to 4 in venous blood, you will see that reported as 14 volume percentage. But also, think about this. If we know flow rate through whatever we're measuring, if it's the whole body or some tissue, then we can just do a little math and then we can measure just the amount of O2 utilized rather than a percentage which obviously is very useful because that's an easier way to think about it. But also in some studies you're going to see reports of saturation and we're going to talk more about the blood probably in the next episode. So arterial blood like 98, 99% saturated or something like that but mixed venous blood you'll see it get down to like 75% at rest. Might go down to like 25% saturated at max exercise intensity. So when we actually get to looking at our in-depth study later in this episode, what we're going to be doing is just reporting a percentage, which means of the blood that is sent to a certain point in the body, we're going to be looking at how much of it is utilized, which is actually the easiest way to report. and think about a VO2 difference. Let's say the blood flow coming out of the heart is saturated with 20 milliliters of oxygen per deciliter of blood or a 20 volume percentage solution of oxygen. So coming out of the femoral vein, which is actually how the study measured it, let's say we get 10 volume percentage. So we would just report this as a 50% utilization rate. And if it came out of the femoral vein as 5%, we would report this as 75%. So just know that when we get to talking about it later, this is exactly what we're talking about. This is the AVO2 difference. And the reason that it disappears and we call it utilization is because it is used by the muscles that the blood perfuses through. Now we need to talk about hemoglobin and myoglobin because these are the two main globins that we want to think about. as opposed to all of our other well-known globins. So myoglobin occurs in muscles, myo meaning muscles, and hemoglobin occurs in the blood. Globin actually refers to a structural motif, not because it's a glob or a globular protein. So myoglobin and hemoglobin in the blood are going to... So myoglobin and hemoglobin are going to reversibly bind... molecular oxygen. So Kyle, is reversibly binding oxygen by iron a strange thing? Or, you know, because don't we normally think of that as rust? Yeah, so it's kind of strange. There are, because iron, and because of the electronic shell structure of iron, Iron is very reactive, and it actually can form, I think, like 16 different species of iron oxide, rust being the most common, the one that we're most familiar with. But normally when you see rust, you have iron or steel or something sitting out, and it rusts. It never unrusts. Your car doesn't slowly, randomly one day become less rusty. In fact, it gets worse. You think of rust, rust actually sort of... Once rust starts to form, people think like, oh, you have to get rid of it because it will actually start to spread. And in normally, yeah, normal conditions in our lives, I don't think we ever think about iron and oxygen connecting in a reversible way. Usually rust, you don't, you don't. Somehow scrape the oxygen off of your rusty car. You actually have to physically remove the iron oxide and just get rid of it, right? Yeah. Rust remover is removing material. It is not just removing oxygen. Yeah, right. And we don't have any rust remover in our blood for unloading oxygen. Don't drink rust-oleum. Yeah. Don't drink rust-oleum. Well, so the reason this happens is because hemoglobin and myoglobin contain what's called a heme-prosthetic group. And heme is just this big old flat thing. It's got an iron atom in the middle of it. You can kind of think about it like a globe suspended in a trampoline, but instead of being on top of the trampoline, it's bisected by the trampoline. So the iron's equator is kind of like the heme ring. So the iron atom is actually bonded to five nitrogens because iron is really cool. It can make a lot of bonds, more than the usual four that we know in high school chemistry. So four of them are on the heme group and one is on a histidine that's underneath the trampoline on the proteins. And we're going to get into some really cool stuff about that when we talk about hemoglobin. I'm ridiculously excited for that. Hemoglobin is probably my favorite protein next to complex five of the electron transport chain. Not steak? Well, steak contains a lot of my other favorite protein, which is myosin. So the reason that myoglobin are cool in their reversible binding of oxygen is because they help oxygen's solubility and diffusion capabilities in the body. So if you want to know how much more it helps the solubility, this is just dissolving into a liquid or a solution, about 1% to 2% of all the oxygen in the blood is dissolved in the plasma, and the rest is in hemoglobin. So basically, almost none is actually dissolved in your blood plasma. Yeah, exactly. And the other thing that we're going to get really into right now, which is critical to understanding AvO2 difference and the utilization of oxygen, is that diffusion and the role of hemoglobin and myoglobin and the entire oxygen transport system of the body, it's passive. All of it's passive. There's no energy input at all. So let's talk about passive diffusion. And normally, you know, if you take a science class or those of you who have taken science classes, you remember that transport and diffusion and other such things were taught on probably the first day of class, if not the second. Because they are that important, they are that fundamental. So from your red blood cell in a capillary, oxygen is going to diffuse into the cell passively. So neither hemoglobin nor myoglobin are capable of actually inputting energy into moving oxygen anywhere. So you can't throw it, you can't place it, you can't deliver oxygen anywhere. Or like I've heard one particularly egregious description of it, you also cannot stuff oxygen into your cells, which by the way would lead to oxygen toxicity. So the closest metaphor for O2 delivery, quote unquote, that I can think of is like if somebody's delivering newspapers and they're driving by your house and they open their window in their delivery vehicle and that's it. That's all they do. That's basically the role of hemoglobin and myoglobin. And, you know, the newspaper would need to get sucked out of the vehicle. and fly into your house and land because you had gotten rid of the newspaper that you had from yesterday. Like there would be a newspaper vacuum. There would be a little spot of zero pressure of newspaper. So something would diffuse in and fill in. They just want to uniformly distribute newspapers everywhere. Yeah, exactly. We talked about entropy in one of the last episodes. We talked about it quite a lot, actually. And so that entropy helps that kind of motion, that kind of diffusion. What I'm saying is that the movement of oxygen into the mitochondria for oxidative phosphorylation is determined by its complete and total disappearance from the mitochondria. It's actually a magic trick. I'm kidding. It just gets turned to water. But the oxygen for intense purposes of diffusion disappears. It's kind of cool. It's very cool. So the movement of... Actually, it's basic chemistry. So it's not... Which actually is pretty cool. Basic chemistry is really cool. Okay. So the movement of gases in the body depends on passive diffusion or flowing from high to low concentrations. So atmospheric pressure at sea level is about 760 tor, right? Yeah, 760. So what is tor, by the way? Because I think we didn't really define tor. It's a unit of pressure, but it's not like atmospheres and it's not like Pascals or something like that. Yeah. It's a sort of archaic unit of measure because one tor is one millimeter of mercury roughly. They're not exactly the same, but an old method of measuring pressure was looking at how much in a in a thin glass tube, how much a known amount of mercury would move up and down. Oh, sort of like temperature, but instead of that reacting to the pressure from a closed system. Correct, yeah. And for whatever reason, Tor is defined exactly, one atmosphere is 760. Okay. And... Well, so the reason that we are talking in Tor is because most of the literature talks either in Tor... or kilopascals or much more often than kilopascals is millimeters of mercury. So this is actually a helpful reference. Anytime you see millimeter of mercury, you can very often just assume that one millimeter of mercury is equivalent to one tor. Yeah. And that whoever wrote it probably has gone to med school. And they will also talk about... Milliliters of Oxygen per Deciliter. And that's really, you know, well, we talked too much about entropy in the last episode. So we're going to move on to how myoglobin helps the diffusion and solubility of oxygen in our muscles. So as a protein, myoglobin has a very simple oxygen binding curve, meaning it becomes more saturated with increasing partial pressure of oxygen. Partial pressure, of course, we... mentioned this briefly in the last episode, but it means that it would be the pressure exerted by a gas if it were the only thing in a container of the same size. And this is sometimes called oxygen tension, but it's the same thing. So myoglobin is actually a very good, quote unquote, simple enzyme. And a simple enzyme means something... Where the enzyme has a reactant, so something that it wants to process in its reaction, and then it's going to do the reaction and release it. So the reaction between myoglobin and oxygen is myoglobin binds oxygen, and it's done. And then it releases oxygen. That's it. That's the whole thing. So myoglobin really helps oxygen move smoothly and easily through a cell to areas of low pressure. Animals like whales have tenfold the myoglobin we do, which is, by the way, why the first x-ray structure of myoglobin was from sperm whale. So not only does myoglobin increase O2 solubility in water, it also helps store a bit or a lot if you're a whale. That makes sense. You're a whale. You're not going to be breathing the... oxygen out of the water with gills, so you have to take big breaths of air and then keep diving for a while, many minutes. Anyway, so myoglobin has such high affinity for oxygen that in most conditions it's fully saturated, but let's look at a couple numbers for myoglobin right now. So because of the complexity of the curve, it's most common and easiest to look at the partial pressure at which myoglobin is 50% saturated. And for myoglobin, this is 2.8. Tor, or 2.8 millimeters of mercury. At 8 Tor, myoglobin is about 80% saturated. So you may remember in a previous episode that the partial pressure of oxygen in a cell at about FTP is 3 to 4 Tor. So myoglobin is 50% saturated, or just about, maybe a little more, when you are at FTP. And so it's this binding curve in particular that is going to tell us a lot about how myoglobin does its job. And by the way, it does it passively. There is zero ATP used in this entire process. So now let's illustrate this a little bit. So let's say you're at rest. Your mitochondria are not working. They're just going to be sitting there with no oxygen flux through them, which means The oxygen pressure in your cell is probably going to be the same as in your capillaries. And that means there's going to be no oxygen diffusing into your cells. However, when your mitochondria start working, and especially when they start working really hard, the disappearance of oxygen by turning it to water makes a zero pressure zone in your mitochondria. And zero pressure... is going to mean that oxygen really wants to diffuse. And this low pressure zone, that means myoglobin is going to actually release its oxygen. And this is the reservoir part and also the diffusion part. And so it's when mitochondria really get going and create this low pressure zone, it kind of creates this low pressure wave. And now the low pressure area is getting wider in the cell. And at some point, it's going to hit the end of the cell. Actually, pretty rapidly, it's going to hit the end of the cell. And then, only then, is oxygen going to be able to diffuse from your capillaries into your cells. And this is what we are going to be measuring as a VO2 difference. So really, we're measuring the, you know, quote-unquote disappearance of oxygen into our cells as they are active. I think this is interesting to think of in terms of the whole Fick equation itself as well because the first time I saw the Fick equation and maybe a few of you have thought, why is it cardiac output? Why isn't there anything to do with lung volume? And why isn't there anything to do with other things that potentially would be able to increase our VO2 max? The reason for this is because if we don't use oxygen first, Then we are not going to be able to take in any oxygen. Like if all of our mitochondria stopped working right now and we magically stayed alive, what would happen is we would have an O2 uptake of zero because we're not using any, therefore nothing is coming in. We are just going to have a lot of hemoglobin and a lot of myoglobin sitting around with a lot of oxygen and there's nowhere for it to go. We need to have a pressure gradient. from the atmosphere all the way down to our mitochondria. So the last thing about myoglobin, actually, before we get into the last thing about myoglobin, let's think first about a VO2 difference's role in VO2 max, because we've been on this tangent for a second. So you can see how now increasing a VO2 difference would leave, you know, quote unquote, space. in our blood for the uptake of oxygen because if there isn't that space, we're not uptaking anything. So as a potential limiter to VO2 max, we can easily see the appeal of a VO2 difference because if we use more by making our mitochondria more active, we will be able to have that flux down the passive chain to our mitochondria and we will have a higher O2 uptake. At least that is the theory. So the last thing to know about myoglobin is that it doesn't really move itself. It won't go like from the edge of the cell next to the capillary, grab an O2 and then water into the cell and be like, hey mitochondria, do you need an oxygen? I got one. It doesn't do that. They just sit there in solution. And they also don't quote unquote bucket brigade it either. Which would imply a physical handoff. But that's the metaphor that I actually see used most commonly. Although I think a lot more sources now are kind of debunking that. Now that education on this kind of stuff is getting a little better on the internet. It's not a baton. It's not like a relay handoff on a track or something. Yeah. Yeah. Myoglobin continually binds and dissociates from oxygen. So is it totally necessary to use myoglobin for diffusion? No. So check this out. This is where we're going to start really getting into it. If you knock out myoglobin in rats, not all are going to survive, but the ones that do will have very slim muscle fibers and a ton of capillaries. So let's talk about capillaries and the role that they have. So they're critical structures involved in oxygen transport and oxygen diffusion. So the obvious place to start would be, you know, just the structure of the cardiovascular system. So the heart pumps blood into arteries, which get smaller and smaller until they're really, really tiny, seven to nine micrometers in diameter, though they can get down to as small as three micrometers. That's very small. Yeah, which is, yeah, and red blood cells are on average actually a little bit wider. than a lot of capillaries. So they actually have to fold to go through, but they do go through one at a time, which is really, really cool that we've evolved to have such precision in our circulatory system. I love that. So control for flow going through the capillaries actually happens a bit earlier in the blood vessels. So just going forward in this episode, know that not all capillaries are always wide open because if they were, Well, that would be catastrophic. And we'll talk about that probably in the next episode. But also, if you want to know why controlling blood flow is really important, I suggest listening to Watt's doc number 13 on oxygen and why it's potentially so bad for you and why we need to control its flow so much. So the number of capillaries around each muscle fiber matters a great deal. But also the number of capillaries in a given space matters. And we're going to talk about this in a little bit in great detail, don't worry. So as we get to the end of a capillary, and capillaries re-merge with others and become veins returning to the heart, remember that this is the mixing of Venus blood. So you've got blood from... tissue that's not being an oxygen hog, and then you've got blood from working muscles that are being oxygen hogs. And so this all produces really an in vivo measurement, an in-body measurement that's kind of difficult to get some exact numbers sometimes. We have to resort to some pretty advanced methods in order to get good oxygen pressure readings in things as small as cells. So the structure of capillaries around muscles is important because it tells us about oxygen flux. under maximal and submaximal conditions. So at least in theory, once we see the components of flux and flux rate of oxygen from the capillaries into cells, we're going to be able to tell if that is indeed a limiter. And this aspect of a VO2 difference would actually give us a trainable target to increase our VO2 max. And we're going to see whether or not that's true. But first, we do need to understand a couple things. So let's talk about flux from the capillary into a cell. Remember that this is all passive. There's no energy input. So this is another equation from Adolf Fick. So flux equals, on the numerator we have D times A times the difference between C2 and C1 all divided by T. So in the numerator, D is the diffusion coefficient, and we cannot change that. It's related to solubility and molecular weight, so that's given. A is area, so area of oxygen to diffuse through. C is concentration, so the difference in the concentrations from where oxygen is going to diffuse. Oh yeah, and T is time, so it's all Per Unit Time. So this is a pretty simple equation too. So what the equation says in its simplicity is distance, difference in concentration gradients, area through which flux can occur, and time. These are the variables that we are concerned with and that we can potentially change with training. So we should, in theory, get better oxygen flux to the area of zero pressure in the mitochondria, you know, where oxygen disappears. and becomes water. We could get better flux there if our fibers were smaller and if we had more capillaries around them. So this is going to be two of the components of the diffusion equation. So let's think about two extreme examples to illustrate what we're talking about here with concentration, gradient, and area, and time. So let's imagine first a very thick muscle fiber with tons of capillaries around it. Big, wide muscle fiber with one mitochondrion in the center. Or if you want to think about it like a cylinder, we've got a row of mitochondria in the center, but they're all equally distant from the sides. They're right in the middle. However, we have a lot of capillaries around this muscle fiber. Let's call it like 20. It's an unrealistic number. It's way too many. But let's pretend it's 20. So they're all basically bumped into each other. Now, let's think of scenario two. We have a really narrow muscle fiber with only one mitochondrion in the center, or we can do the cylinder thing again with a row of them, but we only have one capillary to supply it. So in the first example, with the big fiber with a lot of capillaries, we have a large distance for oxygen to diffuse through. But we also have a lot of surface area of the capillaries. So these are two of the things in our diffusion equation. But in example two with the narrow muscle fiber, we have a really short distance, but the surface area for oxygen flux is also very small. So we have a small distance, but we also have a very small area for diffusion. In reality, Obviously it's kind of in between. So muscle fibers are actually surrounded on all sides by capillaries. And so because capillaries sit between muscle fibers, they can also be shared by different fibers. So one capillary can be serving two different muscle fibers. And because of what we know in the size principle, they may or may not both be working. So remember capillaries also have length. And so when we look at slides of muscle cross sections, we can forget this sometimes because we just see a little dot. But the length is critical. So as blood flows down a capillary's length, the pressure difference gets smaller as oxygen diffuses into the cell, and so the diffusion becomes slower. Because remember, let's remember the diffusion equation. It's a constant times the area times the difference in concentrations, all divided by time. So we can make the number larger if we have more, not only more area, But also more time. Because if the blood is in your capillaries longer because you've got more, then you are going to have more time to diffuse. Actually, it's such a common thing and such an important thing that studies that look at AVO2 difference actually have a term for this, a dedicated term. It's called MTT or mean transit time. Meaning, you know, we're looking at an increased amount of time because every researcher looking at this knows that diffusion equation. Actually, if we look at the cover art for this episode, it's a muscle capillary bed. And it's a 3D one. It's not just a cross-section. So you'll get a really good idea for the length of muscle fibers and what we're talking about here. How and where and why does increasing the size of a capillary bed in the muscle have an advantage? So here's our first study, which we've actually already seen. This is a multifaceted study called Determinance of Endurance in Well-Trained Cyclists. The reference is in the show notes up on the empiricalcycling.com website. And we've seen it. Yeah, a classic. It really is a classic. And we saw it in Watt's doc number 11. So the study looked at 14 cyclists of approximately similar view to max in liters per minute. also many of their characteristics therein, including FTP, fiber type, size of fibers, and capillarization. And what they did in the study, they had divided the groups into those with their FTP as a high percentage of VO2 max and those with a low percentage of VO2 max. The range is 59 to 85 percent. And the cutoff from the low to high group was about 72 percent. So in this study, In Table 4, we see approximately the same capillaries per fiber in the high group as in the low group. But there's 15% less fiber area in the high group. So the high group has an average of about 7,000 square micrometers. The low group has a fiber size or fiber area of on average 8,000 square micrometers. This means that there's more capillaries per square millimeter. The average is 405 in the high group versus 327 in the low group. Now, just this cross-section lets us see that the distance that the oxygen has to diffuse is smaller. Now, you might be thinking other things on other ways to measure this and other measurements of capillaries between the high group and the low group were not terribly different. 2.75 versus 2.55 capillaries per fiber for the high and low groups. So capillaries per fiber doesn't change, but here we see that smaller muscle fibers are beneficial. And also, not only that, but if you're wondering about mitochondrial enzymes and activities and such, both groups had the same Activity of aerobic enzymes found in their mitochondria, citrate synthase, which is used in the Krebs cycle, and beta-hydroxyacylcoid dehydrogenase, which actually has several names, that second protein, and that second one is involved in the oxidation of fatty acids. But having said that, we can definitely see that having more capillary density per square millimeter is associated with what's called higher fractional utilization. So that's a fancy way of saying how much your total oxygen capacity you can use, quote unquote, effectively. Or at least that's going to be our definition. In reality, we could do any task and assign a fractional utilization. So we could look at fractional utilization during a four-hour endurance ride also. But this is what we're doing for our purposes. So this study shows that... Higher capillary density is related to higher fractional utilization or higher FTP as a percentage of VO2 max. And they actually plot this in a way. They plot it in a way because we've gone over in Wattstock number 11 one of their endurance tests. And we won't bore you with the details. More capillaries per square millimeter. correlates, not perfectly, but does correlate with your FTP being a higher percent of your VO2 max. Yeah, R is about .75, especially right in the middle. There's a wide range of FTP as a percentage of VO2 max. So it's not a tight correlation, but it certainly helps. And this has also been seen in other classic studies as well. Well, so can we train fractional utilization? Kind of. We can certainly train the number of capillaries that we have. And in some athletes, we will see FTP go up. In other athletes, we may not. So the second study in the show notes is called Hemodynamic Adaptations to Exercise. So this is an older study, but it's also a classic. And they had, quote unquote, five healthy men and otherwise more or less untrained people. They basically did like one by 40 minutes at FTP sweet spot-ish, 80% of their VO2 max, four times a week for eight weeks. And they gained on average 20% capillary density. It's a lot. It is a lot. Yeah. It's called angiogenesis. So pretty much all aerobic training is going to improve capillary density. And as I've said many times, you know, there's only so many intervals that you can... do and successfully recover from. But after that, general volume becomes a really good way to add this stimulus. So here's an anecdote I have is that I have one particular athlete who's always very limited on time. And whenever this athlete goes to a training camp and rides like 30 hours, which is pretty unusual, normally I have not so much time, there is an immediate increase in FTP by like 15 or 20 watts. It's awesome. Obviously. Not everybody's like this. I can think of a couple athletes offhand where this happens. I can also think of a couple athletes offhand where this does not happen. So it's obviously more complicated than just having more capillaries per square millimeter in your legs, although it's certainly not a bad thing. Also keep in mind that This isn't necessarily – it's not a permanent change. Like once you make those new capillaries, like you're not endowed with them for the rest of your life. Yeah, you definitely have to maintain the stimulus in order to maintain them. And the FTP that we see from these training camps doesn't last forever, especially if these athletes – who get this effect cannot keep training at that same volume, which almost none can. And I'm sure we would see more athletes this happens to, but at the same time, we also don't a lot of the time get the chance to go ride for 30 hours a week, you know, if you even can. So that's FTP. What about VO2max? Well, in this study, All of the participants were actually chosen for a specific reason because they were all very closely matched except for a couple different characteristics. So one of the things that they were closely matched for was actually VO2 max. They were all at... 4.7 to 5 liters a minute oxygen uptake. They were all at different weights, but we see that their capillary density actually didn't have anything to do with VO2max or an extremely minimal effect if it had any at all. So I know that was a long walk for not much, but in a way that's science for you. And at least in terms of the Fick equation and the AVO2 difference, the benefits of having better capillary density as measured in this study did not have a measurable effect on VO2 max. This is because diffusion itself probably isn't a VO2 max limiter, as has sometimes been proposed by some previous and even current physiologists, as well as some coaches and others out there. But... That is not the end of the road. There is more to capillary density, especially in relation to blood flow. And let's also remember that the subjects had similar aerobic enzyme activities as well. Maximal rate of aerobic metabolism is a much more commonly proposed potential limiting factor. for not only the rate of diffusion, but also for a VO2 difference and its relationship with VO2 max, because remember, it's passive diffusion and you can only take in as much oxygen as you use. So let's keep both of those things in mind as we get into the next study. This one is called One-Legged Endurance Training. Leg, blood flow, and oxygen extraction during cycling exercise. So... Imagine you were one of the 12 endurance trained subjects that did this. One-legged cycling for four times a week for seven weeks at about 70% of your maximum one-legged heart rate. That's very strange. Yes. A lot of one-legged cycling, but they really want to see, well, we'll see. Let's get into it. So, AVO2 differences between control and Working Legs after the training intervention at low and moderate intensity, the AVO2 differences were zero. But after the training intervention, the trained leg had on average a 30% higher citrate synthase activity and this was one of the two enzymes that was measured in the last study that we looked at in detail where Everybody had about the same activity of enzymes. Well, in this case, the trained leg had a lot more enzyme activity and so a maximal higher metabolic rate. There was a slight improvement in AvO2 difference as the intensity got higher and by about 4.2% at high intensity. However, this is a big however. There was an even greater increase in blood flow. On average, 16%. Interesting. This altogether resulted in an 8.1% difference in VO2 max. So the trained leg, by doing single leg cycling, had an 8.1% higher VO2 max than the untrained leg. Interesting. And so if you... Yeah. So... I'm going to give you guys a hint is that we cannot separate the delivery of oxygen from the utilization of oxygen because they are linked. Now, this means that we can only assume that the trained leg had, as the authors note, more capillaries and therefore a better diffusion capacity. They didn't actually measure this. But they also discuss how red cell deoxygenation leads to a downstream effect of vasodilation in the distal, meaning further from torso, parts of the microvasculature, and this vasodilation effect from the distal portion also feeds back into the more proximal parts closer to your torso of the microvasculature, controlling capillary blood flow. So, in other words, there's potentially a positive feedback loop for increase of blood flow. So the nice thing about increased blood flow or perfusion is that can lead to more diffusion, which means it might not be about how many capillaries we have so much as how many are open. And if we can get better perfusion, blood flow, through feedback loops and aerobic mitochondrial activity, we now have a plausible mechanism for the increase in VO2 max. The other reason I like this study is that the authors spent a good part of the discussion on other studies using similar methods and the AVO2 differences that they found in trained cyclists. So the other reason that I like this study is that the authors spent a good part of the discussion on other studies using similar methods and the AVO2 difference that they found in trained cyclists. So here we saw a 4.2% increase in AVO2 difference. And that was in the mid-80s. Yeah, so it was about 85% to about 89% between the quote-unquote untrained leg to the trained leg. They look at other studies reporting values as high as 92% to 93% in elite cyclists and cross-country skiers. But they also note that comparatively, leg VO2 was 2 to 2.5 times higher in those studies than in this one. Wow. And yet, utilization was only a couple percent higher. Ah, so. Yeah, hint. So, in fact, one of the other things that's really cool about this study is that they went so far as to test two-legged cycling for the same subjects as well, thankfully both before and after. In your mind, decide right now whether the increased blood flow and oxygen utilization of the trained leg will increase their cycling VO2 max for both legs. And I'll give you a hint that during this study, all of the participants were asked to maintain their normal endurance regimen, their normal training load and activity outside of this study. So this study... their single leg cycling was on top of their normal training. So it's not like their other leg detrained. So, Kyle, do you think that the increase in AVO2 difference, the increase in utilization and potentially the increase in blood flow to the trained leg was contributed or not to a VO2 max of both legs. I would bet that it did, but only a little bit. The difference between the two legs was measurable, but it wasn't like it was the super well-trained leg was 10 times better or something, right? Yeah, so it was only like 4 or 5% better, a VO2 difference with a concomitant increase in blood flow. With the cycling ramp test and VO2 max test, with both legs, they remained entirely unchanged. And I don't mean it changed a little bit, but not statistically significant, unchanged. So 3.92 to 3.91 liters per minute VO2 max pre to post for two-legged cycling. And a three-watt difference at the end of the ramp test. Yeah, that's, uh... Almost Measurement Error? Yeah, that's like not even measurement error. That's an average of the subjects. You know, the standard deviation didn't change much either. Also, let's compare that to single leg cycling, where the VO2 max for the train leg went from 3.06 liters a minute to 3.25 liters per minute. and the ramp test final value for the trained leg went up 17 watts on average compared to the untrained leg. Okay. So that is a, you know, less than 10% improvement in VO2 max. But it was on top of their normal training regimen. Yeah. Think about those VO2 max values, Kyle. I'm going to give them to you again because single leg, Okay, let's go for the higher one. Single leg cycling of 3 to 3.2 liters a minute. Yeah. Two leg cycling, 3.9 liters a minute. Yeah, those are very, very close to the same number. Like, it is not half. Like, maybe naively you think, oh, if I'm just cycling with one leg, my one leg VO2 max would be half of what my two leg VO2 max is. Right. But it turns out. Yeah. Right. And so the single leg cycling VO2max as a percentage of two leg cycling VO2max improved from 75 to 82%. You know, might we infer that there's like, you know, a minimal amount of mitochondrial activity in order to elicit actual VO2max? So let's think about what VO2max is for a second. You know, it's an increase in workload that is not followed by an increase in oxygen. Oxygen uptake will plateau because it is at maximal. One of the things that happens in a lot of these single limb studies or like small muscle group studies like knee extensions, so you're just using your quads, is that when you do a ramp test to try to find VO2 max, you don't actually find VO2 max. You don't get a plateau. I don't think I've ever seen one in one of these studies. You get a peak. It stops. What can we infer about this? We can infer because it's not active, the diffusion, the movement, well, the heart pumps, but that's the only active part in the whole thing. And blood vessels dilate and close and whatnot. So what we're really seeing happen is the muscle is getting as much blood through it as possible. And all of the oxygen that can diffuse is diffusing. And obviously not all of it's going to diffuse. Why is that? Because it's not active. We cannot actively get to 0% in our blood for oxygen content. Yeah, you'd have a bad day. Yeah, you'd have to have a bad day. You'd have to have the worst day ever. And you'd also have to have an active mechanism to get oxygen. out of your blood and into your cells. But we don't have that. What we have is like the best that we can do in a very, very, very, let's pretend that your blood is sitting in a very, very long line of capillaries. And every heart pump, it moves a little bit and then stops. And it moves a little bit and then stops. And what's happening is if there's more capillaries, you can get down to whatever the pressure is in your cells because it's going to equilibrate. between the cells and the smallest value in your blood. It's like Xeno's Paradox, but for blood. Yeah. Except in Xeno's Paradox, for all intents and purposes, we get to infinity all the time. Like, I can touch my desk. I just violated Xeno's Paradox, didn't I? But with diffusion, since it's passive, we couldn't. Yeah, you never actually could. There's not enough time for you to actually sit there and wait. Yeah, and your mitochondria would have to like literally use every single drop of oxygen, or molecule rather, that is between it and effectively like whatever can diffuse, like you would have to be empty of oxygen completely in order to get that. This is a relatively short timeline here that we're looking at when your body is working and you're inhaling oxygen and your blood is pumping it through your system. Like you're only talking like minutes, you know, minutes here for these ramp tests, minutes for a lot of even maybe an hour, right, for FTP tests. So it's if you're just relying on diffusion, you know, how long does it take? for, never mind. Actually, no, you're coming up on an idea that we're going to talk about probably in the next episode or two, which is that the time that the blood has to do certain things between pumps of the heart matters. Now, what we've seen here in this study is that blood flow regulation and opening up more capillaries and getting more blood flow through the muscle is going to increase oxygen utilization. And, you know, we can get close to 100%, but we're never really going to get to 100% because of the passive thing. And so that's why AVO2 difference is linked to blood supply here. And so the reason that we're talking about all of this is that, so we can tell you that AVO2 difference or oxygen utilization is not a large factor in VO2 max. Otherwise, two-legged cycling would have improved due to the trained legs increase in utilization potential, which was clearly not being used when cycling with both legs because it was limited by blood. So this means in the true long term, you know, the major limiter of VO2max is blood and heart. To tie this to the Fick equation where we started off, VO2max equals cardiac output times oxygen utilization. So heart rate times heart stroke volume times oxygen utilization. Because remember that if we don't utilize oxygen, there's nowhere to put the new oxygen. So to answer that question that we started off with, no, the parts of the Fick equation are not equal in terms of their contributions to your VO2max. Cardiac Output is far away the major limiter. And we're going to see that. And also it's intertwined with your blood. So one of the reasons that we see the plateau in O2 uptake during a true VO2 max test is that the heart cannot pump any more blood and or there isn't enough blood available. So we can contrast that with the fact that there's no plateau in single leg or isolated muscle group studies. where there's more than enough blood to power your quads. You're not going to get to that plateau because your muscles fatigue and it's not- Before you get to maximum. Yeah, before you get to see that plateau. So if you're just doing leg extensions, if you're just doing quad extensions and trying to get a pump in the gym or whatever you're doing with that, you're never going to see that plateau because if you could make your muscles contract more, and use more oxygen, you would have it. It would be like, yeah, we got it. We got plenty. Have at it. Your legs, your muscles in general, not just your legs, are more than happy to use almost all, 80, 90% of whatever oxygen they are given. Yeah, exactly. And remember, that was measured in the femoral artery. And so there's a lot of other tissue in your leg that's not working when you do that kind of stuff. So this also explains Why we can see a VO2 max increase, this is, I have not seen a study on this, this is just me putting together information I have, so I could be totally wrong on this, so this could potentially explain the increase in VO2 max from swimming to cycling to running, because even if we can get to a plateau, it'll be a different AVO2 difference, a difference in utilization. Yeah. Yeah, I think so, because if you think about swimming, like, you do use a lot of muscle groups, but you are not using your legs, your quads, glutes, hams, things like that, as much as you are in running, as you are in cycling, and therefore the utilization is just going to be lower, and then when you get to these exercises that actually use... You know, larger range of motion, larger muscles for a longer time or a majority of the effort than you would sort of naturally expect to see a higher amount of oxygen total used because you're supplying all that oxygen to more muscle mass that is more than happy to use whatever you're giving it. Right. And so you might get to the same cardiac output, but you will have a lower AVO2 difference with swimming than with running, for instance. Yeah. And VO2max, as we talked about before, is a number that is sports-specific or discipline-specific. So you see differences for the same athlete running on a treadmill for a VO2max test versus riding a stationary ERG bike for VO2max. Right, exactly. And so that's why we posed the question several episodes ago, what do we do with this information? And this potentially explains that information. You know, originally, remember, A.V. Hill, and his co-author was Lepton, I think, said that there had to be a large amount of muscle mass used. And this is one of the reasons. But, you know, at the time, I don't think they knew that if he used even more muscle mass, you know, it would change this part of the Fick equation. Anyway, so now, I hope everybody listening has a lot more knowledge. than they did before. And I know a couple of you listened to episodes a couple times, so I hope that we've explained things sufficiently. As always, you can email me with questions and comments. But now that you understand all this stuff, so when you hear quotes that are inaccurate, and it'll make you cringe, like it makes me cringe. So here's one that I heard on a podcast recently. Quote. We're putting more oxygen to the muscle than it knows what to do with, and that is a stimulus for adaptation. And a second later, quote, we're going to dump oxygen in there, and if you don't have the ability to process it, we need more capabilities, make it happen, unquote. So, what's wrong with that statement, according to what we've discussed today? Well, it would appear that actually the opposite is true. We are not actually ever really able to dump... more oxygen into our muscles than they know what to do with. Exactly. Even in when you're looking at those very isolated exercises like knee extensions for VO2max, which sounds terrible, by the way. And so the stimulus for adaptation must be coming from something else and not just the fact that your muscle all of a sudden sees oxygen just sitting on its doorstep. Yeah. And also, you know, We're going to talk about the mechanisms for aerobic adaptation down the road. And I know a lot of them. Obviously, nobody knows all of them. But I'm quite familiar with all of the big ones. And it doesn't happen like this. And if you want to know why having excess oxygen, as we mentioned before, in your muscles is bad, I really refer you to Wattstock number 13, What's So Radical About Oxygen. It might be one of the coolest episodes I think we've done. Not one of the most popular, for sure. Yeah, so also you can watch for phrases like, quote, VO2 max is the point at which we can't use more oxygen, unquote. That's silly, because of course we could use more oxygen. We just can't get enough to the places that want it, because oxygen is the limiting reactant or limiting reagent. So in any chemical reaction, like the... Like the turning of oxygen into water that we need for aerobic respiration. If there's a lack of an ingredient, then it's the availability of that thing that determines the rate of the reaction. And in this case, it's the rate of oxygen getting to complex four of the electron transport chain to become water. We got lots of the other stuff around. We don't have that much of the oxygen around. And it's a passive process to get it there. and we're going to see a lot more about that in the next couple episodes. The statement that your VO2 max is the point at which you can't use more oxygen or that your body is delivering more oxygen to your muscles than they know what to do with is patently false and if it were true then certain doping techniques would not be as particularly effective as they are. So as always, I want to thank you guys all for listening so much and this is probably going to be our longest episode yet and I hope that you guys all have enjoyed it. and we've got more to come on VO2Max and VO2Max training so please subscribe to the podcast give us an iTunes rating and a review if you are enjoying the show and remember that we're an ad-free podcast so if you want to donate to the show and support us you can do so at empiricalcycling.com slash donate we've got the show notes on the website with the picture of the rectangular hyperbola we also have all of the studies there linked as well and if you have any coaching and consultation inquiries or questions and comments you can email empiricalcycling at gmail.com we also have merch empiricalcyclingpodcast.threadless.com with that we'll see you all next time thanks everyone