Welcome to the Empirical Cycling Podcast. I'm your host, Coley Moore. Joined as always by my co-host, Kyle Helson. Thank you, everybody, for listening as always. Please subscribe to the podcast, yada, yada, and of course, share it. That's probably the most important thing that you can do if you want to help out the podcast. So share us. And also remember that we're ad-free, so if you want to donate to the show, you can do so at empiricalcycling.com slash donate. We also have the show notes up on the website, so we have some figures, we have some paper references for this episode. And of course, if you have any coaching inquiries, consultation inquiries, We are always taking on athletes and we're always doing consultations. So that goes for coaches and it goes for athletes as well, all disciplines. So if you want to reach out, do so to empiricalcyclingatgmail.com. So if you have any questions or comments, you can shoot a couple comments there. Say hi. And, of course, on Instagram on the weekends in the AMAs and the stories, doing those every weekend. So come participate in that. We had a lot of questions last weekend. That was a lot of fun. Thank you, everybody, for participating. And hopefully we get a lot more. We've been discussing metabolism lately in the podcast series. And so we've covered a lot in terms of understanding carbs and fats and kind of what limits them both. And we've found that it's complicated, right? So we can make it even more complex now. So we're going to start by adding muscle fiber type into the equation. So I think by the end of this episode, I think at the very least, you know, everybody will understand why I always talk about larger motor units and I never say your fast twitch fibers. You know what I mean? So Kyle, give us a couple of thoughts on all of that and kind of where we're going to head in this episode. I think most people who have paid any attention to popular exercise literature or even just vaguely remember a biology class or any sort of – maybe some sort of physical education class have probably heard of muscle fiber types. And typically at the coarsest, those are – classified into slow twitch and fast twitch and i think just the names slow twitch and fast twitch are actually pretty illustrative when you think about it in your head like oh you think slow twitch that kind of or maybe i'm just poisoned by exercise and popular fitness but you think oh slow twitch you think marathon runners or ultra endurance or iron man or stuff like that Versus fast twitch, you think Usain Bolt, like Flojo, like people running track and field or weightlifting, like Olympic weightlifting, not just like bodybuilding, like bro weightlifting. And aside from that, that's maybe all as far as people get. And maybe people think a little bit more like, oh, if you can think about. road cycling you think about people being sprinters versus climbers or time trialists and so maybe you start to think oh yeah those sprinters you know must have more fast twitch fibers and then those climbers or maybe those time trialists must they must be more slow twitch right because that seems to make sense based on the minimal level that you know that if slow twitch is more endurance and fast which is more sprinty and in terms of fats versus carbs I think you can actually sort of tie in the same analogy that you think, oh, endurance means slow twitch, which must mean fat burning, right? And fast twitch, you think glycogen or ATP use, so that must be sprinty. And so carbs, you think glycogen, carbs. One of the major ways that you refill your glycogen stores is by eating carbs and sugar and starches. So perhaps then you might get to the point where you think, oh, it must be that fast twitch fibers, if you have a lot of fast twitch fibers, you must therefore be good at burning carbs. And if you have a lot of slow twitch fibers, you must therefore be good at burning fat. All of that might seem to play together nicely at this very rudimentary level or this sort of shallow understanding of this dichotomy or this apparent dichotomy between you are either a slow twitcher or a fast twitcher. I'm maybe giving away some of the plot here. Maybe, but I think a lot of people coming into this who have been listening to the podcast regularly are already kind of aware where this is going to go because we did cover this a little bit a long time ago in another episode, but that was really early on. I don't usually do this, but I went back and I listened to a little bit of that. And I thought we had gotten further in depth with this skier paper than we actually did. Because in my head, it was just, oh, yeah, we went really in depth with that. And I kind of mentioned it in passing. I was like, oh, okay, this is probably worth really getting into in a lot of detail. And, of course, there's been another paper that Andy Cognos tweeted a couple times recently. He usually deletes his tweets, so I was lucky enough to catch it once. Somebody was like, hey, did you see this one? And it kind of fit here, and so I figured it's probably worth getting into just to really, really throw all the monkey wrenches into this as humanly possible. So I think let's kind of roll this back just a little bit. What are we talking about when we say twitch? Like what is a fiber twitch? So if you have a fiber in a Petri dish and you zap it with electrical current, it contracts. And the rate of contraction... is what determines twitch speed. So from zero to peak force, how long does it take? A slow twitch fiber takes longer to contract. A fast twitch fiber will contract faster. And typically, the fast twitch fibers will produce more force as well. Of course, this depends on a lot of the contractile apparatus inside, the sarcomeres and whatnot. But let's now get into a little bit of review on fiber type. hype beyond just the twitch. So usually we see fiber types for humans given as three types. So type 1, type 2A, and type 2X. And sometimes if you look at the older papers, you'll see type 2B. And a lot of times in humans, in the older papers especially, you'll see people say type 1, 2A, and 2B. But 2B doesn't actually exist in humans. If you take like a rodent, muscle and you grind it up and you put all the myosin heavy chains onto a gel and you run that gel and you put it next to a human, you're going to see three bands that line up between the human and the rodent. But there's going to be a fourth band in the rodent that we don't have in the humans. That's a type 2B properly. So the other thing about this is that when we look at articles, charts, media, We usually see fiber types that are equated to things like certain training levels or power zones. And by the time we get to the end of this, we're going to see that that's not real. But I think probably more importantly, the same things often equate a fiber type with either a fuel source, like you said, type 1 fibers burn fat, type 2A fibers will aerobically burn carbohydrates. Or something like that. Or they'll say they're aerobic. Or they'll say they're anaerobic. Or whatever. So these are not quite right. But if all you've got is one chart to get across all the information, and you're allowed like three words, I see why they make these compromises. So I'm not trying to say that these things are bad. They're a good place to start. But here's an example. I found a slide from something that said, Doing a marathon or a half marathon is a good way to, quote unquote, emphasize your type 1 fibers. And a 400 and 800 meter sprint, these are a good way to emphasize type 2A fibers. And 100 and 200 meter sprints are good ways to emphasize type 2X fibers. So this kind of seems weird on its face, right? I've seen articles saying that there's reliance on certain fiber types with certain training zones. Does this kind of make sense to you, Kyle? Yes, I think it is. I actually think that stuff like that, like you mentioned, where a certain discipline or a certain modality of exercise is equated, makes maybe, it's an easy comparison to draw. It's an easy idea to illustrate. Like, oh, yeah, that makes sense, right? Like, oh, if you're going to do really short activities like a 200-meter sprint, that must mean that it's... this type of fiber or so this type of training, like you must be a way to improve or express that type of fiber or something like that. Yeah. But we can't also ever forget the size principle, which means that, um, you know, if you are, let's, well, let's. Just do a real quick example here. So let's say you are 50% fast twitch and you're 50% slow twitch fibers. And you're doing something that requires 75% of your muscle mass, which is a lot. You are primarily relying on slow twitch fibers because it's 50% that and you're using only 25% of your fast twitch fibers, right? But this gets a little more complicated as we really dig into it. Real quick, let's – well, we kind of went really in-depth on this in one stock number 12. So I don't want to recap the entire thing. But let's go through a couple of review points. So the first one is that when a fiber gets the signal to contract, remember that calcium is released and that contracts a protein away from the passive actin chain. And this allows the active myosin protein to bind, and then it uses ATP to make the contraction happen. It's that ratcheting mechanism. And remember that contraction is all or nothing, and either the fiber contracts or it doesn't, which means that the determinant of the rate at which the contraction happens for the muscle fiber, not the whole muscle, but just for the individual fiber, is highly related to the type of myosin fiber, right? This is what we're talking about with twitch characteristics. So we've got fast versus slow. This is just one way to classify muscle fibers is in twitch characteristics. We have a bunch of other ways to do it as well. So for instance, histochemical staining. This is where we take a cross section of a tissue slice and a really, really thin one, and we bathe it in different chemicals. Most common, we see pH. But there are others. But I think actually in the last episode, Watt's Doc 12, we didn't really get into how this works or we may have, but I think it's interesting how it works. So what happens is acidity inhibits the ATPase, which is the business end of the using ATP to make the myosin contract. Acidity inhibits it in type 2 fibers, but not type 1. And so adding base, on the other hand, will inhibit... the type one ATPS fibers, not the type two. And so when we do this and we inhibit these fibers and then we do one or two other steps, the phosphate ions that are used with the contraction are used, like they're precipitated out and that's what makes the muscle fiber dark in terms of the, when we look at it under a microscope. Dark meat. Yeah. Sorry. So we can actually make type 1 fibers darker or type 2 fibers darker. And this is really just showing that that fiber was active. So anyway, I thought that was really cool. But we can also classify a muscle fiber. based on myosin heavy chain or MHC. And like I said, this is the main unit for contraction. This does the ratcheting action. And myosin heavy chains are typically classified as type 1, 2A, and 2X. But again, if you look at older papers and stuff, or if you look at people who don't really get into the nitty gritty with muscle physiology. you know, there might be some confusion. But generally speaking, it doesn't always mean that they don't know what they're talking about. Sometimes we can just go, oh yeah, they mean this, and they might have a lot of other good stuff to say. So we don't have to throw out the baby with the bathwater here. But there's also the biochemical way to classify muscle fibers, and that's what we're going to focus on today. So typically we see fiber types classified as SOG, FOG, and FG. So slow oxidative Fast oxidative and fast glycolytic. These are the three things. And the O is usually like oxidative. So fast oxidative, glycolytic, for instance, like, okay, this is a fast fiber that will primarily or some might expect entirely burn carbohydrates in order to supply oxidative metabolism, et cetera, et cetera. But, you know, like I said, If you read the real muscle physiologist authors, they take a long time to kind of express the nuance, and there are entire papers devoted to this level of nuance. Let's get right into this paper, because I love this paper. This is a paper that looks at very elite cross-country skiers. And it compares myosin heavy chain types to the metabolic capabilities. And it also compares the arm versus leg muscles. And I kind of like this one because there is a good deal of difference between the arm and leg muscles. But we're not going to get too into that because we're cyclists. We've got T-Rex arms. Even somebody like me, I left weights. I've still got kind of T-Rex-y arms compared to a lot of people in the gym. Sorry, what was that called? I was going to say it's like turkeys where... Turkey has white and dark meat because turkeys don't really fly, so the breasts and wings are all very, very not good at and not often used, and that's why the legs come out dark meat, just like cyclists. And that's why something like a heart is the darkest meat of all because it's constantly contracting. All right, so here's a question. Why is it good that we're looking at skiers for this? And skiers are a good group for this because they have high and rapid force requirements during short ground contacts when they ski. So immediately, this is very different from cycling, where we can make the pedal stroke last as long or as short as we want to by changing our gearing. Not only that... do they have high force requirements? We have low force requirements. But they have a large endurance and oxidative component to their training and racing. And so we have two things that we don't typically get when we look at a lot of athletes. So if we look at weightlifters, for instance, we have very high force requirements and we have very low oxidative requirements. But if we look at marathon runners, we have just about exactly the opposite. So this is a really interesting kind of ground in the middle. And so they also have much more equal training between limbs. So if you like compare a cyclist arm versus legs, you're going to find that the arms are probably a good deal. Well, who knows? I don't know. They're, you know, besides not big. So, yeah. They're going to default more to whatever your natural off the couch distribution of, we'll say, abilities. versus your legs, which are very much trained to whatever you're doing because of your sport. Yeah. Yeah. And so I think that there's a really interesting thing about this study, which is that cross-country skiers have very unique requirements that is not typical for a sport. In a lot of sports, it's either one or the other. This is one of the reasons that this study is so cool. And, you know, we might be able to find something similar for like rowers, for instance. But anyway, so this study compared samples directly from triceps and the vastus lateralis from 10 elite Norwegian cross-country skiers. All national team members, a lot of them race World Cup, average age 22 years, 181 centimeters, 80 kilograms, VO2 max of 69, nice, training age of 11 years. What did they look at to determine the skeletal muscle characteristics? So they did myosin heavy chain composition. They ran the myosin heavy chains isolated on an SDS page gel. They looked at enzyme activity of HAD, or 3-hydroxyacylicoidehydrogenase. Right? Oh, yeah. We all know that one. Classic. Classic enzyme. Oh, my favorite. It's my favorite. Yeah. Yeah, it's my favorite, too, because it's needed for fat breakdown. And if you want more on that, check out Wattstock number 29. So, HAD activity levels is an indicator of the muscle fiber's ability to oxidize fats for aerobic energy production. It's not, you know, it's not a direct... correlation, because obviously there's a lot of other stuff that limits fat use, but it's a really good indicator. Then they also looked at citrate synthase, which is the entry point enzyme for the Krebs cycle, the main quote unquote entry point. And check out Wattstock number 32 for more on that. So this is an indicator of the muscle fibers total aerobic capacity, because if we look at like Wattstock number 32, we see that at the citrate synthase point, Carbs and fats are just about equal, and it doesn't care where it came from. It just wants to use that acetyl-CoA. So this is used as a very general stand-in for mitochondrial density. But we'll also see something like succinic dehydrogenase get used commonly because... Well, I think it's less common these days because SDH is actually bound to the inner mitochondrial membrane as part of the electron transport chain and part of the Krebs cycle both. And so I'm guessing based on my own personal frustrating experience working with membranes, it might be harder to measure that. I might be wrong, but it might be harder to measure. I haven't personally done that in the lab. So HAD and CS, so HAD and citrate synthase are both in mitochondria. So higher activity levels of both, for instance, in a certain amount of muscle, like per gram or per kilogram or whatever, we're looking at how much of this specific reaction happens to indicate activity level. Instead of just isolating out the protein individually, there is a greater error associated with that. It's probably easier for us to look at what is the activity level in a certain sample. So for instance, Something like a high HAD activity level and a high citrate synthase level means that a muscle fiber can highly oxidize both fat and carbohydrates and lactate in the Krebs cycle. However, something like a low or non-existent HAD activity level and a high citrate synthase level means a fiber has some low or non-existent ability to oxidize fats, but a really good ability to utilize carbohydrates and lactate for aerobic ATP production. So does this all make sense so far? Yes. And I think it's – and maybe this is something that might be slightly missed if you're not a little bit familiar with the events that are very common in cross-country skiing. But they do compete in a wide range of events. And maybe you just watched the Winter Olympics and you saw this where you either have – you have both events that are longer, like well over an hour, like up to two hours. for men and like an hour and a half for women. And then you have other shorter events, which are only about a kilometer and a half, but you race that multiple times in like a bracket basically throughout the day. So you would potentially see the same athlete doing both of these events and have to do everything from effectively like a pursuit all the way through. your longer crit road race type duration. Yeah. And so in terms of energetic demands, besides the force demands, these are a really good way to potentially look at the metabolic demands on the muscle fibers of endurance cyclists, competitive cyclists. And so they did two other things to classify the muscle fibers. for these skiers. One is they did histochemical staining, so they did the ATPase fiber typing, and they also looked at muscle capillarization. So we're probably not going to get too much into that one, but just to know that they did it, I thought that was interesting. So if you want to look at the paper itself, it's definitely worth a read. They also did electron microscopy. So they were looking at location of mitochondria and lipids. in the muscle. And we're definitely not going to go into depth in this episode, but we might, if I decide that it might be interesting to look at cellular organization. Because I think a lot of people think that cells are just kind of, you mix a bunch of marbles in a sack and then you mix it up and everything is randomly distributed. There's actually a very high degree of organization in a muscle cell. It's not the watery sack. And I've actually seen papers where people will you know, put stuff like, you know, like make a solution of whatever and call it physiological solution. And it's like, it's more organized than that, guys. It's just, try it again. But those are older papers, typically. Typically. Anyway, enough of my grousing about some things. So what did they find? What is here for our purposes? This is not really relevant to us, but the legs. had way more myosin heavy chain type one than two. So it was 58% versus 40% on average. But I do want you to remember that a little bit. 58% type one fibers in the legs, 40%, so 18% difference. But this is relevant for the study, so keep this in mind because we're gonna bring it up a couple times. But the ranges in the legs were 34 to 69%. And in arms, it was 24 to 57%. So it was a pretty big range. They also looked at enzyme activity. So they looked at the maximal citrate synthase activity between arms and legs, right? And so we know that arms typically have a little bit less slow-twitch fibers and legs have a little more slow-twitch fibers. So given this fiber type distribution, though, remember it's... 34 to 69% in the legs, type 1. 24 to 57% in the arms, type 1. So what do you think had the highest citrate synthase activity, arms or legs? I am going to guess the legs. Barely. Oh, interesting. Yeah. Average citrate synthase activity between arms and legs was basically the same. Average was 118 in the legs and 111 in the arms. Oh. But why? Yeah. That's interesting because, like, I, you know, okay, for cross-country skiing, yes, you have to use both your arms and your legs, but you imagine that it is not 50-50 in terms of force production. Just sheerly based on muscle size, right? Like, if you watch someone classic or freestyle cross-country ski, like, yes, their arms are doing a lot, but it does not appear to be equal, right? Yeah. Yeah, and so that's something that we can talk about in a little bit. So next thing here that I think is interesting is HAD activity. Remember, this is the enzyme that we're using to determine the muscle's ability to oxidize fats. HAD activity was 45% higher in legs than arms. Hmm. That's a lot. Yeah. That's not a little bit. That is. Yes. So yeah, the average in the legs was 144 activity units, let's call it. And in the arms, it was 95. So the legs with much more slow twitch fibers are relatively more able to oxidize fats, but not proportionally to the type one fiber content. Yeah. So the legs do have more slow twitch. So on average, at least. So in an absolute sense, you would think they would be better. But, and that's true. Yeah. But when you consider the fact that they have more slow-twitch fibers than, as a ratio, they're not otherworldly. Yeah, and speaking of ratios, the authors also looked at... HAD to citrate synthase activity ratios, which was 45% higher in legs than in arms, which means the takeaway here is that having more myosin heavy chain type one is a good indicator of HAD capacity. But because we're looking at the distributions here, it's not an ironclad predictor. Because, remember, it's not like there's zero fat-burning ability in the arms. It's not like type 2 fibers have zero fat-burning ability. They have a good ability to burn fats, just not as good. I think that's an important point to make, though, because I think people do think that type 2 fibers must only be good at just churning through ATP or oxidative burning of glycogen. Yeah. Yeah. And so, you know, let's talk about the average activities here. So if you look at the paper, if you're reading along in the paper, check out figure two. So in the arms, the range of HAD activity in the arms was pretty wide. It was 25 to 90. The units here is micromole per gram per dry weight per minute, whatever. But HAD activity in the legs distribution was 45 to 115. Like, you know, it's... Like, there's a good range of overlap. It's not like these two are totally separated. You know what I mean? It's not like... And it's not totally ability to relate to the type 1 or type 2 fiber distribution. So... Yeah, it's not as if you would put like a world-class bench presser set of arms on top of it. Yeah. On top of like a cyclist's lower body or something. Yeah. And so here's the most interesting part of this for me, which is what is the average citrate synthase activity between the arms and legs? It's about the same, right? And so what can we take away from this is the relative oxidative capacity for fiber type. It's actually fairly well predicted by fiber type. as the authors note. So HAD to citrate synthase ratio might actually be one of the better predictors that we have for fiber type. And it shows how much fat burning capacity there is relative to total oxidative capacity, right? So remember the arms had a HAD to citrate synthase range of 0.4 to 0.9, somewhat evenly distributed. But the legs had a distribution. from 0.6 to 1.2. But the low one is an outlier, though. So the legs, if we exclude the low outlier, the low number is actually 0.85 to 1.2, as opposed to the arms 0.4 to 0.9. So except for that one data point, there's actually not much overlap in the relative amounts for the more fast twitch arms versus the more slow twitch legs. But still not a perfect predictor. It's a good predictor. It's not amazing. Can fast twitch fibers still oxidize fats? Yes, absolutely they can. And so I think it's also interesting the way they plotted a bunch of this stuff out because we just went through figure two's graphs. And we just went through the y-axis variables, but they actually plotted this for each sample against the amount of myosin heavy chain type 1 as their x-axis variable. And so there are some distinct separations between the arms and the legs in terms of the myosin heavy chain type 1, but it's still not perfect. This would be illustrated by looking at, for instance, you know, figure 2A. So 2A is HAD activity versus MHC percentage on the x-axis. So the arms are in the open white circles. And, you know, we would expect if it were a perfect relationship, obviously it's not going to be, it would be perfectly linear. As you go up, as you go to the right, the activities of the HAD get higher and higher and higher. This is actually not the case because like for the 40% in the arms, activity ranges from like 25 to, you know, damn near 100. And it's just that the cluster of activity for the legs is actually smaller. Starts a little higher and it ends a little higher, but it's smaller. And there's a good deal of overlap. So we've got one, two, three, four, five, five or six samples from the arms that are within the HAD activity range of the legs. So it's just that the legs are more predictably able to oxidize fast. It's not that the MHC type one is a perfect predictor, like we said. So check out the show notes where I'm actually going to put up figure two. So head to empiricalcycling.com and check out the podcast notes for that one. So if you want to look at this yourself and make some conclusions on your own, please do. So the authors put this whole thing succinctly in the discussion. Quote, a divergence between fiber-type pattern and aerobic metabolic capacity. So they note that this contrasts a canonical 1971 study from Burke, not Louise Burke, by the way, who showed a pretty strong correlation between fiber type and the normally expected muscular metabolic characteristics in the cat gastronemius. The laziest animal on the entire planet. Yeah, where's Millie and Kelvin? I should have told you to get them ready. So when you look at the muscular characteristics of a cat, it's a little bit different amount of activity compared to elite cross-country skiers. And if you think, just what do cats like to do? when they're very active, right? They like to sprint around and they like to chase things and pounce and jump. That is much more like being a sprint athlete or a thrower or something like that, or high jump, long jump, for example, than it is a... hour and a half long jog out, you know, half marathon type jog or something like that. Yeah. Like I think if a cat were like, either you're going to have to run for an hour and a half or you're going to get eaten, the cat would be like, well, just salt and pepper me, baby. Just I'm done. Unless that hour and a half is like 15 seconds to try to find a very tall tree and then sitting up in the top of the tree for the rest of the hour and a half. Yeah, and so what we're going to talk about in a little while is actually a predictor of muscle fiber type composition and metabolic requirements are what are the force requirements and what are the metabolic requirements that you have historically experienced, and also what are your genetics too, of course. So I think when we look at... like a cat and the fiber type and the metabolic characteristics of the fiber types. You know, if you look at somebody who's untrained, for instance, which we will in a little bit, you're going to find the more canonical version. But once you start training people, everything starts to change rapidly. Or maybe rapidly in terms of metabolic characteristics, maybe not so rapidly in terms of fiber type. So anyway, the cross-country skier paper. The authors note that Essen, another author of another paper, found an equally high succinate dehydrogenase activity in histochemically determined ATPase for type 1 and type 2 fibers in endurance runners, highly trained endurance runners, VO2 max average over 72. And so this kind of confirms their finding that when somebody's highly aerobically trained, you are going to find... even in your type two fibers, you're going to find that they have a lot of aerobic capacity in these fibers. And I'll put a link up to that, the Essend paper in the show notes as well. But what they found more importantly, perhaps, is that untrained people had half the succinate dehydrogenase activity of the runners, as you might expect, just like a cat. Yes, because the untrained person sitting on their couch next to their cat has a very similar level of activity. Sorry. Yeah, no. So I think despite what one might think, we cannot reliably correlate a fiber type with any kind of metabolic ability or disability, especially for trained people. Maybe it's easier for untrained people, but again, this is one of those things where... we can never really say for certain. And so what we can reliably conclude from this is that type 1 fibers are somewhat better at burning fat, but sometimes type 2 fibers can get pretty close. Right, yeah. Yeah, substrate use, right? That's super interesting. I think we have talked, we talked about this a little bit before in the episode where we talked about like, oh, it turns out that you are not, like a muscle fiber type is not necessarily just type one and only type one and like type fiber type conversion can happen. But also the fact that it's not like if you have type two, you have, say you're a majority type two, right? If it were the case that they were not good at burning fat at all and just terrible, you could imagine a situation where you could go throughout your day and you would just be tired all the time because your muscles just weren't good at burning fat. That's what you're going to mostly burn when you're just living your daily life. You would definitely be... a cat or like a sloth or something with just a completely different metabolism where you're very slow. You want to really conserve all of that energy all the time. You know, I think that there's been some research lately showing that, you know, people who are fairly inactive have a fairly high reliance on carbohydrates at rest. But also like the metabolic requirements of being at rest are very different than during activity because at rest, a lot of what you burn depends on what you eat. And so if it were like you were saying, they would be finding that regardless of what people ate, it would be their fiber type that determined what fuels they used. And this turns out not to be the case, right? And it's not quite the same once you start exercising. And of course, then that gets into the big question of being trained and what counts as trained and how trained are you, training age, and all that kind of stuff. But I think oxidative capacity. like slow-twitch fibers versus fast-twitch fibers in trained people, I think we can pretty reliably say that it's probably almost exactly the same. Depending on the normally distributed blah, blah, blah, we can pretty much call it, if you're well-trained, your fast-twitch fibers are probably very well aerobically trained as well. Maybe not the best fat burners as your type 1 fibers. But, you know, still pretty good. And actually, I think – this isn't in our notes here, but I think it's interesting to note that when it comes to fiber size, they actually found that between type 1 and type 2A fibers, they were actually basically identical, basically about the same size that I recall. Interesting. Yeah. Yeah, you – I think people might imagine that, oh, lifting weights and – This must mean you have some amount of hypertrophy and you think lifting weights, well, that must be fast twitch. So it must be that fast twitch fibers are the ones that look really big and you look at big bodybuilders or even track and field sprinters where a lot of them probably not doing a ton of upper body lifting, but a lot of them have fairly developed upper bodies even though they're not. using their upper body that much for their sport at least not in high force production um so that is that's also pretty interesting i think yeah you're right because um you know canonically the uh as the fiber type goes from type 1 to 2a to 2x and even in other animals to 2b um you know canonically they get larger and so this is one of those things where okay yeah we're finding out that that's also not the case so kyle uh Having said all that about the cross-country skiers paper, and there's more to that paper, a lot more, but we're going to stop there because that fulfills our purposes. One might think that low-intensity or endurance-type efforts might only recruit type 1 fibers. Think about size principle and whatnot. And something like VO2 max work might get into type 2A fibers in larger motor units, and that anaerobic and sprint work. might get into type 2X fibers because, you know, we were just talking about, you know, these people being well-trained, but they have, you know, if you're doing a cross-country ski race, you know, you're putting out a ton of force. So you're recruiting a lot of muscle fibers. So now if we're on a bike and we're doing like an endurance ride or like a moderate tempo ride or a threshold ride or whatever, you know, Do you think that it would be reasonable to conclude that kind of distribution of fiber type versus power output? I think at first it maybe does seem like, oh yeah, that would be a pretty good rule of thumb. But if you think about it a little bit more, firstly, if we want to follow the size principle, you're not guaranteed that... all of your smaller motor units that you're using while you're putting out lower force are all necessarily one type of fiber, right? You can say that it's likely that more of the smaller motor units have more slow-twitch fibers. But then also, if you think about the size principle, as you get tired, even if you're getting tired doing your long Zone 2 ride, As you get tired, you're going to start recruiting larger and larger motor units, even to produce that lower level of force. And so this would naturally mean that you're going to start getting into some of those faster twitch fibers that are probably more common in those larger motor units. And furthermore, it... For the anaerobic and the VO2 max and the sprint type work where you definitely have to produce a lot more force, keep in mind that when you use larger motor units, it's not like you stop using all the smaller ones. So you recruit all of the ones below that threshold as well and however many more you need to keep producing that higher level of force. It's not like you turn off the small ones. Yeah, and so if we want to think about... muscle fiber requirement, duration becomes the third axis on it, not just how much power is required, like how much force at what velocity. We also have the duration component, the time component of how tired are you from having done all these contractions. And so if we kind of want to take that stuff out, we can actually look at this study that Andy Coggin has tweeted a couple of times now. It's called Progressive Metabolite Changes in Individual Human Muscle Fibers with Increasing Work Rates. So there's a link to it in the show notes. Go check it out if you want. I'm not saying that you should use Sci-Hub to find all these papers, open text, if they're not open text. So that really cuts into the pockets of these big, rich publishers. And I just find that to be, well, it is a choice that you could make. Elsevier is listening. So this is a classic paper from 1987. And I've always kind of thought about using this paper in the podcast, but I wasn't really sure what a good time for it would be. So I saw Andy tweeting it. And well, if Andy thinks it's kind of significant, well, let's take a shot at it. Let's see what happens. So what were they looking for in this study? I quote from the study intro section, the present study was designed to determine if the lactate threshold corresponded to an abrupt increased reliance on fast-twitch fibers for energy production, unquote. Because they had mentioned later in the paper that, I think at the start of the discussion, that a paper that was recent to them had seen an abrupt increase in EMG signal at lactate threshold. And they said, oh, this is probably from increased, this is where you start getting into type 2A fibers. So let's see if that holds true. And also, so that's them subtweeting other authors in their direct time. But now here we are many years on from that. Let's see what we can get out of this for our own purposes. So what they did was they biopsied the vastus lateralis, that's your outside quad muscle. after different levels of work in a ramp test. They also note in the paper, I just went over that. Side note, by the way, to confirm kind of what we had seen with the cross-country skiers, they had also found that citrate synthase activity for the trained runners that they had in the study were about the same between fiber types, which confirms the result of the other papers that we had mentioned. So again, type one, type two fibers. just about equal oxidative capacity. So if you think type 2A fiber is like, oh, maybe more reliance on anaerobic metabolism or whatever, not quite the case. So they had four subjects and three out of the four of them were well-trained. They had two well-trained runners, a well-trained swimmer, and someone who was basically inactive. And they did a graded exercise test, a ramp test to failure. with lactate samples and VO2 max determination, but they had one minute stages. So typically I would say this is a little short, but regardless, we're going to go with it and we'll kind of see the drawback of a short stage in a second. So they rode at a work rate, well, one of their work rates, not work rates, like basically, The first thing they did that got biopsied afterwards to see what kind of muscle fibers were activated, they did a work rate. They started at zero watts and they did a ramp in one minute stages up to two ramp stages below their lactate threshold. And so because the lactate threshold was, you know, this is determined in one minute stages, they probably all underestimated it. But regardless, they then took a rest and then they rode from zero to three ramp test stages above their lactate threshold. And then they rode to exhaustion. And so these are the three things that they did. So they've got four muscle biopsy samples at rest after going up to two stages below LT, going up to three stages above LT, and then going all the way up to exhaustion. That's the protocol here. That's a pretty intense protocol. They got a lot of rest in between stuff. Yeah, and it's not like... Yeah, I hope so. Yeah, so... And they give plenty of rest, plenty of chill, and, you know... I thought it was a good protocol other than it underestimated lactate threshold. But as we see, because if you're riding two stages below your determined LT, you are well below lactate threshold. And if you go to three stages above that, you're probably pretty safely to say that you're over it. So I thought that was, because of the way they designed it, it doesn't matter that much. So what were they looking for specifically in these biopsies? They were looking for... metabolic changes in certain chemicals that signify that the fiber had been recruited. And they took muscle biopsy, they looked at individual fibers, they typed them, and they looked at the changes in these things. So they looked at malate, lactate, glucose 6-phosphate, ATP, phosphocreatine, One other thing? I don't think so. So basically, changes in these things can indicate fiber recruitment. And things like malate, lactate, glucose 6-phosphate, these things, and also ATP, and phosphocreatine, of course. So these are good but not great indicators, and some of them are better than others, and we'll talk about that in a second. So why are these good but not great? Because there's flux through these intermediates. The idea, of course, is homeostasis in a cell, right? Yeah. Okay, so for instance, if we have flux through glycolysis, that does not necessarily mean that glucose 6-phosphate pool is going to increase. Typically, I would expect it to increase well over threshold or over threshold, but below threshold, maybe not so much. But anyway, that's my expectation going into this study. When we look at things like malate, this is a Krebs cycle intermediate. Also, this can also have the same challenges of looking for changes. Like if somebody's really good at maintaining that homeostasis, somebody's well-trained, you may not see their malate level change. There's just flux through it, but the level stays the same as it would at rest. We're going to find that something like ATP or phosphocreatine, now these can be much better signifiers of muscle recruitment. But let's kind of go through the study and look at what happens. So what did they find with the main intermediates? And now I actually suggest everybody goes and spends some time with these graphs. They're a little unwieldy at first. They look like a mess at first, honestly. But after a little bit, you stare at it, and then you go, oh, okay, I get it. So go check out those graphs if you want. But you're going to have to, if you're just listening, kind of trust my analysis on this. If you look at it and go, oh, my God, I spent too long looking at these things. But almost all fiber types, but not all fibers, mind you, almost all fiber types that were sampled showed signs of use. to some degree, even at low levels. Even from zero to two levels below lactate threshold, they found that almost all fibers sampled had signs of utilization and recruitment. Interesting. Even the fast-twitch ones. Even the fast-twitch ones. Not the super-fast-twitch, most likely, the Type 2X, but again, like... Well, we haven't really discussed this previously, but typically once somebody starts doing any kind of training at all, weightlifting or endurance training, the type 2 X fibers convert to type 2A fairly rapidly. Anyway, so let's back up this conclusion here. So if we look at lactate, for instance, in subject B, who's a trained runner, we see that lactate in the sub LT steps is significantly higher than even in the next step over. LT, over lactate threshold. So this is one of those things where depending on when you biopsy and how fast the cells get aerobically warmed up to oxidize that lactate, you can see that bump happen. We've talked about this several times in the podcast. So that's probably what they found, which they didn't find in the subject A, who is the other train runner. So we get some interesting things to look at with this kind of stuff. And this is why, like I said, there are some drawbacks to looking at these things in terms of saying, okay, this definitely shows fiber recruitment. So this here in subject B shows fiber recruitment and subject A, just the lactate itself does not show fiber recruitment, I would say. It may have been recruited, but it's not ironclad. And so anyway, and so actually pretty much one instance in all of the data provided. There's only one instance where it looks to me like type one fibers were recruited when type two were not. And they've got like 16 graphs up. And so in every other case than one, if it looks like type one fibers were recruited, type two fibers were recruited. Like only one exception. And that was in a trained athlete. So no, untrained athlete, sorry. Oh, yeah. The exception was from the swimmer, who's – Kyle, how much do you use your vastus lateralis when you're swimming? It greatly depends on what your stroke you're doing, but not nearly as much as if you're running. Yeah, running or cycling, for instance. Yeah. So the exception here – Oh, yeah. Sorry. Go ahead. No, like you – the other thing is just for most swimming – kicks other than other than breaststroke kick just the general range of motion that your legs go through is significantly smaller than for running and cycling right so you can just imagine that if if even though you are using your legs and especially if you're sprinting and swimming you are using your legs a lot it is not nearly as intense of a usage as cycling and running yeah it's it's like a moderate isometric contraction more or less right Maybe. We'll look at that in depth one day. We'll look at time under tension and stuff like that. So anyway, continuing here, what they found was in subject C in the swimmer, the malate levels were higher in type 1 fibers in the above LT ramp step and not as much in type 2 fibers, although it was definitely well above resting levels. That's the only exception that I see here where it looks like different amount of recruitment. or potentially different amount of oxidative capacity, at least, via these pools in type 1 fibers versus type 2. Or something else that I found was interesting is subject A, trained runner, the sub-LT step, you know, zero to two steps below LT, they found that the glucose 6-phosphate levels actually dropped below resting levels in a good number of fibers. seemingly mostly in type 2 fibers, actually. So this is a sign typically that, for instance, whatever lactate is being made in somebody who's well-trained, it's probably being fully oxidized. So the lactate pool doesn't go up, but we see glucose 6-phosphate dropping a little bit. That might be an indicator of flux through this pathway. It's not ironclad, like I said. So is this making sense so far, or is this just confusing to listen to? I think it makes sense, but I think it is a little confusing, especially if you're not holding the paper. And I think this is a lot of times why I think it's nice that you do tell people to go actually look at the paper if they do have questions or if... just hearing it in words is not quite making it click, then rereading it or reading it at all can be really helpful. Yeah. Although I think, generally speaking, people who have been paying attention and learning from the podcast so far definitely have, it might take some focus. And I know a lot of people can't really listen to this podcast while they're doing intervals. Sorry about that. But we appreciate everybody who listens multiple times or who writes in or who reads the papers. And even if you disagree with our conclusions, that's great. It's just awesome that you're thinking about it. So anyway, so what can we use in this analysis for our own purpose? And this is not conclusions the author had in the paper, because all the stuff that we just mentioned, all the authors had noted. And on the other hand, though, in some of these charts, glucose-6-phosphate amounts are tending to be much higher in the swimmer and untrained subjects in the type 2 fibers. especially for the step at exhaustion of the RAM test. As in, you know, the levels go from about three to six in the type one fibers, but they go from three to 12 in the type two fibers for glucose six phosphate. But in the trained subjects, there's not much difference and the distributions are basically the same. And so what we can kind of conclude here is that people with more well-trained vastus lateralis probably have better ability to oxidize glycolysis products than their type 2 fibers. Voila. Simple as that. That makes sense. I mean, yeah. If we've said before that, you know, type 2 fibers can and are not restricted to, what do you call it, to just burning carbs only. carbs or only being anaerobic or something like that, then yeah, you can imagine that there is a spectrum over which these fibers can be better or worse. It is not like they either can or they can't. Like lots of things we've learned about muscle physiology, there's this huge, huge, huge range, a spectrum of things that you can do. Yeah. Okay. And so let's sow some doubt now because this isn't complicated enough. Now, like we said, the distribution changing from resting levels, of these metabolic intermediates, glucose 6, phosphate, malate, lactate, these are a pretty good indicator. And of course, obviously, once we get into the over lactate threshold step and the full ramp exhaustion step, we see, of course, both type 1 and type 2 fibers getting recruited and lots of lactate and all that kind of good stuff. We also see weird things like we know that type 1 fibers were definitely recruited with some exercise intensities. If we know that... Type 1 fibers were recruited, and we don't know if Type 2 fibers were recruited. We look at the metabolites in Type 1 fibers, and we cannot say for sure, based on these measurements, that they were used at all. Why do you do this to us? Why? Right? Because we know if we get up to that two levels below LT step in the ramp, and then we take a biopsy, we know that those type 1 fibers have been used, but we don't see evidence of them being used in the lactate, malate, G6P, or any of that kind of stuff, those metabolite pools, right? So, because I think of this as like a Wasson selection task, because we not only need to prove our theory of like, this is definitely a thing, we also need to look for something that would confirm or deny the corollary that would possibly disprove what we have. So instead of just saying, no, this doesn't happen, because how do you prove it negative, we have to look at proof that something would disprove us and confirm it, possibly. Yes. I mean, you can try to prove a negative. You're going to be spending a lot of time. Okay. So now let's get into the thing that I think is the best evidence here that that type 1 and type 2 fibers have been recruited at all stages for just about everybody. We're looking at ATP levels now. And so this to me, well, I would also accept calcium presence as well, but it gets mopped back up. So not a great indicator. So in subjects A, B, and C, we see a definite change in ATP levels from resting to the sub-LT level. an equal change in both type 1 and type 2 fibers. In some subjects, it goes up. In some subjects, it goes down. A, B, and C subjects, some are up, some are down. In subject D, it's actually straight across the board. But for subject D, the untrained person, we see that through the metabolic intermediates that they definitely had type 2 fiber recruitment at the sub-LT level. So why does it go up in some people and why does it go down in some people? It's because once you start contracting your muscles, the ATP levels in the cell are going up and down and you are trying to maintain homeostasis. And so you can actually get an overshoot, especially once you stop pedaling to get the biopsy, you can get ATP levels that go up. And sometimes if you get it quick enough, ATP levels may not have gone up. et cetera, et cetera. So change from rest is a great indicator of recruitment. And so we see that in type 1 and type 2 fibers in both the trained runners who have very well-trained bassist lateralis. Again, it's not a huge change for some of these things. We're talking like 12 versus 15 millimole per kilogram dry weight or like 25 to 20, but there are small error bars. Yeah, it's measurable. It's statistically significant, but small. Yeah. Or a small in an absolute sense, but not small in the measurable sense. Yeah. Yeah. And so there you have it. Even at relatively low exercise intensities, we can do recruit some type two fibers, even in well-trained people who probably have a lot of type one fibers, but right. Do we have more type 1 fibers? Do we not? Kyle, what do we know about fiber type distribution in trained people? Well, we've mentioned before that you do not have a lot of type 2 X fibers. If you're trained, you don't have a lot of type 2 X fibers, but you actually cannot necessarily say that, you know, I don't know, that a super, super... If you only ever do, say, cycling or something like that, you can't necessarily say, oh, this person is definitely always going to be type 1 dominant or something like that. Or make the claim that, oh, they only do cross-country skiing. They must only have type 1 fibers. Yeah, or type 2 fibers with the high force requirements. Or type 2 fibers, yeah. Yeah, it's complicated, right? Yeah. Yeah, you have these two things where it's probably some sort of Gaussian distribution because a lot of traits of humans fall on this nice Gaussian curve, and that's wonderful and all this stuff. But it can make it... depending on how big that Gaussian is, it can make it really hard actually to say anything, even though on average, you might, on average, quote unquote, you might have 50-50 or something along those lines. Well, you mean by that, you mean like how wide the tails are for like fiber type distribution between people? Like what's the standard distribution? Yes. Like what's the width of it, right? So for instance, somebody like me, I'm probably, well, I'm probably what, like 20% type one fiber by now? Hard to say, maybe, yeah. Yeah, I mean, I don't think I was ever that type 1-y when I was doing a lot of endurance training. But anyway, so let's get into this real quick before we start using ourselves as guinea pigs again. Because every time I use myself as a guinea pig, if I'm actually in the gym, I end up hurting myself. Whee! So this is nice number guinea pigging. This is nice and safe. So what I really wish that they had reported in that previous study, by the way, That's why Andy Coggin probably had been tweeting it a lot, because when you really get into that paper, it does show that, yes, at low exercise intensities, people are recruiting type 2 fibers. Like I said, I wish that they had reported overall fiber type distribution that they were looking at in these athletes, because we can actually now look at... a really good study that we've looked at before that did report on, I don't know, everything. Um, so we're going to go back to the determinants of well-trained cyclists that we've looked at, I think in two or three episodes before. So if we look at table four real quick, they show type one and type two fiber type distribution in their participants. They had, what was it? 16 participants or something like that. And they had divided them into a high group and a low group where the high group is having an FTP. greater than 72% of your VO2 max. And the low group had an FTP below 72% of their VO2 max. And the range of type one fibers in the high group is 55 to 85%. The range of type one fibers in the low group is 34 to 60%. So that is a pretty big distribution. It is a pretty big distribution. And so there's not much overlap. We've got 5% overlap between the two. Yeah. I mean, 34% to 85% is a big gulf. Yeah. Yeah. And so we've got 34% to 85%. And all of these people had been training for a couple years at least. And so these are all well-trained people. And their fiber types are all over the map. And so we can use where is your FTP relative to your VO2 max as a general indicator of your fiber type distribution, but not a great one. And if you've got WKO5 and you know that this is a pretty common metric that people look at, two things about this. First, it's an estimate only. And I'm not entirely sure it's that accurate for a lot of people. It might be really accurate, but also there's a fiber type. function also in WKO5. I looked at mine and for 2021, my fiber type distribution is WKO5 estimates me at 17% type one. So that is me with a 2200 watt sprint and a 240 watt FTP. Yeah. Crushing it. Right. And you know, again, I'm very well strength trained at this point, poorly endurance trained. So But here's the thing, though, is if, for instance, at endurance pace, you're recruiting 50% to 60% of your muscle mass, for a lot of people, you are almost definitely using some type 2 fibers, if not a lot of them, right? Right. Yeah, that's a good point. That's a good point. Yeah. I'm curious if the range in the high group was 55% to 85%. what that distribution actually looks like. Obviously, this is getting off into the weeds, but of those people, how many were, you would expect, obviously, not as many people to be in the 34% and not as many people to be in the 85%. And there is a little bit of overlap, that 55% to 60%, but you'd expect most of the people, therefore, say, to be within the 40% to 70% range, which means, yeah, a lot of people, perhaps a majority, are using type two fibers all of the time. Yeah. And, you know, the question is like how much fiber type or how much muscle fiber, how many motor units, or would be another way to put it, are we actually using at different levels of intensity? And, you know, here's where I think that we can throw the kitchen sink at you. Because one of the first things that I haven't mentioned yet in that last paper, by the way, is that the required cadence was 60 RPM. So it was a little low, right? But the reason I'm bringing it up now is because this could be a good reason that people were getting into type two motor units. Now, we don't know the fiber type distribution they had, obviously. So we cannot say for certain if, well, we're gonna talk about recruitment of muscle mass in a minute. But for now, I think the important thing is that, so cadence manipulation can add or subtract motor units. But more important than the cadence, more important than the cadence is the fact that the pedal stroke is not even. And so if we say we're doing 100 watts at however many RPM, if we look at the force that would be required per second, which is how we measure power, of course, that's an average. It's not the peak and the valley of force on the pedals, right? And so if we are going to have that as our first complicating factor, the second complicating factor is that typically at lower cadences, the peak force typically tends to be higher. So at normal cadence, like let's call it 80 to 90 RPM, the peak force per pedal stroke, especially if you're an experienced cyclist, may only be 20 to 30% higher than the average all the way around. In a less experienced cyclist, it might be 30 to 40% higher or more. In lower cadences, the peak is typically even higher than that, especially if you are sprinting. Like if you're sprinting at 60 RPM at 1,000 watts, your peak pedal force is probably 150 to 200% of your average for your pedal stroke. And so that's a complicating factor. I'm going to throw another one at you, which is that when we... pedal a bicycle, we're using a lot of muscles at once. We're using, you know, pretty much all of the muscles in our lower body and they're contributing to different degrees. And so if we just think about our legs as a muscle and we think about how much, you know, how many motor units am I getting into in here? Am I getting deep enough into this muscle, this, this a leg muscle in order to get into type two motor units? That's a good question. But the reality, of course, is that we have a lot of muscles and our hip and knee extensors make up most of the power. And so even then, they're very large groups of muscles themselves too. And so some of them have a larger burden than others, metabolically speaking. And so this means that, and not only metabolic is speaking, of course, in terms of force and recruitment. So some muscles are getting recruited to a very large degree at low power outputs and even moderate and normal cadences, and others, not so much. And that's just the reality of it. So there are so many complicating factors about getting into different motor units that I would say it depends on the muscle, depends on the cadence, it depends on the fiber type distribution, it depends on your bike fit, and it depends on, I bet we could name like four or five other things, but that's kind of besides the point. So let's get into something a little more concrete now. Besides the fact that, Kyle, when you and I do pretty much anything, we're getting into type 2 fibers. So people who are really strong like you and me, here's a good question that I don't quite know the answer to because it probably changes so much for person to person. If you're a lot of type 1 fiber and you don't have very big muscles, maybe you're recruiting 70% to 80% of your muscle mass or something like that when you're at threshold. You know what I mean? Just all the time. But they're so well endurance trained that you can just use almost all of your muscles almost all the time. Well, we're not going to get into that paper right now, but there is a paper that concludes that. And it's a pretty good one as far as I recall. Maybe we'll do that next episode or a couple episodes from now. But here's a question that I really don't know the answer to that's really going to throw a monkey wrench into things. So Kyle, you and me, we're really well-strengthened right now. If we ride at 200 watts now, and let's compare us now to where we were like five years ago when we had like, I don't know, 150, 200 pounds less on our squats. If we ride endurance pace now, or we ride at 200 watts now, is that the same amount of muscle mass? Are those the same motor units as we used five years ago to ride at 200 watts? Yeah, that's a good question. I think you can come up with reasonable sounding explanations for both yes and no. You could say yes because you've gone... To now where you have way less slow twitch fibers, so you're going to naturally just be using larger motor units because you're going to fatigue faster. You don't have as much slow twitch, so you're going to tap into those bigger motor units. Or you could say no because you were so well trained that you had a lot more. endurance capacity in those, yeah, it's just- Yeah. Well, here's the thing is that the smaller motor units, the prevailing theory is, and we might end up being wrong on this, but it sounds reasonable to me that the smaller motor units are so well-trained because they get recruited all the time. So metabolically and in terms of experiencing tension that would lead to hypertrophy of these motor units, they don't experience- much adaptation here because they get recruited all the time. They're experiencing tension all the time and they just don't get bigger. So that's interesting. I mean, maybe that, that this is getting way off topic. Bring it. That's not different for us. Uh, this, there are things that people run into when they get seriously into bodybuilding, where they run into trying to say gain. forearm and calf size. Feel free to make all the jokes you want about gaining forearm size. Why? What kind of jokes do people make, Kyle? Sorry. Go ahead. Because you do use your forearms and your calves all of the time in just daily life, writing, picking up things, walking around, opening doors, all this stuff, they... do experience tension all of the time. So when people try to train them, they find that they have to do, not all the time, but some people find it quite effective to do something that is very, very different, i.e. like very heavy, very hard work on your calves or your forearms because you get sort of low, high rep work all the time. Yeah. Yeah, no, well, we can kind of table this for a discussion on hypertrophy. stimuli later. But I think kind of one of the takeaways to think about here is that the force produced to the amount of muscle mass, actually the force produced to the amount of motor units recruited is not linear. And so remember, as we get into bigger motor units, each new motor unit enervated by a nerve, each nerve, as we get into larger motor units, has more and more and more endings. And so small motor units only, like, they only activate a small amount of muscle mass. And as we get into bigger and bigger ones, they gradually start increasing more and more muscle mass. And then at some point it shoots up, I would say like parabolically, exponentially, like what would you describe that curve as? Yeah, it looks quadratic or parabolic or something where in the low regime, you kind of squint and it's like, oh yeah, it's a little bit, it kind of looks linear if you ignore sort of the larger part of the curve. But then yes, very quickly, it is not the same slope throughout and you get into really big motor units and they started recruiting. Like the next level of motor units recruits more than... the previous amount increased in amount of muscle fiber. So it's definitely not linear. Yeah. And so here's another way to complicate this even more. So as we get into larger motor units, not only are they larger, but they produce more force. Right. Because they are getting into fibers that have more type 2, minus, and heavy chain because there are hybrid fibers. Remember, what was the distribution of hybrid fibers? That was hilarious. It was like 5% to like 70%. or something like that, are estimated to be hybrid type 1 and type 2 myosin heavy chain. Right. So there is kind of something to it when we think of low intensity as primarily using type 1 fibers and moderate to slightly higher intensity as using more type 2A fibers. But, you know, and even the highest intensity using type 2X fibers should we have any. we can't reliably say this all the time for everyone, especially because what we just saw in that study is like, it looks like that type two fibers get recruited in a lot of people. And I would guess because these athletes may have had a decent amount of fast twitch muscle fibers, but we can't really say for sure. And it might be worth real quick discussing a drawback. of the histochemical staining when it comes to classification because it really does not tell us about hybrid fibers, right? So, excuse me. So for instance, if we have a particular stain where pure type one fibers will be clear because they are fully inhibited, hybrid fibers that have type one and two myosin heavy chains and ATPases, these will be gray. And then we have the pure type 2 will be black because they're completely uninhibited. And so when we precipitate out the phosphate from the active ATPases, we're going to call the gray fibers type 2A. We're going to call the black fibers type 2X. We're going to call the white fibers type 1. And this is pretty typical, right? But... Now, if we think about hybrid fibers and there potentially being a lot of hybrid fibers as we get into larger motor units, now we're really screwed because we can definitely not say now by histochemical staining, what is a type one fiber? What is a type two fiber? This turns into more of a philosophical question or like what arbitrary threshold do we assign? And then we have to grind everything up and run out on a gel instead of stain it, which is the... easy way in a lot of ways because otherwise you've got to like pull these fibers apart like a string cheese and then you've got to grind them up and run them on a gel and that takes a lot of time rather than taking a thin slice and you know putting a solution on it letting it run for a bit put another solution on it and then put it under a microscope um so less precise but The question is now, how precise do we want to be with our type 1, type 2 thing? Because the existence of hybrid fibers really is the final monkey wrench, at least in terms of what is a type 1 and what is a type 2 fiber. So I'm sorry to do this again. So let's get back to something practical for a second. Metabolic demand, I think, is one of the big takeaways here. Does not necessarily mean that you require a certain fiber. because the metabolic demand meets an expected ability of a certain type of muscle fiber. Try that again. So the metabolic demand, so that means whether it's endurance pace or all-out sprint. Right, so let's say, okay, we're going to ride at, let's call it zone two, even though I don't use the word zone two that much, if ever. So let's say we're going to ride at endurance pace. Now, endurance pace, we would expect that somebody's going to use a lot of fats and not so much carbohydrates. So one would expect, because type 1 fibers can burn a lot of fats, we would go, oh, we're burning a lot of fats, so therefore we have to be using type 1 fibers. This is not actually how the relationship works. Yes, that makes sense. So it only works one direction. Type 1 fibers... use a lot of fats, but you cannot say because you're doing something that you think would burn a lot of fats means you're only working type one fibers. Right. At FTP, for instance, people can be burning a large amount of fat. And if at FTP- Or a large amount of carbs. Yeah, or a large amount of carbs. Most people with a really short TTE burn a lot of carbs. So if somebody's got a really long TTE and they're burning a lot of fats, that does not mean that like at- you're using a lot of Type 1 motor units. I mean, you are. You're using probably all of your Type 1 fibers. And then you're into a lot of Type 2 fibers that are burning a lot of fats because you've done awesome training. Congratulations. You know what I mean? Yeah. Yeah, so it's not as cut and dried as just being able to say that one type of fuel... dictates what type of fibers. Yeah, it would be convenient. And if you're untrained, it may be a much more reliable relationship. But as we get better trained, it really becomes less reliable, especially if we're both strength and endurance trained. Like, I want to see what Ed Clancy can do, please. Yeah, like peak Kittle. Oh my God, yeah, please. Okay, so what determines fiber recruitment? Force demands, that's it. Never forget that the size principle is still undefeated. So this is where, of course, motor units recruited progressively. And again, we cannot selectively recruit a fiber type because of the nature of how threshold works. So if we get a signal that's, say, 70% of maximal voluntary contractile force, everything that requires 10%, 20%, 30%, et cetera, et cetera, et cetera, will also activate because your signal is well over their threshold for activation. Like we said before, you don't... you don't magically turn off those smaller ones because you have to recruit more fibers. Yeah. And so if you hear somebody saying that like a certain level of power or something relies largely or primarily on a certain fiber type, that may not necessarily always be true. And I would say that often enough, people's fiber type distributions are probably wide enough that it's probably not true. And that doesn't mean that whatever they have to say is wrong. It just means that if anything gets built on that assumption, then it's worth a second guess. And it may be right, you know, maybe metaphorically false, literally, or literally false, metaphorically true, but you know, it's worth going, Hmm, I wonder. Hmm. Yeah. And that does be definitely, and we talked about this last episode, it seems like one of those things where on average, it could be true, but individually for you, that definitely may not be true. Yeah. Yeah. And so, you know, when we think about, Actually, maybe let's just do one more little aside because I think this is really fun. I like to think that low force motor units are clustered together, like in terms of not having much more force than the ones before, because this is actually how we get fine motor control, right? So if force were linear, we would have to exert better control in fine levels of... neural drive, which we apparently are not good at. And so if we want to have fine motor control, our neural drive is a very rough instrument apparently, and we can actually get better gradations of force by having all of these small low force things that we can do as clustered together in small motor units, a lot of them. And so for instance, just estimating here, but let's say It's a logarithmic relationship. Base 10, where let's say 50% of force available would require something like 70% of your total motor unit pool, which would be just log 5. So if we go smaller, 30% of the force available requires 50% of your motor unit pool. And of course, again, this is likely a large variation in a lot of people. And so how much force of your total force output can we use? We can never actually say for certain until these studies get done, and I'm sure that we're going to find a very wide relationship. So for instance, if I take some data and I work it backwards, somebody with a 350-watt threshold... If I make some very rough assumptions about their pedal force and relative load on the muscles, I could pretty easily conjure up a number saying that 85% of somebody's vastus medialis and glute maximus, for instance, would be activated while riding at 350 watts, depending on a lot of factors. But it makes some sense. But again, these numbers cannot be relied on. We actually need to do some real measurements for this kind of stuff, right? Yeah, and it's just that the big ones bring in that very high last chunk, let's say, or something. But again, as you fatigue... and you get into larger and larger motor units, you're probably using a whole lot more than that. And that's just a for instance. Do not quote me on this. This is all just for instance because I thought it was really interesting to chase this little rabbit hole down. So let's talk a little bit about a big take-home from today. So obviously, the first one is we already mentioned that just because an activity has an expected metabolic requirement does not mean that it's going to recruit a certain type of muscle fiber just because that muscle fiber happens to match the metabolic requirement. Like when you sprint, believe me, your type one fibers are going through glycogen too. Like they're contracting as fast as they can. It's all hands on deck, like we've said before. But what type of myosin heavy chain you have and what fiber types you have in the distribution, seems to be determined by two things, genetics and mechanical force requirements, or perhaps even metabolic or oxidative stress requirements. Yes, for sure. And so what about metabolic requirements? Metabolic adaptations and fiber type classifications may not necessarily be correlated one-to-one. So if we recruit a fiber a lot, we are probably training it better aerobically. And if we're not recruiting a fiber so much, like people who have no use of their legs are almost entirely type two fibers. Yes. It's just like fresh off the couch. You said, we said before, you haven't converted those type two X fibers. Yeah. Lazy cats, just a lot of fast twitch. That's why some cats can jump so freaking high. And also they're really, really springy. Yeah. In terms of their... Also, their body weights are not so high, but then even very heavy cats that could eat you can still jump very high. So, okay. I guess the last thing to think about here is when would we expect a shift in fiber type? I would just say you would get probably a very slow shift from type 2 to type 1 with a lot of oxidative and metabolic stress and very low force requirements. Um, but again, I would expect that to be determined a lot by genetics as well. Uh, and I would expect to see type one to type two with high force requirements, like in cross country skiing, like, you know, versus like an elite cross country skier versus like Chris Froome or Egan Bernal probably has a ton of type two fibers, right? Yeah. So again, type two fibers can be highly oxidative and they can burn a lot of fat. They can, doesn't mean they will. Depends on your training status. Depends on a lot of other things. I would guess in a lot of elite endurance cyclists that you might find much higher fat oxidative capacities in their type 2 fibers, should they have many. But these are studies that haven't really been done yet. And so instead of speculating here, all we can really say is that we, at the moment, do not know for sure. about any of this. But other than we can reliably say well-trained type 2 fibers have high oxidative capacity. That is an absolute certainty. Nothing like spending an hour and a half saying it depends. 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