Beyond the Basics: Everything You Need to Know About Muscle Fibers

Gear up for a new Fast Talk Podcast series. First up: what are muscle fibers, what do we mean by slow-twitch and fast-twitch, and what are type IIa fibers? Our hosts cover everything you need to know about these all-important fibers.

nerd lab FT ep 293

All too often our hosts have thrown out terms like “type IIa fibers,” “oxidative phosphorylation,” or “VT1” and just assumed that everyone listening knows what is being talking about. Understanding terms like these can help you effectively navigate exercise science research, and therefore, empower you to drastically improve your training.  

Today’s episode is the first in a series of episodes where our hosts will explain some of those key basics of physiology – the terms and concepts you keep hearing us refer to during the show. But don’t worry about nodding off, this isn’t that typical boring science lecture. We’re not just explaining the basics, we’re giving you examples and showing you how can immediately apply this information to improve your training. Essentially, this series entails our hosts giving basic science gems that pack instant gratification. 

Today’s “Beyond the Basics” conversation is on muscle fibers. We’ll explain what they are – and more importantly – what is meant by terms like slow-twitch, fast-twitch; and types I, IIa, and IIx fibers. In high school you were probably taught that the difference is their speed. There’s actually a lot more to it, and those bigger differences have a huge impact on your ability to perform as an endurance or strength athlete.  

Finally, we’ll talk about how we recruit muscle fibers and why that determines how hard we can go and how long we can go at different efforts. Think about it this way – you can walk for hours but sprint for only a few seconds. That has everything to do with the muscle fibers that you recruit.  

Let us know what you think of this new series, if you’d like to hear more of these types of episodes, and if so, what topics you’d like us to cover next. You can email us or post your thoughts on our forum at   

So, pull out every fiber you’ve got for this one, and let’s make you fast! 

RELATED: Episode 281: Nerd Lab: How Dr. George Brooks Revolutionized our Understanding of Lactate 
RELATED: Episode 251: Time at VO2max: An Important Metric You Probably Haven’t Heard of 


​​(ANDERSEN, KLITGAARD, & SALTIN, 1994; Bottinelli & Reggiani, 2000; Costill et al., 1976; D’Antona et al., 2006; Gollnick, Armstrong, Saubert, Piehl, & Saltin, 1972; Ingalls, 2004; Jansson, Sjödin, & Tesch, 1978; Luden et al., 2012; NEARY, MARTIN, & QUINNEY, 2003; Talbot & Maves, 2016; Timmons, 2011; Wilson et al., 2012; Zhelankin, Iulmetova, Ahmetov, Generozov, & Sharova, 2023)​ 

Episode Transcript

Trevor Connor  00:04

Well, welcome to another episode of Fast Talk. Rob, it’s just you and me here alone on a chilly fall morning.

Rob Pickels  00:13

It’s been a while we should have a fire going some blankets reading—no wait—podcasting. We should be podcasting right now.

Trevor Connor  00:20

Yeah, we need a little fireplace, and this could be kind of fun.

Rob Pickels  00:23

It would be dangerous.

Trevor Connor  00:26

Yeah. So I don’t know about my mental state this morning because I will admit I was a little late. Because I got up. I was rushing to get to this recording. I realized I needed to take my garbage out. I took all my garbage out and then realized it was Wednesday, not Thursday and took all my garbage back in and then was late I went, that’s kind of where my brain is at today. But Wednesday

Rob Pickels  00:49

It’s garbage day at the office. And so in approximately 15 minutes. There’s a garbage truck that’s going to be going

Trevor Connor  00:56

by our window. So we do like to record Wednesday mornings on the garbage truck.

Rob Pickels  00:59

I love recording Wednesday morning. It’s my favorite morning to record. There we go. I actually like it too. Hey, it’s chilly this morning, Trevor, you rode your bike. And the work I did. I pulled in the parking lot and cursed you out. I was like this jerk. never shows up on time. And then you were sitting here so I figured you rode your bike.

Trevor Connor  01:14

I rode my bike. I actually spent the entire weekend completely rebuilding my bike because the brake was broken. The handlebar tape was gone, the tires were gone. The wheel needed to be rebuilt. It was basically a deathtrap brakes

Rob Pickels  01:28

only slow you down. Yeah, that’s fair. You know, what speeds you up? What’s that? Muscle fibers.

Trevor Connor  01:36

And that’s what we’re talking about today. Good transition. Rob.

Rob Pickels  01:39

I’m proud of myself. For that one.

Trevor Connor  01:41

That was a good one. So we’re doing a new type of show. And please give us your feedback on this one, whether you like it. But we have heard from people saying you cover really interesting topics, some of the time maybe on what’s new, what’s current, what’s going on in training in the science. But often we refer to things like muscle fibers. And we don’t really explain what that is. So we’ve had this thought about let’s do some this kind of new series of just the basics, let’s go back, cover some of these fundamental things that you really need to know in order to understand what we’re talking about in training science, and just kind of have some fun covering it. I hope this doesn’t turn into a physiology 101 lecture that puts everybody to sleep. Hopefully Rob and I can give you some good, interesting information. But keep it fun, keep it entertaining, and teach you a few things along the way. At

Rob Pickels  02:40

the very least you don’t have to sit and watch a professor write this all out on a blackboard.

Trevor Connor  02:45

Yeah, we should just get the chalkboard and make that sound while we’re talking.

Rob Pickels  02:48

I just want to say Tom Swenson who was my undergrad, you know, physiology teacher at Ithaca College. Thanks, Tom legend, everybody who knows him he’s a legend. The guy could write and talk at lightning speed. It was absolutely incredible. I don’t possess those skills. I could never be a professor.

Trevor Connor  03:07

My organic chemistry teacher took pride in the fact that he had one of the lowest grade point averages of any Origo course in the country. Wow. Like he wrote the big textbook that a lot of schools use. So he was kind of a big name.

Rob Pickels  03:20

I didn’t know sadism was part of organic,

Trevor Connor  03:22

what he would literally do. And you know, this is before all the technology days when we were literally on a chalkboard, yeah, he would sit there with his right hand writing. And he would intentionally put his head in front of what he was writing. And then he would have his left hand with the eraser. And he would write and erase at the same time with the intention that half of the class would not be able to see what he wrote.

Rob Pickels  03:46

That’s funny. That’s funny. Let’s see my physiology, man. Let’s just reminisce about old days. It was the dual chalkboards that you could lower and raise. And there we go. So like you would write on one and then like, raise it up and be riding on the next one. And I remember Tom would be, Professor Swenson would be writing like so fast and furiously, pieces of chalk would just be exploding out of his hand hitting people in the front row. I had to sit in the back, I was afraid of losing an eyeball. The good old days. Now, you can just watch a YouTube video or Khan Academy or listen to fast doc labs.

Trevor Connor  04:18

Yeah. So let’s go to the good old days and let’s get into some good basic physiology. I was after a transition. I think that that worked years was better, that yours was better. That’s okay.

Rob Pickels  04:30

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Trevor Connor  05:01

So we’re talking about muscle fibers. And let’s get into the really well, as you put my notes, let’s cover the boring stuff first and get out of the way. So here is your absolute basics. All muscles are made up of fibers, those fibers are not uniform, am I doing Sephora,

Rob Pickels  05:23

I’m picturing them wearing a jacket. Except I’m not because I can’t picture things. That’s true,

Trevor Connor  05:27

you can’t. So they’re not uniform, you have different types of muscle fibers and most muscles in your body will have a mix of the different types of fibers, it’s not like any one muscle is just gonna be all type one or all type to be, it’s gonna have a mix, but the proportions will vary. So, in humans, we have three main fiber types. And we’re gonna dive a little deeper into this in a minute. But it’s type one, also known as your slow twitch muscle fibers. And then you have your fast twitch muscle fibers. And there’s two types, there’s two a now here’s the first interesting thing for you to x, where the heck did X come from? Right, you will read all the time, type two B, you will hear people talking about type two B. But for reasons we’ll probably cover in a minute, humans actually don’t have type two, B, we have 2x. Thank you, that’s what I was looking for. To be are basically pure anaerobic muscle fibers. So continuing with the basics of the physiology, here, we’re going to cover what differentiates these these muscle fibers. But really think of those slow twitch tend to rely a lot more on aerobic metabolism. Those type two fibers tend to rely more heavily and will this is simplification tend to rely a little more on anaerobic metabolism. When you have a to b fiber, it’s pure anaerobic two acts aren’t. They have both the aerobic and the anaerobic machinery. And so I remember reading one of my physiology texts that humans are one of the purest aerobic animals on the face of the planet. And this is the part of the reason why we don’t have pure anaerobic fibers, all of our fibers are capable of working aerobic ly,

Rob Pickels  07:17

there you go. Yeah, the the fiber types and how they’re found across the body has always fascinated me, Trevor, so I want to touch on that a little bit more. You know, if we think about your lower leg, your calf muscle, right, your calf muscle is actually made up of a few different muscles, with the major ones being the soleus and the gastroc. We don’t really need to get into the anatomy of this. But because the soleus is used more in walking and maintaining balance, it’s kind of always functioning at a low level, you tend to find more slow twitch fibers in your soleus. The gastroc is the big outer portion. And it tends to be more involved when you’re sprinting, jumping, running. And because of those higher force situations, the fast twitch tends to be more populated there. But what’s interesting is, if you take a muscle fiber, no matter where it’s found, a slow twitch type one fiber is the same fiber. Characteristically anywhere in the body, it can come from your tricep or your big toe, it doesn’t matter. Characteristically, they’re all the same. And

Trevor Connor  08:23

I love that you went there because you can contrast humans to rabbits. I remember my, one of my physiology professors explained this to us and it was kind of neat. We’re capable of walking, like, oh, you can go outside and just go, I’m going to just do this slow methodical walk. And that’s because we have all those slow twitch fibers that you were talking about. Watch a rabbit even trying to move slowly because they proceed all the time, their hind legs, their muscles are primarily those two b’s. And those fast twitch really anaerobic muscle fibers are also very strong. So a rabbit can’t just slow walk, keep it smooth through, you know, through the field or whatever it has to hop because those muscles just can’t not contract with a lot of force. Well,

Rob Pickels  09:08

I’m gonna I’m gonna step outside of our human physiology element here for a second and I’m sure what I’m going to say is wrong, but it’s going to sound good enough that people believe me. So if I am wrong, don’t don’t write in. I know that this could be incorrect. I will see if I believe you. But chickens. Chickens can fly, right? We don’t really think of chickens flying but they can fly but they can’t fly very far distances. What do we know about chicken breast meat? It’s white. Yes. And Trevor, why is it white meat? Because

Trevor Connor  09:38

that’s your to be fibers primarily. You got it and they don’t need blood supply because they’re anaerobic. They lack a lot

Rob Pickels  09:47

of myoglobin. They lack mitochondria, they lack all of these things. And that’s what makes them fast twitch right and white meat. That may also means the chicken can’t fly very far because they’re not using in a robic ox sedated muscle to do it.

Trevor Connor  10:01

As we all know, Thanksgiving dinner, the white meat is far better if it is

Rob Pickels  10:05

destined gravy. I agree. That’s fair. Yes. But gravy is the best part of Thanksgiving. So we’ve covered fibers. There’s another topic in here concept. That’s really important, Trevor, and that’s motor units.

Trevor Connor  10:17

Yes. So take it away, Rob.

Rob Pickels  10:19

Knee, take it away, you take it away that was it that?

Trevor Connor  10:21

Are you fine. Okay, you’re doing so good. Yeah, you’re doing actually good throw. So I gotta give you that one, I’ll take this one. So these fibers have to be able to get a signal from the brain, or at least from the spinal system to contract. And the way they do that is there are nerves that connect to these muscle fibers and they send the signal until the fiber time for you to contract. So what you have in these muscle fibers is motor units or motor neuron units. So basically, you have a nerve that comes from the spine, that will then branch out and it will innervate multiple fibers. So I don’t think I’ve ever actually looked this up. But I don’t think you will ever see a one to one where you will have a nerve that will just innervate a single muscle fiber, I think a always innervate multiple fibers,

Rob Pickels  11:11

I think that the lowest I’ve ever seen is a motor neuron innervating, like three to four individual fibers for the very finest motor control. And that might be happening, say in your eye, if I remember correctly, but oftentimes that nerve is having a lot more fibers contract under its will, under its domain. Yep.

Trevor Connor  11:32

But this innervation we’re actually going to get to this in a minute. When we talk about recruitment of muscle fibers. It’s really important. But before we get there, let’s continue with some of the basics. And by the way, I should have given this qualifier at the very beginner. But I’m going to give this right now Rob and I were talking about this, we want to explain these basics. And before we started this episode, we’re like, when was the last time you just read something straight about muscle fibers. It’s been

Rob Pickels  11:58

so long that this episode has scared the bejesus out of me, it


was the same. I actually went back, pulled out a bunch of my textbooks last night and just reread my notes, my highlights from the textbooks. And this is like from 1520 years ago. So one thing we’re gonna say everything we’re talking about, we both have remembered reading. We were taught this, but we were taught this years ago and there’s always this concern of our we remember this right? So we’re gonna have this qualifier here of memory is not perfect.

Rob Pickels  12:31

My fear on this is like the chicken thing that my my memory has filled in some blanks in the meantime, and I don’t know if I was ever taught some of this. Or if I’m just talking, you know, pulling it out of thin air. But, you know, this is all well researched, well known information. We’re not reinventing the wheel on this one, which is sometimes the most dangerous information to share. So yep.

Trevor Connor  12:52

And to that point, I imagine the example that I have always told people, my college admission essay paper, I’ve always told people what I wrote about in that paper, because I was always very proud of it. I mean, chat GPT didn’t write it for you. Yeah, no, we were on chalkboards back then remember? I actually literally typed it on typewriter when I was applying to college. Yep, that old. So I have always told people at that paper is about and about a week ago, I actually went back somebody was asking me about it, and reread it and was like, wow, this is not at all what I’ve been telling people this paper was for 10 years. It was the same general subject. But I remembered it completely wrong. So there’s our qualifier. We were taught all this. We could be remembered some of this a little wrong, but still gonna be fun. And I’m glad to give the qualifier here because this isn’t their member from college stop hedging. Trevor, get back to the episode. Why are they called fast and slow twitch? And here’s my big qualifier. I don’t think really that’s the appropriate names anymore. Yeah. Right. So a long time ago, when they were first studying muscle fibers, they did studies on frogs. And what they were doing was literally taking the muscles out of frogs. And they would connect them to a little apparatus so that the muscle fibers in the muscle would be hanging. And they’d have a little weight connected to the muscle actually can’t remember they took out individual fibers of the took out the whole muscle, I think it was the whole muscle. And then they would get them to contract. They’d zap the crap out of them. Exactly. And what they discovered was some contracted really fast, some contracted really slow, or not really slow, but slower. And so they named them fast and slow twitch muscle fibers because that’s what they saw about them. I’m not going to say this is wrong. Yes, these 2x Two a tend to contract faster than your type ones, often two to three to four times faster, as I remember. But I would say there’s more important characteristics that differentiate these muscle fibers and why is that important? I remember being on a ride not too long ago and hearing Two riders talking about at what cadence do you need to be riding at to go from using slow twitch muscle fibers to fast twitch because they were thinking it had something to do with leg speed. And if you’re pedaling slow, you’re using slow twitch muscle fibers. And if you’re pedaling fast, you’re using fast twitch muscle fibers. And that’s why I don’t like the terms fast and slow twitch, because that’s not at all the way it works.

Rob Pickels  15:23

Yeah, Trevor, it’s, I think it is important that you bring this up. Because let’s be honest, when we’re talking fast and slow, we’re talking milliseconds for both of them, right. So a fast twitch muscle fiber is contracting in about seven milliseconds. And a slow twitch very slow twitch is contracting in 100 milliseconds, right? It sounds like it’s a heck of a lot more. But again, it’s still milliseconds. But I do agree that, hey, in the beginning of research, when you’re just hanging this muscle from a frog, and seeing how quickly, maybe contraction speed is the only thing that you have, and that ultimately sets the nomenclature moving forward. But at this point, I think that we know a lot more. And because it’s a bit of a misnomer. People do take this the wrong way. Right.

Trevor Connor  16:11

So continuing with that, there are better ways of differentiating these muscle fibers right now. And this is something I actually did read about last night. And we’re not going to dive too deep into this, because you don’t need to know about myosin, ATPase, and all these sorts of things. But what they’re tending to do now, in order to differentiate the different types of muscle fibers is it’s a biochemical process. So it’s a histochemical. staining, they basically take a small piece of the muscle, they stain it, and then they’re basically staining for different chemical properties in the muscle fiber. And that allows them to differentiate and there are different methods. And what I didn’t know until last night is there are three main methods will really just talk about the one that’s used now, to differentiate these muscle fibers and they don’t perfectly line up. So one of the first ones was the ones that identified the type one, the type two a type two B, it actually identified seven others. So there was also a two C, one C, think there was a two a C, and kind of goes on like that. But that’s when they originally said hey, there’s these type two B’s in humans, the newer method that’s now kind of the gold standard is they look at what’s called the myosin heavy chain in the muscle. So MHC. And with that, they discovered, hey, we definitely have the type one, we definitely have the type two a, we definitely see an animal’s the type two B, but we have never seen a to b in humans. And so what you see in humans is a 2x. Yep.

Rob Pickels  17:46

And that myosin heavy chain that Trevor was talking about, you can kind of think of it as the structural backbone, the spine, so to say, of each muscle fiber, right, and we don’t necessarily need to get into contraction or the structures any deeper than that. But it is a physical structure within the fiber itself.

Trevor Connor  18:03

But I would say that the best way to look at these muscle fibers is how they produce energy. There’s other ways to look at them as well, we actually have a big chart here. So different ways to differentiate our size of the motor unit resistance to fatigue. Basically, how long it takes them to fatigue, how much force they can produce, their mitochondrial density, capillary density, oxidative capacity, but what I really want to dive into is that how they produce energy. So we’ve said on the show again, and again, and again, you are never just producing energy of robotically, you’re never just producing energy anaerobically you’re always using a mix of these systems. But when you dive into individual muscle fibers, it gets closer, I’m still not gonna say it’s black and white, but it gets closer to being black and white. So when you are diving into that type one fiber that slow twitch muscle fiber, it has a whole lot of aerobic machinery. It has to have some anaerobic machinery because you need the end products of anaerobic glycolysis to power oxidative phosphorylation, but that slow twitch muscle fiber is a huge aerobic animal doesn’t really try to rely too much on anaerobic metabolism. When you get to those two b’s, that humans don’t have, my understanding is they have no aerobic machinery. They are purely anaerobic. And then when you’re talking about you know, 2x is the closer to the two B’s so it has a lot of anaerobic machinery, but it still has some mitochondria, it still has some oxidative potential. And then you have those two A’s, which I was my professor described it as the mimicker. It has both and it can kind of go back and forth. It can become more anaerobic if you’re doing a lot of strength, explosive type work, or it can become more aerobic. If you’re doing a lot of endurance work. Yeah,

Rob Pickels  19:55

those two ways I think structurally they tend to resemble more like To be right in terms of they’re a bit larger in terms of their cross sectional area, their Twitch speed, so on and so forth. But machinery, as we’re calling it in terms of metabolism, they’re a bit more similar to the type one the slow twitch fibers, which means that they have the ability, especially with training, to do things in an aerobic oxidative manner.

Trevor Connor  20:22

Yep. So we talked about this in a past episode. And this was something only read a few years ago and found really interesting as they were looking at what was happening physiologically in the body, when somebody tapered for an event when an endurance athlete tapered for an event. And one of the biggest changes that they saw was in these two a fibres, when you tapered, their aerobic potential went way up. And in this study, at least, they said, that’s really what you’re seeing with this taper, why you’ve suddenly had this this ability to go harder that you didn’t have before the taper.

Rob Pickels  20:57

Yeah, it’s interesting. Let’s, let’s do a little bit more comparison here. And when I talk about the metabolic machinery, then let’s use the two a fibers, I think, to help outline the differences in to be the type one slow twitch, when we talk about things that are going to increase your aerobic ability, right, we’re talking mitochondria, and then the capillary blood supply. The type one and the two a both have relatively high amounts of mitochondria and the blood supply to feed them where that to be. It doesn’t have much of that going on at all. When we talk about myoglobin, right, which is the molecule that is essential for moving oxygen. It’s like hemoglobin in your blood, but it’s the hemoglobin that’s in your muscles. So it’s called myoglobin, again, an oxygen carrying device is a high in the type one and high in the to a but low in the to be. So that’s where they’re very similar in terms of their metabolic machinery.

Trevor Connor  21:56

So I’m going to do a little tangent here, just because I think this is really cool. And this is a theory. But I really liked this theory of how we got mitochondria, because in these muscle cells, basically mitochondria these, and there’s disagreements on what mitochondria look like. But they’re generally drawn as these little oval shaped things, but they’re almost kind of a mini cell within the cell. And they have their own membrane, you know, they have a, they actually have some of their own DNA. And that is where all of the aerobic energy production occurs. So outside in the cell, that’s your anaerobic machinery. And then it’s inside the mitochondria that you have the aerobic processes happen. And the theory behind this is, you have to look way, way back in the earth history. There was life on Earth for a long time before the Earth had an atmosphere. So the original single cells that you had in that primordial goop were entirely anaerobic, they had no aerobic machinery at all. Then the earth developed an atmosphere and the theory here is the first organisms to develop a roving machinery were bacteria. And at some point, that bacteria managed to work its way into these anaerobic cells. And they developed a symbiotic relationship. So they stayed within the cells. And mitochondria are what remain of those bacteria. And that’s the explanation why mitochondria actually has some of his own DNA.

Rob Pickels  23:24

No, I’ve heard the same, I have no way of going back in time and proving that it’s an interesting theory. But I don’t know what to do with that.

Trevor Connor  23:32

It’s kind of a cool theory, it is kind of, we’re not going to do anything with it. Just a cool theory.

Trevor Connor  23:36

Yeah. So let’s get back to these fibers. Because I think that functionally, the biggest difference that people need to know about is the strength of the fiber array travel. That’s

Rob Pickels  23:48

another big difference. And really, when

Trevor Connor  23:51

we talk about slow twitch fibers, we’re talking about fibers that are not very strong. And when we’re talking about fast twitch fibers, we tend to talk about fibers that are more strong now of the fast twitch, our 2x are going to be the stronger of the two. But what I found really interesting, and I didn’t realize this until I was just doing my research, per size of fiber, all the fibers are the same. Ultimately, what’s different is that our fast twitch our two a and our 2x fibers are bigger than our slow twitch fibers and cross sectional area. And so they’re they’re stronger because of it. I always assumed that the fibers were stronger, regardless of their size, the like, you know what I mean? And so that was really that was a really interesting finding for me. So I’m glad that I went back and looked at all this your

Trevor Connor  24:42

actin, myosin crossbridge is that are all basically the same. It’s just how many of them you have correct? Yep, no, definitely. So another quick tangent here. One of my all time favorite interviews was caterpillar and was talking to him about a bunch of things about training. He was a very smart guy, but never studied physiology in his life. So it was great hearing him describe things. And as if he had anything else to say. And he goes, Yeah, this thought I’ve always had that, right. It’s like, I know my accents are horrible. Go with me. Because right is like me. I think we have small muscle. Well, a sprint us have real big muscle. And I’m like, are you talking about slow twitch muscle fibers and fast twitch muscle fibers? He goes, Yeah, I don’t know what that is. But small foot muscle muscle.

Rob Pickels  25:29

Perfect. And he’s not wrong necessarily. Right? No,


he’s not. The other fun thing about hearing him say that is looking at a picture the guy he did not have small miles. No, he was not smallest. Yeah,

Rob Pickels  25:41

I agree. So but yeah, I mean,


these are the better differentiators between the muscle fibers versus the describing them as fast twitch and slow twitch. Even though that’s accurate, I would say these are the things to think about when you’re trying to differentiate the fibers. How strong are there, how good is their aerobic versus anaerobic machinery? Another place where this is really important, because we’ve been talking about this a little bit, we’ve been talking about lactate. And this is another big differentiator. slow twitch muscle fibers love lactate, because lactate is that precursor to the whole oxidative phosphorylation. So they are a consumer of lactate your slow twitch muscle fibers. If they see lactate in the blood, they’ll suck it up and go thank you, I need this. Those big anaerobic, more anaerobic fibers. So your your 2x they produce a lot of lactate, because that’s lactate is I, there’s another debate, but I’m gonna say lactate is the primary end product for glycolysis. And they got nothing to do with it. So they pump it out of the cell. And then the slow twitch muscle fibers take it up and say thank you very much.

Rob Pickels  26:47

Yeah, Trevor, you’re talking exactly about lactate shuttle theory that we talked about back in episode 281. If anybody wants to listen to that theory that was developed by George Brooks. And then the work was further continued with Indigo, San Milan as well. Yeah, you’re entirely right. And it’s interesting, I think, for people to see now with the fiber level, how that is occurring, and how we’re able to shuttle out of a predominantly anaerobic based fiber, a fast twitch fiber into a slow twitch and oxidative and aerobic fiber. And this


is really important because if you take a basic physiology class or biology class, they’ll always draw on the chalkboard or put up a picture of glycolysis. And the short the the end product go glycolysis is lactate or pyruvate. The reason I say the primary is lactate, because pyruvate can’t cross the mitochondrial membrane only lactate can. So if it’s going to go into aerobic metabolism, it has to at some point be converted into lactate. And then once it’s in the mitochondria gets converted back to pyruvate. But the important thing is they’ll always show you a picture of glycolysis and then an arrow pointing down to the Krebs cycle and then oxidative phosphorylation. And the implication is this is all happening in the same cell. But it’s really important to understand that there’s a little bit happening in the same cell. But really, you need this lactate shuttle, because you have some muscle cells that are producing a whole lot of lactate and they can’t use it. And you have other muscle cells that don’t produce a lot of lactate, but they can use a lot of lactate. Before we move on. I think there’s a couple last characteristics point out. One is fatigue ability, slow twitch muscle fibers. And this is a simplification. Technically don’t fatigue, they can keep going. And there are some slow twitch muscle fibers where that is true. Think about your heart. Your heart is basically all slow twitch muscle fibers. And if they ever fatigued, you are in big trouble because then your heart stops. So those muscle fibers are contracting your entire life. So twitch muscle fibers in your muscle. Technically, yes, they don’t fatigue. But realistically, they’ll start over time experiencing muscle damage, they’ll start having fuel issues so they can fatigue but they can go a very long time. When you’re talking about the two acts, they are very powerful, but they fatigue really fast to a kind of in between. They can certainly last more than a few seconds or a minute. But they can’t go the hours and hours and hours that a slow twitch muscle fiber can go. Anything to add to that Rob?

Rob Pickels  29:20

No, I don’t think I have anything to add to that. But I do want to address a question that I think a lot of people are probably thinking and asking themselves right now. And that is we’ll Gosh, all of these different muscle fibers have all of these different traits and characteristics? What’s going on inside of me, right? Do I have different muscle fibers than you do? And what I’ll say is that most people, if you take just a cohort of individuals, hundreds and 1000s of people, and you look at they’re fast, and they’re slow twitch muscle fiber distribution. These are just normal people. They’re not World Champion sprinters. They’re not World Champion marathoners. Most people I think tend to have a relatively 55 If the distribution of these fibers throughout their body, but as we’re going to talk about in the future training can certainly influence that, at least the expression of that what I was trying to do in the background real quick was, look, I have a 23andme account, and I believe in there somewhere is, you know, a guess as to whether or not unlikely to have more fast twitch fibers and slow twitch fibers. Genetically, I think that we can certainly tease out some differences between people. But what I’ll always say with genetics is it is very, very complicated. And we can’t necessarily just look at one part of your DNA, and know if you’re going to be a world class sprinter or not. And so I would caution people away from doing things like


Yeah, and it also gets complexes as you pointed out, different muscles are going to have different compositions and everybody, you talked about the soleus and the gastroc. And everybody, whether you’re a sprinter or marathon runner, that’s still going to hold true that the one has more slow twitch than the other. I’m actually looking at a really interesting chart from one of my old textbooks here, that talks about the distribution you can see in different athletes. So athletes who tend to be higher ratio of slow twitch muscle fibers tend to gravitate towards endurance sports, athletes that have higher ratio of the 2x. Two a tend to gravitate towards the sprint sports but even saying that they have this graph here. And what’s really interesting is that they don’t say which particular muscle they’re talking about in this graph. So I don’t know what muscle the timeout, but they’re, they’re showing the percentage of slow twitch muscle fibers in some muscle, I’m assuming some somewhere in the legs. And you look at long distance runners, they tend to be in the range of 50 to 80%, slow twitch muscle fibers. But let’s go all the way down to the bottom and look at Sprint runners like 100 meter sprint runners, they tend to be 35% to 65%. Yeah, so less. But it’s not like you’re saying the long distance runners 100%, slow twitch and the Sprinter is like 5%. slow twitch Correct? Yep, there’s a lot of overlap there. For both beginners and veterans polarized training is the best way to get and stay fast year after year. And this is the perfect time of year to be thinking about how polarized training can help you in our new guide featuring Dr. Steven Siler explore fascinating and helpful topics, like how polarized training is different from sweetspot, how to bust out to performance plateaus, how to polarize all season, how to build durability, and how to time your high intensity work. With the complete guide from Bastok labs, you’ll have everything you need to polarize your training like a pro, and unlock your elite. Learn more fast talk

Rob Pickels  32:45

Well, Trevor, let’s get to some more fun stuff. recruitment. Yeah, let’s recruit people to the muscle fiber army. Absolutely. So


I love learning about the recruitment of muscle fibers, this is kind of cool to me. So really important thing to understand about all these muscle fibers is a muscle fiber is an all or nothing thing. A single muscle fiber can either contract maximally or not contract at all, it can’t go I’m going to do a 50% contraction here. And to take that a step further, you’re talking about the whole motor units. So if the motor unit innervates, six muscle fibers, and that motor unit activates, all six of those muscle fibers are going to contract. And they are going to contract maximally the best way I had it described to me which I always loved to use. But this was a professor teaching this in a class, he picked up a pencil and then lifted it and said, So muscle fibers contract maximally, why didn’t I just punch myself in the head? How can I control the effort that I put in, and that is the whole theory of recruitment, you control the strength of the contraction of the overall muscle by how many of these motor units contract. So if you’re just picking up that pencil, maybe one or two motor units are contracting, so it’s just a few fibers that are going to contract and it’s going to be a slower, easier effort.

Rob Pickels  34:07

Yeah, I’ve always explained this by using an example of a car on a hill. And let’s say that car broke down, and you have to push it to the top of the hill to get repaired. Well, if it’s just you, and you’re behind that car, you are not strong enough, right? As soon as you take the brakes off that car, it’s going to roll backward over you, no matter how hard you’re pushing, because you’re pushing 100% with all of your strength. Yep, that’s how we lower a weight down. We don’t recruit enough muscle fibers and the weight is just stronger than the muscle is and so gravity pulls it down. So now you got a friend and the two of you are behind the car and you have just enough strength when you’re both pushing as hard as you possibly can that you just hold the car in place on the hill. You want to push that car up the hill, you get a third friend. Now the three of you are slowly pushing the car up the hill. You want to run that car up there. Hildon you get 100 friends and all 100 of you pushing move that car so fast, it’s like it’s not even there, by changing the amount of fibers or motor units that are being asked to help out. That’s how we’re able to grade our strength. You know, the

Trevor Connor  35:15

biggest issue with that car was what is vegetable oil, called back to our fringe Episode

Trevor Connor  35:21

Two weeks ago, I had to do it. It’s early in the morning. Next, yeah, so that’s exactly the way it works. And now here is what is important. And to me, this is one of the biggest, most important thing to know about the different types of muscle fibers is there’s a recruitment order. So if you’re only picking up that pencil, and recruiting one or two units, you’re not recruiting those 2x units, you are going to start with the type one fibers with those slow twitch muscle fibers. And basically, as you’re increasing the effort, you’re going to recruit more and more and more slow twitch muscle fibers until they are all recruited, then you’re going to start recruiting the two A’s, then you’re going to start recruiting the 2x. And there’s a reason for this again, remember the the type ones don’t fatigue. So there’s no cost of using those. You want to save your two acts for when you have to do that big effort. So your body’s been smart and saying, Okay, I don’t have a big effort here, let’s use the type ones because then I’m still going to be fresh as a daisy if I have to do something else. And it’s only going to bring in those stronger, more anaerobic, more fatigued double muscle fibers when it has to.

Rob Pickels  36:36

Yeah, and that it makes a lot of sense, when we do think about this from the strength and the fatigue ability side of things that we’re using the less strong fibers initially, no reason to go straight to the super strong ones. And then at the end, when you’re working as hard as you can bring in the weak ones. That doesn’t make any sense there. But what’s interesting to me, Trevor is, and I didn’t know this stat either, at any one time, right, we would assume based on what we just said that when you are pushing as hard as you possibly can, then you’re recruiting 100% of your muscle fibers, not the case, at any one time, we can maximally recruit only about a third of our muscle fibers. But that makes sense. Because if we were able to recruit absolutely everything at once, than we would get one super strong impulse, we would never be able to sustain that force or our movements would be really Herky jerky. And so by recruiting some fibers here switching off to other fibers, replacing them as we start getting a little bit tired, because we’re trying to hold up the bed so that my son can get his toys out from underneath it. That’s neither here nor there. I don’t know why I had that example. But this is also explains why at times, and in certain circumstances, people can have superhuman strength, right baby stuck under a car, you can lift that car off the baby, because when adrenaline is flowing, and you have all of these other neural things happening in your body, you’re able to recruit more fibers than you typically could it for a one off feat of strength, but that’s going to open you up to things like injury because now you’re putting so much force through your ligaments and tendons, so on. And so

Trevor Connor  38:19

you are going to be doing some tearing, it’s gonna be bad,

Rob Pickels  38:22

but you’re gonna lift up that heavy thing because you have to. So the

Trevor Connor  38:25

one image I can’t get my head when we talked about this recruitment and using the type ones first to save the big strong fibers. Was going back to your analogy of pushing the car up the hill and just picturing this 100 pound nerd push in the car with Dwayne Johnson on the side. We’re gonna go

Rob Pickels  38:43

Yeah, I guess that’s true, right. And that is probably a more accurate way to describe this. I’m glad that you added that in because I’ll add that into my explanation that the weight lifters and the football players they’re just sitting on the sidelines waiting to be called on just laughing watching those weird fibers

Trevor Connor  39:02

drives over by a car.

Rob Pickels  39:04

But but that skinny person, I’m not going to call him a nerd that skinny less strong person can push for a really long time without getting tired, Trevor just saying

Trevor Connor  39:15

my condom nerd because let’s face it, Rob, we’re nerds.

Rob Pickels  39:18

I’m not a nerd. Okay,

Trevor Connor  39:20

you can deny it. I have accepted my nerdiness a long time ago.

Rob Pickels  39:24

Well, you are like 100 pounds compared to me. So yeah,

Trevor Connor  39:27

there you go. Remember, I used to be a football player. I was literally I was a football lineman in high school. Nobody believes me on this. Trevor.

Rob Pickels  39:34

Before we move on, I want to touch back on a statement that you had made earlier an example of the conversation. At what cadence do we switch over from our slow twitch to our fast twitch fibers. And that is important because well, as we mentioned before, it is just not the case. Right? In all honesty, let’s be honest, when we’re talking about 6070 8090 RPM we’re really not talking about A very big change in speed. And so there could be some movements that are so fast that they have to be fast twitch fibers. But running cadence cycling cadence, that’s not it. Ultimately, what we’re doing is this at low workloads, we do not have to put a lot of force into the pedals to push on that pedal. When we’re walking, we don’t have to be putting a lot of force into the ground to go walking speed. And so based on what we just discussed, the initial fibers at low forces that we’re going to recruit are those a robic type one oxidative fibres, as we pedal harder to bring our Watts from 100 to 150 to 200, whatever it is for you, as we move into a more fast running speed, we have to be pushing harder. And eventually we get to a point where those type one fibers were asking all they can give and they just have nothing else to give, right. So if we need to push a little bit harder, we need to begin recruiting those stronger muscle fibers into our to a fibers, right, those are the ones that are metabolically like the slow twitch, but structurally like the 2x fast twitch fibers. This continues on until we are recruiting the biggest baddest fibers that we have, because now we’re trying to push 600 Watts Up a hill because it’s so steep because we’re trying to sprint 100 meter dash as fast as we can. We need all the fibers to do that.

Trevor Connor  41:35

Yep. And I think that segues to a theory here. And Rob mentioned, he looked for any research on this last night, and he couldn’t find it. I do remember reading about this years ago. So my memory might be faulty. But this at least has some intuitive sense. So we’ve talked a lot about the fact that there’s two kinds of breakpoints as your wattage increases, or if you’re out running as your speed increases. So there’s that VT one, are your aerobic threshold, and there’s that VT two and your anaerobic threshold. And if you look at a lactate curve, your lactate are very, very low. And then when you hit that VT one, that’s where you start to see the lactate curve, kick up, and Vt to or anaerobic threshold is where really, the curve really kicks up. And you really start producing a lot of lactate. And one of the theories behind this is, those are the break points of when you start recruiting the different fibers. It’s not perfect, it’s not like you’re only just recruiting slow twitch muscle fibers. And then all of a sudden, you’re recruiting two A’s. But as you remember, we just talked about the fact that your slow twitch muscle fibers produce very little lactate, so they’re not pumping any lactate out into the blood. Two A’s will produce lactate, and they will pump some out into the blood. So it makes sense that as long as you’re mostly just using slow twitch muscle fibers, you’re gonna see no increase in your blood lactate. Once you start recruiting those two A’s, blood lactate, they’re gonna start going up a bit, because they’re pumping it out, when you hit those 2x. They are giant lactate producers, and they can’t use it. So they’re pumping a ton into the blood. So that’s when you’re really gonna see those lactate levels go up.

Rob Pickels  43:21

Yeah, I think it’s important to ever to say that it’s not a perfect line. But it does kind of intuitively make sense. We know that there’s some other things going on there too, right? Because the liver and the kidneys, they’re able to take up some lactate as well. And at higher intensities blood flow there slows down. And that also is part of the reason why the graph gets so steep at the end. But it does kind of make intuitive sense. And I’ve always worked off of that theory. But I’ve never looked until last night for the research to back it up. And I personally couldn’t find any. So I’m going to do a little bit more digging. Hopefully I can find something. And if I do, we’ll make sure that that stuff gets into the show notes. Yep. You know,

Trevor Connor  43:59

another thing that I think is really important here that you can understand in the context of muscle fibers is we’ve talked a lot in the show about the importance as endurance athletes of developing fatigue resistance. So remember, so twitch muscle fibers are essentially in fatigue bubble, but they can get damaged, they can start having fuel issues. So if you are not a well trained athlete, and you’re going out and doing a long bike ride or a long run, there is a point where let’s say you’re just going at a steady pace or a steady wattage, there’s a point where those slow twitch muscle fibers aren’t gonna be able to produce all the power for you anymore. And you’re gonna have to start recruiting those two A’s. And if you’re really out of shape, those two A’s are going to start fatiguing and you’re going to start having to recruit those 2x. So even though if you’re on a bike and the power was steady the whole way, at the start of that ride, you’re just using slow twitch muscle fibers. You’re down in that zone ones oh two. By the end of that ride you recruiting a whole bunch of type two a As 2x fibers, and you’re hitting a whole different system, you’re not training the same thing at the end of the ride that you were at the beginning of the ride. And we talked about this before that that’s actually one of the benefits of a long ride is forcing those two A’s to work aerobic ly and training their ability to not fatigue.

Rob Pickels  45:19

Yeah, Trevor, another interesting thing that ties recruitment and training together is if you think back to our episode 251, which is the time at VO to max episode that we did based on Dr. Ben Ron stats research. A big part of the reason he’s a proponent for time at VO two Max is that. And this is where the recruitment is tied together at VO two Max is an intensity at which we are maximally calling on muscle fibers. Maybe not maximally I shouldn’t say maximally. But we’re calling on significantly more muscle fibers, which is bringing more fibers to be trained into the equation. If we’re only ever spending time at low intensities, short of going for a very long duration like you’re talking about, then we’re kind of training the same fibers over and over again in a very moderate sort of manner. And we have to be at these vo to max level intensity so that we’re recruiting all of our aerobic fibers together to accomplish that work and giving them all a training stimulus. Right.

Trevor Connor  46:21

And you know, one of the biggest tricks I think, in really producing training games. Going back to that whole study about taper and showing the the biggest thing you see in taper is getting those two A’s to work aerobic ly. I think this is something you generally want to do in your training. As we said, those two A’s are mimickers, they can be more anaerobic, or they can be more aerobic, and you want to get them to be more aerobic.

Rob Pickels  46:47

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Trevor Connor  47:16

So going back to I was a football player in high school, and then became an endurance athlete. I noticed that issue and fatigue ability. I was actually explosive as a football player. But when our coach made us go out and do a two mile run, it killed me. Looking

Rob Pickels  47:33

at you today, Trevor, there’s no way you’re a football player. I literally don’t believe that that is part of your history. And the only thing that can explain this to me is answering this question. Can muscle fibers convert? Are you no longer the same person that you were when you were a football player?

Trevor Connor  47:53

And here’s the key question that there is probably some debate over when I first started studying physiology many, many years ago. I was taught, they really can’t convert. But that was a while ago, what I have personally been seeing and Rob Lutz addresses I know you were looking into this last night is yes, they can convert. So no, you can’t be a football player, go and spend a week going for long bike rides and all of a sudden you’re all slow twitch muscle fiber, that’s not going to happen. Nor can you be that skinny little endurance athlete go into the weight room for a week. And all of a sudden now you’re buff and you got a whole bunch of those two axes. It’s not quick, it’s not simple. But they can convert.

Rob Pickels  48:39

Yeah, I think that that’s the same sentiment that I have based on my research. And this is an area where I think that I’m kind of a living example of the ability to convert, right? If you really were a big bulky football player, I’m still waiting for pictures. I’m still waiting for some character witnesses. At least today you are no longer a big bulky athlete and you’re a pretty solid A robic functioning individual right, you’ve reached a relatively high level in cycling. My background to you is is somewhat similar. I was a hurdler in jumper High School in college, very spring focused, and I have been able to improve my ability aerobic ly I really enjoy doing long multi day mountain bike races, so on and so forth. But I still retain a lot of my sprint capability. Right? You know, I I cannot train to sprint at all and still hit easily over 1400 Watts, right. I know that that’s not a lot in the whole scheme of things. But it’s a lot more than most people and certainly more than most people that train for that. But I have not been able to shift myself quite as far as you have. But I’ve definitely been able to shift myself quite a bit. And so there is certainly a range I think in what is achievable through training in terms of a fiber type conversion or at least characteristic conversion. Yep,

Trevor Connor  50:00

So I’ve seen this in textbooks, and this might be outdated, but it’s something I’ve always said to people, which is, the expression goes all training causes a conversion from fast twitch slow twitch. So training causes conversion from 2x to two B to type one, which is why you probably can put out your best sprint power coming off of the couch. You know, I’ve always seen this when I take my offseason and I don’t get on the bike, my ability to time trial, my endurance are all tanked. But I can always put out my best sprint power. So it’s really you want to get those 2x requires a lot of rest. And the example I always point to is, look at the best sprint animal in the world. The cheetah spends a lot of time sitting around. Yeah,

Rob Pickels  50:46

I think it’s interesting. I looked through Gosh, I don’t know, almost a dozen studies last night on on this particular topic. And they I mean, in my mind confirmed that conversion is possible. But what that conversion is looks very different based on the situation. One of the early studies looked at riders doing on a cycle ergometer, I believe like five minute efforts at like 95% of their view to max power or whatever pretty standard vo two max level workout. And they saw conversion of muscle fiber from 2x to two a but also from type one to two a right. So we’re kind of all roads are leading to that to a situation. I looked at another study that looked at beginning individuals who were not necessarily athletes, they had kind of a 5050 fiber type distribution. And they I think, if I remember correctly maintained their two A’s really reduced their two x’s and increased their type ones, right. So two A’s stayed kind of the same based on that training. They’re really strong, fast twitch fibers came down, and then they’re slow twitch came up. So it really depends, like you were saying before Trevor, the types of training that people are doing, and the duration that they’re doing that training for, right. And that’s where I think maybe some of these studies were a bit confusing, because if the study isn’t set up correctly, it’s very easy to say, oh, there was no change, there was no difference. Right? And so if you do it with the appropriate subjects who are in appropriate types of workouts for an appropriate duration, then I do believe we get a lot of good science that says, hey, yeah, we are definitely seeing some structural changes, and the muscle fiber characteristics changing along with that. Yep.

Trevor Connor  52:31

So I think another thing that people get confused, you know, we’re talking about this fiber conversion. As I said, it takes a long time we’re measuring this in years, I think when you see that shorter term transitions, I mean, these things that we were just talking about, I haven’t seen a study to say this, absolutely. But this is just intuitive, that more of what you’re seeing is those two A’s with their capability to mimic. So if you’re seeing, hey, this person is really moved from a much more explosive anaerobic type phenotype to that more endurance type phenotype, I think probably what you’re seeing is a lot of those two A’s moving from being heavily anaerobic to be much more aerobic and their capabilities. Well, and

Rob Pickels  53:15

I think that this backs up something that we talk about a lot here, and that is, if we look at research on high intensity interval training, we can see relatively large changes in a relatively short amount of time, especially because that research is being done on maybe an active but said otherwise sedentary population, not a highly trained population anyway. And so we’re able to see that relatively fast conversion of the 2x to the two a, because the equipment is all for the most part there already. But the longer term stuff that we’re talking about, right, we talk, you know, it takes years and years and years of steady bass steady zone two training to really reach your potential in terms of an A robic individual. And that is probably again, related not just to the training of those muscle fibers that in itself might be able to happen quickly in terms of mitochondrial biogenesis within a type one fiber. But over years and years, we’re getting conversion of that fast twitch fiber to slow twitch and that’s the continued improvement that we see that really makes somebody you know, an appropriate endurance athlete. Yep.

Trevor Connor  54:26

Now the last thing I’m gonna throw into those, even though Rob and I have been sitting here saying, yeah, they can convert. I read a actually a very recent study a couple nights ago, so that they took endurance athletes and they took strength athletes, and we’re looking at the the differences in their fiber type. But what they dived into is differences in genetics, differences in the DNA, and differences in micro RNA. And they did show yeah, there are differences. It is somewhat hard coded in your genes. So I think the answer here is can conversion happen? Yes, but we are all born kind of more one way or more or the other. And if you are born with the genetics to be that big, explosive, anaerobic animal, you’re probably never going to win the Tour de France, I would probably agree with that. And vice versa.

Rob Pickels  55:19

Something else I think that’s interesting to point out is that, as we mentioned before, at this point in history, we tend to be calling muscle fibers based on that myosin heavy chain nomenclature, right? The structural, but it is possible that we’re seeing fibers have metabolic changes without the structural change. And so we might say, have performance improvement or performance changes. But our fiber typing doesn’t necessarily change. Because the structure of the fiber is the same, even though its metabolic activity is a little bit different sound, perhaps depending on the type of staining or the method that’s being used to classify, we may or may not see some changes, and that ultimately, accounts for different research results.

Trevor Connor  56:05

Now, I think there is one last thing to cover here. And this is an effect that you see with aging. This effect is called motor unit remodeling. So as we were talking about early on, you have this motor unit, which is a nerve that then branches out, and it connects to multiple fibers. And as we said, when you recruit a motor unit, you recruit the whole thing, so that nerve fibers and all those fibers will contract, what you see over time, is that you see basically orphaned fibers, they disconnect from their motor unit, for a variety of reasons. And then you have a fiber that cannot contract. But it will eventually re Innervate. So this is the remodeling, it will connect to a another motor unit. And for whatever reason, and I haven’t dived deep enough into this research to understand why almost invariably, you see it connect to a slow twitch motor unit, not a fast twitch. And so something you do see in aging, particularly, you see this endurance athletes all the time, the first thing you lose is your explosive sprint power. But older endurance athletes are just animal. I mean, this is why people who do those alter endurance events like Race Across America, you’re considered in your prime in your mid 40s, because you just become these aerobic monsters. And this is part of the explanation behind that is what you’re seeing is this denervation. And some fast twitch muscle fibers are connecting to a slow twitch unit. And then they do convert to a slow twitch muscle fiber.

Rob Pickels  57:51

And it’s funny because Trevor, I had no background knowledge of this of this concept at all, because frankly, I’ll never age and so it doesn’t matter to me that this happens. Like kid, I’m getting old real fast. But I did know that and I don’t even know how I knew this that if you connected a fast twitch fiber to a slow twitch motor unit, that fast twitch fiber would then convert into a slow twitch fiber, which is just like baffling and mind blowing. But I think ultimately, it makes sense based on the previous conversation we’ve had.

Trevor Connor  58:23

So here is where exercise is so important for health. Because what they have shown is that when you are training and training regularly, you see a lot fewer orphan muscle fibers. So you can you know, you can’t stop it. But you can slow down this effect. The other important thing to remember here is what happens when you have a lot of this remodeling is you start losing motor units, what will happen is, it will connect to an existing motor unit. And that motor unit will start increasing the number of fibers connected to it. And what you will have in any given muscle is fewer motor units that are innervating a lot more fibers, which means you lose fine control. Yep. So then when you go and try to pick that pencil up and pick it up slowly, you might not be able to quite pick it up at the pace you want.

Rob Pickels  59:18

Yeah, or just all of your movements are going to be less smooth, right? Because you’re not able to be grading that for us, maybe there’s going to be a little bit more of a shutter in somebody’s movements interesting.

Trevor Connor  59:30

And so this is something you see an aging with people who aren’t physically active, is you see that reduction in the number of motor units. And so this is why you see them. Elderly people who unfortunately haven’t been too healthy, they tend to shuffle. They tend to have those problems with the fine movement. This is one of the reasons behind that at least from what I have read. And so this is what makes exercise really important. And if you want to avoid becoming that pure, slow twitch That’s all yours innervation type person, this is why getting in the weight room, particularly as you get older is so important because you need to protect those fast twitch motor units. Interesting.

Rob Pickels  1:00:10

Yep. Wow. heck of an episode. That was fun. Did

Trevor Connor  1:00:13

we leave anything out?

Rob Pickels  1:00:15

Well, I mean, I think that we explained about 1% of it. So yeah, we left 99% out point, let’s be honest there. But no, I think that we cover that 1% pretty well. And I learned something it took until the very end of the episode for me to learn it, Trevor, but I learned something today. I


hope everybody got something out of this. I hope we didn’t get too deep in the weeds. As you said, you know, we were trying to get the basics here. We probably at the end got into a little little heavier on the science. But you’re right. You know, when you start diving into this, it gets complicated. You know, realistically, we could talk about type one, two, a 2x. But there’s all sorts of variations within there. You can get real complicated with

Rob Pickels  1:00:54

this. All right, well, maybe we should do this again.

Trevor Connor  1:00:57

I think so. So give us your feedback. If you liked this, let us know and let us know other basic topics in the physiology you’d like us to cover and we can do more of this.

Rob Pickels  1:01:07

Yeah, let’s talk about excitation contraction coupling. That sounds great. Let’s do that next. No, not basic.

Trevor Connor  1:01:14

Had to think for a minute.

Rob Pickels  1:01:16

We should probably do it.

Trevor Connor  1:01:19

I need to re familiarize myself. So again, that was another episode of Fast Talk to subscribe to Fast Talk wherever you prefer to find your favorite podcast. Be sure to leave us a rating and a review. The thoughts and opinions expressed in Fast Talk are those are the individual. As always, we’d love your feedback. Tweet us at @fasttalklabs, or join the conversation at And of course you can learn from our experts at as well. For Rob Pickles, and I’m Trevor Connor. Thanks for listening!