Fast Physiology with Dr. Phil: The Size Principle

Summary

In this episode of Fast Physiology, Dr. Phil Batterson and Dave Schell discuss the size principle in muscle fiber recruitment. They explain that the body recruits muscle fibers based on the force required, starting with slow-twitch type 1 fibers and gradually recruiting type 2a and type 2x fibers as the force output increases. They also discuss the impact of cadence on fiber recruitment and fuel utilization. They emphasize the importance of sport-specific training to prepare for events that require different force outputs and motor unit recruitment. The key takeaways include the need for consistent and specific training to optimize physiological adaptations.

Takeaways

• The body recruits muscle fibers based on the force required, starting with slow-twitch type 1 fibers and gradually recruiting type 2a and type 2x fibers as the force output increases.
• Cadence affects fiber recruitment and fuel utilization, with lower cadences requiring higher force output and more recruitment of type 2a and type 2x fibers.
• Sport-specific training is crucial for events that require different force outputs and motor unit recruitment.
• Consistent and specific training is necessary to optimize physiological adaptations.

Transcript

Dave Schell (00:03.406)
Welcome back.

I'm going to go back for another episode of Fast Physiology with Dr. Phil Batterson. Dr. Phil, thanks for coming back. Thanks. Thank you for having me, Dave. Sorry for interrupting you. I was just so excited to be here. Well, I'm excited, too. I look forward to these. These are always fun. And I think what's kind of funny with this is that when we first talked about doing this series of Fast Physiology, we have a Google Doc, and we had all these.

ideas for topics. And I think out of all those ideas, we probably use the first three. And then every other week, it's just really whatever's been top of mind, whatever we think is interesting and like, Hey, I want to like, let's discuss this a bit more. And so it's been more fluid. And I think that makes it really exciting. Yeah, it is fun, because then it keeps us kind of, you know, gives us the ability to keep our finger on the pulse of like, you know, what are people asking? Where's the where's the industry industry kind of trending that you know, right now and?

and other things like that. So no, it's it is certainly fun. And I have been really enjoying it. And one of the things that that I'm doing kind of in the background is trying to develop a few courses for people. So hopefully, talking about all these sort of topics and everything like that will give me a little bit better of an idea of like, how to develop the courses because I want to do an exercise physiology one exercise testing one and then like a nears one. So

I know this wasn't isn't on my podcast, but for people who have been following me for a long time, they've been asking for courses and those sort of things. So I'm hoping that just by letting this go into the ether that it gives me more motivation to actually get it all done. Yeah, you've said it. So now you have to somebody's to it. Yeah, crap. Now I got to do it. No, I think that's a great strategy. So we did a podcast recently on muscle fiber type. And

Dave Schell (02:02.894)
You can go check that out. That was also on Training Babble. The other Fast Physiology episodes can be found on Critical Oxygen podcast, which is Phil's podcast. Today we're going to kind of continue the conversation about muscles and muscle fiber types and stuff like that and talk about the size principle. boy. So to set this up, why don't you start by telling us what is the size principle? Yeah, absolutely. So...

I think I think before we actually get to that, let's just let's just do a quick review, right? So our our muscles are made up of fibers. And each muscle is made up of a different proportion of these fibers. But we have our slow twitch type one. Fatigue resistant fibers, we have our type two a, which are kind of our intermediate fatigue resistant sort of fibers. And then we have our type two x, which are our

high power, highly fatigable fibers. And the, the reason why I say type one, type two, a type two X is because they're delineated most of the time by their ability to generate force. Type one has the lowest ability to generate a force and force over time, which is power type two, a is in the middle. And then type two X is kind of like, you're, you're really, really, powerful.

very, very fast force production type fibers. So what's interesting about that is that your body is actually really, really good and really, really, really fine tuned at contracting or engaging those fibers. So when our, our body senses that a muscle contraction needs to occur, it sends signals from our brain to the muscle and

Every single one of our fibers or groups of similar fibers. So groups of type one fibers, groups of type two, a groups of type two X fibers are all innervated separately. So that means that our brain has a different control line to each one of those types of fibers. And what our brain does is it tries to, predict or it knows how much force output that is expected here. And it is only.

Dave Schell (04:29.678)
going to communicate to those fibers that will get us to the force output that we want to get to. So for example, if we are walking, our brain is saying, okay, I only want to recruit the amount of fibers that are necessary to get this walking task complete. And what happens is our brain is it's actually, this is actually pretty cool mechanism. So our brain is actually really good.

at saying, okay, low exercise intensity, we're going to recruit the fibers that have the lowest force output. So the lowest force output ones are our type one fibers, right? So what we just said, if we took that walk to a jog, then our brain would be like, shit, we got to create more, more muscle contraction here, more force output. So we're going to recruit

more of those type one fibers. But eventually get to a point where you've already recruited all your type one fibers, essentially. So then what happens? Your brain is like, okay, well, I'm just going to recruit the minimum amount of fibers that are necessary in order to get this force output done. So then what it does is it graduates, it continues to recruit those type one fibers, but then it graduates to recruiting those type two A fibers. So those intermediate force contraction fibers.

And then as we run faster and faster and faster, we recruit type one plus more and more and more type two a fibers. And then eventually if we do stuff like sprinting or jumping, like jumping is really, really powerful, requires a lot of force over a short period of time. That's where we start to recruit our type two X fibers. So this is what, this is kind of what the size principle is, is that you recruit your type one fibers first.

And then you keep recruiting those. And then you start to recruit your type two a fibers, and then you keep recruiting those. And then you start to recruit your type two X fibers and you keep recruiting those until you can't generate any, any more force. So conceivably at the very, very, very, very highest outputs, you're recruiting all of your muscle fibers. And it's called the size principle for a reason, because the amount of fibers that are within the bundles within the communication networks are actually.

Dave Schell (06:56.046)
smallest in your type one fibers and they get larger and larger as you go up. Because when you're doing something like a jump, your body just wants to say, I just want to be able to call in all the type two x fibers. So they're there can be a lot of fibers that are, you know, under that communication range umbrella. And so that would be a large motor unit. So it goes from smaller motor units to larger motor units.

And typically that's the same size as our fibers as well. Yeah. And so I think you did a great job explaining that, which I guess I would expect you to considering what you do. So as you were kind of going through that, I think where my mind starts to go is we're talking about these motor units are recruited based on the force required. And as you said, power is rate of work.

And so whether like if we're cycling or if we're running, the more power you're doing, the more force. And so if we break that down, power is force times velocity. Right. And so the more force you're putting out, the more larger motor units you're going to use. And so I just think about how this impacts like fueling and things like that, or if we're talking about the duration of the exercise. So if you're riding for 30 minutes.

you're going to be able to generate a lot of force for a small amount of time versus riding for six hours. And so how we talked about on the previous podcast about these different fibers and kind of what their fuel sources are and stuff like that. And so how could, how would that impact your fueling based on the type of power you're out, out putting? Yeah. Yeah. So, so first I just want, I just want everybody to understand that.

even at VO2 max, we're still only recruiting 40 to 60 % of our fibers at the at VO2 max because that's maximal aerobic power, right? We still have a whole heck of a lot of fibers that we can recruit for maximal force output. And if you think about it, right, even riding, say, say you did a VO2 max test and you were riding at 450 watts,

Dave Schell (09:17.966)
which requires a lot of force per second. But if we compare that to true, say, sprint capacity or weightlifting capacity or whatever that is, that still really pales in comparison to what we can actually get up to. So I want to frame that saying, like, even at VO2 max, we're still only probably recruiting into our type 2A's, maybe a few type 2X's, you know, depending on how much force we're actually generating.

And then if we start to think about that.

Then we can start to say, okay, well, what happens when we lower our cadence? What happens when we raise our cadence? What happens when we extend periods of time, those sort of things, because if you recall, okay, so let's just start with the cadence idea. So if at a cadence of 200 watts, or sorry, at a cadence, at a 90 cadence, 200 watts, right? That's gonna require a specific force.

per second to make that 200 Watts. And because you're contracting so quickly, 90 RPMs, it's gonna be less force per pedal stroke in order to make that 200 Watts. Now, if we dial that down to half, if we say we're gonna pedal at 45 RPMs, that's going to essentially double our force output in order to be able to make those 200 Watts actually occur.

By doubling our force output, we're going to recruit more muscle fibers because the fiber recruitment is based off of force output. So, and if you're recruiting more fibers, then most likely you're kind of transitioning into type 2a, maybe type 2x again. And by recruiting more type 2a fibers, you're probably going to then be switching to more glycolytic fibers.

Dave Schell (11:17.198)
or non -aerobic means. So most likely you're going to be using more carbohydrates if you're pedaling at a lower cadence, but same power output versus if you're pedaling at a higher cadence in the same power output. So I just want to frame this too, is when we're talking about force and I know this is another episode you want to do at some point.

And so if we're talking about force, and like you said, at 90 RPMs, you're pedaling at a certain force, and then you go to 45, and you have to double that force. But when we talk about doubling that force, it's still relatively small. And when we're measuring force for cycling, we're talking about like Newton meters, which just isn't a lot at all. And so can you, like off the top of your head,

talking about pedaling a bike at 90 RPMs and 200 watts or whatever, and like ballpark it versus doing a deadlift or something like that, or a squat Newton meters. Like what would that look like? I don't know. I don't know the Newton meters and I don't know the comparison, but it's like, if you think about how many pedal strokes you have to do per hour in order. So I think it's, it will liken it a little bit more to like a squat. Okay.

because I think those are a little bit more similar in terms of the movement. But if you're doing so, so say, say you did a hundred RPMs for an hour. That's a hundred times 60. That's 6 ,000 reps of single leg, you know, like, like a lunch, right? Right. Now go and try to do 6 ,000 body weight lunges and let me know how you're feeling. Yeah. 60 into it.

So, so, so that's just body weight, right? Your body has an incredible amount more. I think it would be probably if you're talking like one rep max or maximal power, maximal strength, you're probably on the order of maybe like 20 to 30%. Yeah. Maybe. And it's probably lower than that. Right. So it's really low. Like it is really low in terms of maximal force that you can generate. Yeah. So we'll, we'll, we'll park that for a moment and, and.

Dave Schell (13:38.734)
use that for a future episode because I know that's something you want to talk about. So let's think about now going back to this idea of specificity we talk about in almost every episode and

If we're training for something like a mountain bike race or a trail run or something like that, there is going to be a lot of vertical. And typically when you're ascending, it's going to be more force, lower K and stuff like that. And so what does that mean in terms of motor unit recruitment and training? No, I like this question because this then starts to get even, you know, we were, we were just talking about how, you know, in the realm of endurance,

you know running is treated as all the same or cycling is treated as all the same. It's like no, no, no. A flat, like all out time trial on a bike versus a hilly, like you're going up, you're going down, you're doing all this crazy stuff. I'm like during a mountain bike race. Those are two different, you know, events that you have to train for. Like it's akin to, you know, like you just going and riding on your trainer every single day, you know, erg mode.

150 Watts. That would be more of like the flat time trial. Right. Whereas like during a mountain bike race, you're going up, you're going down. Sometimes you're even holding, you know, you're doing like that static sort of like load because you're trying to just guide your bike downhill and you don't want to crash. You're not pedaling then. So the, the, the type or so what's happening when you're just keeping a consistent, you know, like 150 Watts or whatever it is on a flat road, your body.

is going to be using or recruiting, you know, more type one fibers because your, your cadence is probably higher. Maybe you're 90 to a hundred. I guess that's typically what I prefer. So force per, pedal stroke is going to be a lot lower. And as long as you're efficient in that cadence, then you know, you're, you're just going to be recruiting your type one fibers. Now, when you do go to a mountain bike race though,

Dave Schell (15:52.494)
there's there's a lot of fluctuation right you could you could have a point in the race where you're essentially creating just the force to hold your body up but then you could transition to a hill and during the first few pedal strokes going up that hill you might hit 500 to a thousand watts right like I don't I don't think that's out of the question no not at all so if you think about it from that perspective your cadence is probably gonna be pretty low right it might be like 60

So you essentially just, so just for ease of ease of use, if we said we're going 120 RPMs at 150 Watts or we'll say 200 Watts again, to make the math easy. And then you go and you say you're at 60 RPMs and a thousand Watts. First of all, you just increase your Watts fivefold. And then you also doubled the force or yeah, you doubled the force because you cut your RPMs in half. So.

you essentially increased your force output by almost 10. Yeah. So so from that perspective, your body's gonna be like, holy crap, we just went from, you know, doing like, you know, little easy half lunges to almost a full, you know, like, full requirement of lunges. So that's going to require you to recruit a lot of those type two A and type two x fibers in order to overcome that, that power output, because your body is trying to recruit enough.

to maintain ATP, but also maintain the force output that you're that you're going to be doing. And this is where I think mountain biking gets can get challenging. And this is exactly what you were talking about offline is that, you know, you can train really as much as you want on the trainer, you know, in the winter, but unless you're adding in those like super punchy efforts where it's like, okay, you're going to just be going along riding along riding along 500 watts for five seconds, come back down.

riding along, riding along 500 watts, you know, to kind of simulate those, those climbs. You're never not never, but you're, you're very rarely going to be recruiting those type two X fibers, which are now going to be engaged a lot during a one hour mountain bike race where, like I said, you're going up, you're going down, you're doing all this crazy stuff. Yeah. Yeah. And I think you and I did a, an episode on cramping. And one of the things we discussed was that,

Dave Schell (18:16.462)
One reason you might cramp is that you're just kind of pushing your muscles past the point of their current conditioning. And so you and I, we talk about specificity all the time. And so that is something where it's like, if you haven't prepared those motor units for the demand of the event, and then all of a sudden you're calling on them and they just don't have the endurance capacity for an eight minute climb at 60 RPMs and 300 Watts or whatever. Like.

it's gonna piss them off a little bit. Yeah, to say the least your body because again, your body adapts to the the specific nature of whatever you are doing. So you know, this could come from adaptations with ease of recruitment of these fibers. If you haven't recruited those fibers, and we see this in old age is actually like the the innervation of muscle fibers and like the the motor unit becomes smaller.

because the tendrils of the actual neuron start to shrink back because the muscles are just never recruited anymore. Yeah. So I think we talked about this in the previous podcast. But again, talking about from an evolutionary standpoint, our bodies are lazy. It's always trying to save energy. It's trying to find the path of least resistance. And I went down a path at one point studying the brain and stuff.

Anyway, and so something that's super interesting is our brains compartmentalize based on what's being used and what's not because it wants to save energy. And so like one of the examples were these monkeys. When the monkeys, the monkey would have a separate spot on each part of their brain for each of their digits, their fingers, but then they'd sew them together and it would kind of like come together and condense because it didn't want to waste energy, that spot on the brain.

And then they'd separate them again. And then it would separate back into those. And so our brains do the same thing. Like, I just love this because it's beyond metabolism and stuff like this. It's your brain talking to the muscles. And so the more you use it, the faster those signals get like, it's almost like a wire that has really good like shielding on it or whatever. Exactly. And so it gets faster, it gets more efficient, but you don't use it and it starts to save energy and.

Dave Schell (20:42.51)
you know, use it or lose it. Yeah, no. And this is like this. This is exactly we can't discount the fact that by not doing anything, you're actually reversing adaptations or having maladaptations actually occur because your body is constantly adapting to whatever whatever stresses you're putting it under and no stress is also a stress for adaptation. Right. So so this is where this is where for me, whenever I coach an athlete, I like them to do high intensity stuff. I like them to do low intensity stuff like

all throughout the year. And the way that I sprinkle it in is obviously going to change depending on what point of the season we're in. But I don't have any athlete who doesn't at least do some level of sprints or strides or something like that, who doesn't do accelerations, who doesn't do like I all my athletes do all of that all the time. Right. And it's because once those pathways are actually ingrained, it's easier to keep that pathway going than it is to start from scratch.

And this is exactly what you should be doing if you're trying to prepare for an outdoor mountain bike race where you know it's going to be extremely hilly, extremely punchy. You got to practice that. Yeah. And you might be too young to remember. But it's like when I first started coaching or first started training, there was like this.

very rigid idea of what periodization looked like. And it's like, first you build your base and then you move onto this and then you move onto this. And like, as you just said, you have to, if you're only training those, like at certain times of the year, you're going to lose it. Like it's going to take more time to get there and then gain it back. And so like, really you should be working on everything a little bit all times of the year, but the distribution you're working on, it might be more or less depending on where you're at in the season.

I think. Yeah, no, no, exactly. So I think, I think my biggest takeaway from this is that, you know, it's like, there is so you there is no way for you to specifically target just type 2x and just to type 2a because you're always recruiting type one, then type 2a then type 2x and they're they're recruiting on top of each other. So so there's that you can.

Dave Schell (23:03.342)
kind of specifically target your type one fibers that's at, you know, your, your really low intensity is more of your zone two sort of stuff. But in order to actually allow your body to recruit the fibers that are necessary for force output, especially as a mountain bike or a trail runner, anything that is not just a constant grade the entire time, you have to do sport specific work for that, whether it's a punchy effort efforts,

up a treadmill or going outside and running hills or, you know, just doing something along those lines. You have to continue to do that continually throughout the year in order to be able to do it when the time comes. Fantastic. And I think my big takeaway, and I think this encompasses many of our episodes is we as athletes and with all this access to information and stuff, it can be, we can get in the weeds trying to be very specific with.

physiology and stuff like this and thinking, we're doing this. And really at the end of the day, it comes down to training for the demands of the event and doing things that are specific to what you're going to have to do in the race, whatever. And so it does, I guess this would be my thing. Racing is messy. And so training can be messy, you know? And so that might be something where you're out doing a endurance ride and over the course of the endurance ride, you're doing random sprints and you're like punching it up a hill and you know, like doing tempo for a bit and just like,

doing a variety of things to prepare your body for that. Right. If you train for the requisite event, your physiology will follow. That's kind of, that's kind of what I just came away with. and I am a huge proponent of physiology testing and all of those sorts of stuff. But first and foremost, you need to do the training that is going to be the most specific to whatever rate is whatever event you're trying to do.

You cannot expect to be good at a mountain bike race if you only run. And I think, I think people are like, yeah, yeah, that makes sense. Like, you know, maybe you could be okay, but like, if you never get on a bike, you're, you're not going to get the specific adaptations for whatever event that is. So, so, so that's, I mean, we, if anyone's been listening to us for any period of time, it's like, it always comes back to those basic training principles, specific.

Dave Schell (25:29.422)
Yeah. Consistency, specificity and progression. And it's, it's just one of those things where it's like your physiology will follow trust in the process. If you're staying consistent, then you're managing your stress really well. So all of those things always play into it. It is nice to know the physiology though, because then you can say, okay, well that really helped, you know, this aspect of my physiology or this part of my physiology is lacking. so, so.

Don't throw out the physiology, but train appropriately and your physiology will follow. Awesome. I love it. And where can people find you? As always, people can find me on Instagram at critical02. That's where I do most of my business. So reach out to me there, comment on my posts and other things like that if you have any questions. And there's a lot of content there that hopefully will answer a lot of those questions. So feel free to scroll.

up and down as far as you want because there's a lot of really good information on there. Awesome. Thank you very much for your time. Always a pleasure.

Thanks Dave.

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