If we’re not strength training, we’re probably not getting to that point where we recruit those type 2 muscle fibers.

We lose our type 2 muscle fibers as we age. Those are the ones most responsible for our strength, our balance, our stamina.

It’s either on or it’s off. The muscle fiber is either recruited or it’s not recruited.

Welcome to the Strength Changes Everything podcast, where we introduce you to the information, latest research, and tools that will enable you to live a strong, healthy life. On this podcast, We will also answer your questions about strength, health, and well -being. I’m Amy Hudson. I own and operate three exercise coach studios. My co -hosts are Brian Sagan, co -founder and CEO of The Exercise Coach, and Dr. James Fisher, leading researcher in evidence -based strength training. And now for today’s episode.

Welcome back, everybody. We are continuing a series on principles of exercise design, where we break down components of a workout, why those components are important, and what they mean in terms of your overall results from the exercise that you do. Today’s topic is muscle fiber recruitment. Why is muscle fiber recruitment important during your workout? We’re going to address something called the size principle and we’re going to talk about the benefits of continuous muscular loading without taking breaks. Your workout needs to include these things in order for you to achieve the most benefits.

So today we’re going to learn all about this this concept. Dr. Fisher is here and he’s going to jump right in today to explain what we mean by muscle fiber recruitment and why this concept is so important in our workout. So, hey, Dr. Fisher, how are you doing?

I’m doing very well today, Amy. I’m excited to get into this. How are you today?

I’m doing well. I just came off of a very challenging workout where I gave a lot of effort. And so I’m excited to sort of pat myself on the back. on the back today with understanding kind of what I was doing behind the scenes talking about this. So where would you like to start?

Yeah. So before we get into the size principle, there’s a couple of kind of theories we’re going to discuss here. And they’re relatively proven theories, but to some extent, they’re still theoretical models. So the first of which is called the all or nothing theory or the all or nothing concept. And basically what this says is the motor neurons and the corresponding muscle fibers are either switched on. So they’re recruited or they’re not recruited.

So they’re switched on or they’re switched off. And what that means is when I pick up something like a pencil or a cup of coffee, then I only recruit the muscle fibers required to perform that task.

And those muscle fibers are, are just switched on.

That’s it. So what some people have thought historically is that we recruit a lot of muscle fibers partially, but that’s not the case. We recruit only a few muscle fibers and only the ones that are required, but fully. So the way I often explain this is we can think of it like a light switch without a dimmer. It’s either on or it’s off. So a muscle fiber is either recruited or it’s not recruited.

And the reason that we have this is for small, low effort tasks, the body will only send a small impulse. So a low intensity signal. And that will cross the low threshold motor unit. and only recruit typically type 1 muscle fibers. So type 1 muscle fibers are low force and high endurance or fatigue resistant. And the reason that we recruit those first is because we can use them and then reuse them and then reuse them and so on and so forth.

So there’s not much cost to using them. If we want to get to our type 2 muscle fibers, well they’re high force and they’re also low endurance and they’re high fatiguing. So those type 2 muscle fibers are quite expensive to use. Once we’ve used them, they take time to recover. They take time to be able to be reused or re -recruited.

So We have to save those for later and we save them ultimately for a high enough impulse or a high enough signal from the brain. And that’s what leads us into the size principle, as you said at the start. And the size principle, which originates from two people called Denny Brown and Pennebacker in 1938. So it’s long, long time back, nearly a hundred years ago. They discovered that motor neurons were recruited from the smallest to the largest, which is exactly what we just said. Small motor neurons need less excitation or less stimulus to be activated than larger motor neurons.

Okay, so small motor neurons are our low threshold, and large motor neurons are our high threshold. So there was a really nice paper by Ralph Carpinelli, who was a colleague of mine years back, a wonderful man, and he summarized this in 2008, and he said, When the central nervous system recruits motor units, it begins with the smallest, most easily excited, least powerful motor units.

And it progresses to the larger, more difficult to excite, most powerful motor units to maintain or increase force.

Okay. So we have this sequential recruitment of muscle fibers and motor units that occurs when we, when we exercise or when we provide a stimulus. So I’ve put together a few slides for anybody who’s now watching on YouTube. We’ve got our first slide. just come up now and you might do a better job than me of describing this but I’ve done some artwork and I’m not known for my artwork but I’ve done some artwork which shows a muscle fiber or a muscle and within it we can see muscle fibers and the white muscle fibers are unrecruited or they’re switched off for all intents and purposes and the red muscle fibers are switched on. And I’ve given the example of if we exercise with a moderate load, so somewhere between 50 and 80 percent of our one repetition max, which is normally about 12 reps at a normal pace or about 90 seconds under tension, If you’re moving a bit slower, um, you know, it equates to around 90 seconds, let’s say.

So with our first three reps, we only recruit a few muscle fibers. So we recruit, it’s a low stimulus. It’s, it’s quite still quite an easy weight. Um, the, the weight of course, doesn’t change, but it’s still quite easy for us because we’re not fatigued. So we only recruit those low threshold motor units, probably. predominantly type one muscle fibers.

Now, as we move on, you can see in the next slide, I have upgraded us to reps four to six out of 12 rep max. So this is out of a maximum of 12 repetitions. So we’re about halfway there. And now we’ve recruited maybe a third to a half of muscle fibers, but they’re still only or still predominantly are type one, low threshold muscle fibers. Okay. Now as we move on and we move to reps 7 to 9, so kind of with each successive rep we recruit more muscle fibers.

So now I’ve recruited more and I might have started to tip into some of the type 2 muscle fibers as well to be able to continue to lift this weight, continue to exercise with this weight. And when I finally get to the last few reps, so I’ve said here reps 10 to 12, so rep 12 of a 12 rep max would be the maximal effort. So let’s call this the maximal effort that we can give. We’re now recruiting as many muscle fibers as we can recruit voluntarily. Now in some cases that’s still not all of our muscle fibers we still hold a couple in reserve just because our body likes to do that it likes to keep some back but ultimately we’ve recruited as many as we can and this is important because recruitment is our stimulus for adaptation.

Ultimately a muscle fiber is not going to get stronger and it’s not going to get bigger if we don’t recruit that muscle fiber with a stimulus to change. Absolutely.

It’s kind of like your body is trying to conserve energy and it’s doing all it can to avoid fatiguing itself too much until you give it a proper stimulus where it has no other choice, right? Yeah, that’s exactly the case. Our body is desperately trying to preserve everything because we have, of course, our fight or flight response. So it will always try and keep something in reserve just in case of emergency, just in case we have to do something. So it, and hence why it will always recruit the type one muscle fibers first, because They’re easy. We can recruit them and then we can re -recruit them and reuse them over and over again.

And we can recruit them and we can use them for a prolonged period. A good way to think of this is a lot of our postural muscles, our trunk muscles, our abdominal muscles and our low back muscles, for example, are predominantly type one. And that’s because we use them over and over again through the day. And for example, right now I’m stood up. So they’re helping me to stand and maintain posture. So they’re doing a lot of work, but it’s not a high force to do that.

It’s just a continued contraction. do that. So they’re low force but also low fatigue or high fatigue resistance. They’re resistant to fatigue. So that’s if we were to do a single set of exercise to a sufficiently high degree of effort. Now we can also think of this in terms of what if we did multiple sets because lots of people will talk about single and multiple set strength training.

And it’s quite nice to think about muscle fiber recruitment in that context. So our next slide now talks about a multiple set approach. So in set one, we have effectively the same thing that we had in our early repetitions. With our single set approach, we recruit a series of muscle fibers. And the problem with a multiple set approach is we don’t often train to the highest degree of effort. We kind of train to a given number of reps and I’ve taken the magical three by 10 approach in my example here.

So. Our body is physically capable of doing 12 reps, but we only do 10. So we have what’s called incomplete recruitment. So we’ve recruited some muscle fibers to do the job because we’ve been doing exercise. It’s been progressively getting harder with each repetition. My effort level has gone up.

The stimulus that I’m sending, the impulse that I’m sending from my brain to my motor neurons has increased. It has recruited some high threshold motor units, but not all of them. But the problem therein lies, because after my 10 repetitions, I put the weight down and I rest and I recover. And in recovering, I’ve allowed those type one muscle fibers to completely rest and completely recover to be able to be re -recruited. So now I begin set two, but actually I’ve kind of taken two steps forward, but one step back. So I’m going to do.

allowed some muscle fibers to recover. So now when I start the exercise set, I’m effectively starting over. So it takes me another 10 reps to get to a point of similar recruitment. I do 10 reps, even though I could do 12, and I still have incomplete recruitment. So I’ve done two sets. I’ve done twice as much work as I would have done beforehand, but I still haven’t got to the same place of recruitment because I’ve allowed muscle fibers to recover.

So now I get to my third set. And again, between set two and set three, I might take a break. I might take typically two minutes. And again, I’ve allowed some of those muscle fibers to completely recover. So now, again, I’ve taken two steps forward, but one step back. So I begin my exercise set, and now I might do my 10 reps. And because of fatigue of some of the other muscle fibers, I might still get to the same point of stimulus, the same point of recruitment as I would have with my single set to momentary failure or to sufficiently high effort.

But it’s taken me three times as long to get there because I kept stopping at submaximal effort level with incomplete recruitment. And then I allowed muscle fibers to recover in between the sets. And hence why there’s this kind of debate about single and multiple sets. And as we’ve talked about previously, they actually tend to get us to the same place. There’s almost no difference in our adaptive response. But our problem lies if we have a recovery time between our multiple sets.

We need to repeat the exercise just to get to that high recruitment level.

Perfect. Yeah, that makes a lot of sense. And if muscle fiber recruitment is our goal, we don’t want to stop recruiting muscle fibers multiple times during the exercise before we get to the end where we’ve recruited all of them because that is our ultimate goal, right?

Dr. Fisher, are you familiar with the toy Lightbrite?

I’m not familiar with Lightbrite. Okay, so in the 80s, when I was growing up, there was a very popular toy called Lightbrite. And it is a grid of little holes. that comes with little plastic colorful beads that you stick inside the holes and then behind all of the holes is a master light. And so you would make these designs with your beads on the front by placing your beads in different places and then turn the light on. It was such a delight to see your artwork.

So what this visual that you have here in the podcast, and if you’re not listening, if you picture a light bright and you’re familiar with it, picture the whole light bright board filled in with the beads. and turned on with all of the beads lit up, right? It’s kind of like that’s the ultimate goal is to recruit as many as you can and light them all up as much as possible. You don’t want you know, a bunch of them turning out. But when you take breaks or when you let too much time go by, it’s like a bunch of these lights go out. It’s like pulling a bunch out when your goal really is to recruit all of them.

And so that’s kind of the analogy that I that comes to my mind. So, Dr. Fisher, a quick follow up question, too. So your you just explained basically splitting up an exercise into multiple sets. You gave the examples example of three sets of 10 with this. Would this dimming of the muscle fibers or sort of like unrecruitment of the muscle fibers that we’re trying to recruit during a set still happen as well if we took too long of a break within one set?

So let’s say we were doing one set and then we just took 10 seconds off. Yeah, absolutely. So this is, this is where it starts to get interesting. So we can, of course, if you do a repetition and then you pause, uh, for, you know, a couple of seconds and then repeat, there will be some degree of recovery. Uh, now with the exerbotics devices, of course, there’s, if you pause, if you unload the muscle, either the extended or the flexed position at the end of the concentric or the eccentric position, you know, if you see that bar drop right down because you’re not pressing anymore. that’s resting.

You know that you’re doing that, you know that you’re resting. So that’s allowing for a degree of recovery. So there will be some degree of recovery there, but it’s of course momentary. So how much that actually allows a muscle fiber to recover is quite questionable. It’s not the same as taking two or three minutes rest in between sets. Now we can even go one step further than all of this.

And we can talk about the difference between a free weight or a selectorized machine and the isokinetic devices of the exercise coach. Because when we talk about the eccentric component of an exercise repetition, we know that we’re 40 % stronger in the eccentric phase. So even when I’m pushing almost as hard as I can to lift a weight, when I lower the weight, I’m much, much stronger. So the weight is still sub -maximal on the way down. So some muscle fibers are derecruited in that time, they’re allowed to switch off and essentially recover. But with the exerbotics devices, as you will have guessed, noticed in your workout earlier today, in the eccentric phase of the workout it’s actually harder that the motor plate is coming back towards you with more force.

so you can now push on it harder, and it’s around 40 % more effort. But that maintains that high level of recruitment, and in fact, it maintains the high level of type 2 muscle fiber recruitment, which are the key muscle fibers that we’re trying to stimulate. to stimulate to get stronger and to grow bigger. And they’re the ones that we typically lose as we get older. And they’re the ones that we obviously want to train and provide stimulus to, to adapt, to retain. So somebody the other day asked me about going for walks and going for a jog about, you know, staying fit and recruiting muscle fibers.

And I said, you know, Of course, we should definitely go for a walk and definitely go for a jog, they can be good for you. But if we’re not strength training, we’re probably not getting to that point where we recruit those type two muscle fibers. And as I said, they’re the ones that we lose.

So this method of strength training, specifically with exerbotics, but for the most part with high effort is what recruits those type two muscle fibers to adapt, to get stronger and helps us to keep hold of them. Yeah. At our exercise coach studios, oftentimes we’ll educate clients. You know, you just mentioned we lose our type two muscle fibers as we age, and those are the ones most responsible for our strength, our balance, our stamina. And so when we notice a lack of strength as the older we get, It’s because we are losing those type 2 muscle fibers. I mean, we still have our type 1 active to get around from move to point A to point B, right, to take that walk or to go to the grocery store.

And we’re not losing that capability that quickly as we get older. But what we’re losing is the strength, which we have to recruit type 2 muscle fibers for. One other thing we often point out, which is what you were just talking about, too, with the eccentric demand on our muscles where we are stronger. Oftentimes, we love to point out the first time somebody does a leg press exercise, they complete their set, they give a little bit higher effort on that eccentric because they just can. They can give a little bit more effort because they’ll be stronger there.

At the end of the set on the screen, they’ll be able to see their concentric average effort and their eccentric average effort. And oftentimes we will see, just for an example of like a young woman, for example, maybe she’s at 75 average strength index on her concentric, but she’s like 105 on her eccentric. And so a very simple comparison we can point out is, you know, if you were doing this, like press on a weight stack machine where you had to just choose one weight to work at the entire time, you would have to select 75, just to begin the exercise, but on the way back, you could be working at 105, but you’re leaving that on the table because you’re stuck at the 75.

And that really helps people to understand like, okay, I’m activating more every single second of this exercise, which is part of the reason it makes me stronger faster.

And so I hope that helps you, you know, picture If you are an exercise coach client, if you experienced the power of eccentric training at a studio, why that is.

Fantastic. Fantastic. So my takeaway points from this, uh, this session were that our stimulus is recruitment and it’s essential for adaptation. It’s strength training to high effort that recruits all available muscle fibers. And multi -set strength training allows recovery between sets.

So if we’re lifting the same weight, it just allows some of those muscle fibers to recover, so we end up re -recruiting them, and it just delays full muscle fiber recruitment. So, you know, it’s hopefully a relatively simple concept, and hopefully the slides will have helped the viewers to understand all of that. Yeah, they were awesome, Dr. Fisher. Thank you for breaking this down for us. Remember, when you work out, Get those muscle fibers recruited. The best way to do that is by loading them continuously at the right level of effort.

So go get your workout in today, and we will see you next time on the podcast. Until then, we hope you remember, strength changes everything. Thanks for listening. If you enjoyed today’s episode, please share it with a friend. You can submit a question or connect with the show at strengthchangeseverything . com.

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