The Activator of Muscle Fibres

Muscle growth has a signal, this is easy to understand.

If you’re reading this hopefully you’ve read all about the signal for growth here.

Stimulus isn’t as simple as a signal though. A stimulus is made of two parts, the signal, and where the signal is. This will be all about what recruits muscles’ which experience the signal for growth.

Let’s break upon the second part of the muscle growth stimulus.

By now you know a lot about the signal

• You know the two types of mechanoreceptors and how they’re triggered

• You know the two different ways hypertrophy is expressed

• You know ways to manipulate these two different stimuli to get more/less

• You know that only active fibres experience hypertrophy and there’s a missing piece you need to understand

Let’s get that understanding.

Similar to the post about mechanical tension this will cover both types of contractions, but first.

There’s a few things I’m going to teach you about to fully understand motor unit recruitment (in a way that benefits your training):

• What motor unit recruitment is and how it actually turns on muscle fibres

• How motor unit recruitment appears (or doesn’t) in concentric, and eccentric contractions

• The things that can reduce the amount of motor unit recruitment possible

• The things that can increase the amount of motor unit recruitment possible

• The difference between motor unit recruitment, and motor unit recruitment in a muscle/region.

Motor unit recruitment is a simple system which allows our body to perform complex movements.

In order to be efficient the body has a system which provides regions of muscle with motor neurons which are in charge of muscle fibres.

When muscle is required the central nervous system will send electrical signals (called action potentials) to motor neurons and the muscle fibres they govern will activate according to the speed and intensity of the electrical signals. this is something that can be triggered with external electrical signals as well which is cool

A muscle is either on, or off. There’s no 50% activation. It’s always 100%

Side note, this is the reason why you need to be seeing involuntary concentric slowdown no matter what, even if you’re intentionally doing slow reps.

So, you understand that muscle fibres are going to be contributing the same amount no matter what, now there’s another piece.

If a movement requires more force, motor unit recruitment will increase in order to fulfil the demand.

This is where you’ll see how efficient the body is.

You have two types of muscle fibres in the body

• Type 1 fibres are called slow twitch, they don’t experience fatigue easily, and don’t have as high of a force output potential as type 2

• Type 2 fibres are called fast twitch, they experience fatigue easily, but have a high force output potential

It would be inefficient to use these type 1 fibres when there’s low demand, they’d suffer fatigue and damage for no real reason.

This is why the body is setup to recruit type 1 fibres before type 2.

Even in explosive situations, the slow twitch fibres are still being recruited, the body doesn’t have a way to signal out fibres from the low end of the motor unit pool, the only difficult part is reaching up onto the high end of the motor unit pool.

Here’s what’s so interesting. Motor unit recruitment isn’t even always voluntary.

You might have not noticed it but throughout the day you’ve been keeping your head up, keeping your spine straight, walking around, supporting yourself.

Standing up doesn’t have any voluntary effort unless you’re intentionally changing your posture, this is because there is a level of involuntary motor unit recruitment which keeps all of these muscles at a level where they support your posture.

The super cool thing about this is that more weight/moving around doing things we don’t even consider as working out ends up providing a signal to those active fibres.

Here’s where it gets even cooler.

The force velocity curve I showed you focuses on the concentric side (what we know to care about most) but it goes a step further when we take a look at the entire curve.

Regardless of the weight you’re resisting, an isometric is when the motor unit recruitment recruits the exact amount of muscle fibres required to prevent any movement at all.

The force from the muscle is equal to the force acting on it.

But remember this is just what turns on a muscle fibre, we explored the signal of muscle growth in a previous article here.

Even if the fibre is active, doesn’t mean you’ll grow. Explosive movements like sprinting or jumping can have almost maximal motor unit recruitment, but that doesn’t mean you’re going to be growing like crazy from them, they need that growth signal.

So this section has taught you a few things:

1. Motor unit recruitment recruits motor neurons which govern muscle fibres

2. Muscle fibres are either all on, or off, there’s no in-between

3. Henneman’s size principle describes how the motor neurons govern muscle fibres, depending on the effort required more fibres are recruited, with all the ones below it on the motor unit pool staying active

4. An active muscle fibre is not enough to make the muscle fibre grow

5. Motor unit recruitment can be both involuntary and voluntary

It’s time to go over concentric and eccentric contractions.

When we spoke about the growth signal we looked into how these two differed.

By now we know exactly how motor unit recruitment works with concentrics.

• When we resist a force we recruit motor neurons and activate muscle fibres due to motor unit recruitment

• If we exert more effort we’ll have more motor unit recruitment

• If we exert less effort we’ll have less motor unit recruitment

• These activate fibres can experience growth if they experience enough mechanical tension

Where it gets a little more complicated (again) is with eccentrics, although it shows even more pitfalls (again) on something we don’t want to target (again).

If you remember, eccentric contractions involve Titin.

This is a spring like structure which can generate passive tension.

In an eccentric there will be some voluntary motor unit recruitment which resists the force, but there’s also a passive recruitment of muscle fibres which skips the whole motor unit recruitment stuff…

Yeah this sucks 😭 

See the passive tension generated by Titin to contribute to the movement is done kinda, randomly?

The muscle fibre either does it, or it doesn’t. It’s really based on how much it’s lengthened, and if that’s challenging enough.

The problem here comes in if we wanted to target that lengthened adaption of sarcomerogenesis we mentioned the other day, because it doesn’t stop there.

Not only do we get involuntary passive Tension from Titin and have no control of which muscle fibres contribute, our actual motor unit recruitment here is a lot lower too (because you’re stronger in an eccentric by about 100%) which means we end up getting half the activation…

You might remember this being one of the reasons I mentioned to not worry about this type of adaption, it’s because it’s just a pain.

Yet again, if you want to get this adaption don’t worry about motor unit recruitment, instead just focus on passive stretches.

Now that I’ve got this part out of the way we can talk about what’s actually hurting, motor unit recruitment, and how you can get more. (fun stuff that helps you)

Things that Reduce Motor Unit Recruitment

The simple way to answer this is with one word fatigue.

That doesn’t really mean much to anyone though.

Fatigue can show up in many different ways. It can be caused with different mechanisms. It can last, and effect, differently. it can even be imperceptible

We’ll split fatigue into two broad categories, short-lived exercise induced, and sustained exercise induced.

Trust me, you’ll be thanking me on the distinction later.

Short-lived exercise fatigue

The funny thing about short-lived fatigue is that it’s what most people consider to be the things going on with sustained fatigue. 🙄 

I’m going to give you the knowledge to stop you from making the same mistake.

Ever thought, “today’s workout was super fatiguing and hard, that’s going to last a while because my CNS is fried”.

Probably.

You’d be dead wrong.

This isn’t what sustained fatigue is, and I’ll cover that next.

For the most part when you talk about a workout that’s fatiguing, a set that’s hard, or a weight that challenged you, you’re talking about your perception of effort.

This is what supraspinal fatigue (CNS) is.

1. When your CNS sends electrical signals to recruit motor neurons which activate muscle fibres it’ll create a signal called corollary discharge.

2. This corollary discharge is a signal which travels to a different part of the brain and increases your perception of effort.

   1. Interestingly, the muscles don’t have anything travelling to the brain to say “this is hard”, it’s the fact you’re sending signals to get them to work in the first place that trigger that feeling.

3. This perception of effort starts to reduce your motor unit recruitment (in most cases) as exercise performance is HEAVILY motivation driven.

4. This corollary discharge is cumulative, when we reach a place where we’ve met the highest effort we can perceive as tolerable, we can no longer continue.

Here’s what you might find shocking about supraspinal fatigue.

The majority goes very quickly. i’m talking minutes.

In a vacuum, this is the main thing that’s causing you to end your sets, but we have a few more things in practice.

1. If you start to feel the burn, or pain in a muscle your perception of effort will increase, and will potentially limit your motor unit recruitment.

2. If you start to get high cardiovascular demands, your perception of effort will increase, and will potentially limit your motor unit recruitment

Notice how none of these things are actually a problem with the muscle.

Even the cardiovascular side is never reaching a point where the lungs/heart is incapable of continuing, they’re just making it feel harder.

Look, this is a big part of why we take rest breaks.

Get rid of the corollary discharge, get rid of the burn, have your cardio recover, and you’re ready to go!

But sustained workout fatigue will also have a part to play here.

Sustained exercise fatigue

One quick thing to point out before I open this can of worms, sustained exercise fatigue doesn’t have to be from strength training.

A bulk of it will be, but cardio, everyday activity, etc, can effect as well.

Sustained workout fatigue is where mainly muscle damage lets out inflammation markers called cytokines into the bloodstream. These markers influence the CNS by limiting it’s ability to send motor commands, and increasing perception of effort.

If you do a super damaging workout one day, and then go to the gym and try and train a different muscle part, you’ll notice it’s not as productive as usual.

This is one part of what people think is happening when they have an insanely difficult workout and assume that the perception of it being hard is what causes prolonged fatigue (when in reality is was the muscle damage).

Muscle damage has a great effect on subsequent workouts, it’s something you can directly test. It limits your ability to recruit more muscle fibres.

Less muscle fibres = less growth, that’s plain and simple.

Here’s some of the big things that cause more muscle damage.

• More reps

• More lengthened position work

• Slower eccentrics

• Slower concentrics

• More sets

• More exercises

• Super damaging cardio (eccentric heavy stuff like running, running downhill, or running fast)

• Training closer to failure

• Having more motor unit recruitment yes really, however, not bad thing inherently. 

See we can have more muscle damage for good reasons, and bad reasons, it’s just about making sure we’re not seeing a level of damage that actually takes away from our ability to get maximal motor unit recruitment.

Some of this stuff is good albeit most aren’t.

Okay fine I’ll pick out the good stuff just to make sure people know.

• More reps if it’s allowing us to get more stimulating reps

• Lengthened position work if it’s to better target a muscle

• More sets if we’re talking about in a set range that actually makes sense

• Training closer to failure if we’re talking about being able to get the growth signal

I don’t need to explain the having more motor unit recruitment one…

The thing that people don’t always understand here is that certain muscles (especially the smaller ones) enable you to have a higher motor unit recruitment because there’s less muscle fibres you need to recruit (whoops that’s spoiling the next section). These exercises can actually end up being super damaging (not something to spam).

Bare in mind that the reason I give certain recommendations are for this exact reason, sustained fatigue is one of the bigger things that prevent a good stimulus.

And, no, muscle damage isn’t “I don’t feel sore/I feel good”, most of the time it’s imperceptible.

So this section should have taught you a few big lessons:

1. An exercise feeling difficult doesn’t actually mean it’s going to create a ton of long term fatigue

   1. Reducing the burn or cardio demand can help keep motor unit recruitment high

2. Rest periods are important in you workout to make sure you keep motor unit recruitment high

3. The biggest limiter for motor unit recruitment is muscle damage, to keep this low I should learn all about

   1. Stimulating reps

   2. Volume

   3. Reps

   4. What increases motor unit recruitment (next 😉)

Things that Increase Motor Unit Recruitment

This is the fun bit where we actually get to talk about fun ways to improve a workout, less the stuff that ruins it.

For starters I’m going to talk about a couple of things that I could have got away with throwing in the previous section.

1. Mental effort

2. Muscle mass involved

You’ll see why it’s probably better to put it in here.

See everything before was about stuff you shouldn’t do/should do less of (other than rest breaks) okay maybe I just suck at structuring things 🙄 

Point is this is going to be about things you can implement.

Some lifts have a higher mental effort, if your feet don’t touch the ground, if you have to balance more, if the movement is more complicated, your perception of effort actually increases.

If you make exercises more stable, or less complicated, this mental effort goes away.

This isn’t the only way instability/coordination demands effect things though.

Some exercises will cause us to have to involve more muscle mass, conveniently goes into our next point.

When you recruit more muscle mass even if you have the same levels of motor unit recruitment you’ll have to divide this motor unit recruitment up between different muscles (meaning less active muscle fibres in the area you want to grow).

However you’d likely see less motor unit recruitment as this would appear in exercises that are less stable/need more coordination.

What I’m really trying to say here is pick stable exercises that are brainless.

This can even be something as simple as adding chest support/holding onto something with a free hand to brace yourself.

Now for PSYCHOLOGICAL WARFARE

Let’s increase your motivation.

The best way to increase motor unit recruitment is by being highly motivated.

Perception of effort limits motor unit recruitment, what you deem to be the most you can tolerate effort wise stops you, and this is entirely psychological.

Your brain is thing that governs motor unit recruitment, if you’re not motivated enough, if you don’t want it enough it’ll be less, plain and simple.

Step 1. want it.

It is that simple but I’ll break upon other stuff we can do.

1. Listen to music, pick a song that’s upbeat and has a high volume/tempo. I shouldn’t need to tell you how impactful this is.

2. Competition/socialise, if you’re with a group of friends, or trying to beat someone, you’re instantly highly motivated. Think of all the WRs that were pulled off purely because they had a live audience hyping them up, cheering them on.

3. Autonomy, if you have more control over your exercises, or your timing/diet etc, if you have this control in your journey, you’re motivated more, you’ll be stronger. It’s why I’m giving you a workout skeleton.

4. Performance accountability/feedback, if you record your workouts and show people, they’ll judge you. You’ll instantly up the effort, weight, intensity. You won’t be sandbagging. Even if you keep this private you’ll be judging it.

5. Goal weights, imagine you have a goal for a workout. X weight Y reps. If you tell your self that you need that, that you’ll get it, that you’ll fight for it, you’ll make yourself achieve it. Even if we throw on extra steps like reps from failure, or form, you’ll understand they’re important to achieving the goal, and follow the parameters. Track your workouts.

6. Mission/goal, if you have a clear goal in the gym, there’s a mission you’re working towards, you’re leveraging it with mantras, you’re building everything around a concrete, clear goal, that matters, it’s going to happen.

If I forced someone to sit down and read only one part of this email, it’d be the part on increasing motor unit recruitment.

Although muscle damage and fatigue sucks and will reduce it, focusing entirely on this side will be what makes the most difference.

For our finale there’s one last thing to cover, I’ll have a super long big email explaining this in great detail, but that’s not important for this one email.

Neuromechanical matching.

This is a principle that suggests that the CNS recruits muscles/regions of muscles based on their leverage in order to minimise effort.

This is a principle I wholeheartedly believe in. I’ll speak way more about it over and over, and have already in secret on my emails, for now here’s what you need to know when it comes to motor unit recruitment.

As explained when talking about how multiple muscles can effect the amount of motor unit recruitment a target muscle gets, the same goes for regions.

In some muscles you’re able to bias regions (chest/lats/etc) even if motor unit recruitment stays the same.

This is an important thing to start thinking about as understanding motor unit recruitment means knowing that even if the deadlift uses all muscles, it’s not going to be good for growth.

Most of the time, you’re not even activating all possible muscle fibres of a SMALL muscle like the biceps in a bicep curl. Even if we have maximal stability, low coordination, and high motivation.

This is something to bear in mind as you program, and chose exercises. There’s only so much motor unit recruitment to go around.

As important as the growth signal is to the stimulus, you might consider this wayyyy more important. i don’t because they’re equal but everyone has a different perspective

You need to at least be paying attention to the psychological side of training, it’s not all about moving heavy weight.

Remember:

• Only active fibres can experience growth

• Active fibres don’t care what the weight on the bar is, just what they resist

• The amount of fibres you activate are heavily influenced by fatigue, and motivation

• The type of exercise will determine how the motor unit recruitment gets divided between muscles

Your Hypertrophy Hero,
Fletcher

P.S. This was a super long video so I really hope you enjoyed. If you’re looking for something to implement immediately, first make sure fatigue is in check, then focus as much as possible on motivation. More motivation = more motor unit recruitment = more muscle damage, so you don’t want a bad program to be involved here. Go follow my skeleton and build a great one.