Nervous System Training – Muscle Fiber Recruitment and Rate Coding Explained

By on April 23, 2019

Have you ever thought about what happens when you hold an isometric contraction?

Pick up a dumbbell in one hand, then hold it with your arm at a 90-degree angle. What is actually going on inside your muscle to make this possible?

Muscle Fiber Recruitment

You may be forgiven for thinking that your muscle is holding ‘half a contraction’ – that is is exerting just enough force to close those muscle fibers ‘half way’ to fix the arm in that position.

But NOPE. The human body is far more complex than that, and understanding why that is can help to give you an advantage in the gym, in the ring, and on the street. Maybe not that last one.

Basically, by understanding how your muscle works, you can train smarter.

Tong Zi

A Bit of Muscle Anatomy

Inside any muscle, you have hundreds of thousands of tiny muscle fibers (study), that are just thinner than human hairs. To simplify matters a little, these fibers contract by working a little like telescopic poles. That is to say that there are sheaths (actin) that can close over the top of the inner bits (myosin). It’s really more of a chemical reaction but this is easier to visualize.

To trigger this contraction, a nerve needs to transmit a small electrical impulse to those fibers across the neuromuscular junction. This impulse is called an ‘action potential’. Of course not every single muscle fibre can be controlled by individual nerves, so instead a single nerve will take care of a group of fibers: collectively referred to as a ‘motor unit’. Motor units generally only contain one type of muscle fibre: whether that is Type I, Type IIa/b.

Biomechanics

The key fact to consider here, is that action potentials, motor units, and muscle fibers are binary. That is to say ‘all or nothing’. Only once excitation passes a certain threshold – once the stimulus exceeds a certain point – will the nerve ‘fire’ and create an action potential (the technical term for an electrical signal).  This action potential is what triggers a contraction. When that happens, all of the muscle fibers belonging to that motor unit will contract, and then they will instantly stop.

So the notion that you can hold a weight half-mast by ‘half’ contracting muscle fibre is incorrect. It is all or nothing. And there’s no way for muscle fiber to ‘remain’ at a mid-way point.

Childish movements Shaolin

Not only that, but the action potential will only last for around 10-100ms depending on the muscle group in question.

So the idea that your fibers are ‘half contracted’ and staying there in order to hold a weight in place, is the wrong way to conceptualize this. In that case then, how the heck does this work?

Motor Unit Recruitment

When you lift a heavy weight, or attempt to jump off the floor, your brain will ‘recruit’ as many motor units as necessary to produce that amount of force – but never 100% at all times.

As the amount of force required increases, so do the number and size of motor units being used. In agreement with Henneman’s Size Principle, you will start by recruiting the smallest motor units first – which utilize fewer, slow twitch muscle fibers – and only progress to the larger ones as the challenge demands it.

Neurons brain plasticity

This is how we are able to smoothly increase the amount of force we exert. By gradually increasing the number and size of the motors involved in the movement. This also means though that the step change becomes more sudden as you reach higher levels of force: you can increase the force you use to move a tool to minute degrees. But if you’re already giving 80% of your max force during a bench press, the next ‘step up’ is going to be much larger. It’s why you couldn’t pain the Mona Lisa using a 50KG pen.

There are other reasons that wouldn’t work too…

Smaller motor units that control slow-twitch, oxidative muscle fibers also do not fatigue in the same way as explosive, fast twitch fibers. This means that you can sustain a contraction much longer, and in fact these motor units fire randomly all throughout the day to provide muscle tone. This prevents atrophy, helps us to maintain our posture, and also reduces slack in the muscle to allow us to exert force more quickly. Normally the resting position of a muscle is also its strongest position.

Jump height

During explosive movements though, your ability to recruit motor units is just one of the factors contributing to your overall strength; and the ability to recruit more of your larger fibers can essentially allow you to tap into additional strength that is already there. By improving recruitment, you should be able to exert considerable extra force. This is why you can increase your strength without necessarily increasing muscle size.

Except recruiting 100% of your muscle fibers at a time actually isn’t necessarily the most efficient way to exert force. This would lead to very rapid fatigue, and it may even cause injury seeing as muscle responds to training faster than tendon.

Jump height Nightwing

Tetanic Contractions

So that’s how you can gradually increase force, but how do you go about holding something in place? If an action potential is only able to last that long though, how are you able to maintain a single position for minutes at a time?

When a muscle contracts in response to an action potential, it is called a ‘twitch contraction’. This lasts around 100 milliseconds and includes a short latent period (a roughly 10ms pause prior to the contraction) and a roughly 60ms relaxation period as tension is released.

What’s really going on when you hold something in place, is that your motor units are twitching extremely quickly in order to provide continuous tension. In fact, they can twitch before they have even finished completely relaxing. Meanwhile, asynchronous firing of different muscle fiber throughout the muscle helps to ensure there is no interruption to the signal. This may be why synchronized contractions across all motor units actually don’t yield the most force as you would expect (study, study).

Overcoming Isometric

The result of this rapid pulsing is what is known as a tetanic contraction: the sustained contraction. And that’s why if you look closely, you aren’t actually locked in place, but rather twitching and vibrating as your muscles fire rapidly to hold the position.

If you are using a yielding isometric contraction – attempting to hold a moderately heavy weight in place – then as your motor units are fatigued, your body will gradually begin to recruit other varieties of muscle fiber: such as fast twitch fiber. Now the weight begins to shake and wobble, as large, explosive contractions attempt to hold it in position. Eventually those fatigue too, and the weight begins to lower in an eccentric movement as you no longer generate enough force to keep everything in place.

The speed of these contractions is another factor that contributes to overall strength then. This is rate coding – the way in which the central nervous system encodes intensity using a digital system. The more rapidly your nerves fire (discharge), the greater the overall force. This can actually be trained, and resistance training has been shown to increase discharge rates after just four weeks of isometric contractions (study).

Rate coding is also used to convey information regarding intensity across the sensory neurons in the brain. The stronger a stimulus such as light, the more rapidly the neuron in the eye will fire. Rate coding is not unique to muscle then, but is simply the way the nervous system codifies intensity.

Putting This Into Practice

So now you know all this, how does it impact on your training?

Well, we can easily see that the effect that something like a plyometric movement will have on the neural component of training will be completely different from the effect of a heavy eccentric.

In order to recruit the largest, fastest motor units – which will lead to the most hypertrophy and increased mind-muscle connection – we need to exert maximum force. Those large motor units have higher excitation thresholds, meaning that they need a big challenge in order to be recruited. This normally means either using explosive movements, or very heavy weights (usually >90% 1RM).

Intermittent Fasting Testosterone

This is also why overcoming isometrics – pushing or pulling against immovable objects – are great for increasing muscle fiber recruitment. You are attempting to produce maximum force, which requires the engagement of the largest motor units and their sustained tetanic contraction.

Another way you can engage those motor units though is by using something like a drop set, or a very lengthy yielding isometric exercise. We’ve seen that by fatiguing the slow twitch fibers, you are this way able to force the body to recruit the larger motor units – even when using lighter weights. But the recruitment pattern here is entirely different, as you are using a smaller percentage of your motor units at any given time in a bid to maintain a more constant amount of force. This is why as you tire, you will struggle to curl a barbell slowly, but may be able to ‘speed’ your way through it by calling on those as-yet-unused fast twitch fibers. It’s like a CNS form of pre-exhaust!

When performing yielding isometrics this all happens in an automated fashion. You simply try to maintain the position, and your body calls on everything it has to help you do that. This is a useful skill in itself – fine control over the amount of power you exert – and it’s one very much worth training with isometrics. Remember too that most yielding isometrics will end with an eccentric contraction as you slowly yell “noooooo”.

When using something like a slow eccentric movement – gradually lowering a very heavy weight – the pattern of recruitment is different again: here you’ll gradually be disengaging motor units to reduce the amount of force exerted. Because you’re able to use weights heavier than you could lift (called supramaximal training) this is also excellent for increasing your maximum strength and may help to develop stronger tendons. ‘Functional isometrics’ which are very short range of motion movements, similar to heavy partials, can do the same thing.

Plyometrics on the other hand will help you to more quickly recruit fast twitch muscle fiber to increase your starting strength. Can you build muscle and increase explosive strength just by moving your limbs extremely quickly without any resistance at all? Probably not, due to the lack of cross-bridging that occurs at the very maximum speeds. The precise relationship between force and velocity is referred to as the force-velocity curve.

These types of training all meanwhile have entirely different effects on hypertrophy (muscle growth). Fast movement alone is not enough to encourage hypertrophy due to the absence of muscle damage, metabolic stress, or tension necessary for hypertrophy.

Then there’s the effect they have on tendon hysteresis, on the conversion of muscle fiber types between type IIa and IIb (and possibly I and II). Then there are the differences in the ways that different types of training will fatigue the nervous system.

Electromyostimulation

The take home then is that thinking about your central nervous system while training is important if you want to see the adaptations you’re aiming for. Something as simple as changing the speed of your contractions, or isodynamics into your sets can drastically alter the control you have over your muscle.

I’m going to be discussing all this in more detail soon, as there is much more to talk about – such as the effect that electrical stimulation (EMS) could have on recruitment and rate coding. This means running a current through the muscles artificially to stimulate a contraction. As you can imagine, this will work entirely differently to an authentic ‘endogenous’ contraction, as it will cause indiscriminate recruitment and predefined rate coding.

Some interesting research suggests that EMS can actually increase the number of acetylcholine receptors at the neuromuscular junction, thereby perhaps enhancing the mind muscle connection (study). Stay tuned for more on that, as well as more forms of CNS training.

Your Body Is Amazing

Finally, just take a moment to appreciate the amazing complexity of the human body. Something as seemingly simple as reaching for a glass of water in fact involves the contraction of thousands of muscle fibers across multiple muscle groups. Your brain handles the necessary muscle recruitment pattern to apply the correct torque and force to multiple joints, such that your hand can move accurate through space. It then recruits only the necessary motor units in the hand and arm, firing at a constant rate to allow for the necessary tetanic contraction such that you are able to grip the handle. Countless systems are in place to reduce the computation load necessary for such a feat: the myotatic response that allows muscle to shorten and maintain its resting length, low-level firing of slow twitch muscle that allows it to maintain tone, and the size principle that automatically recruits only the necessary amount of force.

We think that we control our skeletal muscle in a conscious manner, but in fact we have no idea!

 

 

 

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About Adam Sinicki

Adam Sinicki, AKA The Bioneer, is a writer, personal trainer, author, entrepreneur, and web developer. I've been writing about health, psychology, and fitness for the past 10+ years and have a fascination with the limits of human performance. When I'm not running my online businesses or training, I love sandwiches, computer games, comics, and hanging out with my family.

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