What You Didn’t Know About Muscle Fiber Types and Your Central Nervous System

By on September 3, 2015

I know you’ve probably heard it all before, fast twitch and slow twitch muscle fiber yada yada… It’s been done to death and for the most part it doesn’t much affect your training. Right?

Well actually, I’d wager there may still be a bit you don’t know about your muscle fiber… and that some of that stuff is very relevant to your training if you’re at all interested in strength. Did you know for instance that the speed with which you can recruit motor units is more of a limiting factor than the number and type of muscle fibers you possess? Did you know that you can create new muscle fiber? Or that you have fast and slow twitch nerves as well?

Read on to find out just how much there is you don’t know about muscle fiber and how you can use this to potentially, greatly  increase your strength.

Note: I have just updated with a few references at the end…

Muscle Fiber Type Explained

Before we get into the more advanced stuff, let’s get the basics out of the way first…

Our muscles are made up of thousands of tiny bunches of fibers, which in turn act like little pistons with a telescopic compression and extension ability mediated by actin and myosin. Sounds complicated but all you really need to know is that muscle contractions come from thousands of smaller contractions controlled by these fibers.

These muscle fibers are not all born equal though and can be broadly divided into type 1 (slow twitch), type 2a (intermediate) and type 2b (fast twitch). Sometimes ‘fast twitch’ will refer to both type 2a and 2b, with 2b being referred to as ‘super-fast twitch’. Other times, type 2b is called ‘type 2x’. As though this wasn’t all complicated enough already… It’s all to do with the method of ‘muscle fiber typing’ used. And while type 2x is the more accurate description for what occurs in humans, the majority of articles simplify it as type 2a and 2b, so that’s what we’ll go with here…

Essentially, type 1 fibers are the kind that specialize in low force, power and speed but have very high endurance. In other words, you can use them all the live-long-day without getting tired out. Type 2b meanwhile are used only in moments when you need explosive, rapid power. These specialize in power through acceleration – such as when you need to leap into the air. Finally, type 2a are somewhere in the middle and are used for things like weight lifting if you’re at 80% of your 1RPM.

Different Muscle Fiber for Different Occasions

One many people don’t realize about muscle fiber is that it either fires, or it doesn’t. In other words, there is no such thing as a partially firing muscle fiber or a ‘half firing’ muscle fiber. This is a binary signal and the strength of your lifts and punches is only controlled by the number of the fibers that fire at once and their type – not by ‘how far’ they are contracting. Muscle fibers meanwhile are clumped into groups known as ‘motor units’. These are small bundles of fibers that always fire together. The more motor units fire at once, the more power you’ll exert.

What’s also key to realize is that your body only uses the fibers it needs and always tries to be as energy efficient as possible. So for instance, if you were to turn the page of a book, you would recruit just a few smaller motor units comprised of mainly slow twitch muscle fibers. This would then produce just enough force for you to turn that page. You could do this all day as the slow twitch fiber rarely ever fatigues. If you were trying to get a barbell off the ground though, only then would you recruit your type 2a and type 2b fibers and the bigger motor units. You could do that for a few reps, create some microtears (to thicken those motor units) and then tire out after 8-10 reps as the fast twitch fibers would give in. Even after that though, you’d still be able to lift the page of your book, because the slow twitch fibers are just that effective.

A sprinter, martial artist or a powerlifter would want as much type 2b fiber as possible to perform, fast, explosive movements. A long-distance runner on the other hand though would need more slow twitch fiber in order to run long distances without fatiguing. Bodybuilders would want fast twitch fiber mainly, as this is the type of fiber that is most susceptible to microtears and most likely to grow. Everyone needs both types of fiber though – and interestingly we also require different muscle fiber types in different parts of the body. The pecs for instance are made up of fast twitch fibers largely for pushing and punching things, whereas the erector spinae (the muscles that keep our backs up straight) are made up of slower twitch fibers so we don’t collapse after a long commute.

Thus a lot of sports coaches and athletes focus a lot of energy on trying to convert their muscle fiber types. While it’s (probably) not possible to convert type 2a fiber to type 2b, what you can do is to turn type 1 into type 2a. Additionally, you can make type 2a fibers a little ‘quicker’ or conversely you can make type 2b fibers a little ‘slower’. Instead of thinking of these types as being rigid categories then, it makes more sense to think of them as points on a spectrum. Either way, your body will adapt to the type of training you use and this (combined with genetic predisposition) is why sprinters have 75% fast twitch muscle fiber whereas long-distance runners have 75% slow twitch muscle fiber. All of us do have a genetic preference for certain types which dictates roughly 40% of the variance. 45% meanwhile is thought to be changeable via exercise and lifestyle factors.

Converting Muscle Types

So how do you increase your fast twitch muscle fiber? The main strategy is to combine power with explosive speed. Normally weight lifting does not incorporate enough explosiveness to really challenge the type 2b fibers – type 2a will do just nicely. To really challenge the body to be explosively powerful, you need to train in that manner. One way to do this is with plyometric movements – exercises like box jumps and clapping push ups that challenge you to really explode into the motion. Lifting close to your 1RM (1 Rep Max) in an explosive manner does something similar.

The other strategy is to use ‘speed lifting’. This means performing a set of exercises as quickly as possible. So instead of having a 2-1-3 cadence (two seconds on the concentric portion of the movement, one second pause at the top, three seconds down on the eccentric), you now have a 0-0-0 cadence. Now you are training for force and velocity.

Bruce Lee actually used this type of training and would perform a relatively standard ‘bodybuilding routine’ but with an added element of speed and explosiveness. Bruce Lee quite likely had more fast twitch muscle fiber in his upper body than pretty much anyone – hence his ability to steel a coin from your palm and swap it for another coin before you could close it. Bruce Lee’s training is outlined in the book Art of Expressing the Human Body, a collection of his training notes and many of his ideas were leaps and bounds ahead of his time.

You may also be wondering at this point whether you can create new muscle fibers. And if so, what type of fibers would those be? Until recently, it was believed that muscle fibers never increased in count, only in surface area – that they could get stronger but never increase in number. Interestingly, this has more recently been called into doubt. As is so often the case, science underestimated human potential it would seem (neurogenesis anyone?). Today, most experts believe that we can and do increase muscle fiber count through training; a process which is known as ‘muscle hyperplasia’ (as opposed to hypertrophy). This has been demonstrated by counting muscle fibers in just a couple of animal studies and by some observations of the changes in muscle of bodybuilders.

How do you encourage muscle hyperplasia you ask? I’ll be discussing that more in a future article but one interesting potential method is weighted stretching, which is gradually gaining popularity. It also appears though that regular weight lifting may also trigger hyperplasia to a lesser extent.

What’s particularly interesting about this, is that it shows we can increase our strength and endurance simultaneously. In the past, running long distances has been thought to lead to the conversion of muscle fiber to type 1 – thereby negatively affecting your explosive power potential. If you can create new muscle fiber though, you could theoretically increase the amount of fast twitch and slow twitch muscle simultaneously. Specialization is for pussies.

Anyway, focusing on fiber type alone is a mistake as there’s actually something much more crucial involved in increasing your explosive power and pure strength…

The Nervous System

Sorry to ruin the punch line with that heading there but the crucial factor in question is none other than the nervous system. Why? Because it’s the nervous system that allows you to recruit those muscle fibers in the first place.

“It takes anywhere between .4 and .6 seconds for the nervous system to contract all available motor units and this is how long it takes you to reach maximum strength. But a vertical jump only takes .2 seconds roughly…”

Here’s something you don’t hear often: we can only recruit a maximum of 50% of our motor units at once anyway. This means that there is a hell of a lot of untapped potential in your muscles that you simply aren’t able to use. And while it’s slightly higher for an athlete, most people will be able to engage even less than that 50% – closer to 20%!

What’s more, the speed of the nervous system also plays a big role. It takes anywhere between .4 and .6 seconds for the nervous system to contract all available motor units and this is how long it takes you to reach maximum strength. But a vertical jump only takes .2 seconds roughly… so what does this mean? It means that only a fraction of your available power is going into your ability to jump.

So really muscle fiber type is only a small part of it. What’s possibly more important is your ability to quickly engage as many motor units as possible.

And there are a few unique individuals who possess a unique ability to do just that. One example is Dennis Rogers. Dennis Rogers was featured on Stan Lee’s Superhumans for his ability to pull vehicles using chains and to bend steel. His feats of strength are genuinely superhuman and in tests it was found that this ability comes from ability to recruit more muscle fibers more quickly. Dennis doesn’t look particularly strong but he can generate over 1,000 pounds of force and he’s considered to be pound for pound the strongest guy alive. He’s a World Arm Wrestling Champion and Grandmaster Strongman and most likely his abilities come down to the genetic lottery.

Crisis Strength

The other scenario in which motor unit recruitment exceeds normal levels is when we find ourselves in a life-or-death situation. We’ve all heard stories of mothers with the ability to lift cars off of their children trapped underneath, or stories of rock climbers throwing huge boulders off of themselves. It appears that under these circumstances an extreme fight-or-flight response (caused by the release of catecholamine neurotransmitters) unlocks our hidden potential to allow us to recruit a much larger percentage of our motor units. This is what is sometimes referred to as ‘crisis strength’ or ‘hysterical strength’.

crisis strength

The reason we don’t normally engage 100% of muscle fiber is that there’s a large chance we’d injure ourselves. What’s more, we would exhaust the motor units in an entire area and this would leave that limb practically useless until we had recovered. Finally, inefficiencies in our nervous system prevent the signal from getting through as clear as it would need to to trigger 100% activation.

Increasing Motor Unit Activation

So how do you increase your motor unit recruitment without taking your youngest child and throwing them under a bus?

I’m very curious as to whether it would be possible to use CBT and meditation to get yourself into a mental state where you could throw hugely heavy objects around… but more on that in a future article.

A more practical method is to try and focus on strengthening your ‘mind muscle connection’. In other words, your ability to precisely control the muscles in your body. And to do that you can use a number of strategies. One is to practice tension and try tensing all the muscles in your body. The more you practice this, the more used to contracting those muscles your body will become.

Guess who used this old technique? That’s right: Bruce Lee. Bruce called it the ‘iso-tension’ technique (though it’s also called ‘iso contraction’) and used it for ten seconds after completing a set. This also helped with something else: time under tension. Time under tension represents the total amount of time that the muscle spends ‘working’ and by using iso-tension for ten seconds following each set, Bruce could increase the ‘occlusion’ of blood and metabolites in the muscle to stimulate more growth. Bruce Lee would also contract the rest of his body during any isolation exercise (so his core would be tensed for instance throughout a set of press-ups) and this allowed him to further gain control over his CNS. You can also use dynamic tension which means contracting the muscle while performing the range of motion needed for that exercise.

Another strategy is to use ‘overcoming isometrics’. ‘Iso’ in this context means static, and thus ‘isometrics’ are the opposite to plyometrics. Instead of launching and exploding, you’re instead holding a position. An isometric hold against sub 100% 1RM (called yielding isometrics) involves holding a dumbbell at 90 degrees for instance, or holding a handstand position. That’s not what we’re talking about here though. Instead, we’re talking about overcoming isometrics which involve pulling or pushing against an immovable force (+100% of 1RM). This is another one Bruce Lee would use – this time by trying to curl a barbell attached to the floor via a chain.

Why is this effective? Because as far as the body is concerned, it’s the same as trying to lift 150% of your 1RM. In other words, your body will need to try and recruit every single motor unit including those big ones made up of fast twitch muscle fibers. And the more you practice recruiting all your muscle fibers, the more you will strengthen that mind-muscle connection. Powerlifters increase their mind muscle connection to a great degree because they constantly try lifting close to 100% of their 1RM – thus engaging more and more fibers. The real strongmen who bend steel though, use overcoming isometrics and that’s pretty much what bending steel equates to – at least when you start out! (This also happens to be excellent training for your grip strength, which drastically improves your overall

Finally, it’s also worth noting that you can improve your control over your central nervous system through practice. Like the rest of your brain, the nerves connecting your muscles are plastic and can be reinforced and altered through training and repetition. Every one of your motor units is connected to your brain via a nerve and each of these connections is then mapped out through your motor cortex. And like your muscle fiber, your nerves range in speed as well (you have ‘fast twitch’ and ‘slow twitch’ nerves).

When you learn any complex movement, your brain changes shape via the release of brain derived neurotrophic factor (BDNF) and nerve growth factor. Meanwhile, the nerves and brain cells involved in the movement will become more conductive via myelination each time you rehearse that movement (increasing ‘long term potentiation’ or ‘LPT’). The result is that you can perform the movement more efficiently and powerfully over time – and as demyelination doesn’t tend to occur, once learned, these movements will remain somewhat imprinted. The more you practice any movement, the more graceful, powerful and fast that movement becomes.

What’s that? You were hoping for a ‘quick fix’? You’re in luck! The best way to quickly improve your control over a movement is to perform a ‘warm up set’. That’s a set of that movement performed perfectly without weight. Why does this work? Because it ‘potentiates’ the neural network involved in the movement – once a neuron has fired once, it becomes more inclined to do so again subsequently. So if you perform one perfect set, you’ll prep your brain and nervous system to better engage the muscles in your following sets against resistance. This is a technique used by many a bodybuilder to great effect.

So there you go, that’s a fair bit of information you likely didn’t know about muscle fiber types. This is a far more complex subject than many people realize and if you really want to maximize your strength and force, you need to consider factors such as hyperplasia, your central nervous system and more. Now you know!


Speed training: “Adaptation to chronic eccentric exercise in humans: the influence of contraction velocity.

Muscle fiber conversion: “Increase in the proportion of fast-twitch muscle fibres by sprint training in males.

Hyperplasia: “Skeletal muscle fiber hyperplasia.

Stimulants with a similar effect to hysterical strength?: “Action of the sympathetic system on skeletal muscle.

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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