Understanding Muscle Fiber Type

By: Kelly Baggett

With any discussion of athletic performance one topic that arises again and again without fail is the topic of muscle fiber type. So how important is muscle fiber type? If you are slow twitch dominant is it possible to make yourself fast twitch dominant? Or is your muscle type completely reliant on genetics?

I believe that muscle fiber typing is both over-rated and under-rated. People and coaches tend to fall into one of 2 groups. They either say that muscle type determines just about "everything" when it comes to athletic ability, or they say that muscle type makes "no difference" whatsoever. This article will deal mainly with the 2nd argument, how muscle type is over-rated and how it gets more credit then it deserves. A second article will deal with how a small percentage of people may be able to further enhance the results of their training if they pay attention to and "zero in" on fiber type conversions. Let me say ahead of time that the vast majority of people should pay a lot more attention to this article as the 2nd will incorporate a lot of minutia and only apply to advanced athletes or those who really like to dive deeply into training science. That said lets get to it.

An Overview Of Muscle Fibers

There are three primary muscle fiber types in humans -- Type I, Type IIA, and Type IIB. Type I are referred to as "slow twitch oxidative", Type IIA are "fast twitch oxidative" and Type IIB are "fast twitch glycolytic" As their names suggest, each type has very different functional characteristics. Type one fibers are characterized by low force/power/speed production and high endurance, Type IIB by high force/power/speed production and low endurance, while Type IIA fall in between. These characteristics are a result, primarily, of the fiber's Myosin Heavy Chain (MHC) composition, with Mysosin heavy chain isoforms I, IIa and IIx corresponding with muscle fiber types I, IIA, and IIB.

Individual muscles are made up of individual muscle fibers and these fibers are further organized into motor units grouped within each muscle. A motor unit is simply a bundle or grouping of muscle fibers. When you want to move the brain nearly instantaneously sends a signal or impulse through the spinal cord that reaches the motor unit. The impulse then tells that particular motor unit to contract it's fibers. When a motor unit fires all the muscle cells in that particular motor unit then contract with 100% intensity. So, a muscle cell either contracts 100% or not at all. A motor unit is either recruited 100% or not at all. Therefore, there is no such thing as a partially firing motor unit or a partially contracted muscle fiber.

When you engage in very low intensity activities like lifting a spoon to your mouth, your brain recruits motor units that have a smaller number of muscle fibers and the fibers that make up these smaller motor units are slow twitch, meaning they don’t contract as fast or contract with the same level of force as type II fast twitch motor units and fibers. If they did you’d be knocking yourself in the head with a spoon everytime you sat down to eat!!

These smaller motor units are termed low threshold motor units. As the intensity needed to apply force increases, so does the number of motor units involved in the task, particularly the number of fast twitch or high threshold motor units. The main difference between a slow twitch motor unit and a fast twitch motor unit is the fast twitch motor unit controls more muscle fibers or cells and these cells are bigger. In much the same way, the main difference between a slow twitch muscle fiber and a fast twitch muscle fiber is the fast twitch fiber is larger and can thus produce more force. During an activity such as curling a dumbbell, not only does your body recruit the same motor units as it does when you lift a spoon, but, since curling a dumbbell requires more force, it recruits enough additional fast twitch motor units until enough have been recruited to do the job.

The body recruits the lower threshold motor units first (slow-twitch), followed by the higher threshold motor units (fast-twitch) and continues to recruit and fire motor units until you’ve applied enough force to do whatever it is you’re trying to do regarding movement. When you are lifting something extremely heavy or applying a lot of force your body will contract practically all the available motor units for that particular muscle.

When engaging in high intensity or high force activities you get lots of motor unit activation and thus a lot of force. So how does this relate to the fiber in the available motor units? Well type I muscle motor units contract less forcefully and a little slower then type II fast twitch motor units and they reach peak power slower. They are also highly resistant to fatigue so they have good endurance. This is why you can sit and eat all day or play Playstation all day and never get tired!

The type II motor units are divided into type IIA and type IIB. Both of these sub-groups are capable of greater levels of absolute force than type I and also fatigue a lot quicker. Type IIA and IIB are capable of roughly the same amount of peak force, but the IIA fibers take longer to reach their peak power in comparison to type IIB. Type IIA fibers reach peak power in about 50 milliseconds whereas type IIB reaches peak power in about 25 milliseconds. Because of their greater contraction speeds, the total peak power by IIB can be up to 5 times higher then the IIA's.

Fiber Type--Contraction Speed---Time to Peak Power---Fatigue

I (slow twitch)-------slow------------- 100 milliseconds-------slowly

IIA (fast twitch)-----fast---------------50 milliseconds-------fast

IIB (fast twitch)-----very fast-----------25 milliseconds------fast

Now, when we realize that sports movements usually occur in around 200 milliseconds or less, if you look at the time to peak power of the individual muscle fibers, it should then become obvious that each type (I,IIA,IIB) has enough time to reach peak power production. So, why the superiority in having more fast twitch II B fibers? Well, two things. Since they contract quicker, if you have an advantage for the first tenth (arbitrary) of the movement, it can result in superior performance. Since their total peak power is greater this could also give one an advantage when producing force under high velocity conditions.

This can be documented when you analyze a large group of athletes for vertical jump performance and their style of executing a vertical jump. Athletes with more FT fibers (A&B) change direction a bit quicker during their countermovement (down to up) switch and they tend to use less knee bend. (Bosco) These results can be confirmed by muscle biopsy and even by special force-plate analysis. This doesn’t mean that one with a lower FT fiber% can’t jump even higher, they just tend to do it a little slower and with a deeper knee bend.

Although having a high % of FT fibers may give one an advantage, there is little doubt that the nervous system is actually much more important and should take precedence.

Muscle fibers and nerves

You see, the type of fiber expressed as far as type I vs Type II is controlled by the nervous system. Nerves that control and connect to a group of motor units run from the brain to the motor unit and are hardwired in the brain. Fast twitch motor units are controlled by fast twitch nerves. Slow twitch motor units are controlled by slow twitch nerves.

In the laboratory you can take a nerve from a motor unit that supplies a slow twitch muscle fiber and replace it with one that supplies a fast twitch fiber and the slow twitch fiber will behave just like a fast twitch fiber! The reverse is also true. You can take a slow twitch nerve and connect it to a fast twitch motor unit and the fast twitch will behave like slow twitch. Unforunately, it's impossible to change a slow twitch nerve into a fast twitch nerve and vice versa. However, you can make the Myosin Heavy chain expressed in a fast twitch fiber either more or less fast twitch or a slow twitch fiber more or less slow twitch but more on that later.

Muscle Recruitment

So, aside from muscle fiber involvement why is the nervous system so important? The majority of the time, the real limit to your performance is the number of motor units your nervous system can recruit in the short amount of time you have in a sporting movement and the amount of horsepower (size of the muscle cells) under control of those motor units, not the type of muscle fiber (slow twitch or fast) that comprises those motor units. Remember, the nervous system determines the degree of motor unit involvement.

It should also be noted that with regards to peak "force" production, the only real difference amongst the fibers is their size. Type II's are bigger yet an equal volume of type I's can produce roughly the same peak force. Therefore, for displays of maximum force (strength), fiber type is of little consequence.

Now this next part is important. Recall that the average person can only recruit around 50% of their muscle motor units anyway. It normally takes anywhere from .4-.6 seconds for the nervous system to call on all the available muscle motor units to contract. This is the same length of time it takes to demonstrate max strength or apply maximum force. However, it takes only .2 seconds to perform something like a vertical jump. So the main determining factor is how many of ALL the available muscle motor units one can get turned on in .2 seconds and not necessarily how much fast twitch fiber one has. Therefore, if one lacks fast twitch fiber but also has a very efficient nervous system capable of recruiting nearly all the FT fiber they do have, they will tend to have superior performance in comparison to someone with a less efficient nervous system and lots of fast twitch fiber.

Normally the body inhibits the contraction of all available muscle fibers as a protective mechanism. An example of this phenomenon in reverse can be seen when looking at weight-lifters. Often people can considerably increase their strength without any increase in muscle size. Why is this so? It’s simply because the body becomes more efficient at muscle recruitment and firing synchronisation. By engaging in the correct training programs over a period of time with an emphasis on speed, explosiveness, and power you can better teach your body and nervous system to recruit it's FT fibers.

Slow to Fast conversions

Another reason that fiber typing may be largely disregarded is that studies in both man and animal have consistently shown a fast to slow conversion in response to training of any kind. That is, IIB fibers convert into the slower contracting and less powerful IIA. In fact, guess what group of people has the highest percentage of the fastest contracting IIB fibers?? COUCH POTATOES! With just about any type of training, the higher threshold fibers (IIB) change into slower contracting IIA fibers. When training is ceased these fibers once again revert back to IIB. The likely reason why this occurs is because of metabolic efficiency. The body will deal with stress in the most efficient manner possible and a slow transformation is metabolically more efficient while it still allows the body to adapt to stimuli.

As noted, the main difference between IIA and IIB is their speed of contraction. They contract at about the same force but the IIB/IIx contract quicker and are better at creating force at high speeds. Therefore, with typical training schemes the relationship between IIA and IIB is also inconsequential. In fact the amount of either type II type only becomes even remotely important when a resistance is less than 30% of max.

Running a funny car on the highway

The fast to slow conversion may seem like paradoxical and obviously would be for a speed or power athlete but it makes sense when you consider survival. The body strives to be as efficient as possible in an effort to conserve energy. Fast twitch IIB fibers are fuel hungry machines. They are very strong, fire very quickly, burn a lot of energy per unit of activity, and recover slowly. Therefore they're very inefficient. They're much more like a funny car rather then a Honda Civic. If you tried to take an ultra high RPM funny car out on the highway and run it alongside the economy cars out there what would happen?? It would probably be a lot like taking a powerlifter, shotputter, olympic lifter, or sprinter and putting them out on the highway in a 26 mile marathon race with distance runners! They would cramp up, sputter and run out of gas!! The training that athletes engage in is much like this stress. A slow and economized Honda Civic would have a better chance of survival in the face of large volumes of work therefore this adaptation makes perfect sense even for those who might be engaged in speed training.

Fast twitch fibers don't like high volumes or long durations of work. They don't even like a high frequency of work. If we go back to our ancestral roots, in humans, fast twitch IIB fibers were used only in times of dire circumstances and stress or for "fight or flight" situations. These would include running away from a predator, fighting, chasing food, or other brief explosive muscle action. Therefore, they were only active for a few minutes per day at most. Since they weren't used often the body had no real need to sacrifice them for a more efficient fiber. Sedentary people are the same way and have more fast twitch IIB muscle then athletes as the use of their fibers is limited and there is no need for their bodies to make more efficient adaptations. A faster muscular subtype (funny car) is advantageous for an organism whose main objective is to occassionally battle a predator or protect its children as it might be for a sedentary well fed human.


Fast to slow (IIB to IIA) transformations are also seen in hypothyroidism which is characteristic of the body being in a starved state. When in a food shortage the main thing the body wants is "survival." Thus, the body sacrifices display of FT IIB fibers and adaptations related to the display of fight or flight are done away with because they would use up too much energy. This also partially explains why those who think they can shed a metric crapload of bodyfat in an effort to better display power are often met with less then satisfactory results. They may lose the weight yet, depending on the amount of weight they lose and how lean they get, they will eventually begin to lose speed-strength and strength-speed proficiency.

According to Caleb Stone the reverse is true of hyperthyroidism, hyperinsulinemia, and leptin administration - where slow to fast transformations are seen. What these all have in common is they are characteristic of the body being in an overfed state. Speed, power, and strength thrive off of the fed state! In these cases the need for metabolic efficiency is nonexistent leaving free to display muscular characteristics conducive to fight or flight situations.

Sprinters and fast to slow conversion

The fast to slow conversion has even been documented in elite level sprinters. During intensive training their IIB % actually decreased even though their sprint times improved. If fiber dominance is of such paramount importance how is it possible they still improved their sprint times?? Well you knew you’d hear this again didn’t you!? The nervous system! They became more efficient in the movements. Therefore the main limiting factor is the nervous system as it dictates the speed of motor unit recruitment and the amount of muscle that can be recruited. The next important factor would be how much horsepower is turned on when those motor units are recruited (size of the muscles in relationship to bodyweight), followed by how fast the horses run (muscle fiber type) when they get turned on. Thus in order of importance the main factors would be:

1. Body structure (muscle, tendon, and limb lengths and attachments)

2. Neural factors (muscle recruitment etc.)

3. Relative strength levels (strength per lb. of bodyweight)

4. Muscle fiber type

This is further illustrated if you compare the performance capabilities and physiques of top-level sprinters, powerlifters, bodybuilders, baseball pitchers etc. The research states that the largest, most powerful, and strongest fiber is the fast-twitch fiber. If this were ALL there was to it then an athlete with tremendous muscular size would also be proportionately strong, powerful, and fast. An athlete who could throw fast or run fast would also be big and strong. An athlete who is strong would also be fast and powerful. This is obviously not true.

It should also be noted that having good neural factors correlates with having lots of fast twitch fiber (both type IIA&B). Fast twitch muscle percentage correlates with reaction time. Therefore,when you see studies showing fast twitch fiber to be correlated to displays of sports power what those studies are mainly showing is that good neural factors correlate with displays of sports power.


The point to take home is that if you have less then 3 years of consistent training experience you should be “aware” of muscle fiber type and give it consideration, but don’t obsess about it. Don't put the cart before the horse! Simply learn how to correctly train for performance and your body will take care of the rest as a natural adaptation to your training.

If you've followed solid training systematics and have stagnated and you feel you're ready to obsess about it then read the next installment, "Becoming a Fast Twitch Machine", where I will do a complete 180 and provide you with plenty of information for that purpose.