How important is muscle fiber type? If you are slow-twitch fiber dominant, is it possible to make yourself fast-twitch fiber dominant, or is the type of muscle fiber that you naturally have going to stay with you your whole life?
A: Actually I believe that muscular fiber typing gets more credit then it deserves. 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. Before getting into this, it’s important to understand a little about the function of muscles and how they relate to the nervous system.
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 tell your body to move the brain instantaneously sends a signal through the spinal cord that reaches the motor unit and the signal 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 also fire 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.
During 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 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 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. During very 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 in about 1-tenth (100 milliseconds) of a second. 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 a little bit longer to reach their peak power in comparison to type IIB. Type IIA fibers reach peak power in 30-50 milliseconds whereas type IIB reaches peak power in about 25 milliseconds.
| Fiber Type | Contraction Speed | Time to Peak Power | Fatigue |
| I (slow twitch) IIA (fast twitch) IIB (fast twitch) | slow fast very fast | 100 milliseconds 30-50 milliseconds 25 milliseconds | slowly fast very fast |
Now, when we realize the vertical jump happens in about 200 milliseconds, 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 which they do. So, why the superiority in having more fast twitch fibers? Well, since they contract quicker, if you have an advantage at the very beginning of the movement, it can
result in superior performance. They also contract with a bit more force then the type I’s.
This can be documented when you analyze a large group of athletes for vertical jump performance and the method of executing a vertical jump. Athletes with more FT fibers 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.
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 200 milliseconds you have to produce max force, not the type of muscle fiber (slow twitch or fast) that comprises those motor units.
Remember the nervous system sends a signal for the muscle motor unit to
contract and this determines the degree of motor unit involvement. Now, this next part is important. 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 apply maximum force. However, it takes only .2 seconds to perform the 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 how much fast twitch fiber one has.
Recall that most people can only recruit around 50% of their muscle motor units anyway. Therefore, if one lacks fast twitch fiber but also has a very efficient nervous system capable of recruiting nearly all there available motor units in the available .2 seconds - 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 the movement and muscle recruitment increases. By engaging in the correct training programs with an emphasis on speed, explosiveness, and power (and a de-emphasis on endurance) you can better teach your body and nervous system to recruit it's FT fibers.
So how can you use this information and apply it in the real world? Well take someone who is say 50/50 fast vs slow-twitch. Over time and with proper training if he trains
his body to utilize 90% of all those available those fibers and also increases the size of the FT fibers he does have, he can outperform someone who has say an 80:20 fast to slow-twitch ratio.
Here’s yet another example of how important the nervous system is. 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! Focus should be placed on manipulating the impulse from the
nervous system to create an adaptation favorable to your goals. You can accomplish this by focusing your training on speed, power, explosive dominated activities. By doing this you train your nervous system and all your muscle fibers to behave in a fast twitch manner. The reverse can also occur. For example, if one is blessed with a high % of FT fibers and starts marathon training the FT fibers will begin to behave much more like ST.
Now going back to the original question, can a person actually change their natural muscle fiber type? The answer is a resounding YES! Canadian scientists, Drs. J. Simoneau and C. Bouchard, have estimated that 40% of the variance of fiber type is due to environmental influences (i.e. exercise) while 45% is associated with genetic factors. This means you have about 40% control of your muscle fiber type, the other 45% you
can do nothing about. The ratio of your fiber type is a result of (1) what you were born with, and (2) transformation of slow to fast or fast to slow through training influence. If you were to look at a muscle biopsy of a muscle fiber you’d see both red and white along with various shades of each. The white being fast twitch and the red being slow twitch. Think of eating chicken, the white meat (breast) is fast twitch. The dark meat (legs and thigh) is slow twitch. It’s likely you can't take a completely red (pure endurance fiber)
and turn it into a completely white (fast twitch) IIB fiber but the intermediate fibers are plastic to a certain extent and you can change the intermediate fibers into more of a slow twitch version or more of a fast twitch version. However, this may not even be all that important. Here’s why.
Guess what group of people has the highest % of the fastest contracting IIB fibers?? COUCH POTATOES! This might come as a shock but 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 into faster contracting IIB. The reason why this happens is because the body will deal with stress in the most efficient manner possible. If we go back to our ancestral roots, in humans, fast twitch fibers were used only in times of dire circumstances and stress, such as running away from a predator or fighting. Much like the typical average sedentary person, this didn’t happen very
often. If you’re constantly stressing your fast twitch fibers as an athlete, the body adapts to that stress in the most efficient manner possible and one thing it does with consistency is it makes those IIB fibers a little more endurance oriented, thus converting them into IIA. Fast twitch IIB fibers are fuel hungry and take a lot to maintain. By converting them into IIA this makes those fibers better able to tolerate the constant stress you put them under. This 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 and learned to use ALL their muscle fibers in the task in minimal time.
Now having said all that I do believe that fiber typing, although less important then the nervous system, is still of some significance. What I mean is, with all things being equal it is “usually” advantageous to have a greater preponderance of fast twitch muscle fibers because they do produce more force at high speeds. Yet you can’t draw perfect conclusions on this. The research states that the largest, most powerful, and strongest fiber is the fast-twitch fiber. If this were ALL true then an athlete with tremendous muscular size would also be proportionately strong, powerful, and fast. We know this to not be true.
Just compare the physiques of top-level sprinters, powerlifters, etc, and you will quickly find that size is not indicative of anything, and does little to help us project speed or strength levels. And what about guys like pro baseball pitchers? How are they able to throw so fast being that they are far from muscular nor are they known for their strength feats in the weight room, running speed, or jumping ability. Traditionally, defensive
backs are faster then linebackers but the LB’s are typically stronger. Bodybuilders are bigger then powerlifters yet weaker and slower.
The point to take home is be “aware” of muscle fiber type and give it consideration but don’t obsess about it. Leave the obsessing to the people like me! Simply train for performance and your body will take care of the rest.
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