# Muscle Fiber types and changing them -



## awhites1 (Mar 6, 2012)

I'm studying for a test and getting to the section about muscle fiber types. So far it hasn't got any deeper than Type 1 (fast twitch) and Type 2 (slow twitch), and unless it's much farther ahead in the lesson I don't see where it mentions anything other types. However in my own personal study prior to it I've come across numerous sources stating there are more than that. So I'm taking some issue with the text book.

My question is two fold and relates though to it being possible or not to change a muscle type from one to the other. The book states:

"_It would be nice to pick or change your fibers to match your interests, but fiber types don’t change with training. They will, however, become more efficient at any activity the more you perform it. This suggests a person with a heavier distribution of slow-twitch fibers can still add muscle and increase strength using heavy resistance training_"

but again I've read numerous articles and studies stating the oppesite. So *can you change your muscle fiber type with training, or is it just really becoming more efficient*?

*second question*, I found another forum talking about this samething and it said: _remember from grad school the discussion of where they changed neural stimulation of muscle fibers, and saw the muscle fibers switch from type I to type II and vice versa, so appears neural stimulation is main contributing factor to muscle fiber type_.

*Has anyone else heard this?* It would sort of make sense, if you can change a fiber from one to the other then maybe biologically they're the same thing and it's just the neuron acting on it I guess that gives it it's properties.


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## awhites1 (Mar 6, 2012)

sorry for the lengthy questions . I usually hate half page long comments/questions but there's just no way to really shorten something like that


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## ThreeGigs (Mar 6, 2012)

There are type IIa and type IIb fibers. If I recall correctly, the type IIa fibers can "convert" to either type I or type II. However, if it began life as a type I or type IIb, it stays type I or type IIb.

I'm just going by memory, so you'll have to Google muscle fiber type conversion IIa IIb 
Like this: muscle fiber type conversion IIa IIb - Google Search


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## Kenny Croxdale (Mar 6, 2012)

awhites1 said:


> I'm studying for a test and getting to the section about muscle fiber types. So far it hasn't got any deeper than Type 1 (fast twitch) and Type 2 (slow twitch),



*Type I *

This is Slow Twitch Muscle Fiber. 

*Type II Fast Twitch Muscle Fiber*

There is two type of Fast Twitch Muslce Fiber. 

*Type IIA* 

Type IIA are associated more with strength 

*Type IIB/X*

Type II muscle fiber are the driving force for power and speed.



awhites1 said:


> and unless it's much farther ahead in the lesson I don't see where it mentions anything other types. However in my own personal study prior to it I've come across numerous sources stating there are more than that. So I'm taking some issue with the text book.



*Fiber Types*

There are two types: Type I Slow Twitch and Type II Fast Twitch

Within type Type II there are two sub types:  Type IIA and IIB/X.

However, there are more sub types.  However, I doubt your text book will break down that far.  

*Seven Subtypes*

I, IC, IIC, IIAC, IIA, IIAB, and IIB.



awhites1 said:


> My question is two fold and relates though to it being possible or not to change a muscle type from one to the other. The book states:
> 
> "_It would be nice to pick or change your fibers to match your interests, but fiber types don???t change with training. They will, however, become more efficient at any activity the more you perform it. This suggests a person with a heavier distribution of slow-twitch fibers can still add muscle and increase strength using heavy resistance training_"



*Type IIA can be converted to Type IIB/X  *

Focusing on power and/or speed movements will convert Type IIA Fast Twitch Strength Muscle Fiber to Type IIB/X "Super" Fast Twitch Muscle Fiber.

*Type IIB/X can be converted to Type IIA*

Using heavy loads that move slowly converts Type IIB/X "Super" Fast Twitch Muscle Fiber converts to Type IIA Fast Twitch Strength Muscle Fiber. 

*Type I Slow Twitch Muscle Fiber*

Type II does not convert to Type I. 

*Your Text Book*

Your text book has some flaws in it.  

1) What is the name of the Text Book?  

2) What test are you studying for?



awhites1 said:


> *can you change your muscle fiber type with training, or is it just really becoming more efficient*?



1) You can convert muscle fiber. 

2) Your central nervous system will become more efficient.



awhites1 said:


> *second question*, I found another forum talking about this samething and it said: _remember from grad school the discussion of where they changed neural stimulation of muscle fibers, and saw the muscle fibers switch from type I to type II and vice versa, so appears neural stimulation is main contributing factor to muscle fiber type_.



*Neural Stimulation*

Yes, a motor neuron can determine the speed of contraction of muscle fibers.   

However, you CAN convert muscle fiber but you cannot "rewire" your nervous system.  



awhites1 said:


> *Has anyone else heard this?* It would sort of make sense, if you can change a fiber from one to the other then maybe biologically they're the same thing and it's just the neuron acting on it I guess that gives it it's properties.



It is a bit more complicated.  

*Understanding Muscle Fiber Type*

Kelly Baggett does a nice job of explain muscle fiber in this article.  

Kenny Croxdale


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## camarosuper6 (Mar 6, 2012)

There is no consensus that fiber types can be changed, and until fairly recently they have been thought not to be able to.


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## Kenny Croxdale (Mar 7, 2012)

camarosuper6 said:


> There is no consensus that fiber types can be changed, and until fairly recently they have been thought not to be able to.



*Until Recently*

This information has been around for a while.  

*Type II Fiber*

Again, Type II Fiber has the ability to convert from IIA to IIB and from IIB to IIA.  

*The Conversion Factor*

What cause them to covert has to do with your exercise choices.  

*Type IIA Fast Twitch Muscle Fiber*

Type IIA are predominately developed with Limit Strength movement.  Limit Strength movements are those that involve high load, low velocity movements...1 Repetition Max.  

*Strength Training Percentages*

Thus, training of 85% of 1RM (Repetition Max) will convert Type IIB to IIA.  

*Type IIB "Super" Fast Muscle Fiber*

Type IIB are predominately developed with Power and/or Speed movements. 

*Power Training Percentages*

Power is best developed with moderate loads and moderate velocity.

Training loads of approximately 40-60% or 1RM work best for exercises such as the squat, bench press, etc.  

Load of 70-80% work best for Olympic movements. 

*Speed Training*

Speed is developed with light loads and high velocity movements. 

*Speed Training Percentages*

Training percentages of 10-40% of 1RM work in the development of speed, with the sweet spot being 30% of 1RM.

*Lifting Heavy Weight Makes You Slow. * 

The answer is Yes and No.  

*Strength The Foundation of Power and Speed*

The foudation of power and speed is built on strength.  

Initially to be able to exert more power and increase your speed, you MUST increase your strength.  

*When And Why Strength Training Makes You Slow*

If you perform nothing but Limit Strength Training movement all the time, your power and your speed are going to drop. 

*IIB to IIA Conversion*

What occurs is the body learns and adapt to you training to become stronger, such as powerlifters.  

It does so by converting IIB to IIA Muscle Fiber.  

*IIA to IIB Converstion*

Converstion of IIB to IIA takes place is when you're focus is on speed. like sprinters.

*The Best of Both Worlds*

Olympic Lifters have the best of both worlds.  They possess strength, power and speed. 

*Conjunctive Training*

Olympic Lifter incorporate strength, power and speed movement into their training program.  

By doing so, their develop both the Type II "Strength" Fast Twitch Muscle Fiber and Type IIB "Super" Fast Muscle Twitch Fiber. 

*"A Comparison of Strength and Power Characteristics Between Power Lifters, Olympic Lifters, and Sprinters*"

This 1999 National Strength and Conditioning research article is one of many compelling documents that examines this.  

*Type I Slow Twitch Muscle Fiber Stand Alone*

Type I will not convert to any Type II...Nor will Type II convert to Type I.  

Kenny Croxdale


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## camarosuper6 (Mar 7, 2012)

That being said, I do believe that it is possible, so Im not disagreeing per se..


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## Spaullba (Mar 9, 2012)

From my understanding which is admittedly limited (I am nearing the end of my undergrad in exercise science) there are three main muscle fiber types: 
Type I- slow twitch: aerobic, endurance.

Type IIa -Which are your FOGs (fast oxidative glycolytic). They are a sort of hybrid or intermediate fiber type.  

Type IIa -  Fast twitch muscle fibers.  Forceful, anaerobic.

Now, I could be wrong, but I don't think that you can _technically_ change your muscle fiber types.  The proportions of fiber types you have will always be constant.  With that said, through training you can change how each of these fiber types behave.  This is especially true with your FOGs, with a lot of training you can get your FOGs to behave more like Type IIb fibers.  You can also get your fast and slow twitch fibers to be more efficient through training as well.  They do not change what type of fiber they are though.

Each motor unit only innervates one type of muscle fiber ( it can innervate MANY muscle fibers, but all of the same type), and the ratio between the # of motor neurons (units) and innervated muscle fibers has a direct impact on the speed and force of muscle contraction.  The number of synapses between these motor neurons is something you have control over as well and can improve through training.

There are also 3 main factors that influence muscular force generation from a NS view point, and you can improve all three of them through training as well.

1) Recruitment: the number of motor units recruited.  
2) Rate Coding: the frequency of stimulation.  Or the rate at which you can generate action potentials which are responsible for muscle contraction.
3) Synchronization: Your motor units can fire in a synchronous or asynchronous manner.  Most tasks you do in life are asynchronous.  Synchronous firing really only occurs in maximal exertion situations, so it takes time for your NS to be able to sustain synchronous firing for any extended period of time.

All of these things contribute to being able to "change" muscle fiber types.  In actuality you are not changing the fiber type, only changing how that fiber type behaves.


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## SFW (Mar 9, 2012)




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