# Muscle Fiber Classifications



## BabsieGirl (Nov 11, 2003)

I'll just jump on into the topic.  Just thought people would find this rather handy to know.

Muscle fibers are classified by the characteristics of the motor unit serving that specific muscle fiber. The fibers are characterized on the basis of speed, force and the length of time a muscle fiber can contract without losing tension (endurance). There are two classifications of muscle fibers with respect to speed: fast-twitch and slow-twitch. With respect to force, fibers fall into three classifications: low, moderate, and fast. The two classifications in which muscle fibers fall into with regard to endurance are fatigue-resistant and fatigable.

Although we usually classify muscle fibers, in general, by means of the two classifications of force production (fast and slow), you should be familiar with the fact that there are three major classifications, characterized by the primary energy sources facilitating muscle contraction:


* Slow-twitch, oxidative (energy provided through the use of oxygen) 

*Moderate or intermediate fibers which are fast-twitch, high-oxidative and glycolytic (energy provided through oxygen and the breakdown of sugars) 

*Fast-twitch, glycolytic (energy provided through the break down of sugars). 

As you can see, when discussing voluntary muscle action, we concern ourselves and our training recommendations with three major types of skeletal muscle fibers. And each fiber type is quite different in function than the others. Therefore, I recommend that you know the properties of each fiber type in order to accurately address results-specific training recommendations. 

*Low Intensity (Long Duration)*

Slow-twitch muscle fibers are those which are most activated during slow, long-term movements like marathon running and long-distance cycling. Connecting nerves are thinner than those which are prominent in the fast-twitch fibers, allowing for continuous muscular contractions of low-force output. 

These fibers appear red in color when viewed under a microscope. And although you probably will not be analyzing your client???s muscle tissue under a microscope, it is important that you know the relationship of various muscle fibers to their respective levels of blood flow, and subsequent color representations. Slow-twitch muscle fibers have a larger amount of blood flow through them than the fast-twitch fibers. This should signify to you that they are thus highly-oxidative (perform better with the use of oxygen), making them more aerobic in nature. This increased ability to utilize oxygen (as compared to fast-twitch fibers) makes them more resistant to fatigue and together with the fact that these fibers contract slowly, make them more endurance oriented. Therefore, slow-twitch muscle fibers must be trained by way of low resistance and long duration (e.g. light weights and high repetitions of 15-40 reps per set of a resistance exercise). 

*Moderate Intensity (Moderate Duration*

Intermediate fibers are those which contain both, slow and fast-twitch muscle fiber characteristics, and are the main source of power output in activities like the 800-meter run and lengthy competitive rowing. Varying sizes of nerves innervate these muscle fibers. 

Intermediate fibers are often referred to as red, fast-twitch fibers because they receive a relatively high degree of blood supply (less than slow-twitch and more than fast-twitch fibers). These fibers are resistant to fatigue (not as much as slow twitch), and can be trained to contract with high degrees of force for moderately long periods. These fibers are also referred to as highly oxidative and glycolytic, because they are capable of being fueled by the breakdown of sugars (through glycolysis) and through the use of oxygen (oxidation). 

Intermediate muscle fibers are best trained through the use of moderate resistance and over periods moderate in duration (e.g. moderate weight for moderate repetitions 10-15 reps per set of a resistance exercise). 

*High Intensity (Low Duration Fibers)*

Fast-twitch fibers are those which are highly active in explosive or fast movements like Olympic Weight Lifting and sprinting. They contract much faster than slow-twitch fibers due in part, to thicker nerves innervating these fibers. When viewed under a microscope, fast-twitch fibers appear white in color, indicating to you, the limited blood supply serving them. These fibers utilize energy supplied via the anaerobic energy system (without the use of oxygen), and through the process referred to as glycolysis. Although these fibers fatigue rather quickly, fast-twitch muscle fibers are those which enlarge the most and are most capable of displaying greater degrees of strength, power and speed. 

Training of these fibers is accomplished by use of heavy resistance and low repetitions (e.g. 3-8 reps per set of an exercise). 

*Muscle Fiber is Muscle Fiber is Muscle Fiber...you get it*

Muscle fiber type is a product of heredity. That is, fiber composition (fast vs slow-twitch) is predetermined at birth. And, although fiber types remain unchanged, it appears that fast-twitch glycolytic muscle fibers can become more endurance oriented through endurance training, and display characteristics similar to those of the slow-twitch fibers.2 

*Recruitment*

When a movement is begun, it is the slow-twitch muscle fibers that first become activated. But the more the movement is resisted (e.g. via heavy weights) assistance is provided for by the fast-twitch fibers. As a training effect of lifting heavy weights, an earlier recruitment of fast-twitch fibers is experienced. If slow movements are continued with little resistance, it is the slow-twitch fibers which are called upon to facilitate the muscular contractions. 

In sports activities that require a single, all-out effort, the synchronous recruitment of muscle fibers may be more important than the element of fatigue resistance.3 Research has shown that the synchronous recruitment of fast-twitch fibers exhibits the greatest high peak torque.4 

*Recovery Time*

You should know by now that the greater the blood supply to a muscle, the greater the ability of that muscle to recover (provided good eating habits are practiced). That is, red or slow-twitch muscle fibers recover much quicker than fast-twitch fibers because of this greater blood flow - ???delivery system??? - serving them. 

Blood flow aids in the recovery of a muscle first by removing waste products resulting from muscular contractions, and second, by delivering the necessary nutrients (e.g. protein and carbohydrates) to the worked muscles for recovery. The recoverability of a muscle group helps you determine how frequently a particular muscle group can be trained. Obviously, the shorter the recovery time required of a muscle, the more frequently the muscle group can be trained. And fiber type is only one of several factors affecting the length of time needed for complete recovery. 

REFERENCES 

1. Hatfield, F. Complete Guide to Fitness Training, International Sports Sciences Association, 1991. 

2. Kaijer, L. and Jansson, E. Ef fect of extreme endurance training on muscle fiber characteristics. Proc. Int. Union Physiol. Sci. 11, 723 (abstract), 1977. 

3. Saltin, B., Henriksson, J., Nygaard, E., and Andersen, P. Fiber types and metabolic potentials of skeletal muscles in sedentary man and endurance runners. Ann. N.Y. Aca. Sci. 301: 3-29, 1977. 

4. Thorstensen, A., Larsson, L., Tesch, P., and Karlsson, J. Muscle strength and fiber composition in athletes and sedentary men. Med. Sci. Sport. 9: 26-30, 1977.


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## senimoni (Nov 11, 2003)

Very interesting.....I must be all fast twitch b/c I have the endurance of a goat(why goat you ask, I have no idea it just sounded good)


PS - Must have sleep


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## Dale Mabry (Nov 11, 2003)

Not to over-complicate things, but there are actually 3 classes these days and it has been found that there is a chance that one can transition between the 2 fast-twitch types, but not between fast twitch and slow twitch.  Here is the skinfo...


Like most things, there is the simple story, and the real story. Physiological investigations in the late 60s and early 70s have done a great deal to shape our knowledge of skeletal muscle function and fiber type. The biopsy technique, enzyme histochemistry, and physiological studies all advanced this issue. From this work, we now know the fiber types differ: 1) in contractile speed, 2) in myosin ATPase enzyme characteristics, and 3) in metabolic enzyme profile. From these three differences, three different fiber type classification schemes have emerged.
Dr. Phil Gollnick and colleagues studied differences in contractile speed in different muscles. They found that the fiber types were distinguishable based on the time it took them to reach peak tension when stimulated. That difference is graphically demonstrated here.. They proposed the distinction slow, and fast. This turned out to be an oversimplification.

Meanwhile, even before this study, Brooke and Engle distinguished the fiber types based on differences in Myosin ATPase enzyme activity. They arbitrarily divided the muscles into two groups and called them Type I and Type II. 

Around the same time Gollnick and colleagues were classifying muscles based on contractile speed, Dr. J.B. Peter and his group investigated the properties of the two categories of fibers established by Brooke and Engle. They proposed another set of terminology created by combining tension generating and metabolic properties. Type I cells were termed Slow Oxidative. That was simple. The slow fibers had a lot of mitochondria (containing oxidative enzymes) and capillaries. However the Type II or Fast fibers had to be further divided into two sub-categories. Type II cells were either Fast Glycolytic (FG) or Fast Oxidative Glycolytic (FOG). The FG fibers stored lots of glycogen and had high levels of enzymes necessary for producing energy without oxygen, but contained few mitochondria. The FOG fibers had the best of both worlds, high speed and glycolytic capacity, plus high levels of oxidative enzymes. These INTERMEDIATE fibers were termed type IIA fibers by a fourth research group (Brooke & Kaiser, 1970). The pure fast fibers (FG) were termed Type IIb. This last lingo system seems to have stuck within the physiological research community. 

For the athletic community, the important information is this. It does appear that pure fast (Type IIb) fibers can transition to "hybrid" (Type IIa) fibers with chronic endurance training. Biopsies of elite endurance athletes reveal that after years of training, they have almost no IIb fibers, but often have a significant percentage of the intermediate, IIa fibers. BUT, the majority of the available research suggests that Type IIa fibers do not transition to Type I. This is the more accurate way of saying what I said at the end of Part I of the Fiber type discussion. 
Below is a table depicting the characteristics of the three fiber subtypes described above in comparitive fashion.

The chart is all screwed up and won't copy properly, the websit is:

http://home.hia.no/~stephens/fibtype2.htm


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## BabsieGirl (Nov 11, 2003)

Thanks for contributing


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## vanity (Nov 11, 2003)

so this is where the intellectuals hang out.


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## BabsieGirl (Nov 12, 2003)

Are you learning?


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## MtnBikerChk (Nov 12, 2003)

> _*Originally posted by BabsieGirl *_
> Are you learning?



are you teaching?


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## vanity (Nov 12, 2003)

the cool people always sits at the back.


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## BabsieGirl (Nov 12, 2003)

No, not teaching.  Just adding information to the Knowledge bank here at IM.


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## vanity (Nov 12, 2003)

what exactly does Babsie mean?

I can't get that word outta my head.


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## BabsieGirl (Nov 12, 2003)

My husband calls me Babsie and my dad use to call me babs


lol...can't remove it huh.....hmmm.....I'll take the eyebrows as a good sign


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## vanity (Nov 12, 2003)

I'm doing eyebrow presses.

3 sets of 10 000 reps Mon Wed Fri.


 

maybe I'm overtraining.


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## vanity (Nov 12, 2003)

6001....6002....


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## JJJ (Nov 12, 2003)

> _*Originally posted by vanity *_
> 6001....6002....




I hear you start at 100 just to impress. Otherwise you wouldnt be able to do 10 000. Am I right?


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## vanity (Nov 12, 2003)

shhh.

it's the alternate eyebrow presses that are hard.


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