arm entire anterior muscles sized

What’s Your Type? Muscle Fibers Explained

As most of you know, there are a number of factors that determine the potential action of a muscle. Factors such as the angle of pennation, overall muscle length, mitochondrial density, enzyme activity etc… are initially based on genetics of any individual.

Changes in these factors affect the strength and endurance of a muscle and will be different for most individuals. That gets complicated. In the most simplistic sense, we can look at the differences between the capabilities of two individuals determined by muscle fiber types and concentrations. Generally, muscle fibers are separated into three main groups: Type I, Type IIa, and Type IIb.

Type I Muscle Fibers

Type I muscle fibers are classified as slow-twitch, which means they develop force slowly and relax at a similar pace with a longer “twitch” time in between. These fibers do not utilize much energy from ATP (adenosine triphosphate, the body’s processed fuel). Instead, Type I fibers are more aerobic than anaerobic; they have a readily available supply of oxygenated blood and can function for longer periods of time with repetitive contractions. Essentially, athletes involved in endurance-style activities such as marathon runners, distance swimmers, triathletes, and soccer players will have a musculature made up of a high concentration of Type I fibers.
Type I Dominant Athletes: She probably can’t squat much or jump high, BUT she can run forever. – Marathon Runners

Type II Muscle Fibers

On the other hand, Type II fibers are the fast-twitch, explosive muscle fibers associated with power and explosiveness in athletes. These fibers are “inefficient and fatiguable” (1) They characterize high anaerobic power but very low aerobic power which equates to shorter duration, but more powerful contractions of the muscle as a whole. The main difference between type IIa and type IIb muscle fibers comes from their “capacity for aerobic-oxidative energy supply.” (1) Type IIa muscle fibers have a more efficient means of aerobic metabolism due to a higher number of surrounding capillaries that circulate blood throughout the muscle which results in a higher resistance to fatigue. Athletes whose body composition is mostly made up of Type IIa/Type IIb muscle fibers are the explosive athletes such as sprinters, Football players, baseball pitchers, and power lifters. These mainly ATP fueled muscle fibers create rapid, intense contractions, but cannot maintain action for a long period of time.

Type II are strong…but not for long.  I wouldn’t get this deadlift

Type IIb Muscle Fibers

For most athletes, the muscles are comprised of a combination of these muscle fiber types to allow both explosive movement and the ability to continue those movements over a longer period of time. With training, muscle fiber concentrations can be altered. For example: if a distance runner decides to hit the weight room and develop his/her lower body power, the muscle will start to shift from a dominant concentration of Type I fibers to an even distribution between Type I and II fibers. To further the point, if a football lineman has been lifting heavy during the off season, they will have a higher concentration of Type IIb fibers. When the season begins and the duration of training (~1hour lifts to >2hour practices) a higher concentration of Type IIa fibers will develop because the athletes requires more muscular endurance at this point. In addition, different muscles within the body will have different concentrations. Skeletal muscles responsible for posture will have more Type I fibers to maintain posture throughout the day while muscle groups largely responsible for movement like the quadriceps are not necessarily hampered by such limitations and can develop massive explosive force.

He jump crazy high…but it would be nearly impossible to do back to back due to Type IIb fatigue

Muscle Fibers Summarized

To sum it all up, the muscles of an endurance athlete will be comprised mainly of Type I muscle fibers while more explosive athletes will show a higher concentration of Type IIa/IIb fibers within the muscle. While the concentrations of fiber types for any individual can be changed to some degree, a total transformation of a muscle from type I to type II is less probable (1). A marathon runner is not going to become a world-class power lifter any time soon.

- Joe Hashey, CSCS -

PS.  This article is written by gym intern Aaron Zajac, and I think he did some good research!

PPS.  Even though there is some debate over whether you can convert fibers types, there it is still generally understood that athletes need to train for the demands of their sport (either growing or converting fibers).

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  • Larry

    Reply Reply April 4, 2011

    Great article Joe! Again, always good to get the scientific explanation behind muscles, fitness, and the proper way to train. Is there something that can be done( a test of some sort maybe) to see what muscle type you have and then be able to go from there to train to change to the muscle type you want? Also, you seem like you do a lot of reading about this kind of stuff as you always seem to have a pertinent source for all of your information. Could you put out a list of reading material that someone like myself that’s really interested in learning more about this kind of stuff could go out and get, maybe like a “top 10?” Lastly, I’ll give you my post comment beg once again… “WE DESPERATELY NEED YOU DOWN IN SOUTH JERSEY!!!!” Good quality training like yours is really hard to come by nowadays. A lot of people claim to know what they’re doing and don’t have a clue. Again, great Article keep em coming!

    p.s.- Happy Belated Birthday!

    • Thanks for checking it out Larry! I hope it helps.

      I appreciate the well thought out comment. That’s a good idea about the top 10 reading – although mine always seems to change haha.

      I’ll get something together for you soon,


  • Tim

    Reply Reply June 13, 2013

    Joe- it should be noted that physiologists have observed that muscle fibers themselves cannot shift, or change, to be more glycolytic (ex. type 1 to type 2) but only can shift toward being more oxidative (ex. type 2b->type2a->type1).

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