T Nation

Possible to change muscle fiber type ?!


I have a question concerning muscle fiber distribution. Is is possible to change muscle fibers from slow to fast with training? How can I increase my white fiber content?


Somewhat. If you train more often using fast-twitch protocols (heavier weights, lower reps, more sets and acceleration lifts), you can "train" your muscles to be more fast twitch. Just don't expect to have the massive gains that someone with more fast-twitch fibers will have.


You can't train your fiber makeup to be more than the other. So, if you have a 40/60 slow/fast makeup, you can't train yourself as Nate said to become a more dominate fast or slow twitch by taking slow twitch and converting to fast twitch (thats how I took his reply).

You may attempt to train yourself with fast twitch recruited movements to promote a increase of fast switch by changing your fast pxidative-glycolytic fibers since they have properites of both slow and fast. Sames goes for slow twitch. I say "may" because I haven't done much research in the subject, but I've read before in textbooks that it's possible to increase fast or slow twitch by training to recruit these fog fibers (i think type IIA-B, or just B?)


We are born with a certain ratio of Type I to Type II. That is why people like Lance Armstrong are incredibly good at The Tour de France, while an Olypmic powerlifting probably couldnt complete one stage. If you took a look at Armstrong's Type I fiber ratio they have to be way up there in making up way more than a majority of his muscles.

However, to answer your question can you "train" you muscle to become more Type II. Like previously stated sortof.

Yes, we are born with a set ratio of Type I - II fibers, but there are some fibers which will carry traits of both types. They will have a tendancy to lean more towards one side, but you can sortof make them lean closer to the other side. While not changing there genetic make up, you more or less just reassign those fibers to help you lift more, rather than run farther. However, this is a very minute majority of your fibers, so all you end up doing is training your Type II fibers to become stronger.

If you are really interested in finding out what fiber type you are, go have a muscle biopsy. Fun stuff.

Hope this clears up any misconceptions.



You can influence fast twitch fibers to BEHAVE like slow twitch fibers.

I actually believe there is some research to say that the percentage of fast twitch fibers decreases with training, but with an extended lay off, the percentage can overshoot the original number.


An acquaintance of mine is working on her Masters of Physiology at the University of Alberta studying muscle fibre changes.

Apparently there's like 7 or 9 different muscle fibre types between the extremes of "fast" and "slow" twitch.

She can change their fibre type in culture by mimicing the body's hormonal reactions to exercise.

I'm guessing yes, you can change fibre type.


100% Alberta Beef- So she's found that these intermiediary fibers can be influenced to take on more of the traits of one extreme or the other? Does age play a role in ones ability to cause this to happen?


I actually wish I had more Type I in my legs so that I could do better at biking. Unfortunately, my legs are full of Type II, and thus they are big & muscular, and more suited for sprinting.


I think there are other factors besides fibre type influencing whether or not some one will be a better power lifter/tour rider. Intramuscular fat is higher in distance runners than it is in sprinters. Nervous system, muscle mass and tendon/ligament geometry should also be considered. These things may give an "mistyped" individual more endurance or power. Also, if (a BIG if) people are capable of producing more fibres after puberty (maybe in pro body builders), can one influence what fibre types are produced?

Rage-- I am just the opposite. I can bike/run/backpack forever, but I can only produce so much power with my legs. My vertical jump is way low. Legs have always been one of my less strong body parts and it shows (<20" thigh diameter). Though I guess this is a "good thing" for rock climbing.


Yes, or should I say, sort of.

There are the basic fiber types: I, IIa and IIb (or IIx dependent on who you talk to). Now, there are also various 'hybrid' type fibers that lie in between, such as I/IIa, IIa/IIb, I/IIa/IIb, etc. These fibers show characteristics of the different fiber type isoforms. Untrained people have a larger amount of hybrid fibers us trained folks. It's kind of like your muscles reserving the right to adapt to your training regimen.

As you train, you shift these hybrid fibers. For example, a powerlifter might shift his I/IIa to a more pure IIa fiber type, etc. The purpose of these is to better allow your body to adapt to whatever training stress you place upon it.

Hope this helps.

Stay strong
Mike Robertson, MS, CSCS, USAW
Director, Athletic Performance Center


What's been said appears to be true to me.

My experience with the literature and with professionals in the field dictates that there is no conversion or change in the percentage of an individual's fiber type. That is, the percentage of Type II and Type I fibers does not change with training.

Of course, that's not to say it's not completely out of the question because there's very little, if any, long-term training data examining endurance or strength athletes in conjunction with regular muscle biopsies.

Generally speaking, though, you can change a Type II fiber to a Type I fiber. However, as mentioned, you can shift from a Type IIb to a Type IIa fiber. In addition, you can influence fibers of the Type IIa variety to exhibit more Type I-like characteristics.


One very good thing to remember is that whatever you were born with the same rules apply to getting bigger and stronger. Doing a ton of reps for someone with Type I isnt going to make them the Hulk. The best thing to do is not worry about it.


RS - "So she's found that these intermiediary fibers can be influenced to take on more of the traits of one extreme or the other?"


"Does age play a role in ones ability to cause this to happen?"

I don't think the research has gone that far. I believe she is studying the chemical triggers and processes by which fibres change from one to the other.

Also, she was saying that humans don't even have type IIb muscle fibres.

What's up with that?


I've also read somewhere that the Type IIA fibers can be "taught" to act more like Type IIB or Type I depending on your training.


The muscle fiber types are much more plastic then people realize, fibers are just a combination of Myosin heavy chains, and latest research shows that there are really no pure fibers. To answer your question about creating more IIB or IIX (fastest contracting) fibers this generally doesn't happen with training of any kind. Usually Type I's transfer into IIA and IIB's transfer into IIA, however, the study I pasted below showed an increase in IIB % along with a decrease in Type I % after following a program of heavy and fast eccentrics. This is the only study i'm aware of to show any increase in IIB % following any type of strength training methods. I would say this definitely verifies shock methods, plyometrics, and various other forms of fast/heavy eccentric training. Have a look:

Adaptation to chronic eccentric exercise in humans: the influence of contraction velocity.

Paddon-Jones D, Leveritt M, Lonergan A, Abernethy P.

Department of Surgery, University of Texas Medical Branch, Galveston 77550, USA. djpaddon@utmb.edu

We compared changes in muscle fibre composition and muscle strength indices following a 10 week isokinetic resistance training programme consisting of fast (3.14 rad x s(-1)) or slow (0.52 rad x s(-1)) velocity eccentric muscle contractions. A group of 20 non-resistance trained subjects were assigned to a FAST (n = 7), SLOW (n = 6) or non-training CONTROL (n = 7) group. A unilateral training protocol targeted the elbow flexor muscle group and consisted of 24 maximal eccentric isokinetic contractions (four sets of six repetitions) performed three times a week for 10 weeks. Muscle biopsy samples were obtained from the belly of the biceps brachii. Isometric torque and concentric and eccentric torque at 0.52 and 3.14 rad x s(-1) were examined at 0, 5 and 10 weeks. After 10 weeks, the FAST group demonstrated significant [mean (SEM)] increases in eccentric [29.6 (6.4)%] and concentric torque [27.4 (7.3)%] at 3.14 rad x s(-1), isometric torque [21.3 (4.3)%] and eccentric torque [25.2 (7.2)%] at 0.52 rad x s(-1). The percentage of type I fibres in the FAST group decreased from [53.8 (6.6)% to 39.1 (4.4)%] while type IIb fibre percentage increased from [5.8 (1.9)% to 12.9 (3.3)%; P < 0.05]. In contrast, the SLOW group did not experience significant changes in muscle fibre type or muscle torque. We conclude that neuromuscular adaptations to eccentric training stimuli may be influenced by differences in the ability to cope with chronic exposure to relatively fast and slow eccentric contraction velocities. Possible mechanisms include greater cumulative damage to contractile tissues or stress induced by slow eccentric muscle contractions.