T Nation

Can a FT Fiber Change into a ST Fiber?


(No, google didn't help. At least not in getting a definitive answer)

If I understood correctly, type II can't change into type I, and viceversa, and the differences found in, say, sprinters vs. marathoners are not that much due to variation in number of fibres (of course, a good sprinter has to be born with more type II's, a good marathoner has to be born with more type I's)...

...but due to the relative area of each fibre type (i.e. a sprinter has only developed his type II's, a marathoner has exclusively developed his type I's; so one can say that regardless of number of type I or II fibres one had, the type he didn't train atrophied, the one he did hypertrophied, and as such the relative areas are much lower/greater than in an untrained person).

Or, can type II's actually turn into type I's?

Asking because I had the revelation (yeah, maybe a year since reading 5/3/1) that doing some high rep sets might actually NOT reduce/hinder strength development, and be good for the joints. (hmm...if fibres don't change type, then developing type I's is actually very useful for PLing, right?)

Thank you for reading the wall of text,


From what I can remember, no. Can I back that up with anything though? no. Is this a pointless response? yes.


Yes, but it's not gonna happen from a 100 rep set every now and then. If you run a marathon every day on the other hand, things can happen.

Just lift things up and put them down.


I doubt they can change, you could increase your numbers of one and reduce the other i suppose. But wouldn't type2 changing to type1 be like saying fat turns into muscle??


No. The fiber itself cannot change. It can however, take on certain properties of different fiber types over an extended period of time (think of - FT taking on properties of LT fibers in olympic endurance runners or rowers). Massive amounts of volume over YEARS can force these physiological changes/adaptations but the fiber itself does not change and without the consistent stimulus, the fiber will take on more and more of it's original properties.


Yes, type I's turn into types II's. Dunno about the reverse, but I guess that happends too. However, as stated above, just lift things up and put them down.


What I remember from my gym instructor course is that you have your white fast twitch and red slow twitch fibres and they cannot be converted into each other. However, you also have pink, intermediate fibres that can be influenced either way.


We already know that type 2B will canvert to type 2A after resistance training. Regarding conversion of type 1 to 2 and vice versa I would say over a VERY LONG period of time and the right training that yes it is possible to convert between these fibre types. To what extent, I don't know as most studies only focus on short periods of time.

I have read in supertraining that fluctuations in thyroid hormone have an effect on the fast twitch/slow twitch ratio. In people with overactive thyroids, their muscles will show a tendancy to shift towards fast twitch fibres. People with underactive thyroids will show a tendancy towards slow twitch fibres. However this is only a minor shift apparantly. I would assume that abusing thyroid hormone might have a stronger effect.

I recall reading one study which showed a REDUCTION in the total amount of slow twitch fibres after a period of 8 weeks. The study involved subjects completing four 30 second sprints on a stationary bike with 15 minutes rest between each sprint. The subjects began with a 50:50 ratio (as far as I can remember) and finished with 40% slow twitch 60% fast twitch. So that study did show a conversion from slow twitch to fast twitch. Unfortunately I can't find the study anywhere anymore. I found it on google scholar. You may have better luck finding it.


Also in subjects that have had spinal cord injuries, over a period of time all muscle fibres become 100% fast twitch.
Another thing to note, is that if the nerve which supplies a fast twitch fibre is connected to a slow twitch fibre, then the slow twitch fibre will take on the exact characteristics of the fast twitch fibre. More food for thought.


Only 10 weeks according to Adaption to chronic eccentric exercise in humans: the influence of contraction velocity, study performed by Douglas Paddon Jones, Michael Leveritt, Andrew Lonergan, Peter Abernethy.

EDIT: I read only the first sentence of your post.


So, whether they can or cannot, doing some sets of <1 minute in length a few times a month shouldn't lead to conversion of type II to type one.

Thank you very very much. :slight_smile:



As you age you go from fast twitch to slow twitch.


lol no


Actually, I think it might not matter much whether or not it is or is not possible. I remember reading somewhere that there is not much of a correlation between muscle fiber type and powerlifting-type strength. Instead type 2a/b fibers seem esssential for muscle size or power in explosive events like high jump, sprinting, throwing, and the like.

I could be wrong about that, but I'm pretty certain I've read it before that some study(ies) reported that there is no correlation between fiber distribution and success amongst elite powerlifters. It seems reasonable, given that powerlifters do not need to develop power at very high speeds during their events, whereas sprinters/jumpers/ol lifters do.


OK I'm pretty sure this is the study I was talking about in my original post, although I think there may be another one:


"The proportion of type I fibres decreased from 57 to 48%". I find this very interesting. Most of these studies do show an increase in type 2A fibres, but only from the conversion of type 2B to type 2A. This study showed a DECREASE in type 1 and subsequent increase in type 2A, indicating that the type 1 fibres converted to type 2A.
What can be taken away from this study? Notice the extremely long rest intervals between each sprint, up to 20 minutes. This is why I always feel that "conditioning" work (walk back recoveries between sprints etc) is useless for increasing sprint speed unless you are severely out of shape. If you want to actually get faster then you need to go low volume with full recoveries between efforts (1 minute per 10 metres at least).


Yes, but it doesn't normally happen with any type or amount of physical conditioning. It takes something drastic for the change to occur in that direction, such as a surgical replacement of the motor neuron innervating the fiber. Fiber type is determined by the type of the motor neuron associated with it. There is a procedure called cross-innervation where scientists can take a muscle fiber of one type and connect it to a motor neuron meant for the opposite type, causing the fiber to switch types. ST fibers can also supposedly switch to ST if a spinal injury blocks all electrical signals to the fiber.


No type nor amount of training will convert FT to ST. It will only convert FT to a "slower" subtype of FT.


Then how do you explain these studies?


Is there any chance that the scientists measured relative surface, not number, of fibres?

OTOH, considering that "the neuron makes the fibre", changing type of fibre <=> changing type of neuron, right?


No. Things like motor recruitment patterns will determine efficacy of power generation and/or fatigibility (Yeah, I just made up a word), but have little bearing on the type of fiber. You most certainly do not change the neurons themselves, though.

And type I fibers do not turn into type IIb, there is, however evidence that type IIa fibers can migrate more towards the activities you pursue. They tend to start out more like type I in a lot of ways but if you only lift weights they'll end up more like type IIb (fast twitch, glycolytic).