Heavy strength training and fiber conversion

According to Journal of Applied Physiology, 74, 911-915. prolonged heavy loading of muscles converts most type IIb fibers to type IIa.

Consequently, according to an experiment by scientists from the Muscle Research Center in Copenhagen, a muscle biopsy (vastus lateralis) taken after prolonged heavy loading showed that there was a decrease in the proportion of fast twitch IIb fibers (from around 9% to 2%) during the resistance training period. But after three months of sedentary living, researchers found that the proportion of IIb fiber restoration doubled to around 18%.

Is this accurate?


There is an interesting article related to that subject at www.coachr.org/tpjrs.htm . The article is about strength training and special strength development for primarily long and triple jumpers and this phenomenon is discussed as are the projected implications of that study upon training, etc.

Pretty much.

What you have to realize is that IIx/IId MHC is pretty much a non-factor in trained athletes. Best theory to date is that it contracts too quickly to be of use, explaining why there’s a shift towards IIa in practically every anaerobic athlete.

It seems that in FT fibers (neurally defined), IIx expression serves as a “marker” of disuse…which would explain why it’s often seen in high-threshold fibers, as those are only targeted by specific forms of training, as well as it’s occurance after detraining.

At the peripheral level, MHC expression is little more than a sign of activity-- form following function. Research has shown as well that the shift in MHC expression does not correlate to performance, either positively or negatively. Neural activity is and always will be the defining factor.

The message is don’t take studies like that to mean that there’s a dropoff in performance. The shift to IIa is actually a good thing, because it means that you’re training a wide range of fibers.

Thanks for the link to the article. Other information that I found interesting in that article is RFD info and the development of power with weights of various loads; as long as maximum speed of movement is attempted.

A thanks for the info, makes sense!

Could you also get from the aforementioned study that strength training should be periodic with an off period to allow for performance peaks (via overcompensation of IIb)? Or do you feel that the performance peaks can be realized while maintaining a weight program (via neural activity)?

As far as neural factors go, there’s reason to load and unload…psychological arousal and neural output are linked intimately, and (as a function of the volume and intensity of the output/stress) there will be times that periods of unloading are necessary. I don’t really think that the IIx overshoot phenomenon is that important for strength training, at least based on what I’ve seen so far. It may well be linked, just that I’ve seen nothing supporting it.

Interestingly enough, there was a study I came across recently showing that the calcineurin/calmodulin signaling pathway (which is more or less neurally dependant as it’s downstream of Ca++) is most active in least-used fibers. Calcineurin is at least partly responsible for MHC expression. So that’s at least some of the mechanism by which the fiber shift occurs. If that’s true, it would postulate that at least some IIx shift would occur during deloading intervals; how much I don’t know, and it would be very dependent on how significantly the intensity and volume dropped off, as well as how long. Whether this, being a peripheral factor and one dependent upon central action, is that critical I can’t say for sure but my intuition says no. Neural action seems to be the driving component. I don’t have the references here at the moment, but I can post it when I get home if anybody’s interested.

For hypertrophy gains, that’s a different matter. Detraining for brief periods can have significant effects, mostly seeming to stem from the loss of adaptation to both eccentric damage and inflammatory action. In this instance, IIx overshoot may well play a role, though again this isn’t something I can verify without more time Pubmed-jockeying.


Appreciate the response! Good stuff! Also, if you are able to post any references, I would appreciate it. Nevertheless thanks!

Optimal ages for muscle fibre conversion? Al Vermeil sited a few studies that suggest that athletes between 12-18 are most succeptible to this “conversion”. I think rather it had more to do with power training resulting in there being a larger cross section of white muscle fibre instead of a higher number of white muscle fibre cells.

Type IIb/x overshoot is important and needed. Jonathan Edwards was able to achieve his world record performances according to his coach Carl Johnson, because of the fact that he had epstien-barr syndrome which meant that he had to forceably unload prior to his world record breaking year. If you look at “Speed Trap” I postulate that Ben’s performance was aided by his enforced down grade in volume of training due to his hamstring injury coupled with CF’s unloading /tapering week and Ben’s recuperation with Astaphan. I think there is much to be said and gained from unloading. I ran my fastest times after a very intense week of training involving intense speed endurance sessions, followed by complete rest for approx four to five days. Overshoot will also occur in testosterone levels and adrenal glands, hypothalamus and the master gland. There is much to be said about enforced rest. Do not feel afraid to rest, it takes about SIX MONTHS to loose enzymes gained from sprint training. This does not mean that you stop training for six months, I am not advocating that at all!

Interesting stuff! It would seem that allowing for down time/rest would benefit the organism in many ways (hormonally, physiologically, psychologically, etc.) Thanks!

Recovery periods can be fine, but only after VERY hard work. And don’t conclude Ben did nothing when he was injured or that general work didn’t play a role in his specific results. He had a significant pyramid of uppper body weights as well.
A few facts:
1: He was comming off a significant history of speed development over the previous 3 years, including over 60,000 meters of speed work in 1987.
2: Ben’s phase one was completed successfully and several world indoor records were broken before the first injury in Feb.
3: His upper body weights reached new highs during the injury period. Examples include- Bench 2 x (10 x 365), Incline 1 x (10 x 330) in May.
Bench 4 x (5 x 405) in June! As well he did substantial work on pull-ups, Seated rows, Wide and narrow grip pull-downs, hanging leg raises out to the hands on both sides for sets of 30 (try that some time)


That fits with what I was trying to put across. I wasn’t advocating that nothing is done and total complete rest for weeks on end, as I said a down grade (decrease) in volume of work.


Do you think an unloading phase consisting of little to no weight is necessary for adaptations to occur or is the normal microcycle of 48hrs between lifting sufficient?

For example, I’ve heard it takes 24-48 hours for muscle tissue to breakdown and be excreted and another 48+ hours to rebuild. How does this affect lifting?

The point that should still be stressed, going back to the original topic, is that expression of MHC-IIx is more akin to a symptom of unloading, rather than an actual causal factor in recovery.

Simply put, it just doesn’t appear to be that important once neural and endocrine factors are considered.

An unloading phase will be necessary when the load is very high- whether that is as maintenance, or, at time, as rest.

Ok! thanks a lot for the info. Also, I look forward to your SPP DVD. The best to you and your family.

Too many words have been spend on muscle fibre composition and muscle fibre conversion, since most research articles are based on non-elite athletes and very little work has been done on e.g. elite sprinters. Our group had the unique opportunity to take muscle biopsies of e.g. Nelli Cooman (multiple!), Marlene Ottey and a 400m runner (still Dutch record since 1986- 45.68).
Another interesting article is the research done by taking a muscle biopsy from worldchampion shot put Werner Gunthor from Switzerland.
Not only the fibre type is important, but also the selective hypertrophy of the different fiber types!
A very good article about this subject was written by the doctor who performed the biopsies on both Nelli and Merlene: Bill Laich- M.D.Ph.D: “Fibre-specific: that’s what your training should be for maximum results”; Muscle and Fitness, april, 1989, pg.166-168, 254, 256, 264.

Henkra- you say that too many words have been “spend” about muscle fibre composition and muscle fibre conversion but with respect you are guilty of doing exactly the same thing by doing little more than asserting that “not only fibre type is important but also the selective hypertrophy of the different fibre types”.

What are we expected to make of a reference to an article printed 15 years ago in such an esteemed magazine as Muscle and Fitness?

It’s not a question of where it was printed- but who wrote it, and who the test subjects were. There’s a BIG difference between Merlene Ottey and a “lab rat” who “Volunteers” to take part in a study, so he won’t fail phys-ed!
Drawing conclusions is another matter.

The general points you make about the author, subjects tested and the drawing of conclusions are certainly true but the relevance of where it was printed is that to the extent it purports to be scientific it would not be peer reviewed but more to the point I cannot imagine that many of us would have read or have access to a 15 year old article in M&F.

You can read about findings from a historical perspective, without drawing, or accepting, conclusions. As for peer review- don’t get me started! Just who, exactly, are these peers?
When you begin to move out front, the group of subjects gets smaller than conclusive studies could accept- till you reach a sub-set of ONE. By then you’ve left the peers, with their lab rats and treadmills behind.