type IIb to IIa

What are the likely physiological stimuli for conversion of type IIb fibres to convert to type IIa?

Fatigue of type IIb fibres
Exercising at a high percentage VO2max
Lactic acid accumulation

I would like to learn more about the specific stimuli to prevent over conversion with 200 and 400m training.


(Pushed the wrong button)
Your list is a good starting point. Anyone else with thoughts?

Lack of involvment of type IIbs for power work. (have edited this - see below - was confusing)

Anyone else?

Other known causes include

stress (hormonal and physical)
muscle activation!
muscle tension

Only factors repeatelly shown to cause a shift from IIa to IIb is rest (particuarly after a training block) or denervation! Not ideal for training!

I wouldn’t worry about your fibres too much, rather worry about your performance adaptations. Lots of IIa fibre is not a bad thing - after all there is significant amount of speed endurance required even in a 100m sprint.

I have a question about lactic acid accumulation being on the list of converting type IIb to IIa. Now I wouldn’t think that lactic acid accumulation would encourage transition of the fibers from IIb to IIa. My reasoning being if there is lactic acid accumulation wouldn’t the main energy source would still be from substrate phosphorylation and not oxidative phosphorylation. And it was my understanding that conversion of IIb to IIa was caused by to much aerobic work.

Rossa is right. Set your objectives and, if you meet them, everything is adapting as it should.

Paul - we may be getting too far off on a tangent, but if you are doing work that produces lactate, with short recoveries - as in hard tempo workouts, or any training that causes an accumulation of La with short rests, the way that the La is dealt with or metabolised may increase the aerobic potential of the muscle. This COULD (only speculating!) cause a tendency to shift to IIa characteristics. I am not referring to Special Endurance where you are doing 300 to 600m with a long rest, but units where you have 10 x 400m with 2 min rest (just an eg!) where La clearance becomes important.

I think that Rossa and Charlie are right - don´t get too caught up in details that the scientists are really only speculating on - focus on the objectives, and how to attain them.

Hi guys, :slight_smile: I’m new here… now about fiber type transitions.

Well, it all comes down to gene expression as the final step in regulation of MHC isoform. And MLC. And troponin complex. And… But there are higher-level regulatory factors, one of them (and in my opinion the most important one, at least in training subjects) being neural activity.

So, why would IIx/b fibers convert to IIa? Let’s look from the other side of the problem: To accomplish transition from IIa to IIx or even IIb, neural activity must be phasic in type. That means very, very high frequency discharge rates of action potentials followed by relatively long pauses. That, of course, is not enough. The frequency of that kind of stimulation must be low and each individual session must be short. The tricky part is, the stimulation I’m talking about is electrical stimulation (human, normal muscle) as there is very little research concerning slow-to-fast transition where stimulation is physical activity – training.

Now, we know that changes in structure and dynamics of neuro-muscular system change the metabolic demand of the muscle which is followed by changes in muscle structure and function. If we combine previous and this fact, we have the answer to the original question: It is metabolic demand of training (that results in fatigue of type IIb fibers, lactic acid buildup etc.) that forces the transition from IIx to IIa fibers. That kind of training obviously isn’t »phasic« enough in nature. But this metabolic demand is, as I said at the beginning, directly related to dynamics of NM activation… so it is all in training design. With such a complex situation you can’t isolate one specific factor and try to eliminate it.

I would say that the second strongest stimulus for muscle fiber type conversion is T3.

And one more thing: Do not worry about IIb -> IIa transformation; as Charlie said, if your training is designed properly, there could be no significant changes. But if your training is some “fiction story”, well… start worrying because the transition could become your problem… transition from IIa -> I.

OK, just one more: More important than fiber type transition is de-novo formation, fusion etc. (satellite cells). Just something to think about…

I hope my post is understandable as I’m from Slovenia. :smiley:


Good post gregor

Can you tell me more about fusion & de-novo formation as it relates to sprint training?

Thank you

I appreciate the admontion to concentrate on overall performance rather than physiological details but can anyone explain why-apart from the beneficial effect on speed endurance- we do not need to be concerned about fibre conversion?

The implication of fibre conversion from IIB to IIA appears to some to be that a balancing act needs to be performed between resting/deloading to prevent conversion and the detraining that inevitably results from such rest. Doesn’t Bosco among others take this position?

On drsquat.com Kenny Croxdale has put forward the view that ballistic
lifting avoids the conversion but I am having trouble following his argument and purported evidence. Perhaps others can have a look.

BTW does anyone know precisely how conversion from IIB to IIA measured and do we know for certain that with this conversion the velocity of contraction of those converted fibres is the same as type IIA?


You can drive yourself crazy with these questions- or you can monitor your training results to monitor your progress. you can compare your pure speed to your speed endurance results to see where any modification is required. This method has been described at least since Valery Borzov.

According to the literature MHC IIB is not to be found in human muscle sample, only in rodent. The fastest protein found in humans seems to be IIX. There have been speculations that with further genetic research scientist might be able to implant IIB genes in willing athletes: That would be a tricky one for WADA to detect I’m afraid.

It has been very quit on this forum regarding TGH and other designed drugs. The day I heard of the bust against Balco and athletes using their services was one of the happiest in my 25 year frustrated sprint-coaching career. Hope that there is more of the same around the corner.

Håkan Andersson

QUOTE=Richard Hand]What are the likely physiological stimuli for conversion of type IIb fibres to convert to type IIa?

Fatigue of type IIb fibres
Exercising at a high percentage VO2max
Lactic acid accumulation

>>>>>I think Charlie and Rossa are right but still our Slovenian friend also hit a nerve here. IIb muscle fibres need high intensities to become involved. The more you give stimuli to them, the more they get IIa characteristics as it will become their final adaptation result. I would suggest not to get to that “finish line”. Train IIb but don’t transform them to IIa - if you run the 100m - do so if you compete in the 200m since this event need more sprint endurance.

By the way, wouldn’t be a unique way to stimulate IIb fibres by using Vibration training (Bosco)? :wink:

I like Charlie’s post above -
Quote: ‘don´t drive yourself crazy’ end quote…

with things that you cannot measure! IT´s how fast you run!! I like the KISS philosophy - Keep It Simple Stupid!!

Charlie’s philosophy of doing speed at 90% plus, and tempo at about less than 75% (of Max Speed) will avoid the conversion that we are all sweating about above. Working between 75 and 90% will convert IIb to IIa. And there ARE STILL no biopsy studies to say what a sprinter needs to be world class, in terms of fiber type. I was once pretty proud when as a biopsy guinea pig I was shown to have 80% fast twitch in my vastus lateralis. I told my boss, a great exercise physiologist, and he replied, quite calmly, that fiber type is NOT that important, there are many other factors to consider.

Don’t sweat the small stuff!

I’ve got about 90% fast-twitch in my forebrain…

Carson- I take on board the message about not losing sight of the bigger picture but some of us are just curious as to why:

  1. Conversion does not matter when we are constantly urged to undertake training programs designed to target the “fast twitch” fibers

  2. Some authorities think that “detraining” should be undertaken to prevent conversion.

I am also curious as to your assertion that speed at 90% plus and tempo at less than 75% in accordance with Charlies philosphy will avoid conversion. I do not know if this can be backed up by any studies but its very assertion seems to imply that conversion should be avoided.


Let’s not get ahead of ourselves. I suggest using 95% and up and 75% and down as a method to optimally develop speed. I can then put forward some suppositions as to how a program that has been successful might be explained.

Peter - I don´t have the answers - this is pretty theoretical. The scientists don´t have measurement methods that are fast and inexpensive, and alot of what they are suggesting IMHO is still speculation. A sprinter wants to maximize his anaerobic potential IMO and avoid conversion. But I think we are getting off on a tangent that is not taking us anywhere. I didn´t say that conversion was not important, but we can´t measure it. We can measure max speed, and speed endurance. Program design should target measurable goals.

If you are really curious, get a good exercise physiology text, read up on the characteristics of I and II(a,b,c) fibers, and how they are recruited, and then try to determine how training will target the fibers you want to train. IMO one cannot target JUST IIa, or JUST I but one can create tendencies for recruitment by the training demand.

OK a coupla thoughts, firstly no scientific literature supports that Charlie’s training which excludes running intensities between 75 and 95%is the way to go to maintain IIb/IIx fibres. Having said that there is plenty of anecdotal evidence to suggest that such programming does make you run fast, so again why worry about changes you can’t measure and that aren’t all that closely tied to performance! As far as I know only one sprint study has shown regular training and an improvement in the proportion of IIb fibres (Jansson et al., 1990), however, these results were obtained using histochemistry rather than the more sensitive electrophoretic techniques used more frequently now, and these results have not been replicated even in further papers from the same lab. Nevertheless some individuals in some other studies have also shown increases in the proportion of IIb fibres… perhaps such individuals possess either the prior training background or the genotype for success in sprint events… There’s a talent ID project for somebody!
Carson, you’re right there are no inexpensive or fast techniques for accurate fibre type assessment, however muscle contractile characteristics can be rapidly assessed with electrical stimulation and assessment of the twitch torque characteristics eg peak rate of torque development and/or relaxation. IMO this is probably more informative to an athlete anyway than knowing exact proportions of fibre type in that tiny piece of muscle that is a biopsy, as rapid force/torque production is what sprinting is all about… such assessment can be used to track training or detraining adaptation. Worth consideration anyway…
Finally Peter, yeh we do need to be concerned with fibre type adaptation but I’d worry about a shift from IIb→IIa→I and not so much IIb→IIa←I (particularly if the total amount of type II fibre is increasing in the latter example). And yeh you’re right about detraining for fibre type maintenance/improvement but it comes with the cost of fibre atrophy loss of neuromuscular adaptations…

good post, rossa.