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Fast + Slow Twitch


im not sure, i am 90 percent sure all muscles are fast and slow twitch muscles.

but does anyone know if there are specific muscles which respond better to one or the other..

people say lateral delts, which the exercise for that is side raises, you should do them heavy for lower reps because they are fast twitch muscle fibres..

someone explain some shit to me




According to Charles Poliquin the triceps and hamstrings are the two most fast-twitch dominant muscles in the body. He has also said that delts tend to be more slow-twitch dominant.


There is no definite answer.

This will vary by person to a great degree - and if you do more high speed, power, peak velocity work (employing more glycolytic fibres) then you will have a higher amount of fast twitch fibres (IIRC this is Polymorphism).

That said, generally speaking the groups i have heard of are:

Gastrocnemius - Faster
Soleus - Slower
Bicep brachii - Faster
Hip and knee Extensors - Faster
Hip and Knee Flexors - Slower
Lateral delt - Slower
Trapezius - Faster

I know that the Soleus is taken from rodent studies, who as a group have a different structure function relationship with their lower limb than we do (as a group), BUT it seems to be true for humans too.

But this cant be followed (other IME than the lower limb ones which seem to be very accurate for most all people - due to our common function of support and walking i believe [with women having powerful lower legs due to the shared function of child bearing]) to any degree of accuracy, as for example i find my shoulder girdle and joint respond well to a training stimulus that targets the faster fibres primarily, as do other muscle groups, but this may well (will) differ for others.

As mentioned, there is no handbook, you need to find out for yourself.

I recently read that there is no size difference between oxidative or glycolytic fibres in humans and that the sarcoplasmic volume is no different either (being between 2-6% in slow and 5-6% in fast twitch in animals or certain mammals [would have to look it up again]).
This is important when looking to fast twitch fibres for greater growth potential for this reason, although there are other factors which still make this a truth i believe.

OP - this is a VERY complex subject (one which i enjoy thoroughly) and one that you will find goes right down from the muscle groups you know of down to the actual contractile and structural proteins that make up the contractile machinery of the muscle fibres themselves (on a scale honestly much smaller than comprehension!).

It should be noted (as with everything) it isn't quite as simple as the 3 'types' we know of, but there are not only other types in other types of skeletal muscle (which we dont need to think about for bodybuilding AFAIK), but that those 3 'fibre types' actually interact to make 15 contractile velocities (polymorphism IIRC).. so simply put - you do what works best for you, and keep your eyes and mind open.

This is now and always will be (IMO) the simplest and most effective way to train.

So try high reps for the seated calf raise and try low, each for 2 moths and see which you respond best to - at worst if you just repeat this for a year you will just gain moderately well! :wink:


I know there's a lot of writing out there breaking down what is typically fast or slow twitch muscles, but I always like to think that everyone has a different breakdown, and the only way to know 100% for certain is a muscle biopsy. I recall reading somewhere a long while ago that Skip Lacour was examined once, and was described as a 'fast twitch monster' -lol. Still, It's always smart to err on the side of what the odds state, and if no results come, change things up.



Another article (although ti could be bullshit) claimed that taking up to 200 mg of Co-Q10 can help morph slow-twitch fibers into fast-twitch. Supposedly it was previously believed that Fast-twitch could be morphed into slow and back again due to training, but not the other way around. Again, a lot of this comes from Poliquin.


My latest CSCS text actually touches on research that suggests that doing too much high rep work can actually condition fast twitch muscles to take on characteristics of slow twitch fibers (If I've got this slightly incorrect, someone with a better understanding please feel free to chime in!)



Interesting.... lol


Yeah, I'm studying exercise science and the lecturer explained this. Basically the fast twitch fibres can be either more fatigue resistant or more explosive. They will repond to the stimulous they get and can change between the two accordingly.
From what I understand though this would require a long period of time and prodominantly using one energy system (endurance athletes for example).
So by my reasoning, doing higher reps such as 12-20 is unlikely to make fast twitch fibres more fatigue resistant.
This is all getting a little off topic but its acutally the neural activation that will determine the fibre type.


Ok, I'll try to avoid making this overly complex, as I usually do.

A nice, simplified summary of muscle fiber types are type I, type IIa (fast twitch, fatigue resistance), type IIab, type IIb (fast twitch, glycolytic).

Summary points.

There is NO EVIDENCE to suggest that you can switch fiber types from type I to type II.

There is very good evidence to suggest that in SOME muscles, for example the paraspinals, the type I fibers can be bigger than type II.

WITH TRAINING, ANY FORM OF TRAINING, the predominant phenotypical shift is to decrease the relative percentage of type IIb fibers, and increase the relative number of type IIa fibers.

With strength training, this occurs with concurrent HYPERTROPHY of the muscle fiber cross sectional area (all fiber types).

Although NOT PROVEN in humans, it does seem we may have the potential to increase the total number of muscle fibers, which is HYPERPLASIA.

Most of the hypertrophy measured is directly due to increased fiber size, not sarcoplasmic hypertrophy.

If you do extensive training in both an endurance and strength context, while you can increase the relative proportion of type IIa fibers, the impairment in protein synthesis, the so-called INTERFERENCE PHENOMENON, will mean that gains in cross sectional area will be impaired.

STEROIDS tend to increase an individuals capacity to hypertrophy their type I muscle fibers to a greater extent than individuals not using AAS (yes, helps type II hypertrophy as well, but the big area of influence that is disproportionate to non-AAS lifters seems to be on hypertrophy of type I fibers).

Type II muscle fibers get bigger when they are exposed to a heavier load, type I fibers tend to hypertrophy regardless of load intensity.

Fiber type composition in each muscle is very individual, and, for training purposes, IS OVERRATED.

What is more relevant is the inherent architecture, and the potential for change.

Only continue reading if you don't mind thinking a bit about muscle structure/function. If not, STOP NOW.

A muscle with a penniform arrangement, where the fibers are aligned at an angle to the tendon/line of pull, have greater force producing capability at LOW VELOCITY. As you increase movement velocity, they lose their effectiveness. This would suggest that training a muscle group, such as the quads, at high velocity, is a waste of time. Pennated muscles also have greater potential to increase their cross sectional area as you can essentially pack more in, as well as a greater ability to change the angle of pennation to pack even more in.

A fusiform muscle is where the fibers are aligned parallel to the line of pull. They are generally smaller, as you can't pack as much in with the way the space is utilized. These muscles tend to be more efficient at maintaining a maximal level of force generation across a range of movement velocities. This would suggest that a muscle group, such as the hamstrings or biceps for example, will probably respond quite nicely across a range of training velocities as it can generate similar high forces regardless of speed.


But it IS theorised isn't it, and you must agree that a lack of evidence does not prove anything.

Really? that is very interesting - especially as it is the absolute opposite to popular belief.

Is this because the latter is generally more efficient? If not, why?

Again, a lack of evidence is not the same as evidence disproving the occurance (i am not so much stating this for your benefit as you clearly have opinions of your own on the mater, more for some of those reading).

Measured when? In rodent soleus cross sections? Not being picky, but where (what measurements) do you mean?
Also - for my clarification only, do you mean sarcomere hypertrophy? :slightly_smiling:

What would be the reason for this?

Which is direct relation to the size principle is assume?

I don't agree personally, i find having a knowledge of such things assists my training greatly, as opposed to stabbing around in the dark.

Nice post :wink:



Doesn't seem like anyone cares about this thread, so I'll answer the most interesting one of your points.

It has been shown specifically in the upper traps and vastus lateralis (lateral quad) of individuals on AAS that the area of type I fibers is substantially larger than in non-users, and that this was proportionately greater than the difference in type II fiber area.

It has been shown that type I fibers are more sensitive to anabolic agents than type II fibers. Furthermore, dosages of 300-600mg of testosterone seem to elicit increases in type I fiber area, whereas type II fibers enlarge after doses in excess of 600mg (from Sinha-Hikim, 2002; Am J Physiol).

The mechanism behind all this isn't clear. Could be increased satellite cell proliferation that responds to different doses in the different fiber types, could be something hormonal.

HOWEVER, the one I like the best is that increased levels of test helps to downregulate levels of cortisol (either absolute amount or relative balance to testosterone, which is often talked about as being a critical indicator of an overall anabolic environment, the T/C ratio). Since cortisol is involved in the breakdown of protein when we need fuel in certain situations, this is especially important for type I fibers. Type I fibers tend to utilize REDUCTIONS IN PROTEIN BREAKDOWN rather than increased protein synthesis as a primary mechanism of hypertrophy.

Oh well, see if anyone bothers to read this, bit too much science that will bore most people here to death.


Am I the only one left reading all of this? COWARDS!! -lol.



LOL, you rock Stu :slight_smile:


This thread took a turn for the best. Favorites...added


I'm reading as well.....


<- Also reading


Wow that was awesomely informative! I always did wonder about that topic...


I am definately still interested, thanks for the detailed response. I hope the disscusion continues.

WITH TRAINING, ANY FORM OF TRAINING, the predominant phenotypical shift is to decrease the relative percentage of type IIb fibers, and increase the relative number of type IIa fibers.
With strength training, this occurs with concurrent HYPERTROPHY of the muscle fiber cross sectional area (all fiber types).[/quote]
Thanks for clearing that up, I was just taking a stab in the dark (I hope that was clear).

This is interesting, my Prof brought this up in a lecture and he explained it is theorised that muscle cells can split in a way in which one or more neuclei break off to form a new muscle cell, but no new neuclei are formed. Have I understood that correctly? I wonder if this would have an effect on the function of the two daughter cells.

Ok, just tell me if I am just being too lazy and I will search this out, but is the imparement due to higher cortisole due to the endurance training?

In a sense I agree, trying for yourself and getting the basics right is key. I find it interesting to discuss though and today's grey area (theoretical stuff) might lead to solid aplicable information in the future.

Do you base your training around this at all? for example do you use a slow tempo on lateral raises? Also as penniform muscles produce their greatest force within a shorter range of motion, would reduced range of motion reps be more beneficial?


more ,more ,more !!!
i like this post :wink:)