T Nation

Muscle Fibre Types

Looking for some advise or direction on different muscle fibre types. Not just the difference between Type I (slow-twitch) or Type IIa/Type IIb (fast-twitch), but how they relate to specific muscles and muscle groupings. I’m sure there’s some info out there on it but I can’t find anything really good.

Little help!

This is to the best of my knowledge:

Primarily slow twitch:
biceps, lower back, glutes, hamstrings

Primarily fast twitch:
Triceps, chest, shoulders

I’m not sure of other groups.

I was under the impression that hams were mostly fast twitch (or predominantly so).

Gastroc=fast
Sloeus=slow

Not absolutely either way though, it’s as much an individual thing percentage-wise as anything can be.

My research showed that you can actually change the relative fiber make-up with specific training.

“The hamstrings are built for speed. As a result, they tend to be more fast-twitch dominant than most muscle groups.”

Quoted from Thibaudeau

yep hams respond to very low reps/high weight.

Soleus slow, gastroc faster (but still predominately slow IME)

Quads Slow

but as derek mentioned - the ratios and proportions of fibre types are very personal, like a finger print.

This is one of the reasons why BB’s take YEARS to find out what works for them, and why one program works for one guy but not another.

I personally get better growth in my Bi’s from heavy weight than low… slow or not.

Gastroc and soleus both respond massively to low weight.

These last 2 examples are personal to me of course… you may be the same, maybe different.

Derek, i believe it is true that fast twich type II (a or b, cannot remember) can be made a more Type I fibre, but not the other way around.

i have been reading about such matters in the ACSM Advanced phys. (great book)

JJ

Yeah, I see where you guys are coming from. A persons’ genetic make-up is going to ultimately define what their workouts should consist of. Whatever works for them.

Putting that aside, I remember hearing something about how muscles move against gravity and how that can help define muscle fiber types. I don’t know, I may be way off on that.

If you could confirm Derek, JJ’s point about specific Type II fibers being more likely to addapt to Type I types, that would help a lot.

You can naturally be dominant in one or the other. this is genetics, but you can increase the number of fast twitch if your slow twitch dominant, at least enough to be considered mixed fiber makeup.

If youve been doing bench press for a long time but never worked your legs except for running or cardio, your going to most likely be fast twitch or mixed fiber in your upper-body but slow dominant in the lower because the stress needed to cause the hypertrophy of fast twitch hasnt been applied.

However after a couple years of squats, you may consider you lowerbody to be mixed or even fast dominant. if your fast dominant, it is not likely that you will become slow-twtch domnant by doing lots of distance running, because slow fibers are not as easily stimulated for hypertrophy. thats just my opinion and the way i understand muscle fibers.

Specific kinds of training can apparently cause a conversion among FT sub-types of fibers (e.g., Type IIB and Type IIA), but the available evidence does not suggest that Type I and II fibers are interconvertible.

(Experimentally, the switching of motor neurons supplying fast and slow fibers has resulted in the gradual reversal of the speed with which the fibers contract.)

It has been suggested that, on a practical level, the transformation from one muscle fiber type to another is impeded by a number of natural conditions. In the case of the transformation of slow twitch into fast twitch fibers, any transformation stimulated by training may be countered by the use of the trained muscles for postural reasons.

(The low intensity and long- term kind of muscle action that is needed to maintain posture stimulates the slow-twitch qualities of the muscle, perhaps offsetting any stimulation for those muscles to transform into FT fibers.)

The transformation of FT fiber types to ST is probably impeded by the fact that considerable effort is required in order to reach the threshold necessary to activate the FT muscles often enough to transfer them to a slower type.

Several studies have shown the conversion of type II fibers to type I.

Under conditions of chronic electrical stimulation the Extensor Digitorum Longus in rats (which is primarily fast twitch) becomes almost entirely slow twitch, indicating a considerable conversion of type II to type I.

Also, hyperthyroidism in rats can convert the almost entirely slow twitch soleus muscle in rats to primarily fast twitch. If we immobilize a limb we can also observe a conversion of fast twitch fibers to slow twitch fibers. In fact, there are a host of different treatments (drugs, metabolic perturbations, hormones, activity patterns) that can enduce a large scale muscle fiber type conversion.

The fact that we can shift a muscle from 98% type I to better than 50% type II indicates that some fibers are shifting from type I to type II.

The conversion probably proceeds from type IIb to type IIa to type IIx then type I. In fact, evidence now indicates that the fast twitch contractile proteins (type IIa and probably also IIb) are constitutively expressed (expressed by default in the organism) and that it takes some kind of stimulus to express the type I contractile proteins.

This is supported by the numerous studies on muscle adaptions to disuse. If muscles are not allowed to contract (eg, by severing the nerve or physically immobilizing the limb) the muscles begin to express the fast contractile proteins. The longer the immobilization, the more fast twitch the muscles become.

Dr. Giarnella is correct to the extent that it has been difficult to demonstrate a conversion of type I fibers to type II due to exercise training. The current theory proposes that type IIb fibers are converted to type IIa during strength training and type IIb are converted to IIa and type IIa to type I during endurance training programs.

Years ago it was assumed that muscle fiber type was fairly stable and genetically determined. This has been disproven in recent years by literally hundreds of studies indicating that muscles posses an incredible degree of plasticity.

Mike Prevost, Ph.D.

soluble in-soluble

oh wait, wrong fiber.

[quote]derek wrote:
Several studies have shown the conversion of type II fibers to type I.

Under conditions of chronic electrical stimulation the Extensor Digitorum Longus in rats (which is primarily fast twitch) becomes almost entirely slow twitch, indicating a considerable conversion of type II to type I.

Also, hyperthyroidism in rats can convert the almost entirely slow twitch soleus muscle in rats to primarily fast twitch. If we immobilize a limb we can also observe a conversion of fast twitch fibers to slow twitch fibers. In fact, there are a host of different treatments (drugs, metabolic perturbations, hormones, activity patterns) that can enduce a large scale muscle fiber type conversion.

The fact that we can shift a muscle from 98% type I to better than 50% type II indicates that some fibers are shifting from type I to type II.

The conversion probably proceeds from type IIb to type IIa to type IIx then type I. In fact, evidence now indicates that the fast twitch contractile proteins (type IIa and probably also IIb) are constitutively expressed (expressed by default in the organism) and that it takes some kind of stimulus to express the type I contractile proteins.

This is supported by the numerous studies on muscle adaptions to disuse. If muscles are not allowed to contract (eg, by severing the nerve or physically immobilizing the limb) the muscles begin to express the fast contractile proteins. The longer the immobilization, the more fast twitch the muscles become.

Dr. Giarnella is correct to the extent that it has been difficult to demonstrate a conversion of type I fibers to type II due to exercise training. The current theory proposes that type IIb fibers are converted to type IIa during strength training and type IIb are converted to IIa and type IIa to type I during endurance training programs.

Years ago it was assumed that muscle fiber type was fairly stable and genetically determined. This has been disproven in recent years by literally hundreds of studies indicating that muscles posses an incredible degree of plasticity.

Mike Prevost, Ph.D.

[/quote]

Thankyou Derek.
Although i am a little peeved that he needed someone else to confirm my post… hmm… ;p