- Bodybuilding Forum - Bodybuilding Forum (
-   Training (
-   -   Fiber Types, Training, and Hypertrophy (

EricT 12-20-2006 02:10 PM

Fiber Types, Training, and Hypertrophy

I decided to make a thread of some of the things I've posted about this subject.

I got into this notion of isolating fiber types through different reps and speeds but once I began to learn about how muscle fibers are actually recruited I dropped it.

This excerpt is the kind of thinking this leads to and, indeed, this is exactly the kind of thing many personal trainers have been brainwashed into thinking:

Originally Posted by Jason R. Karp, M.S
Train according to genetic predisposition

For maximum results, try to train your clients according to their genetic predisposition. For example, someone with a greater proportion of slow-twitch fibers would adapt better to a muscular endurance program, using more repetitions of a lighter weight, or cardiovascular exercise at low intensity for longer periods. Likewise, someone with a greater proportion of FT fibers would benefit more from a muscular strength program using fewer repetitions of a heavier weight. Of course, the clientıs goals and needs should be considered as well. You wouldnıt want to train a 40-year-old female client to develop larger muscles if thatıs not her goal, even if she does have a greater proportion of FT fibers.

Except for the part about training based on goals, the rest is complete bullshit. EVERYONE can grow some muscle. It’s SUCH BS it pisses me off.

Fiber Types, Training, and Hypertrophy
From the Hypertrophy-Specific Training Series
By Bryan Haycock, Editor-in-Chief

The issue of fiber type that continues to resurface in the discussion of different training methods. Some people insist that in order grow to get maximum growth, muscles must be trained according to fiber "types". A look at what determines a fiber's "type" should help clear up the issue and help you make a decision as to its relevance to training specifically for muscle growth.

"Fiber types" and how they are classified.

Muscle fibers were first classified according to their "function". Fast-twitch and Slow-twitch are the two basic types. It was later discovered that the twitch (twitch = contraction) characteristics were the result of different kinds of contractile proteins. Some proteins were good at contracting quickly, and were also dependant on "fast oxidative" pathways (ATP, and fast glycolytic pathways). The other type, slow-twitch, has contractile proteins that were different than those in fast-twitch fibers, and were dependant on "slow oxidative" pathways (beta-oxidation, fatty acid oxidation).

The two distinct metabolic profiles of fast- and slow-twitch fibers, give them distinct fatigue profiles. Fast-twitch fibers fatigue rapidly because their fuel source, ATP, is depleted rapidly. I use the term "depleted" loosely. Slow-twitch fibers fatigue slowly because their fuel source (fatty acids) take a long time to deplete.

There is another factor in the fatigability of fast- and slow-twitch fibers. The amount of power they are able to generate. Because fast-twitch fibers contract quickly, they are able to produce more "power" than slow-twitch. So fast twitch fibers use their available more quickly because the "motor-units" are larger. A motor-unit is a group of fibers connected to a single motor neuron. Keep in mind that power is a function of work over time.

The purpose of each different type of fiber?

Fast-twitch fibers are used to move your body mass quickly. This is important for running, jumping, and reflex movements (e.g. pulling your hand away from a hot stove). This requires short burst of relatively high force (but with low precision). Slow-twitch fibers are used to support the body posturally. This requires long/sustained contractions of relatively low force (but with high precision).

You will find a high proportion of slow-twitch muscle in the calves, and trunk (spine) and in the forearm predominantly. This makes sense when you think about it. Your calves, which contain your toe and foot muscles, are constantly working to balance your body while standing and walking. They are contracting constantly when you are standing. Your trunk muscles hold you upright when you are standing or sitting unsupported. Your forearms house your finger and hand muscles. These are used to hold things. Holding requires constant contraction of your finger muscles.

Isolating fiber types in training.

Forget about the notion of isolating fiber types while training for hypertrophy. You can't isolate fiber types per se when lifting a weight sufficiently heavy to cause muscle growth. Let me explain. Your brain activates muscle fibers in a specific sequence and manner based on the kind of movement it desires. This progressive activation of muscle fibers is called recruitment. Small "motor units" (motor neuron-muscle fiber unit with a low threshold of activation) are activated first to produce precise movements. These small motor units use slow-twitch fibers.

If activation of the inductive small motor units is insufficient to produce the desired movement, the brain activates progressively larger and higher threshold motor units. These larger motor units involve fast twitch fibers.

So, slow-twitch fibers are recruited first, followed by fast twitch fibers, based on the needed amount of strength (force or power). Because of this recruitment pattern, you could theoretically isolate small slow-twitch fibers, but you couldn't isolate fast twitch fibers because your brain activates slow-twitch first during any contraction. The greater the force of contraction, the greater the number of fast twitch fibers will be activated, but only after all slow twitch-fibers are activated.

So picture in your mind a dial that goes from 0 - 11. The numbers indicate how much force you want the muscle to generate, 0 being none and 11 being maximum intensity contraction. ON the dial, going from 1-5 the body will activate an increasing number of small motor units (slow twitch fibers) until it has activated them all. From 5-11, the small motor units will remain activated, but the body will add to them, large motor units (fast twitch fibers) until the desired muscular force is achieved. You progressively fine motor control as the amount of force goes up. This is a manifestation of the recruitment pattern just described.

Fiber type and muscle hypertrophy

Both slow twitch and fast twitch fiber are able to hypertrophy when exposed to overload. In a study by Hortobagyi, muscle fiber size of the quadriceps were compared after 36 sessions (12 weeks) of maximal isokinetic concentric or eccentric leg extensions. Type I fiber areas did not change significantly, but type II fiber area increased approximately 10 times more in the eccentric than in the concentric group.

There is a tendency for fast twitch fibers to experience more damage from training, thus fast twitch fibers tend to hypertrophy "more readily" to heavy resistance exercise. Nevertheless, both fast and slow twitch fibers hypertrophy. If you look at a bodybuilder's cross section of muscle fibers, you will find both fiber types hypertrophied, this being due to the inclusion of both concentric and eccentric contractions under load.

In conclusion fibers are classified into two different types, fast and slow. The distinction between the two types of fibers is based on both their contractile properties, as well as their metabolic properties. Slow twitch fibers, associated with small motor units, are activated first when a effort is applied against an object. Once all small motor units have been activated large motor units, involving primarily fast twitch fibers are activated.

All exercises performed by a person trying to build muscle are, of necessity, performed using sufficient weight to activate all slow twitch fibers and most fast twitch fibers. Both slow and fast twitch fibers will then hypertrophy. Fast twitch fibers will hypertrophy first, and to a greater extent, due to their susceptibility to cellular micro-trauma during the eccentric portion of every rep.

When trying to grow muscle, it is worthless to try to adjust the program to "stimulate" or "isolate" any specific type of fiber. Recruitment patterns involved in lifting weights heavy enough to cause hypertrophy activate all fibers, both fast and slow.


Cope, T. C, and M. J. Pinter. The size principle: still working after all these years. News Physiol. Sci. 10: 280-286, 1995

Hortobagyi T, Hill JP, Houmard JA, Fraser DD, & colleagues. Adaptive responses to muscle lengthening and shortening in humans. J. Appl. Physiol. 80(3): 765-772, 1996


About the Author

Bryan Haycock M.Sc. is an exercise physiologist and NPC judge. Bryan has been bodybuilding for over 20 years and holds certifications with the NSCA, ACE, and is a member of the American College of Sports Medicine. Bryan is currently the Editor in Chief of and is the founder and CEO of Bryan is a highly sought after authority on the physiology of muscle growth and fat loss. Bryan also specializes in the management of type-II diabetes through diet and exercise.

EricT 12-20-2006 02:11 PM

Muscle Fiber Recruitment

Muscle Fiber Recruitment Again

Dr. Darden’s Fiber Recruitment (originally posted by Madcow

I haven’t read through these yet but I’ll post anything by Kelly Bagget:

Understanding Muscle Fiber Type by Kelly Bagget

Fiber Type II: Becoming a Fast Twitch Machine by Kelly Bagget

hrdgain81 12-21-2006 05:40 AM

I never saw much reason to concern myslef with the fiber type and recruitment crap you see every now and again.

To me, even if it were fact, it would only serve to explain what I've been doing all along. And that never really concerned me.

besides you know how I feel about hypertrophy anyway.

EricT 12-21-2006 08:19 AM

^^^^Good for you!


Originally Posted by hrdgain
To me, even if it were fact, it would only serve to explain what I've been doing all along.

But I don't think your understanding what the thread is addressing. It's not meant to dwell on fiber rectruitment for the sake of it. It's debunking "training to fiber type". Hopefully, you didn't mean that's what you've been doing all along (j/k). Orderly recruitment as explained in the material I gave is the truth to our current level of understanding.

If you research and read like I do, you'll know that there are many so-called experts out there who COMPLETELY ignore the reality of orderly recuitment and tell people the only way to do it is to fatigue this fiber through a certain rep in order to "get" to that fiber, etc., and so on. There have been workouts posted here advocating that very thing. Or you even have people saying that certain individuals can't gain muscle at all due to predominant fiber type.

Check out the shit a, for example. And people could easily be taken in by it, because it sounds sciency and smart.

I have been asked these questions myself. The purpose of the posting was for those people who may be drawn in by such thinking. Not just beginners, I've seen more experience people drawn in by it to. For most of us, as you say, it is not something you need to think about. But I put this here to show people the way it DOESN'T work, as much as anything else.

BTW, I put hypertrophy so more people might actually read the stuff :biglaugh: . Someone has to post the non-flashy nuts and bolts things. Your muscles work the same way when your strength training.

EricT 12-21-2006 08:32 AM

Here is an example of what I'm talking about:


Training to Maximize Your Muscle Fiber Types
By Nick Nilsson

Your muscles are made of 2 different types of fibers.
Find out what they are, what your personal fiber make-up
is and how to train for maximum results.

Knowing your personal muscle fiber make-up can be an invaluable aid when it comes to properly targeting your training program. If you're working your muscles in the wrong way, you'll be cheating yourself out of hard-earned results.

Every muscle in your body is made up of a bundle of small fibers. In each bundle, you have two main types of fibers: slow twitch and fast twitch. I will explain exactly what these are in a moment. The percentages of these different fiber types that your muscles are made of can help you determine exactly how you should train each particular muscle group in your body.

Slow Twitch: These are also known as Type I or red muscle fibers. They are responsible for long-duration, low intensity activity such as walking or any other aerobic activity.

Fast Twitch: These are known as Type 2 or white muscle fibers (divided further into A and B). They are responsible for short-duration, high intensity activity. Type 2B fibers are built for explosive, very short-duration activity such as Olympic lifts. Type 2A fibers are designed for short-to-moderate duration, moderate-to-high intensity work, as is seen in most weight training activities.

By looking at elite athletes in different sports, you can see extreme examples of each make-up of muscle fiber. At the slow twitch end is the endurance athlete, such as the marathon runner. These athletes can have up to 80% or more of slow twitch muscle fibers in their bodies, making them extremely efficient over long distances. At the fast twitch end is the sprinter. World-class sprinters can have up to 80% or more of fast twitch muscle fibers in their body, making them extremely fast, strong and powerful but with limited endurance.

How to Find Your Muscle Fiber Type:

To find the predominant fiber type in a particular muscle in your body, we need to test the repetition limits of a muscle compared to its maximum strength. Keep in mind, these limits can be altered by your training and are, therefore, just rough estimates.

First, determine your one rep max (known as the 1 RM) for an isolation exercise for that muscle group, e.g. the dumbell curl. Find the weight you can only do one rep with. You want to use an isolation exercise because any exercise that uses any other muscle groups will skew the results.

Once you've figured out your one rep max, take a weight that is 80% of it (multiply your max weight by 0.8 to get this) and do as many reps as possible with it.

- If you can do only 4 to 7 reps with 80% of your 1 RM, you
have mostly fast twitch fibers in that muscle.

The reason you will only be able to do 4 to 7 reps with
80% of your 1 RM is that fast twitch muscle fibers are
strong but don't have great endurance. You will be able
to lift more weight but you be able to do as many reps
with it.

- The ability to get approximately ten reps with 80% of
your 1 RM is the typical fiber-type mix for a muscle.
This works out to about a 50/50 split between fiber types.

- If you can do 12 to 15 or more reps with 80% of your 1 RM,
your fiber make-up is probably mostly slow twitch fibers.

Slow-twitch fibers are not as strong but have excellent
endurance. This means you won't be able to lift quite as
much but you'll be able to do a lot more reps with it.

Repeat this procedure for each muscle group you wish to determine a type for (it will vary from muscle to muscle). By knowing what type of muscle fibers you have, you can tailor your training towards developing them to their maximum potential.

Though there are always differences in individuals, there are some general similarities in fiber types in muscle groups from person to person.

For example, in most people, the outer, visible muscle of the calf (the gastrocnemius) is primarily made of slow twitch fibers while the soleus (which lies underneath the gastrocnemius) has a higher percentage of fast twitch fibers.

Two more examples of this similarity between people include the abdominals and the hamstrings. These two muscle groups are both generally made primarily of fast twitch fibers.

How to Train Your Muscle Fiber Type:

When you're training with weights, your goal is to work as many muscle fibers as possible. Affecting more muscle fibers means greater gains in strength and muscle mass.

If your fibers in a particular muscle consist primarily of slow twitch fibers, in order to affect the greatest number of those muscle fibers, you'll need to train that muscle with higher reps, shorter rest periods and higher volume. This is because they take longer to fatigue, they recover quickly and they require more work to maximize growth.

Unfortunately, slow twitch muscle fibers are limited in their potential for growth so even if a muscle group is primarily slow twitch, you should definitely include some lower rep training to maximize the fast twitch fibers you've got in that muscle.

If you find you have a hard time gaining size in a particular muscle, it could be because it has a predominance of slow twitch muscle fibers. Higher reps (e.g. 12 to 15 reps), higher volume (more sets) and shorter rest periods (30 seconds to a minute between sets) can help you to maximize those muscles. This doesn't mean you should use light weight, though. You should still strive to use weights that are as heavy as possible that will cause you to reach failure in those higher rep ranges. If you don't use heavy weights, you won't give your muscles a reason to grow.

If your fibers in a particular muscle group consist primarily of fast twitch muscle fibers, you're one of the lucky ones. You'll have a much easier time building mass in that muscle - fast twitch muscle fibers have greater potential for size than slow twitch. The more fast twitch fibers you've got, the greater your ultimate muscle size can be. These muscles are most likely your strongest and quickest to develop.

To maximize your muscles with fast twitch fibers, you'll need to train with low to moderate reps (e.g. 4 to 8 reps), rest periods of around 1 to 2 minutes and a moderate training volume (too much volume will compromise recovery).

If your muscles have a fairly even mix of fibers, you can evenly divide your training between focusing on the lower-rep, fast twitch fiber training and the higher-rep, slow twitch fiber training. This will help you to develop all the fibers in your muscles, maximizing your ultimate development.

EricT 12-21-2006 08:57 AM

Another example of this has to do with rest periods. Someone at some point quoted Arnold's book on this. Part of it is the idea that if you allow your muscles to recover too much between sets you will not be able to "recruit" all your fibers. It's just that recruiting a fiber doesn't mean you've "stimulated" a fiber. Hence more than one set, in a nutshell.

hrdgain81 12-21-2006 09:28 AM

hahah i was actually agreeing with you eric.

I've never heard it said that some people cant gain because of fiber bias, thats a new one to me.

EricT 12-21-2006 09:33 AM

^^^LOL, your wording through me off :) . I wasn't sure which side of the coin you were playing!


Originally Posted by hrdgain
I've never heard it said that some people cant gain because of fiber bias, thats a new one to me.

Oh yeah. I've seen it on numerous occasions. Pretty much any inane thing you can think of, someone will write an article on it!

All times are GMT -8. The time now is 03:40 PM.

Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2017, vBulletin Solutions, Inc.