Types of Muscle Fibers and How to Train to Maximize Your Growth

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In this article you will discover how your body uses different types of muscle fibers and how to take advantage of this knowledge to optimize your muscle mass growth.

Not all muscles are created equal. Moreover, not all muscle fibers in the same muscle are created the same.

Simply put, you could say that there are two categories of muscle fibers:

  • Slow-shrinking fibers – also called tonics, oxidatives, reds or type I – used to generate low to medium tension for long periods of time, such as maintaining posture, walking slowly or running marathon-type.
  • Fast-twitch fibers – also called phase, glycolytic, white or type II – used when a much greater force is needed (such as running a sprint or moving a significant weight) but they get tired much faster due to the energy processes they require.

Depending on the proportions of fibers you could say a muscle is tonic or phasic. For example, the triceps, which has almost 70% fast-twitch fibers, is a strong or “phasic” fiber, while the suboccipital or spinal erectors are postural muscles with a predominance of slow-twitch “tonic” fibers.

This is just a general ranking. In reality, things are more complicated, and scientists recognize many more types of muscle fibers, which differ from each other depending on:

  • Nerve activity – the size of the motor units that stimulate contraction
  • Metabolic processes – the way energy is produced, using the aerobic (oxidative) or anaerobic (glycolytic) system
  • Capillary density – the amount of blood that reaches them
  • Mitochondrial density – the number of mitochondria (that part of the cell that produces energy)

Thus, based on different combinations of these characteristics, there are many intermediate groups and subtypes of muscle fibers.

Type I fibers are easily recruited due to the small size of the motor neuron, can slowly produce a much lower force, but can keep it for a long time due to vacularization, the number of mitochondria and the use of the oxidative system.

They do not have a high growth capacity and are associated with weak people who participate in endurance activities.

However, even if the top marathon runners have a higher proportion of type I fibers and very little muscle mass overall, this does not mean that you cannot develop the type I fibers significantly enough to increase your weight even more. muscular.

Type IIb fibers are recruited by the nervous system only in cases of maximum need, requiring a higher stimulus to activate the motor neuron. They produce a very high force, very fast, intervening in intense and explosive efforts such as sprints, jumps and lifting maximum weights.

Basically, in order to cut and develop them, you will have to lift something very hard or very fast.

They get tired very easily, but have a high capacity for hypertrophy (growth) and therefore should be the main focus of a training program for muscle development.

Type IIa fibers are quite similar to those of IIb, but due to the capillary density and the possibility of using oxygen in energy processes, it resists more effort.

Having a smaller motor neuron, they are more easily recruited and are the main “target” of classic bodybuilding workouts with 8-10 repetitions per series.


All people have a certain mixture of types of muscle fibers.

Indeed, studies show that top powerlifters have a higher percentage of type II fiber and more marathoners type I, but this is not necessarily related to genetics, but also to the way they have used their body for years. .

An interesting study from 2003 and 2004 shows that the best bodybuilders in the world do NOT have a different fiber distribution than an ordinary man. In other words, while a powerlifter has more Iib type fibers and you might be inclined to think he was born with that potential, a top bodybuilder has about the same proportions of fiber as a row type, but they are ALL much more developed.

Knowing this, one of the myths of bodybuilding is eliminated. Certainly genetics play an important role, but in terms of fiber, you also have the potential to significantly develop your muscle mass as long as you train intelligently, stimulating all types of muscle fibers.

Muscle Mass Maximization Solution

Knowing the above, in order to put on as much muscle mass as possible, you will need to train all types of fibers:

  • Type I – which exerts less effort for longer periods of time, can be maximized using a resistance (weight) that allows you somewhere between 10-15 reps, sometimes even 20 or more for certain muscle groups.
  • Type IIa – the one who is typically trained in a traditional program can be stimulated as much as possible using resistance that allows you somewhere between 6-10 repetitions per series.
  • Type IIb – the most “strong” of all, will need to be trained and maximized using very high resistance that does not allow you to do more than 3-5 repetitions.

Basically, in order to stimulate all your fibers and increase your muscle mass as much as possible, you will have to train using a wider range of sets, repetitions and weights:

  • from 8 sets x 3 reps,
  • at 5 sets x 5 reps,
  • at 4 sets x 6-8 repetitions,
  • 3 series x 8-15,
  • 2 x 15-20
  • and all in between.

Obviously, each performed with a weight that does not allow you more repetitions than the target number or interval.

And for all this you will have to take into account the target muscle groups and the movement used.

For example, you would never want to do 8 sets of 3 repetitions of side shoulder fluttering or leg extension to the machine. Those muscles have mostly type I fibers and respond better to more repetitions, and using such high intensities you will put a lot of stress on that wrist significantly increasing the risk of injury.

Training organization

There are several ways to organize all of this into one training program.

A simple option is to divide the week into sessions focused on type IIb in which you do only compound exercises for 5-8 sets of 3-5 repetitions, sessions focused on type IIa in which you work 3-5 sets of 6-10 repetitions, and type I focused sessions in which you perform 2-3 sets of 10-15 or even 20 repetitions for smaller muscle groups.

Another (more efficient) way to organize all this in a well-thought-out program that maximizes your muscle mass gains is the one I describe step by step in the “Muscle Mass Secrets” system:

  • Divide your whole body into 3-4 basic movements that work as many muscles as possible at once and allow you to move significant weights without putting too much stress on your wrists: straightening, kneeling, leaning, pushing from the chest, etc.
  • You start each training session with such a movement lifting weights as high as possible for a small number of repetitions, thus targeting type IIb fibers.
  • Continue the session with another 2-3 exercises at an intensity that targets the type IIa fibers, then finish the training with 2-3 light exercises that target the type I fibers.

Thus, you will take advantage of the energy from the beginning of the session to be able to move significant weights, to recruit and develop type IIb fibers. Then, as fatigue occurs, you will switch to isolation exercises with lower weights, performed for a large number of repetitions, which will also train your type I fibers, leading to as much hypertrophy as possible.

In the training plan from the “Muscle Mass Secrets” system, I have already chosen all these exercises for you so that you have a well-balanced development and speed up the recovery from one training to another in order to grow as fast as possible without overloading yourself.

In addition, we have introduced an advanced training periodization strategy that will allow you to lift heavier weights, get over any “plateaus” and grow impressively in muscle mass.

If you want to follow a well-thought-out training program, proven to quickly develop the muscle mass of hundreds of guys before you, read more about the “Secrets of Muscle Mass” system.

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