Muscle Fibers vs Athlete Performance

Muscle Fibers VS Athlete Performance

Before the subject of muscle fibers, first let’s take into account that the increase in training volume is unlikely to be possible, and that performance improvement can only be achieved by better training effectiveness, which comes to the more obvious conclusion that something different has to be done.

Until this moment, the endurance training methodology is based on maximum oxygen consumption and on the metabolic capacity of the body. For that reason, many athletes eventually find that they are not born for triathlon, precisely because these physiological characteristics of the body have a lot of genetic influence. But, the main problem has never been about how to increase (VO2max), but how to increase the speed at which the athlete reaches this plateau. This means, looking for increase the ability of slow muscle fibers to work on an extended regime with greater force. So, it’s better to understand the characteristics of all muscle fibers and how to train them first.

What’s are muscle fibers?

A muscle “fiber” is a muscle cell. And muscle cells contract and relax in response to messages from the central nervous system. Additionally, the body have three different kinds of muscle. 

Skeletal muscle accounts for more than a third of body mass, and it can be broken down into three distinct types of fibers:

Every skeletal muscle (calf, hamstring, etc.) contains muscle fibers of all three types. But not all athletes have the same percentage of each. For example, elite distance athletes have high percentages of slow-twitch and intermediate fibers. In contrast, sprinters are rich in fast-twitch. And despiste genetics determines the percentage of fiber types in our legs, but training can alter how these fibers function.

How do muscle fibers work?

In the first place, understanding how the body uses nerve and muscle fibers to create motion provides the foundation for creating a better training program.

Moreover, It all begins in the central nervous system. The messages originate there, before being sent via nerves to targeted muscles throughout the body. So, a nerve cell and the muscle fibers it controls a “motor unit.” 

By all means, a motor unit can contain more than a thousand muscle fibers (all of the same type: slow-, intermediate, or fast-twitch), and these fibers contract in unison. In addition, groups of motor units work together to contract an entire muscle. And, for one muscle to contract successfully, another muscle must relax. 

For example, consider the upper arms. In essence, for your biceps to flex, your triceps must relax, and vice versa. Must be remembered that many pairs of muscles contract and relax to create the running stride. Consequently, improved coordination of this contraction and relaxation makes the athlete more efficient.

How to train muscle fibers?

There’s no aspect of muscle fiber recruitment and activation that can’t be improved through training. A great endurance coach called Tom Schwartz was very happy to relate some key points related on how to train different types of muscle fibers.


Fibers contain small myofilaments called myosin and actin; weaker myofilaments are damaged through training. And the body responds by replacing these damaged myofilaments with stronger ones, leaving with fibers that can better handle the stress of running.

Also, as the intensity and volume of running grow, the number of these myofilaments increases, causing muscle fibers to swell. It’s this increase in the size of muscle fibers (called “hypertrophy”) that leads to visible enlargement of our muscles. Adn the combination of stronger and larger fibers allows the muscles to produce more force, while simultaneously increasing their resistance to damage and fatigue.

Maximum Recruitment

Knowing that  muscle fibers can be strong, it stands to reason that we’d want to strengthen all of them. Athletes who focus solely on volume miss the opportunity to strengthen intermediate and fast-twitch fibers.

Recruiting and then strengthening all three types of muscle fibers is essential to achieving the best effort. But of course, there is have to be sensibility in how to structure this type of training.

Recruitment Patterns

Athletes need to experimenting with a wide range of pace, effort, force and technique. In doing so, adaptations can be created to improve the body’s function in the following areas:

Reciproval Inhibition

When one muscle contracts, another must relax. But if relaxation isn’t complete (e.g., if we flex our triceps while trying to flex our biceps), then movement is inhibited. And by improving the ability to coordinate contraction and relaxation, inhibition is reduced.

Opposing muscles aren’t in sync. So, in order to lift the knees, the opposing muscle must relax. What’s important is reducing this inhibition. This reduces the energy expenditure, to move farther faster.”

Fiber Conversion

There’s an ongoing debate as to whether one type of fiber can fully convert to a different type. But there’s no doubt that faster fibers can be trained to take on characteristics of slower fibers — vital for distance runners looking to maximize both force and endurance.

To be continued…

In the next post, I’ll be writing what can be done in practice, what types of training sessions, and what muscle fibers these training sessions will affect. Excellent material for assembling your own routine.

To learn more about this subject, I recommend the book by writer, runner, and coach Pete Magil, Build your running body.


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