Muscle Fiber Vs Athlete Performance

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athlete performance

In the context of elite athletes, if we 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, then we come 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 our body. For that reason, many athletes eventually find that they are not born for triathlon, precisely because these physiological characteristics of our body have a lot of genetic influence.

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 menas, looking for increase the ability of slow muscle fibers to work on an extended regime with greater force.

So, we first need to understand the characteristics of each muscle fiber and how to train them.


What’s a muscle fiber?

A muscle “fiber” is what we call a muscle cell. Muscle cells contract and relax in response to messages from our central nervous system. Our bodies have three different kinds of muscle. Cardiac muscle is found in our heart. Smooth muscle controls involuntary functions like digestion and blood pressure. And skeletal muscle moves our body — making it the muscle we focus on when training.

types of muscle

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

muscle fibers

  • Slow-twitch fibers (Type I): contract more slowly and less forcefully than Type II fibers. This type of fiber is replete with mitochondria, myoglobin, and capillaries – ingredients that ensure the constant supply of oxygen and energy we need to perform aerobically.
  • Intermediate Fast-Twitch (Type IIa): share many of the aerobic characteristics of slow-twitch fibers, but they can contract faster and with more force.
  • Fast-Twitch (Type IIx): are the speed demons of muscle cells. They contract fastest and most forcefully of the fiber types. But they’re also limited by an inability to function aerobically, providing only short bursts of anaerobically fueled running.

Every skeletal muscle (calf, hamstring, etc.) contains fibers of all three types. But not all athletes have the same percentage of each. Elite distance athletes have high percentages of slow-twitch and intermediate fibers. Sprinters are rich in fast-twitch. Genetics determines the percentage of fiber types in our legs, but training can alter how these fibers function.


How do muscle fibers work?

working system

Understanding how our bodies use nerve and muscle to create motion provides the foundation for creating a better training program.

It all begins in our central nervous system. Messages originate there, before being sent via nerves to targeted muscles throughout our body. We call a nerve cell and the muscle fibers it controls a “motor unit.” 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. Groups of motor units work together to contract an entire muscle.

For one muscle to contract successfully, another muscle must relax. Consider your upper arms. For your biceps to flex, your triceps must relax, and vice versa. Many pairs of muscles contract and relax to create our running stride. Improved coordination of this contraction and relaxation makes us more efficient athletes.


How do we 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 to how we can train different types of muscle fibers.



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

Also, as we increase the intensity and volume of our running, 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. The combination of stronger and larger fibers allows our muscles to produce more force, while simultaneously increasing their resistance to damage and fatigue.


Knowing that we can strengthen our muscle fibers, 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 fasttwitch fibers.

Recruiting and then strengthening all three types of muscle fibers is essential to achieving our best effort. Of course, we have to be sensible in how we structure this training


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

  • The sequence in which we recruit muscle fibers;
  • Coordination of fiber recruitment at specific paces and fatigue levels.


reciprocal 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. By improving our ability to coordinate contraction and relaxation, we reduce inhibition.

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



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.




In the next article, 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.