The Phenotype of a Dominant Athlete

In Part I and Part II of this series, we discovered the most effective and efficient way to develop an athlete’s explosive power: increasing the maximum strength of the athlete.

In this installment, we’ll talk about something else that provides a significant competitive advantage—an increased phenotypic expression of fast-twitch muscle fibers.

The body of research on muscle physiology points to the inescapable conclusion that all else being equal, the athlete with the greater fast-twitch fiber expression also has the greatest capacity to express explosive power during competition.

What Are Fast-Twitch Muscle Fibers Again?

It is well known in muscle physiology that muscle fibers are not merely differentiated into fast and slow twitch. There are Type I, Type IIa, and Type IIx, but a lot of people don’t understand that there are a lot of hybrid fibers in between and beyond.

Even then, it’s a bit of a misnomer that there are even distinct “types” of fibers, because the majority of muscle fibers can actually change characteristics as an adaptation to the specific demand placed on them.

That is, while there are some muscle fibers that will always be slow-twitch (lower relative force production and greater relative endurance) and some that will always be fast-twitch (higher relative force production and lesser relative endurance), there are mostly intermediate fibers that can phenotypically “express” as different types based on the athlete’s training.

So is there a type of training that can preferentially recruit these “hybrid” intermediate fibers into the powerful, explosive fast-twitch fibers that can make an athlete better? It turns out there is.

This review from The American Council on Exercise lists these eight elements that are primary for understanding fast-twitch muscle fiber:

1. Fast-twitch fibers can be further classified into (1) fast-twitch IIa – fast oxidative glycolytic, because they use oxygen to help convert glycogen to ATP, and (2) fast-twitch type IIb – fast glycolytic, which rely on ATP stored in the muscle cell to generate energy.
2. Fast-twitch fibers have a high threshold and will be recruited or activated only when the force demands are greater than the slow-twitch fibers can meet.
3. The larger fast-twitch fibers take a shorter time to reach peak force and can generate higher amounts of force than slow-twitch fibers.
4. Fast-twitch fibers can generate more force, but are quicker to fatigue when compared to slow-twitch fibers.
5. The phasic muscles responsible for generating movement in the body contain a higher density of fast-twitch fibers.
6. High force demand training can increase the number of fast-twitch muscle fibers recruited for a specific movement.
7. Fast-twitch fibers are responsible for the size and definition of a particular muscle.
8. Fast-twitch fibers are called “white fibers” because they do not contain much blood, which gives them a lighter appearance than slow-twitch fibers.

Additional Read: How to Build Your Fast & Slow-Twitch Muscle Fibers

Adaptive Resistance™ Exercise Maximizes Fast-Twitch Fibers

When the athlete’s muscle fibers are made to contract with high magnitudes of force during training, the fibers can change their expression to be more fast-twitch over time.

What type of training involves the highest-possible force demand? As we can see from our strength curve data, it’s the eccentric phase of contraction (an active muscle lengthening) that offers the greatest capacity for force production—if it’s properly loaded—and thus the greatest capacity to stimulate the differentiation of the intermediate fibers into a fast-twitch phenotype.

And what’s the best tool for safe, quantifiable, max-force-demand eccentrics?

That’s right, it’s Adaptive Resistance™ Exercise (ARX). Properly-loaded eccentrics on ARX—which are dangerous and impractical with traditional tools—preferentially recruit the fast-twitch fiber expression necessary for optimal athletic performance

This study out of Germany, for example, showed a significant increase in muscle fiber cross-sectional area and jump height for concentric + eccentric training with eccentric “overload,” with benefits above and beyond traditional concentric-eccentric training with static (weighted) loads.

Another interesting finding was an increased expression on mRNA for MHC IIX and glycolytic enzymes along with an increased cross-sectional area for type IIx—fast twitch/explosive— fibers.

What does this mean? It means that after the training period of the study, the muscles exposed to greater eccentric loading had transformed into faster, more explosively-powerful muscles! The study authors draw the conclusion that this type of training is good for explosive and fast sports.

Adaptive resistance on an ARX machine = the highest-possible force demand on every rep.

Higher force demand = greater expression of fast-twitch muscle fiber over time.

Greater expression of fast-twitch muscle fiber over time = more explosive and powerful athletes.

It makes sense to us, not sure what the controversy is about ¯\_(ツ)_/¯

Must Read: Concentric vs. Eccentric Muscle Contractions: What’s The Difference?

Another Win for ARX’s Adaptive Resistance™

So aside from maximum strength stimulus, this is the second way ARX optimizes an athlete’s power and explosiveness.

All things being equal, an athlete with more fast-twitch fiber is a faster and more powerful athlete, and the adaptive resistance available through the use of ARX—especially during the safe, maximally-loaded eccentric contractions—provokes the most rapid and most potent increases in fast-twitch muscle fiber expression.

Stay tuned for the next installment in the series exploring the necessity of ARX in a 21st-century athlete’s training: maximum safety.