Muscle Fiber Types and other Basic Essentials for the Virtual Cyclist

With an understanding of muscles and how they work, you will be able to make educated training decisions and maximize your virtual cycling potential.

The difference in the distribution of muscle fiber types from person to person, determined mainly by genetics, explains why some cyclists are strong sprinters, time trialists, or climbers.


Sprinters have a higher percentage of fast-twitch muscle fibers and larger muscles.  Climbers have smaller muscles with a higher ratio of slow-twitch fibers that offer the best efficiency and fatigue resistance. 


Have you ever wondered what has made you the type of cyclist you are?  That answer is in part by learning about your muscle fibers.

Classification of the Different Muscle Fiber Types

Muscle fiber types are classified based on how fast they contract and how they produce ATP.  Adenosine triphosphate, also known as ATP, is the primary energy carrier used during all cellular activities, including muscle contraction. Thus, ATP provides the energy for muscle contraction. 

The three mechanisms for ATP regeneration are creatine phosphate, anaerobic glycolysis, and aerobic metabolism.

Muscle fiber contraction velocity depends on how quickly muscle fiber proteins break down ATP to provide the energy for movement.  In general, type II (fast-twitch) fibers utilize ATP twice as rapidly as type I (slow-twitch) fibers.


Muscle fibers can produce ATP through multiple pathways and are classified based on their utilization of oxygen.  When a fiber makes ATP using oxygen through aerobic pathways, it is oxidative.  Glycolytic fibers do not use oxygen and produce ATP through anaerobic pathways.


Oxidative (type I) fibers can produce more significant ATP per metabolic cycle and are more resistant to fatigue.  Glycolytic (type IIb) fibers produce less ATP and fatigue more rapidly.  Fast Oxidative (type IIa) fibers fall in between and are termed intermediate fibers.

Kreb Cycle: The sequence of reactions by which muscle cells generate energy during aerobic respiration. It takes place in the mitochondria, consuming oxygen to convert ADP to energy-rich ATP.

Type I Muscle Fibers - Slow Oxidative (SO)

Contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP and are fatigue-resistant.

Aerobic metabolism occurs in the mitochondria of muscle cells; therefore, oxidative (type I) fibers contain many more than glycolytic (type IIb) fibers.  A large number of mitochondria allow type I fibers to produce more significant amounts of ATP and are capable of contracting for long periods.  However, type I fibers have a relatively small diameter and do not produce a large amount of tension.


In addition, type I fibers have extensive capillaries that deliver oxygen from the blood and sustain muscle activity without fatigue.  They also contain myoglobin, a molecule that carries oxygen similar to hemoglobin in the red blood cells, which stores oxygen in the fibers themselves. 

Type 1 fibers predominate when it comes to endurance. Training of these fibers will lead to increased aerobic capacity, increased fatigue resistance, and greater efficiency shown in the ability to recover from more challenging efforts while maintaining zone 2 (endurance) and 3 (tempo) power for more extended periods.

Glycolysis: A process in which glucose (sugar) is partially broken down by cells in enzyme reactions during anaerobic respiration and is one method that muscle cells use to produce energy.

Type IIa Muscle Fibers - Fast Oxidative (FO)

Have fast contractions and primarily use aerobic respiration, but since they can switch to anaerobic respiration (glycolysis), they can fatigue more rapidly than SO fibers.

Fast Oxidative (type IIa) fibers are known as intermediate fibers because they share characteristics of the other two types.  They produce ATP more quickly than type I fibers and thus can generate relatively high amounts of tension.  

They produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue as quickly as type IIb fibers, but can’t sustain muscle activity as long as type I fibers because they lack significant myoglobin.  


Type IIa fibers produce more tension than type I, but they are more fatigue-resistant than type IIb fibers.

Type IIa fibers are primarily aerobic and utilized in 10-20-minute time trial efforts, but some have glycolytic capabilities for shorter (anaerobic) efforts such as a 1-3-minute climb.

Mitochondria: These organelles are known as the powerhouses of the cell that act like a digestive system that takes in nutrients, breaks them down, and creates energy-rich molecules for the cell in a biochemical process known as cellular respiration.

Type IIb Muscle Fibers - Fast Glycolytic (FG)

Contract powerfully and very fast and primarily use anaerobic glycolysis; therefore, they fatigue more rapidly than the other fiber types.

Fast Glycolytic (type IIb) fibers produce ATP through anaerobic glycolysis, which utilizes the significant amounts of glycogen found in the large-diameter fibers to produce high amounts of tension.  They do not predominantly use oxygen and therefore have few mitochondria and a small amount of myoglobin.  


Type IIb fibers produce quick, powerful movements through rapid, forceful contractions but fatigue quickly.


Type IIb fibers are the pure anaerobic power providers, such as for a 10-second max sprint.  Type IIb fibers use stored ATP, creatine phosphate, and produce some through anaerobic glycolysis of muscle glycogen.

You can burn glycogen faster than you replace it, but fortunately, it begins to be replenished after approximately three to five minutes of recovery.

Myoglobin: An oxygen-binding protein found in muscle tissue that is similar to hemoglobin.

Ask your Coach

When strength training to target type I muscle fibers, train at lower intensities and perform higher repetitions.  3 sets of 12 or more repetitions at 55% to 65% of your maximum.

On the bike, this means training Zone 1 (active recovery), Zone 2 (endurance), and Zone 3 (tempo) or <55% to 90% of your threshold power.

When strength training to target type II muscle fibers, higher intensity, and fewer repetitions is the key.  6 or fewer repetitions for 2 to 6 sets and an intensity of 85% or greater.

On the bike, this means training Zone 4 (threshold) and Zone 5 (VO2 max) at 91% to 120% for type IIa fibers, and Zone 6 (anaerobic capacity) at 121% and greater for type IIb.


Use this as a basic framework.  The variables, theories, and principles are up for debate.  As always, a coach is the best way to go!

Conclusion: Knowledge is Watts

Cycling is mainly aerobic, and training generally results in little to no increase in muscle fiber size. However, resistance training will increase muscle fiber size of all fiber types, with type 2 fibers receiving the most benefit.

When making decisions on optimizing the efficiency of our training and weighing the benefits of adding strengthening exercise to our plan, there is a tremendous amount of information to consider.  Knowing the scientific basics of muscles and their function increases the confidence that you will make the proper decision.

You can’t change your genetic makeup, but you can maximize the potential of your inherent athletic gift through focused training and the identification of weaknesses.  Now that you have a clearer understanding of why you are the cyclist you are, it is time to make yourself the best you can be.

Check out this Zom article on Strength Training terminology to increase your knowledge base.

What type of cyclist are you?

I’m sure you know!  Do you plan your strength training based upon the type of cyclist you are?  Your fellow virtual cyclists want to know.

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[…] For more helpful information on the different muscle fiber types and implications to the cyclist check out this post on The ZOM. […]


[…] a previous post entitled Muscle Fiber Types and other Basic Essentials for the Virtual Cyclist, the basics of muscle make-up and the cycling-specific implications were outlined.  Here is a […]


[…] If you enjoyed the in-depth technical nature of this informative article, then you will really like this Zom article on Strength Training Terms and another on Muscle Fiber Types. […]

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