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Types of Muscle Fibers & Its Influence on Muscle Growth & Athletic Performance

One of the most common fitness goals for many is muscle definition. Muscles generally define body structure. However, there is a multitude of factors on how one can achieve a certain look. Among these are the types of muscle fibers which play a huge role in creating those sought-after curves and shapes. These silhouettes are usually achieved by contracting muscle fibers in a certain way, somehow altering and strengthening those through rigorous and consistent workouts.

If you are interested in achieving results, understanding this complex system could help you create appropriate exercise programs based on your goals. Each muscle fiber responds differently. Knowing how to generate those responses would help you improve your overall efficiency when working out.

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What is Muscle Fiber?

Muscle fibers, or Myocytes, are the basic building block of a muscle. Each of these fibers contains myofibrils, which are strands containing two types of protein — myosin and actin. The interaction of these strands causes our muscles to contract and, thereafter, leads to muscle protein synthesis.

Generally, these fibers control one’s movements throughout the body. Each type and classification plays a different role to help us function.

What are the types of Muscle Fibers?

Muscle fibers are classified based on speed type, size color, its response rate to stimulus, and other characteristics.

There are two main types of muscle fibers: the slow-twitch (type I) and the fast-twitch (type II),
which are broken down into two (type IIa and type IIb).

In a nutshell, slow-twitch muscle fibers help in long endurance workouts. On the other hand, fast-twitch muscle fibers are used in powerful bursts of movements like sprints.

Type I Muscle Fiber Type (Slow-twitch)

Type I (slow-twitch) fibers have a dark and reddish appearance. These also have a higher density of mitochondria, which are good for aerobic metabolism. These also consist of low glycogen, giving it lower resistance to fatigue and tiredness.

Type I fibers cannot contract muscles easily. They typically require more work to build up, making it ideal for endurance activities. These are also highly oxidative. They efficiently use oxygen to generate more adenosine triphosphate (ATP) for continuous, extended muscle contractions for long periods of time.

These muscle fibers are usually associated with lower-intensity, endurance-oriented activities like walking, running, and cycling.

Influence on Muscle Growth: While Type I fibers contribute to muscle endurance, their low force production and slow contraction speed make them less significant in terms of muscle hypertrophy compared to fast-twitch fibers.

Type IIA Muscle Fiber Type (Intermediate Fast-twitch)

Usually referred to as the moderate type I and type II, Type IIA finds a balance of adopting characteristics from both types.

Type IIA fibers have high fatigue resistance with high glycogen content, making it easy to perform long anaerobic exercises. Like the Type I fiber, these also have high oxidative enzymes, using up a lot of oxygen to create energy. They can be involved in aerobic activities as these fibers have mitochondria. They can also be used to produce rapid force for activities that require a high amount of strength or power.

Fast-twitch muscle fibers have a bigger diameter compared to Type I. These make it play a more significant role when it comes to muscle growth.

Influence on Muscle Growth: Fast-twitch muscle fibers, including Type IIa, have a bigger diameter compared to Type I, playing a bigger role in muscle growth due to their ability to generate more force.

Type IIB Muscle Fiber Type (Fast-twitch)

Known as the anaerobic muscle fibers, Type IIB fibers store energy for a short burst of high-intensive activities. This type of muscle fiber uses anaerobic metabolism to create energy and has the highest rate of contraction.

However, Type IIB fibers have low resistance to fatigue due to its limited supply of stored energy. These are usually associated with strength and power activities like sprints and weightlifting.

Type IIB fibers do not have mitochondria. These do not have any color and usually have white or pale appearance.

Influence on Muscle Growth: Type IIb fibers are the most influential in muscle hypertrophy due to their high force production and rapid contraction speed. Their ability to grow in size greatly impacts overall muscle mass.

How do you identify your muscle fiber type?

While everyone has both types, people usually have a predominant muscle fiber. The best way to determine it is through muscle biopsy. It is where professionals collect muscle samples and test it within a laboratory.

Other Methods:

  • Personal Observation: By paying attention to which activities you excel in naturally (endurance vs. power).
  • Exercise Tests: Assessing endurance and capacity to exert energy over different periods can provide insights. For example, if you perform better in long-distance running, you may have more Type I fibers. Conversely, excelling in sprinting may indicate a higher proportion of Type II fibers.

Can Muscle Fiber Types Change?

Yes, muscle fiber types can change depending on training and circumstances. The most common change is the movement of Type IIA to Type IIB. It is where there is an observable increase in the capillary of fiber ratio after following a high exercise training.

Changes from Type I to Type II are very rare. These occur mostly in the case of drastic changes like injuries. There is also some evidence that shows changes from fast to slow. However, the scientific community continues to research and question this concept.

Does muscle fiber type affect sports performance?

Not entirely but it may influence the sports where you are naturally good at. Most Olympic athletes excel at sports where it matches their genetic makeup. While it may be a part of an athlete’s success, it cannot be a sole predictor of performance.

Does Fiber Type Influence Muscle Growth?

Yes. An old study conducted on mice showed that there is significant muscle increase of Type II muscle fiber, compared to that of Type I.

Type I has low resistance to fatigue, meaning that it is likely to produce less force on the muscle, thus, slowing muscle growth. Type II, on the other hand, has low oxidative enzymes and can easily carry out high-intensity exercises. This type has the most effect on muscle growth.

Additional Factors Influencing Muscle Growth and Performance

Nutrition: Proper intake of proteins, carbohydrates, and fats is crucial for muscle repair and growth. Supplements like creatine, BCAAs, and protein powders can also aid in muscle development.

Training Regimen: A balanced approach that includes both endurance and resistance training can optimize muscle fiber utilization and growth.

Recovery: Adequate rest, sleep, and recovery techniques like stretching and massage are essential for muscle repair and growth.

Genetics: Genetic predispositions can influence muscle fiber composition, response to training, and overall athletic potential.

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Many articles about muscle growth would have sidelined on the topic of muscle fiber types. However, most of them only focus on its insignificance to muscle growth. Its effects may not directly reflect muscle growth. Still, it definitely has an important role in determining the right exercises and programs to achieve your best performance. This is not only limited to toning your body. It is also important for other health workers, such as trainers, clinical staff, and physiotherapists, to further assess your performance. This assessment can then help you grow and prevent muscle-related diseases and injuries.


Binder-Macleod, S.A. (2001). Human Skeletal Muscle Fiber Type Classifications. Journal of American Physical Therapy Association. At

Hernandez R.J., and Kravitz L., The Mystery of Skeletal Muscle Hypertrophy, Medicine and science in sports and Exercise, 21 (5) at

James N.T (1979) Studies on the reponses of different Types of Muscle fibre during surgically induced compensatory hypertrophy. J.Anat. 129, 4, pp.769-776 at

Jason R.K. (2001) Muscle fiber Types and Training at

Mallory L.A., et al. (2001). Influence of peak VO2 and Muscle fiber type on the efficiency of moderate exercise. Medicine and science In Sports & Exercise. Retrieved from

McCall, P. (2015). 10 Things to Know About Muscle Fibers at

Ogborn D., and Schoenfeld B.J., (2014). The Role of Fiber Types in muscle Hypertopghy: Implications for Loading Strategies. Strength and Conditioning Journal at

Quinn, E. (2018). Fast and Slow Twitch Muscle Fiber Types at

Rogers, P. (2018). Muscle Fiber Types and Strength Training: Fast Twitch, Slow Twitch, or In Between? At

Zierrath J.R. and Hawley J.A (2004). Skeletal Muscles Fiber Type: influence on contractile and metabolic properties. PLoS Biology, Vol.2, Issue 10, pp.337-348 Retrieved from

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