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Muscle Fiber Types

Often times we hear discussion about different muscle fibers and how they affect our daily lives and/or fitness and performance. Descriptions such as fast and slow twitch or red and white fibers get thrown around, but most of us do not understand what they mean. This article explores the differences between muscle fiber types, how they affect performance, and concepts on how to better train them.

            A metaphor

The white and dark meat of a chicken is a common argument. Many prefer the light meat as it is leaner and seen as healthier. Others prefer the dark meat as it is fattier and more flavorful. Another example would be the difference between the colors of tuna and tilapia. Tuna has a dark fatty presentation as these fish swim great distances, steadily, over long periods of time. The tilapia doesn’t travel distance and generally move in quick bursts either to catch prey or escape predators. Their meat is more flakey and less oily compared with the tuna. These are visual, and flavorful, differences between the muscle fiber types based upon what is happening in the tissue.

            Type I Fibers (Slow Twitch/Red)

Type I fibers are a “slow twitch” muscle fiber. They are called slow twitch as they generate force at a slower rate comparably with type II. These fibers also have a slower rate of fatigue. These muscle fiber types are also sometimes called postural muscles and rely primarily on oxygen and burning of fat for their source of energy (Baechle & Earle, 2008). Due to their dependence on oxygen they have a few key characteristics. Type I fibers have increased blood flow to provide the oxygen required for energy production. This, along with oxygen stored in the muscle cells, is what gives the tissue its darker color. Additionally, due to the usage of oxygen and fat for energy, these muscles contain a greater concentration of free fatty acids to provide the necessary materials for the slow oxygen burn. In order to burn this oxygen these tissues also contain greater concentrations of mitochondria which are the “power plants” of the cell (Baechle & Earle, 2008; McArdle, Katch, & Katch, 2015).

            Type II Fibers (Fast Twitch/White)

Type II fibers are the fast twitch muscle fiber. They are called fast twitch due to their ability to quickly generate force compared with type I fibers (3-5x faster), however they will fatigue at a much quicker rate (McArdle et al., 2015). Type II are considered to be our power muscles and are primarily recruited during high intensity or burst types of movements. Because they are more inclined for explosive movements they depend more on the burning of sugars (glucose) for their source of power. Additionally, they also have greater concentrations of ATP (the energy molecule) and phosphocreatine (PCr) which are capable of generating energy rapidly (Baechle & Earle, 2008; McArdle et al., 2015). Because the burning of sugars can lead to lactate accumulation(more on lactate here), these muscles are also excellent at converting lactate back into usable energy when the workload decreases sufficiently (McArdle et al., 2015).

            Training Basics

All muscles have a mixture of fiber types. This means any muscular activity will be activating both fiber types regardless of the type of exercise chosen. That being said we can work different fiber types with different modes of exercise and elicit different adaptation responses (Farup et al., 2012; Willardson, 2006).

Type I fibers play a more dominant role in endurance exercise. Their ability to maintain submaximal effort for extended periods of time makes them the ideal choice for this type of work. Endurance activities are extended in nature, typically lasting at least 30 minutes and up to multiple hours, include an elevated heart rate (60-80% of max HR), and can incorporate small rest periods such as in interval training (Bompa & Haff, 2009; Hottenrott, Ludyga, & Schulze, 2012). Adaptations to type I fibers include increased capillary density and increased mitochondria (Baechle & Earle, 2008). Some hypertrophy (larger muscle size) does occur but not much when compared with type II fibers.

Type II fibers play a more dominant role in strength and power activities. Strength and power athletes have been shown to have a greater percentage of these fiber types (53-60%) showing genetic variability between individuals (Bompa & Haff, 2009). Higher levels of resistance, often at a level leading to failure, elicits the greatest adaptation response (Craig & Judge, 2009). Long rest sets are often used to maximize recovery. This allows for working with greater levels of resistance for longer durations (time under tension) (de Salles, BF. Simão, R. Mirand, F. Novaes, Silva. Lemos & Willardson, 2009). Adaptations in type II fibers are seen as hypertrophy (larger muscle size) and improved performance with greater levels of resistance (McArdle et al., 2015).


Baechle, T., & Earle, R. (2008). Essentials of Strength and Training and Conditioning (3rd ed.). Champaign, Il: Human Kinetics.

Bompa, T., & Haff, G. G. (2009). Periodization (5th ed.). Champaign, Il: Human Kinetics.

Craig, B. W., & Judge, L. W. (2009). The Basics of Resistance Training Program Design: Where Do I Start! Strength & Conditioning Journal, 31(6), 75–77.

de Salles, BF. Simão, R. Mirand, F. Novaes, Silva. Lemos, A., & Willardson, J. M. (2009). Rest Interval between Sets in Strength Training. Sports Medicine, 39(9), 765–777.

Farup, J., Kjølhede, T., Sørensen, H., Dalgas, U., MØller, A. B., Vestergaard, P. F., … Vissing, K. (2012). Muscle morphological and strength adaptations to endurance vs. resistance training. Journal of Strength and Conditioning Research, 26(2), 398–407.

Hottenrott, K., Ludyga, S., & Schulze, S. (2012). Effects of high intensity training and continuous endurance training on aerobic capacity and body composition in recreationally active runners. Journal of Sports Science and Medicine, 11, 483–488. Retrieved from

McArdle, W., Katch, F., & Katch, V. (2015). Exercise Physiology (8th ed.). Baltimore, MD: Wolters Kluwer.

Willardson, J. M. (2006). A brief review: Factors affecting the length of the rest interval between resistance exercise sets. Journal of Strength and Conditioning Research, 20(4), 978–984.


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