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Muscle Fiber Types: What’s the “Difference” Between “Slow-Twitch” and “Fast-Twitch”?

by Joseph Giandonato, MBA, MS, CSCS, World Instructor Training Schools


 The human body is the most sophisticated piece of machinery in nature. Comprised of an assemblage of intricate systems that work interdependently to facilitate functions and processes critical to sustaining life, the human body is irreplicable, and its marvels have inspired art, engineering, and fashion since the dawn of civilization.

A constellation of systems is supplicated to produce movement required to carry out a continuum of tasks ranging from the rote, such as self-care and activities of daily livin,g to the enigmatic feats demonstrated by society’s “modern day gladiators” competing at the highest levels of their sport.

The systems called upon to produce movement include the cardiovascular, pulmonary, nervous, muscular, and skeletal systems. The latter three systems are colloquially known as the neuromusculoskeletal system due to their conjoined structure and function.

Movement, whether classified by type or velocity, is often characterized as “slow-twitch” or “fast-twitch”. Within the fitness industry and strength and conditioning circles it is common for coaches to describe their athletes as either “slow-twitch” or “fast-twitch” which are both associated with connotations of physiological attributes. Individuals who are considered slow twitch generally lack explosiveness and speed, yet excel in activities requiring great muscular endurance, whereas their fast twitch counterparts exhibit considerable muscular power and are capable of demonstrating immense limit strength, which dwarves their muscular endurance and aerobic fitness.

Muscle Fiber Types: I through II (x, a, and b)

The terms “slow-twitch” and “fast-twitch” are also used to distinguish between their myosin heavy chain expression and attendant ability to produce adenosine triphosphate (ATP) through aerobic and anaerobic mechanisms. Slow-twitch, or slow oxidative fibers, also known as Type I fibers, yield most of their ATP aerobically in the mitochondria and as such, encompass greater mitochondrial density and more developed capillary networks which are instrumental in supplying oxygen to the mitochondria. Conversely, fast-twitch, or fast glycolytic fibers, also known as Type IIx fibers, derive a majority of their ATP from phosphocreatine breakdown and glycolysis and have a greater propensity to fatigue due to lessened mitochondrial and capillary density.

In early literature, a third fiber type, fast oxidative glycolytic, also known as Type IIa was identified, possessing characteristics of both Type I and Type IIx fibers (Brooke & Kaiser, 1970). This fiber type was considered unique as it is capable of resisting fatigue due to its increased mitochondrial and capillary content, yet able to contract quickly and forcefully when stimulated. These muscle fibers possess are most trainable, meaning they will readily adapt to stimuli that is either endurance or power oriented and are capable of interconversion with Type IIa fibers (Wilson, et al., 2012). Additionally, prior research has substantiated that Type IIa fibers encompass great plasticity as their activation was shown to increase when subjected to high velocity movements over a period of six weeks (Liu, Schlumberger, Wirth, Schlumberger, and Steinacker, 2003).

However, through the advent of advanced histological techniques and imaging technology, additional muscle fibers have been discovered among mammals with another fiber identified in humans. This most recently uncovered fiber type, fast glycolytic, also known as Type IIb fibers, possesses the greatest anaerobic capacity, force production capability, and contraction speed of the four fibers (Schiaffino & Reggiani, 2011, Hoffman, 2014).

The distribution of muscle fiber types is largely influenced by genetics and varies across individual muscles. Among males and females, 52% of muscle fibers are Type I, 33% are Type IIa, and 13% Type IIx (Howley and Thompson, 2017). A scant amount of Type IIb muscle fibers are presumed to be present amid a majority of population but are postulated to be found in greater amounts among athletes encompassing exceptional speed and power. Athletes on opposite ends of the spectrum — elite endurance athletes and competitive strength athletes — may have greatly differing muscle fiber distributions (in upwards of 90% of Type I muscle fibers among some endurance athletes and nearly 60% of Type II muscle fibers among strength athletes) and characteristics of Type IIa muscle fibers due to the divergent physiological and biomechanical demands associated with their sport.

Some muscles, specifically the deltoids, have a blended composition of muscle fibers. A study involving digital imaging of muscle fiber morphometry showed a balanced distribution of Type I and Type II fibers comprising the deltoids. A greater proportion of Type II fibers were found in the anterior and lateral portions of the deltoids, whereas more Type I fibers were present in the posterior portion of the deltoids, as the posterior deltoids have greater involvement with lower threshold activities such as assisting in adduction, external rotation, extension, and stabilization of the shoulder, especially when the arm is abducted during higher velocity gait cycles (Bryant & Giandonato, 2019).

Muscle fibers are recruited in concert to facilitate contractions. However, the amount or type of muscle fibers is contingent upon the speed or intensity of the contraction. Ordinarily, the order of contraction begins with the slowest to the fastest fiber (i.e., Type I to Type IIa to Type IIx) (Howley & Thompson, 2017), but among elite athletes who participate in power-oriented or strength sports or activities involving greater rates of force development and individuals with greater neural efficiency, characterized by streamlined muscular recruitment patterns, lower threshold motor neurons innervating Type I fibers are bypassed as higher threshold motor neurons which supply Type II fibers are activated during high intensity activities through a process regarded as a neurophysiological phenomenon known as selective recruitment.


 In summary, fitness, rehabilitation, and strength and conditioning professionals should be cognizant of the characteristics, energetic capacities, distribution determinants and variations, and recruitment patterns of the four muscle fiber types that have been identified in the literature to formulate appropriate programming that will evoke desired adaptations such as improvements in metabolic health, athletic performance, and resistance to injury.


Brooke, M.H. & Kaiser, K.K. (1970). Muscle fiber types: How many and what kind? Archives of Neurology, 23 (4): 369-379.

Bryant, J. & Giandonato, J (2019, December 16). The science of training: Deltoids. JoshStrength.

Hoffman, J. (2014). Physiological aspects of sport training and performance (2nd ed.). Human Kinetics.

Howley, E.T. & Thompson, D.L. (2017). Fitness professional’s handbook (7th ed.). Human Kinetics.

Liu, Y., Schlumberger, A., Wirth, K., Schlumberger, D., and Steinacker, J.M. (2003). Different effects on human skeletal myosin heavy chain isoform expression: Strength vs. combination training. Journal of Applied Physiology, 94 (6): 2282-2288.

Schiaffino, S. & Reggiani, C. (2011). Fiber types in mammalian skeletal muscles. Physiological Reviews91 (4): 1447-1531.

Wilson, J.M., Loenneke, J.P., Jo, E., Wilson, G.J., Zourdos, M.C., & Kim, J. (2012). The effects of endurance, strength, and power training on muscle fiber type shifting. Journal of Strength and Conditioning Research, 26 (6): 1724-1729.

About the Author

 Joseph Giandonato, MBA, MS, CSCS has been a World Instructor Training Schools faculty member since 2010. Presently, Giandonato serves as an Employee Well-being Coordinator at the University of Virginia, where he assists with the design, delivery, and oversight of programming associated with their award-winning wellness program, Hoos Well. Giandonato is also pursuing a PhD in Health Sciences with a focus in Exercise and Sports Science from nearby Liberty University. Additionally, Giandonato serves as an adjunct professor at a number of two and four year colleges and universities where he teaches exercise science electives, statistics, research methods, and anatomy and physiology.