Muscle Fiber Types: What's the "Difference" Between Slow Twitch and Fast Twitch? (2023)

von Joseph Giandonato, MBA, MS, CSCS, Instructor Training Schools of the World


The human body is nature's most sophisticated machinery. Composed of an intricate array of systems that depend on one another to facilitate vital life-sustaining functions and processes, the human body is unique and its wonders have inspired art, technology and fashion since the dawn of civilisation.

A constellation of systems must generate the movement necessary to perform a range of tasks ranging from routine tasks such as personal hygiene and activities of daily living to the enigmatic exploits of society's "modern gladiators" competing at the highest level. large. level of your sport.

The systems involved in generating movement include the cardiovascular, pulmonary, nervous, muscular and skeletal systems. The last three systems are colloquially referred to as the neuromusculoskeletal system because of their common structure and function.

(Video) Types of muscle fibres - fast twitch, slow twitch (GCSE PE)

The movement, whether classified by type or speed, is often referred to as a "slow twitch" or "fast twitch." In the fitness industry and strength and conditioning circles, it's common for coaches to refer to their athletes as "slow twitch" or "fast twitch," both of which carry connotations of physiological traits. People classified as slow-twitch individuals generally lack explosiveness and speed, but excel in activities that require great muscular endurance, while their fast-twitch counterparts exhibit considerable muscular strength and are capable of immense limit strength to demonstrate endurance that surpasses their fitness.

Types of muscle fibers: I to II (x, a and b)

The terms "slow twitch" and "fast twitch" are also used to distinguish between the expression of myosin heavy chain and the concomitant ability to produce adenosine triphosphate (ATP) via aerobic and anaerobic mechanisms. Slow twitch or slow oxidative fibers, also known as type I fibers, produce most of their ATP aerobically in the mitochondria and as such comprise greater mitochondrial density and better developed capillary networks, which are essential for mitochondrial oxygenation. In contrast, fast-twitch or fast glycolytic fibers, also known as type IIx fibers, get most of their ATP from phosphocreatine breakdown and glycolysis, and are more prone to fatigue due to lower mitochondrial and capillary density.

In the early literature, a third type of fiber, rapidly oxidative glycolysis, also known as Type IIa, was identified as having characteristics of both Type I and Type IIx fibers (Brooke & Kaiser, 1970). This type of fiber was thought to be unique because its higher proportion of mitochondria and capillaries allowed it to resist fatigue, but could contract quickly and vigorously when stimulated. These muscle fibers are more trainable, meaning they easily adapt to resistance or force stimuli and are able to exchange with type IIa fibers (Wilson, et al., 2012). In addition, previous research has shown that Type IIa fibers exhibit high plasticity as their activation increases when subjected to high-speed movement over a six-week period (Liu, Schlumberger, Wirth, Schlumberger & Steinacker, 2003).

However, thanks to the advent of advanced histological techniques and imaging technologies, additional muscle fibers have been discovered in mammals, with another fiber identified in humans. This only recently discovered type of fast glycolytic fibre, also known as type IIb fibre, has the highest anaerobic capacity, force generating capacity and contraction speed of the four fibers (Schiaffino & Reggiani, 2011, Hoffman, 2014).

(Video) Types of Skeletal Muscle | Fast Twitch Slow Twitch Fibres | Type 1 Type 2 Fibres | Muscle Physiology

The distribution of muscle fiber types is strongly influenced by genetics and varies with individual muscles. Between men and women, 52% of muscle fibers are type I, 33% type IIa, and 13% type IIx (Howley & Thompson, 2017). Type IIb muscle fibers are believed to be present in low numbers in the majority of the population, but are postulated to be found in greater numbers in athletes of exceptional speed and power. Athletes on opposite ends of the spectrum (elite endurance athletes and competitive strength athletes) can have vastly different muscle fiber distributions (over 90% type I muscle fibers in some endurance athletes and almost 60% type II muscle fibers in strength athletes). . ) and Type IIa muscle fiber characteristics due to the different physiological and biomechanical demands associated with their sport.

Some muscles, particularly the deltoids, have a mixed composition of muscle fibers. A study using digital imaging of muscle fiber morphometry showed an equal distribution of type I and type II fibers that comprise the deltoids. A greater proportion of type II fibers were found in the anterior and lateral deltoids, while more type I fibers were present in the posterior deltoids because the posterior deltoids are more involved in low-threshold activities such as assisting adduction, external rotation, and extension , and shoulder stabilization, particularly when the arm is abducted during rapid gait cycles (Bryant & Giandonato, 2019).

Muscle fibers contract to facilitate contractions. However, the number or type of muscle fibers depends on the speed or intensity of the contraction. Typically, the order of contraction begins from slowest to fastest fiber (i.e., Type I to Type IIa to Type IIx) (Howley & Thompson, 2017), but in elite athletes participating in sports or strength activities or performance levels that involve higher rates of strength development and individuals with greater neural efficiency, characterized by simplified muscle recruitment patterns, the lower-threshold motoneurons innervating type I fibers are neglected as low-threshold motoneurons. The Type II upper threshold is activated during high-intensity activity through a process considered a neurophysiological phenomenon known as selective recruitment.


In summary, fitness, rehabilitation, and strength and conditioning professionals need to be aware of the characteristics, energetic capacities, determinants, and variations in the distribution and recruitment patterns of the four muscle fiber types. Programming that evokes desired adjustments, e.g. B. Improvements in metabolic health, athletic performance and resistance to injury.

(Video) Fast twitch vs Slow twitch muscle fibers


Brooke, M.H. & Kaiser, K.K. (1970). Types of muscle fibers: how many and what kind?Archives of Neurology, 23(4): 369-379.

Bryant, J. & Giandonato, J. (2019, 16. Dezember).The Science of Training: Deltoids. Josh Kraft.

Hoffmann, J. (2014).Physiological aspects of training and athletic performance(2Dakota do Norteed.). human kinetics.

Howley, E.T. and Thompson, D.L. (2017).Fitness Professionals Guide(7Ised.). human kinetics.

(Video) Type 1 “Slow Twitch” Muscle Fibers vs Type 2 “Fast Twitch” Muscle Fibers MCAT Biology & Biochemistry

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

Schiaffino, S. & Reggiani, C. (2011). Fiber types in mammalian skeletal muscle.Physiological overviews,91(4): 1447-1531.

Wilson JM, Loenneke JP, Jo E, Wilson GJ, Zourdos MC. and Kim, J. (2012). The effects of endurance, strength, and resistance training on changing muscle fiber type.Journal of Strength and Conditioning Research, 26(6): 1724-1729.

About the author

Joseph Giandonato, MBA, MS, CSCS has been a faculty member at World Instructor Training Schools since 2010. Giandonato is currently the Staff Wellness Coordinator at the University of Virginia, where he assists in the design, delivery and oversight of the program. associated with its award-winning wellness program Hoos Well. Giandonato is also pursuing a PhD in Health Sciences with a focus on Sports and Exercise Sciences from nearby Liberty University. Giandonato is also an adjunct professor at several two- and four-year colleges and universities, teaching electives in exercise science, statistics, research methods, and anatomy and physiology.

(Video) Muscle Contraction | Slow-Twitch vs Fast-Twitch | Isometric vs Isotonic | Muscle Physiology

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What is the difference in fast and slow twitch muscle fibers? ›

Most muscles are made up of two kinds of muscle fibers that help you move your body: slow-twitch muscle fibers, which move more slowly but help to keep you moving longer. fast-twitch muscle fibers, which help you move faster, but for shorter periods.

Which is better fast twitch or slow twitch muscles? ›

Fast twitch muscles are optimal for short, quick bursts of energy. Slow twitch muscles are better for long-term endurance activities and can improve your heart health. Working out both can give you a wide variety of activities to choose from and increase your overall health and strength.

What is the difference between Type 1 and Type 2 muscle fibers? ›

Type 1 (“slow twitch”) fibers contain more mitochondria, which means they can produce more energy and are better for long, aerobic activities. Type 2 (“fast twitch”) fibers, on the other hand, are suited for short, fast bursts of activity that don't require as much oxygen.

What are the 3 types of muscle twitch fibers? ›

The three types of muscle fiber are slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). SO fibers use aerobic metabolism to produce low power contractions over long periods and are slow to fatigue. FO fibers use aerobic metabolism to produce ATP but produce higher tension contractions than SO fibers.

Which muscles are fast-twitch? ›

The chest muscles, triceps/biceps, and hamstrings are more fast-twitch. Shoulders, forearms, and calves, however, are more slow-twitch, while quads and back muscles tend to be a mix. To train fast-twitch muscles, weight lift with lower reps (5-7) and more weight.

Are slow twitch muscles type 1 or 2? ›

Skeletal muscle fibers are broadly classified as "slow-twitch" (type 1) and "fast-twitch" (type 2).

Are biceps Type 1 or 2? ›

Fast-twitch muscle fibers, more specifically type IIb fibers, dominate over slow-twitch fibers in the biceps muscle. These fibers are categorized as muscle fibers that are able to produce quick and powerful contractions for a period of approximately 30 seconds.

What are Type 2 A and B muscle fibers? ›

Type IIA fibers have high myosin ATPase activity (pH 9.4), are fast twitch, have high oxidative and glycolytic capacity, and are relatively resistant to fatigue. Type IIB fibers have high myosin ATPase activity (pH 9.4), are fast twitch, have low oxidative and high glycolytic capacity, and fatigue rapidly.

Which fiber type gets tired the fastest? ›

Fast-twitch fibers will fatigue quickly, so focus on using heavy weight or explosive movements for only a limited number of repetitions (e.g., two to six) for maximum effectiveness.

What are Type 1 and Type 2 muscles? ›

The two types of skeletal muscle fibers are slow-twitch (type I) and fast-twitch (type II). Slow-twitch muscle fibers support long distance endurance activities like marathon running, while fast-twitch muscle fibers support quick, powerful movements such as sprinting or weightlifting.

What is type 2 muscle fiber? ›

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).

How do I know my muscle fiber type? ›

To find the predominant fiber type in a particular muscle in your body, you can try the following test.
  1. Find your one rep max here for an isolation exercise for that muscle group.
  2. Take 80% of it and do as many reps as possible.
  3. If you can do only 4 to 7 reps with it, you have mostly Fast Twitch fibers in that muscle.
Dec 17, 2018

What athletes are slow twitch? ›

Type I, or slow-twitch fibers, are seen in high abundance in elite endurance athletes, such as long-distance runners and cyclists. Alternatively, fast-twitch IIa and IIx fibers are abundant in elite power athletes, such as weightlifters and sprinters.

Do bodybuilders have fast or slow twitch muscles? ›

Weightlifters and powerlifters have muscle fiber areas composed of roughly 2/3 type II fibers and 1/3 type I fibers. In bodybuilders it's the other way around: 2/3 of their muscles' cross-sectional area comprises slow twitch and only 1/3 of it is fast twitch.

Do slow twitch muscles burn more fat? ›

Slow-twitch fibers happen to burn a lot of calories, which means you'll also burn more fat. Alternatively, traditional strength training hones in on fast-twitch fibers for more powerful muscles and increased speed.

Are glutes fast or slow twitch? ›

The glutes are 50% fast twitch and 50% slow twitch muscle fibers. This means that half of you workouts should be working at 60-80% 1RM for 6-12 reps and the other half should be working at 50-60% intensity for 12-20 reps or even failure!

Are calves fast or slow twitch? ›

The calf muscles are mostly slow twitch muscle fibers, meaning that (again), they're less likely to be damaged and/or sore than other muscle groups. The ankle joint doesn't go through a large range of motion, so it can be difficult to get a big stretch in the calves.

Are people with fast twitch muscles stronger? ›

Fast-twitch muscles are larger and denser than slow-twitch muscles, making them more powerful. They're used for hardcore exercises like high-intensity interval training. And, because we have Type I and Type II muscle fibers at opposite ends of the spectrum, we of course need a middle-of-the-road kind of guy.

Are triceps Type 1 or 2? ›

Even more so than the pectoralis major, the triceps brachii is a performance muscle. It's fast twitch fibers outnumber their sluggish counterparts two to one with approximately 67% type II fibers.

What are muscle fiber types 1 2A and 2B? ›

Human skeletal muscle is made up predominantly of two types of muscle fiber: red fiber and white fiber. Red fibers (type 1) are also known as slow-twitch fibers, and white (type 2) are called fast-twitch fibers. White, fast fibers can also be broken into two types—2A and 2B.

What are the easiest muscle to grow? ›

What are the easiest muscles to grow in later life?
  • Lower Back Muscles.
  • Latissimus Dorsi.
  • Rhomboids.
  • Abdominals.
  • Hamstrings.
  • Obliques.
  • Calves.
  • Forearm Muscles.

What bicep size is considered muscular? ›

12-14 inches (30-35cm): Average size, but you have your work cut out for you. 14-16 inches (35-41cm): Muscular, you're on the way to impressive guns. 16-18 inches (41-46cm): Strong; people know you work out. 18-20 inches (46-51cm): Elite, you made it to the promised land of the gun show.

What type of muscle fiber is lats? ›

Fiber Type Distribution

What we can deduce are the lats are composed of both fast and slow twitch muscle fibers and therefore require wide range of rest periods, reps, tempos, angles, and training intensities. Additionally, since the lats are a big, strong muscle, don't let your grip limit their development.

What are type 2a muscle fibers good for? ›

Type IIa muscle fibers are fast twitch, meaning they fire more quickly. They are also more powerful than type I fibers and are recruited for activities that require more intensity: sprinting, lifting heavy weights.

What are type 2a muscle fibers used for? ›

Type IIA fibers, or fast oxidative fibers, are used more during sustained power activities such as sprinting 400 meters or doing repeated lifts with a weight below maximum (but not with very light weights). They contain very large amounts of myoglobin, very many mitochondria and very many blood capillaries.

What is the difference between slow twitch and fast twitch muscle fibers quizlet? ›

fundamental difference between slow and fast twitch fibers... glycolytic capacity: higher in slow or fast twitch? higher capacity in fast twitch b/c they rely on glycolysis for ATP whereas slow twitch rely on OxPhos (b/c don't need ATP as quickly...)

Why are fast twitch muscles faster? ›

Energy: Fast-twitch muscle fibers produce small amounts of energy rapidly, which works best for HIIT exercises. Slow-twitch muscles slowly produce larger amounts of energy, supporting endurance activities, like long-distance running.

Do fast twitch muscles grow faster? ›

Fast-twitch muscle fibers, also known as type ll muscle fibers, contract faster (hence the name) and have about a 25 to 75% greater potential for muscle growth than type l fibers. This is why they're generally considered helpful for power sports like basketball, football, and sprinting.

What type of fibers are fast-twitch? ›

In the simplest terms, fast-twitch (type II) muscle fibers are built for short, powerful bursts of energy—that's in contrast to slow-twitch (type I) muscle fibers, which are built for endurance activities like long-distance running or biking.

Why are fast-twitch fibers stronger? ›

Because fast-twitch fibers use anaerobic metabolism to create fuel, they are better at generating short bursts of strength or speed than slow muscles.

How do fast fibers and slow fibers differ quizlet? ›

Fast fibers can produce higher mechanical power and contract more quickly, but have a higher ATP expenditure. Slow fibers contract more slowly and generate less mechanical power, but also spend less ATP, particularly in relation to tension development.

Which is the strongest muscle fiber type? ›

Type 1 fibres correspond to the fastest and most powerful of the twitch fibres, and type 3 fibres are the slowest and least powerful.

Can you change muscle fiber types? ›

Evidence suggests that muscle fibers have the ability to undergo fiber type transition, from hybrid to pure fibers, and between fiber types.

Which muscle Fibre is best? ›

The two types of skeletal muscle fibers are slow-twitch (type I) and fast-twitch (type II). Slow-twitch muscle fibers support long distance endurance activities like marathon running, while fast-twitch muscle fibers support quick, powerful movements such as sprinting or weightlifting.


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