Fast and Slow Twitch Muscle Fiber With Performance

The natural makeup of your muscle fibers may influence what sports you are naturally good at and how well you respond to training.

Fast-twitch muscle fibers contract quickly but tire rapidly. Slow-twitch muscle fibers contract slowly but also tire slowly. Having a greater proportion of fast-twitch fibers can help you excel at activities that require bursts of power and strength and may make you a better sprinter or weightlifter. Having more slow-twitch fibers can help you become an accomplished endurance athlete, such as a distance runner or cyclist.

This article explores the types of muscle fibers, how they work, and the potential for fibers to shift with training.

Fast and Slow Twitch Muscle
Verywell, 2017

How Do Muscle Fibers Differ?

Skeletal muscle is made up of bundles of individual muscle fibers called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull. This shortens the muscle and causes muscle contraction.

It is generally accepted that muscle fiber types can be broken down into two main types: slow-twitch (type I) muscle fibers and fast-twitch (type II) muscle fibers. Fast-twitch fibers can be further categorized into type IIa and type IIb fibers (also known as IIx fibers). There are also hybrid fibers with more than one type.

These distinctions seem to influence how muscles respond to training and physical activity, and each fiber type is unique in its ability to contract in a certain way. Human muscles contain a genetically determined mixture of both slow and fast fiber types.

On average, people have about 50% slow-twitch and 50% fast-twitch fibers in most of the muscles used for movement.

Slow-Twitch Muscle Fibers (Type I)

The slow-twitch muscle fibers are more efficient at using oxygen to generate more adenosine triphosphate (ATP) fuel for continuous, extended muscle contractions over a long time. They fire more slowly than fast-twitch fibers and can go for a long time before they fatigue.

Because of this, slow-twitch fibers are great at helping athletes run marathons and bicycle for hours.

Fast-Twitch Muscle Fibers (Type II)

Because fast-twitch fibers use anaerobic metabolism to create fuel, they are better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly. Fast-twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly.

Having more fast-twitch fibers can be an asset for sprinting because they allow you to quickly generate a lot of force.

Fast-Twitch Muscle Fibers (Type IIa)

These fast-twitch muscle fibers are also known as intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of type I and type II muscle fibers.

Fast-Twitch Muscle Fibers (Type IIb or IIx)

These fast-twitch fibers use anaerobic metabolism to create energy and are the "classic" fast-twitch muscle fibers that excel at producing quick, powerful bursts of speed. This muscle fiber has the highest rate of contraction (rapid firing) of all the muscle fiber types, but it also has a faster rate of fatigue and can't last as long before it needs rest.

Hybrid Muscle Fibers

Research suggests that some muscle fibers are pure, or one type, and others are hybrids that express more than one type of fiber. Hybrid fibers may be a combination of both IIa and IIb fast-twitch muscle types or be any combination of both slow-twitch and fast-twitch types. It's also possible that they may shift over time. More research that examines single fibers is needed to better understand the role of hybrid fibers and how they may change over time.

Does Muscle Fiber Type Affect Sports Performance?

Olympic athletes tend to fall into sports that match their genetic makeup. Olympic sprinters have been shown to possess about 80% fast-twitch fibers while those who excel in marathons tend to have 80% slow-twitch fibers.

Research on sports performance within families also suggests that genetic factors may influence about 30% to 80% of the differences among individuals.

However, genetics are just one aspect. Fiber type is part of a great athlete's success, but it alone is a poor predictor of performance. Environment and training matter too. There are many other factors that go into determining athleticism, including mental preparedness, proper nutrition and hydration, getting enough rest, and having appropriate equipment and conditioning.

Can Training Change Your Muscle Fiber Type?

There is some evidence showing that human skeletal muscle may switch fiber types from "fast" to "slow" due to training or transition from hybrid to pure fibers. This is not entirely understood, and research is preliminary and still looking at that question.

One study published in 2018 examined the muscles fibers of a set of twins. One had trained as an endurance athlete for decades and the other was sedentary for decades. Researchers found that the endurance athlete had 55% more slow-twitch fibers than the sedentary twin. Also interestingly, the untrained twin had more fast-twitch and hybrid fibers.

Although no firm conclusions can be drawn from just two people, the twin study does suggest that training may strongly influence shifts in fiber types in some individuals.

Although genetic differences may be dramatic at the elite levels of athletic competition, following the scientific principles of conditioning can dramatically improve the personal performance of a typical athlete. With consistent training, muscle fibers can develop more and improve their ability to cope with and adapt to the stress of exercise. With consistent effort, they may even have the potential to shift types to favor your sport over time.

3 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. MedlinePlus. Is athletic performance determined by genetics?

  2. Plotkin DL, Roberts MD, Haun CT, Schoenfeld BJ. Muscle fiber type transitions with exercise training: shifting perspectives. Sports. 2021;9(9):127. doi:10.3390/sports9090127

  3. Bathgate KE, Bagley JR, Jo E, et al. Muscle health and performance in monozygotic twins with 30 years of discordant exercise habitsEur J Appl Physiol. 2018;118(10):2097-2110. doi:10.1007/s00421-018-3943-7

Additional Reading
  • Clark M, Lucett S, Sutton BG. NASM Essentials of Personal Fitness Training 4th edition revised. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2014.
  • Powers SK, Howley ET. ​Exercise Physiology: Theory and Application to Fitness and Performance. New York: McGraw-Hill Higher Education; 2012.

By Elizabeth Quinn, MS
Elizabeth Quinn is an exercise physiologist, sports medicine writer, and fitness consultant for corporate wellness and rehabilitation clinics.