Sports Performance Muscles

Fast-Twitch Vs. Slow-Twitch Muscle Fiber Types + Training Tips

Stacey Penney
Stacey Penney
| Stay Updated with NASM!

Looking to build endurance? What about power? Do dreams of being an all-star hitter or marathon runner need to be dashed if twitch ratios aren’t ideal? Not necessarily. The types of muscle fibers targeted in different types of training programs can impact sports performance training goals.

In this article, we explore the two types of muscle fibers in detail and discuss how to train each type according to athletic goals.

What are muscle fibers?

Skeletal muscles are made up of individual muscle fibers. And like muscles themselves, not all muscle fibers are the same. There are two types of skeletal muscle fibers, fast-twitch and slow-twitch, and they each have different functions that are important to understand when it comes to movement and exercise programming. 

Slow-Twitch Muscle Fibers

Slow-twitch muscle fibers are fatigue resistant, and focused on sustained, smaller movements and postural control. They contain more mitochondria and myoglobin, and are aerobic in nature compared to fast-twitch fibers. Slow-twitch fibers are also sometimes called type I or red fibers because of their blood supply. 

Fast-Twitch Muscle Fibers

Fast-twitch muscle fibers provide bigger and more powerful forces, but for shorter durations and fatigue quickly. They are more anaerobic with less blood supply, hence they are sometimes referred to as white fibers or type II. Skeletal muscles contain both types of fibers, but the ratios can differ depending on a variety of factors including muscle function, age and training. 

Skeletal muscles contain both types of fibers, but the ratios can differ depending on a variety of factors, including muscle function, age and training. If you are a sports performance specialist, it's crucial to remember the differences between the two muscle types. 

Slow Twitch vs. Fast Twitch Muscle Fiber Types

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. 

Muscle Fiber Type Comparison Chart


Slow-Twitch Type I

Fast-Twitch Type IIA

Fast-Twitch Type IIX or IIB


Marathons, distance running, swimming, cycling, power walking, endurance training

Powerlifting, sprinting, jumping, strength and agility training 

Powerlifting, sprinting, jumping, strength and agility training 

Muscle Fiber Size




Force Production



Very High

Resistance to Fatigue



Very Quick

Contraction Speed



Very Quick









Myoglobin High Medium  Low

ATPase Level




Oxidative Capacity




Slow-Twitch, Type I

Slow-twitch muscle fibers have high concentrations of mitochondria and myoglobin. Although they are smaller than the fast-twitch fibers, they are surrounded by more capillaries (1,2). This combination supports aerobic metabolism and fatigue resistance, particularly important for prolonged submaximal (aerobic) exercise activities. 

Type I fibers produce less force and are slower to produce maximal tension (lower myosin ATPase activity) compared to type II fibers. But they are able to maintain longer-term contractions, key for stabilization and postural control (1,2).


  • Small muscle fibers
  • Low, slow force
  • Fatigues slower than fast-twitch, type II
  • Long-term contractions 
  • Supports fatigue resistance for aerobic activities, stabilization and postural control

Fast-Twitch, Type II

Fast-twitch type II muscle fibers are further divided into Type IIx and Type IIa. 

Typically, these have lower concentrations of mitochondria, myoglobin, and capillaries compared to our slow-twitch fibers, which means they are quicker to fatigue (1,2). 

These larger-sized fibers are also produce a greater and quicker force, an important consideration for power activities (1,2).

Type IIX (also known as Type IIB) fibers produce the most force, but are incredibly inefficient based on their high myosin ATPase activity, low oxidative capacity, and heavy reliance on anaerobic metabolism (1,2).

Type IIA fibers, also known as intermediate muscle fibers, are a mix of type I and type IIx, with comparable tension. Able to use both aerobic and anaerobic energy systems, these fibers have a higher oxidative capacity and fatigue more slowly than type IIx (1,2).


  • Large muscle fibers
  • Greater and quicker force
  • Fatigues faster than slow-twitch type I
  • Two types: Type IIx and Type IIa
    • Type IIx produces the most force but inefficient (fatigues very fast)
    • Type IIa is a mix of type I and type IIx muscle fibers (fatigues slower than Type IIx)
  • Short-term contractions
  • Supports power activities

What’s Your Muscle Fiber Type?

So now that we’ve covered the different types, are you wondering what type you are? What type of muscles are in your hands, biceps, legs, chest and arms?

You and your muscles are not comprised of one type of muscle fiber. All of your muscles are a mix of fast-twitch and slow-twitch muscle fiber types (1). 

Whether you have more of type I or type II depends on your activity level and age

Activity Level

Nonathletic individuals have close to a 50/50 balance of fiber types. 

When you start looking at highly skilled, top-performing athletes, some differences may begin to appear. 

Power athletes have a higher ratio of fast-twitch fibers (e.g., sprinters 70-75% type II), whereas for endurance athletes have more slow-twitch fibers (e.g., marathon/distance runners 70-80% type I) (2). 

Of course, muscle fiber type is not the only factor in an athlete’s success! There are plenty of other variables that take an athlete from good to great.


Age is also a factor for our muscle fibers. 

Aging causes a loss in lean muscle mass, with a decline in our fast-twitch fibers, especially the type IIx, but there is also an increase in our slow-twitch fibers (2-4). 

Recall that the fast-twitch fibers are larger in size than the slow-twitch and are metabolically efficient fibers. Thus, loss of lean muscle mass can contribute to age-related metabolic dysfunctions, body composition changes, even an increased risk of falls (2-5). 

Resistance training can combat this decline.

Training Both Type I and Type II Muscle Fibers

You can modify fiber types through exercise. 

Type I muscle fibers can be developed through endurance training, such as low resistance, high repetition, or long duration, low intensity. (As seen in OPT ™ Phases 1 and 2.)

Type II muscle fibers can be developed through strength training

Resistance training increases the size of both type I and type II muscle fibers. Greater growth (i.e., hypertrophy) occurs in type II fibers and increases actin and myosin filaments. This results in an increased ability to generate force (2). 

Fast-twitch fibers can also recruit slow-twitch fibers: endurance training at high-intensity intervals can be effective in improving aerobic power (2,6).

Tapering during training programs (reducing volume and intensity), can also improve the strength and power of type IIA fibers without decreasing type I performance (9). 

One study investigated muscle fiber changes in recreational runners training for a marathon. After 13 weeks of increasing mileage and a three week tapering cycle, not only did the functions of type I and type IIa fibers improve, but type IIa continued to improve significantly during the tapering cycle (9).

If you are interested in training athletes, becoming a strength and conditioning coach, or getting trained as a performance enhancement specialist, get in touch! 

3 Sports PErformance Blogs to Check out


  1. Clark M, Lucett S, McGill E, Montel I and Sutton B (Editors). (2018). NASM Essentials of Personal Fitness Training (6th ed). Burlington, MA: Jones & Bartlett Publishing.

  2. Powers SK, and Howley ET. (2012). Exercise Physiology: Theory and Application to Fitness and Performance, (8th Edition). New York, NY: McGraw Hill.
  3. Akasaki Y, Ouchi N, Izumiya Y, Bernardo B, LeBrasseur N, and Walsh K. (2013). Glycolytic fast-twitch muscle fiber restoration counters adverse age-related changes in body composition and metabolism. Aging Cell 13:80-91. doi: 10.1111/acel.12153
  4. Narici MV, and Maffulli N. (2010). Sarcopenia: characteristics, mechanisms and functional significance. British Medical Bulletin 95:139-159. doi: 10.1093/bmb/ldq008
  5. Stuart CA, McCurry MP, Marino A, South MA, Howell MEA, Layne AS, Ramsey MW, and Stone MH. (2013) Slow-twitch fiber proportion in skeletal muscle correlates with insulin responsiveness. Journal of Clinical Endocrinology & Metabolism 98:5, 2027-2036. DOI:
  6. Vanhatalo A, Poole DC, DiMenna FJ, Bailey SJ, and Jones AM. (2011). Muscle fiber recruitment and the slow component of O2 uptake: constant work rate vs. all-out sprint exercise. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology. 300:3, 700-707. doi: 10.1152/ajpregu.00761.2010
  7. McGill E, Montel I. (Editors). (2019). NASM’s Essentials of Sports Performance Training (2nd ed.). Burlington, MA: Jones & Bartlett Publishing.
  8. Barh R (Editor). (2012). The IOC Manual of Sports Injuries. Chichester, West Sussex: Wiley-Blackwell/ Jophn Wiley & Sons Ltd.
  9. Trappe S, Harber M, Creer A, Gallagher P, Slivka D, Minchev K, and Whitsett D. (2006). Single muscle fiber adaptations with marathon training. Journal of Applied Physiology, 101:3, 721-727. doi: 10.1152/japplphysiol.01595.2005

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The Author

Stacey Penney

Stacey Penney

Stacey Penney, MS, NASM-CPT, CES, PES, CNC, is the Content Strategist with NASM and AFAA. A 20+ year veteran of the fitness industry, she's worked with the top certification and continuing education groups. At NASM and AFAA she drives the content for American Fitness Magazine, blog and the social media platforms. Stacey received her degree in Athletic Training/PE from San Diego State University and an MS in Exercise Science from CalU, plus credentials in Health Promotion Management & Consulting (UCSD), Instructional Technology (SDSU), group fitness and yoga. Previous San Diego Fall Prevention Task Force Chair, she’s developed continuing education curriculum for fitness organizations in addition to personal training, writing, and co-coaching youth rec soccer.

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