Posted on Leave a comment

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 Reviews, 91 (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.

Posted on Leave a comment

4 Exercises to Improve Your Posture and Spine

Woman working out at home

Good posture is not just important for looking good, but it can also save you from many bone issues—especially those related to your spine. Proper posture also reduces the stress on your muscles and prevents any health risks. But most of us are in the habit of slouching. You must’ve noticed that your muscles start to ache while slouching. Here are 4 healthy exercises you can do to reduce slouching and improve your posture.


Forward Fold

This is a common pose recommended to those that work long office hours. The pose helps release built-up stress in your muscles. But what many don’t know is that it’s also excellent to improve your posture as well.

You simply need to stand upright with your feet slightly apart. Not bend forward and bring your hands to touch your toes. It’s alright if you can’t touch them properly. Hold this pose for 1 minute, then relax for 30 seconds, and repeat.


Child’s Pose

This pose is specifically recommended to straighten your back and improve your posture. Sit on a flat surface with your lower legs folded under and your toes touching the floor. Bend forward while extending your hands. You should try to extend your hand as much as you can to maintain tension. Place your forehead on the floor to avoid neck pain. Hold this pose for 5 minutes while taking deep breaths and repeat a few times a day for surprising results.


Cat-Cow Pose

Personal trainer guiding a man

The cat-cow pose stretches your muscles, massages your spine, and relieves tension in your torso. Balance your weight evenly between your hands and knees. Take a deep breath and drop your abdomen downwards while extending your spine.

Your spine should resemble a smiley face. Now exhale gently while arching your back towards the ceiling and tucking your chin under your chest. Repeat this exercise a few times to relax your muscles.


High Plank

Planks are generally harder to maintain, but with enough practice, you’ll get the hang of it. This pose will also help get rid of stiff muscles and would relax your hamstrings and glutes. In the long run, you’ll also develop good core strength.

Get on all fours, and straighten your legs. Now lift your heels, raise your hips, and straighten your back. Hold this position for a minute and repeat after 30 seconds.

If you want to up your fitness game, become a certified fitness trainer with us. We, at W.I.T.S. Education, offer multiple fitness programs like health trainer programs, group fitness trainer programs, and exercise instructor programs to people passionate about helping others with their physical fitness. Check out our certifications or contact us today.