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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.

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Explicating Concurrent Training

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


 Fitness professionals are faced with a multitude of dilemmas in their practice. Prominently among them lies the controversial issue of prescribing both aerobic endurance training and resistance training simultaneously, which is known as concurrent training. Concurrent training has its roots in ancient Egypt, which boasted the world’s wealthiest kingdom, buttressed by a robust military force that had prevailed over Hittite and Sea People contingents and ousted Hyskos invaders, sustaining a reign lasting over two thousand years. According to scrawlings on papyrus scrolls, Egyptian soldiers performed a variety of bodyweight exercises and running in preparation for battle, helping them surmount their opposition. On the other side of the Mediterranean, Olympic athletes in ancient Greece engaged in concurrent training, often swimming or running to improve endurance and lifting weights and performing resisted running and running in sand to improve strength. Exercises performed by ancient societies serve as the foundation for programming to this day and while an exploration of the literature demonstrates a synergism between resistance training and aerobic endurance training, the greatest challenge is striking a fine balance conducive to elicit desired adaptations, which specifically encompass improved physical preparedness and athletic performance.

Aerobic Endurance Training

 In isolation, regular participation in aerobic endurance training strengthens the myocardium, resulting in increased cardiac output, which in conjunction with improved maximal oxygen consumption (VO₂max), boosts the supply of oxygenated blood to working musculature. Notable adaptations such as decreased heart rates and blood pressure, increased lactate threshold and clearance rates, improved glucose tolerance, insulin sensitivity, free fatty acid mobilization and oxidation, culminating in reduced body fat. Reduced body fat is paramount in lowering cholesterol and optimizing metabolic functioning (Dolezal & Potteiger, 1998).

Resistance Training

 Resistance training, commonly referred to as strength training, entails intermittent exercise of short durations at higher intensities interspersed with varying rest periods during a predetermined period of time (i.e., workout, training session, et cetera). In the past half century, the popularity of strength training has mushroomed as evidenced by a proliferation of health clubs throughout the industrialized world and publications showcasing its health and performance evoking capabilities in a spectrum of populations. It has become common knowledge that people of all ages, activity levels, and athletic backgrounds and goals can derive benefits from incorporating strength training within an exercise program. Regular participation in resistance training confers improvements in muscular strength, local muscular endurance, hypertrophy (Evans, 2019), strengthening of tendinous and ligamentous structures (Brumitt & Cuddeford, 2015), improved bone mineral density (Holubiac, Leuciuc, Crăciun, & Dobrescu, 2022) and coordinative abilities (Carroll, Barry, Riek, & Carson, 2001). Similar to aerobic endurance training, strength training has been shown to improve cholesterol (Mann, Beedie, & Jimenez, 2014), glucose tolerance (Craig, Everhart, & Brown, 1989), insulin sensitivity (Ishii, Yamakita, Sato, Tanaka, & Fujii, 1998), and reduce resting blood pressure (Cornelissen, Fagard, Coeckelberghs, & Vanhess, 2011).

Review of the Literature

 An early investigation by Hickson (1980) garnered concerns about the compatibility of resistance training and aerobic endurance training. Concurrent training which consisted of 30 minutes of strength training five days per week and 40 minutes of endurance training performed six days per week over a period of 10 weeks was found to diminish strength development in comparison to those engaging only in strength training. However, these concerns were rebuked by later studies and a series of recent meta-analyses demonstrating that concurrent training does not compromise muscle hypertrophy and strength (Schumann, 2022) as long believed, and the resulting interference effect is largely dependent upon loading parameters, such as frequency, intensity, time, and type (Wilson, 2012). Lower aerobic endurance training volume, such as distances of 3km or less or at 18 minutes in duration was found not to inhibit strength endurance performance in comparison to greater volume entailing distances of 5 to 7km or at 30 to 42 minutes in duration (Panissa, 2014). Weekly aerobic endurance training volume was strongly correlated with decrements in strength performance (Sousa, 2020). The type of aerobic endurance training activity was also shown to be a determinant in eliciting an interference effect. Cycling was shown to inhibit strength endurance performance more than running (Panissa, 2014).

Concurrent training can be beneficial in increasing total daily caloric expenditure (Poehlman, 2002), which will hasten their resting metabolic rate, and in turn, assist with weight control (Pollock, 2000). Improvements in hemodynamic response, arterial stiffness, muscular strength (Cortez-Cooper, 2005), and submaximal exercise capacity (Beckers, 2008) have been shown among concurrent training samples. Additionally, concurrent training has also been proven effective in cardiovascular disease management (Meka, Katragradda, Cherian, & Arora, 2008) and has been shown to restore impaired aerobic endurance and strength in heart transplant patients (Chtara, 2005). Jointly incorporating aerobic endurance and strength training protocols is also postulated to be effective in improving upper and lower body strength, aerobic endurance, and balance in the elderly (Toraman, Erman, & Agyar, 2004). Further, a recent randomized controlled trial revealed that the concurrent training group significantly reduced peripheral and central diastolic blood pressure and experienced greater increases in cardiorespiratory fitness, upper and lower body strength, and lean body mass in comparison to aerobic endurance and resistance trained groups over an 8-week period (Schroeder, Franke, Sharp, & Lee, 2019). More specific to performance, concomitant modalities of aerobic endurance and resistance training have been shown to produce greater improvements in endurance performance and aerobic capacity (Chtara, 2005; Rønnestad & Mujika, 2014).

Lower volume, high-intensity strength training has been shown to evoke greater improvements in both aerobic endurance and strength performance than moderate-intensity training (Rønnestad & Mujika, 2014). Among endurance athletes, concurrent training entailing heavy or explosive strength training has been shown to improve running, cycling, and swimming economy (Rønnestad & Mujika, 2014; Giandonato, 2011).

Practical Application

 Fitness professionals should be cognizant of the varying physiological and biomechanical demands of their clients, especially those who are engaged in competitive pursuits. For example, a distance running client who competes in local 5k races and half marathons and clients who avid lifters likely possess disparate fitness qualities, biomotor skills, and metabolic profiles. These differences are likely to be more pronounced at higher levels of competition. While both clients can and should be encouraged to engage in concurrent training, preventing undesirable adaptations, specifically reduced muscular power and strength, can be attenuated by accounting for the quaternary of loading parameters: frequency, intensity, time, and type, colloquially known as the “FITT principle”. For distance runners, lower volume, high-intensity resistance training is optimal, whereas those desiring increases in strength should have their participation in aerobic endurance training capped at no more than 18 minutes per session based on aforementioned literature. Though maximal durations of aerobic endurance training sessions may depend on the individual athlete’s training history, cardiovascular health, cardiorespiratory fitness, and performance vectors specific to their event or sport.

Additionally, if sessions are to be performed subsequently on the same day, priority should be given to which fitness qualities and biomotor skills may be insufficient or needing improvement. Further, it should be considered that performing aerobic endurance training will deplete muscle glycogen stores needed to facilitate repetitive high-intensity outputs. This is critical especially if the athlete needs to learn, practice, and ingrain technique on given movements or drills, as fatigue can impede motor learning. Ideally, same day aerobic endurance and resistance training sessions should be interpolated by a 4-to-8-hour recovery interval to reduce the interference effect and strength endurance performance. A review by Eddens, van Someren, and Howaston (2018), reported that resistance training followed by aerobic endurance training is conducive to improving lower-body dynamic strength, which is a worthy finding for clients who participate in endurance- or strength-oriented sports.

Preferably, aerobic endurance and resistance training should be performed on different days. However, if that is not possible, frequency, intensity, and volume of both aerobic endurance and strength training should be undulated throughout the year, especially during a time when the client may be competing in races or events. Additionally, fluctuations in loading parameters should account for a client’s nutritional and hormonal status, sleep quality, available time, training and chronological age, and health and injury status.


 Beckers, P.J., Denollet, J., Possemiers, N.M., Wuyts, F.L., Vrints, C.J. & Conraads, V.M. (2008). Combined endurance-resistance training vs. endurance training in patients with chronic heart failure: A prospective randomized study. European Heart Journal, 29 (15): 1858-1866.

Brumitt, J. & Cuddeford, T. (2015). Current concepts of muscle and tendon adaptation to strength and conditioning. International Journal of Sports Physical Therapy, 10 (6): 748-759.

Carroll, T.J., Barry, B., Riek, S., & Carson, R.G. (2001). Resistance training enhances the stability of sensorimotor coordination. Proceedings of the Royal Society B: Biological Sciences, 268 (1464): 221-227.

Chtara, M., Chamari, K., Chaouachi, M., Chaouachi, A., Koubaa, D., Feki, Y., Millet, G.P. & Amri, M. (2005). Effects of intra-session concurrent endurance and strength training sequence on aerobic performance and capacity. British Journal of Sports Medicine, 39 (8): 555-560.

Cornelissen, V.A., Fagard, R.H., Coeckelberghs, E., & Vanhess, L. (2011). Impact of resistance training on blood pressure and other cardiovascular risk factors. Hypertension, 58 (5): 950-958.

Cortez-Cooper, M.Y., DeVan, A.E., Anton, M.M., Farrar, R.P., Beckwith, K.A., Todd, J.S. & Tanaka, H. (2005). Effects of high intensity resistance training on arterial stiffness and wave reflection in women. American Journal of Hypertension, 18 (7): 930-934.

Craig, B.W., Everhart, J., & Brown, R. (1989). The influence of high-resistance training on glucose tolerance in young and elderly subjects. Mechanisms of Ageing and Development, 49 (2): 147-157.

Dolezal, B.A. & Potteiger, J.A. (1998). Concurrent resistance and endurance training influence basal metabolic rate in non-dieting individuals. Journal of Applied Physiology, 85 (2): 695-700.

Eddens, L., van Someren, K., & Howatson, G. (2018). The role of intra-session exercise sequence in the interference effect: A systematic review with meta-analysis. Sports Medicine, 48 (1): 177-188.

Evans, J.W. (2019). Periodized resistance training for enhancing skeletal muscle hypertrophy and strength: A mini-review. Frontiers in Physiology, 10: 13.

Giandonato, J.A. (2011). Strength training for swimmers: training considerations. Journal of the International Society of Swimming Coaching, 1 (3): 47-56.

Holubiac, I.S., Leuciuc, F.V., Crăciun, D.M., & Dobrescu, T. (2022). Effect of strength training protocol on bone mineral density for postmenopausal women with osteopenia/osteoporosis assessed by dual-energy x-ray absorptiometry (DEXA). Sensors, 22 (5): 1904.

Ishii, T., Yamakita, T., Sato, T., Tanaka, S., & Fujii, S. (1998). Resistance training improves insulin sensitivity in NIDDM subjects without altering maximal oxygen uptake. Diabetes Care, 21 (8): 1353-1355.

Mann, S., Beedie, C., & Jimenez, A. (2014). Differential effects of aerobic exercise, resistance training, and combined exercise modalities and the lipid profile: Review, synthesis, and recommendations. Sports Medicine, 44 (2): 211-221.

Meka, N., Katragradda, S., Cherian, B. & Arora, R.A. (2008). Endurance exercise and resistance training in cardiovascular disease. Therapeutic Advances in Cardiovascular Disease, 2 (2): 115:121.

Panissa, V. Gonçalves, V.L., Greco, C.C., Riberio, N., Julio, U.F., Tricoli, V. & Franchini, E. (2022). Concurrent training and the acute interference effect on strength: Reviewing the relevant variables. Strength and Conditioning Journal, 44 (3): 46-57.

Poehlman, E.T., Denino, W.F., Beckett, T., Kinaman, K.A., Dionne, I.J., Dvorak, R. & Ades, P.A. (2002) Effects of endurance and resistance training on total daily energy expenditure in young women: A controlled randomized trial. Journal of Clinical Endocrinology and Metabolism, 87 (3):1004-1009.

Pollock, M.L., Franklin, B.A., Balady, G.J., Chaitman, B.L., Fleg, J.L., Fletcher, B., Limacher, M., Pina, I.L., Stein, R.A., Williams, M. & Bazzare, T. (2000). Resistance exercise in individuals with and without cardiovascular disease: Benefits, rationale, safety, and prescription: An advisory from the Committee on Exercise, Rehabilitation, and Prevention, Council on Clinical Cardiology, American Heart Association. Circulation, 101 (7): 828-833.

Schroeder, E.C., Franke, W.D., Sharp, R.L., & Lee, D. (2019). Comparative effectiveness of aerobic, resistance, and combined training on cardiovascular disease risk factors: A randomized controlled trial. PLoS One, 14 (1): e0210292.

Schumann, M., Feuerbacher, J.F., Sünkeler, M., Freitag, N., Rønnestad, B.R., Doma, K., & Lundberg, T.R. (2022). Compatibility of concurrent aeronbic and strength training for skeletal muscle size and function: An updated systematic review and meta-analysis. Sports Medicine, 52 (3): 601-612.

 Shirai, T., Aoki, Y., Takeda, K., & Takemasa, T. (2020). The order of concurrent training affects mTOR signaling but not mitochondrial biogenesis in mouse skeletal muscle. Physiological Reports, 8 (7): e14411.

Sousa, A.C., Neiva, H., Izquierdo, M., Alves, A.R., Duarte-Mendes, P., Ramalho, A.G., Marques, M., & Marinho, D.A. (2020). Concurrent training intensities: A practical approach for program design. Strength and Conditioning Journal, 42 (3): 38-44.

Toraman, N.F., Erman, A. & Agyar, E. (2004). Effects of multicomponent training on functional fitness in older adults. Journal of Aging and Physical Activity, 12 (4):538-553.

Wilson, J.M., Marin, P.J., Rhea, M.R., Wilson, S.M.C., & Loenneke, J.P., & Anderson, J.C. (2012). Concurrent training: A meta-analysis examining interference of aerobic and resistance exercise. Journal of Strength and Conditioning Research, 26 (8): 2293–2307.