What's the difference between fast and slow twitch muscle fibers?

Geraldine Campbell, MSc


August 24, 2017

What's the "sitch" with the fast and slow twitch?

The phrases "fast-twitch" and "slow-twitch" crop up frequently in the fitness industry (and throughout our Member's Area) in relation to our muscle fibers. But what are these and how do they differ?

There are three major types of muscle fibers in the human body: Type I, Type IIa and Type IIx (sometimes referred to as Type IIb). They are distinguishable through the level of activity of the enzyme Myosin Adenosine Triphosphase (ATPase) as well as through other characteristics. Your Type I fibers are slow-twitch while both Type II fibers are classed under the term of fast-twitch. The percentage of each found in your body is determined by both your genetics (40% for slow-twitch and 60% for fast-twitch) and environmental factors such as training experience.

Slow-Twitch Fibers

These muscle fibers are an endurance athlete's best friend. They are plentiful in mitochondria, myoglobin (oxygen binding protein), and capillaries which allow for oxygen to be used to produce energy via oxidative phosphorylation. The capillaries transport the oxygen bound to the myoglobin into the muscles where it can be used by the mitochondria to synthesize ATP; allowing you to exercise longer. Slow twitch muscle fibers are therefore highly fatigue-resistant which is why muscles such as your gastrocnemius (calf) contain more slow-twitch fibers innately; allowing you to keep walking, standing and doing all those everyday activities without fatiguing quickly. The major fuel for slow twitch muscle fibers are triglycerides (fats) as this energy source can provide an abundance of ATP. Find out more about how your body uses fat for fuel here.

While slow twitch muscle fibers power you through long runs or rides, they won't be great for explosive lifting or sprints as they are slower to contract.

Fast-Twitch Fibers

The two types of fast-twitch muscle fibers differ from the slow-twitch fibers because they have the ability to produce energy in the absence of oxygen (glycolytic oxidation). This allows them to produce energy quicker using phosphocreatine and glycogen (read more on these energy systems here), to fuel those quick explosive movements such as jumping and sprinting. This means they have faster contraction speeds than slow-twitch fibers. These two types of fast-twitch fibers also differ from each other, with Type IIa fibers displaying some similarities with the slow-twitch fibers.

Type IIa fibers can produce energy through both glycolytic AND oxidative pathways which make them slightly more fatigue resistant than the Type IIx fibers. Type IIa also have more capillaries than IIx; aiding their ability to produce energy using oxygen.

The table below summarises the differences between the different muscle fibers:

Muscle Fiber Table

So what sports are each best for?

Your Type I fibers are perfect for long duration exercise performed at a low to moderate intensity. If you enjoy marathons, half marathons, 10ks, long walks or cycles; these are the muscle fibers powering you through.

Type IIa fibers are for moderate duration exercise at a higher intensity. Events like the 400m and using moderately heavy weights in an 8-12 rep range during your resistance training will predominantly use these fibers.

Type IIx fibers are the experts at short exercise bouts at a high intensity that require that explosive production of force by the muscles. Sprinting the 100m or powerlifting are examples of the kind of sports these fibers thrive in.

What can FitnessGenes tell me about my muscle fiber types?

There are many gene variants that can influence your make up of these different fiber types. ACTN3 is one such example, with the protein that this gene codes for being most abundantly found within fast-twitch muscle fibers. Having the genotype that leads to a knockout of this protein has been shown to be more associated with greater endurance performance than power or speed.

But your genetics only have partial control over your muscle fiber type composition. If you want to train to be a sprinter but have the genetic advantage for endurance, the training you put in to become a sprinter will help your muscle fibers to adapt to higher intensity stimuli and aid your performance!


Edgerton, V.R., Smith, J.L. and Simpson, D.R., 1975. Muscle fiber type populations of human leg muscles. The Histochemical journal, 7(3), pp.259-266.

Thorstensson, A. and Karlsson, J., 1976. Fatiguability and fiber composition of human skeletal muscle. Acta Physiologica, 98(3), pp.318-322.

Bouchard, C., Simoneau, J.A., Lortie, G., Boulay, M.R., Marcotte, M. and Thibault, M.C., 1986. Genetic effects in human skeletal muscle fiber type distribution and enzyme activities. Canadian journal of physiology and pharmacology, 64(9), pp.1245-1251.

Essen, B., Jansson, E., Henriksson, J., Taylor, A.W. and Saltin, B., 1975. Metabolic characteristics of fiber types in human skeletal muscle. Acta Physiologica, 95(2), pp.153-165.

Geraldine Campbell, MSc

After originally joining the company as an intern, Geraldine has progressed through the science team to now oversee all new research and releases. In addition to her MSc in Clinical Exercise Physiology, Geraldine is a certified Exercise Physiologist (ACSM), and is therefore pivotal in the delivery of recommendations. A keen footballer and gym-goer, Geraldine likes to balance her active lifestyle with her love of food.

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