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The Long Read on Muscle Fibers: Types, Strength, Hypertrophy and Training Optimization

Muscles, muscles, muscles! This week's blog post is all about... muscles! I delve beneath what is visible from the outside of a flexed biceps and explore muscle fibers: types, strength, hypertrophy and training optimization.

Have you ever thought about your muscles? Have you considered how muscle contracts, how some people can lift an impressive amount of weight but only a few times, others unable to lift as much are able to endure a moderate load for much longer?

Some have muscles that appear to rapidly grow in response to training, for others there is less obvious external change (yet still much of benefit happening internally).

The different types of muscle fiber and their proportional representation in your muscles play a crucial role in your physical expression.

To get the most out of exercise:

  • Do you need to know what percentage of which fibers you are made up of?
  • What about the fiber type make-up of each individual muscle group?
  • Do you need to tailor your training to target the various fiber types with differing loads, rep schemes and load times (TUL), or is there a handy one-size fits all fibers approach?
  • And while we are at it, how do your fibers actually contract so you are able to do something useful like lift a barbell?

Let’s delve beneath what is visible from the outside of a flexed biceps and take a look under the hood, beneath the skin as we explore muscle fibers: the science, theory and practical outcomes.


The Basics: Three Types of Fiber

Let’s begin with an overview of the fiber types found in muscle tissue. There are three main types of muscle fiber in humans:

  • Type I (also referred to as slow),
  • Type IIa (or fast oxidative glycolytic) and
  • Type IIx (or fast glycolytic).


Muscle fiber types and their characteristics

Type I Type IIa Type IIx
Characteristics Resistant to fatigue. Slow twitch fibers continually switch, so that recycled and freshly charged slow twitch fibers are activated allowing continuation of the activity These fast twitch fibers can generate a fairly high degree of force, instantly, which can then be maintained until about the three-minute mark of sustained activation Fatigue rapidly, in comparison to the other fiber types, attaining peak force rapidly. However by about the 2 minute mark, they can only produce 10-20% of their starting force
Generation of energy Occurs through oxidation of energy substrates (using oxygen to generate more fuel or ATP) Occurs through aerobic and anaerobic metabolism Occurs through anaerobic metabolism, utilizing energy sources, such as creatine phosphate from within the muscle
Low vs. high intensity activity Are selected for activities of lower intensity Are selected for activities of high intensity Are selected for activities of high intensity
Long vs. short duration Are selected for activities of longer duration Are selected for activities of shorter duration Are selected for activities of shorter duration
Force Produce in the region of 15% of the amount of force that type IIa fibers can generate. And only about 5% of the amount of force that type IIx fibers can generate Produce about 45% of the amount of force that type IIx can produce – far more than type I fibers, and yet they retain a degree of aerobic capacity (though a lesser aerobic capacity than type I fibers) Produce the highest degree of force
The effect of resistance training Resistance training has been shown to up regulate the gene expression of type I fibers Resistance training enlarges type II fibers, twice as much as it does type I fibers. Resistance training up regulates the gene expression of type IIa fibers Resistance training enlarges type II fibers, twice as much as it does type I fibers. Resistance training has been shown to down regulate the gene expression of type IIx fibers, converting type IIx fibers to type IIa fibers


What about the hybrids?

In addition to the three main fiber types, there are also hybrid fibers; I/IIa, IIa/IIx and I/IIa/IIx.

These hybrids have the ability to convert to a pure type relatively quickly depending on environment and exercise. The hybrids are a great representation of the plasticity of muscle tissue. Resistance training for example, has been shown to maintain the proportion of type I fibers whilst increasing type IIa fibers, decreasing IIx fibers, and decreasing hybrid types.

Conversely, during extended periods of relative muscular inactivity, (e.g. bed rest or space flight) there is a general shift in fiber type characteristics from slower to faster. Type I fiber proportion decreases whilst IIa/IIx hybrids increase (they are probably in transition to a faster pure type expression). This results in a decrease in muscular strength, endurance and size, such as that experienced by ISS dwelling astronauts.

The above examples indicate the inherent plasticity of human muscle tissue. A plasticity with limits, as genetics dictate the individual’s starting point and cap their range of possible change. Nevertheless this adaptability of muscle tissue is good news for those of us who want to improve our own muscular strength, endurance, size or indeed our ability to perform our best at a given sporting event or activity.


Motor Units or How Fibers Contract

Let’s break this down using the example of the biceps.

The human biceps muscle has been estimated to consist of about 580,000 individual fibers.

When we flex our guns the central nervous system activates the fibers via neurons (nerve cells).

In the biceps one neuron is responsible for controlling about 750 individual muscle fibers. The neuron and the fibers it innervates are known as a motor unit.

The human biceps has been shown to have about 774 motor units (MUs).

774 MUs x 750 fibers = 580,000 total muscle fibers accounted for in the biceps.

The principle is the same for all our skeletal muscles.

Individual motor units are always found in one of three states:

  • resting (prior to activation)
  • activated
  • fatigued/recovering

Once a motor unit has been activated its firing frequency, the rate at which nerve impulses arrive, increases to meet demands, up to the point where the motor unit is fatigued. This increase in firing rate alone can double or even quadruple the force being produced by a given motor unit.


What happens during a HIT set


Let’s consider a set of an exercise taken to muscular failure (MMF) with a load that is relatively light: 40% of a one rep maximum (1RM- the amount of load a person could lift just once). At the very start of this fairly light set the central nervous system will mainly recruit lower threshold motor units that innervate type I fibers (a minimal number of higher threshold motor units will also be recruited, those responsible for innervating type IIa fibers).

As the set progresses there will be continual substitution of the type I fibers, thanks to their quick recovery. Simultaneously an increased number of higher threshold motor units will be activated: initially more type IIa motor units and as muscle failure approaches type IIx units too. This up the ladder recruitment of motor units and muscle fibers is known as orderly recruitment, sequential recruitment or the Henneman size principle.

Towards the end of a set to MMF, when nearly all the available motor units have been recruited, emphasis switches to increasing the firing frequency of the motor units to optimal levels. As fatigue sets in a motor unit’s firing rate will ultimately decrease to the point where it can no longer be activated.

With a 40% 1RM load MMF would typically be reached by around the 2 minute mark or so. When MMF is reached both type IIa and type IIx controlling motor units are temporarily exhausted, capable now of producing less force than the lower order type I motor units can. The continually recovering and substituting type I fibers are still producing about the same amount of force as they did at the outset, a degree of force however which alone is unable to keep the load moving, the set is finished.


With heavier loads, say 60-80% 1RM, there will be an increased degree of synchronous firing of both low and high threshold motor units from the very start of the exercise as the heavier load requires more type II fibers to help from the outset. Indeed, with 80%+ of 1RM load it is likely that all available motor units will be engaged right at the start of the set.


Performance of a 1RM alone, such as in Powerlifting will of course also recruit all available motor units synchronously, however the very limited load time of a 1RM results in minimal muscular fatigue.


Maximal Voluntary Contraction

During a maximal voluntary contraction (MVC, the greatest amount of force you can generate isometrically) all available motor units will be recruited right at the outset: peak force can typically be reached within about 1.5 seconds. When an MVC is sustained for as long as possible, muscle force output starts dipping rapidly due to type II fibers, first IIx then IIa, getting fatigued. Force output can be reduced to just 20-35% of fresh output within 90-180 seconds. After this initial dramatic drop, a baseline is reached consisting of a very stable but much lower degree of force. This baseline force is mostly attributable to hard to fatigue and continually substituting type I motor units.

The degree of force reduction and the time in which it takes to dip to baseline during an MVC is likely individual, based on the motor unit/fiber type make-up of the muscle group in question. Comparatively, a muscle group with a higher proportion of type IIx fibers may reduce in force output more significantly and more quickly e.g. reduce to 20% of fresh output in around 90 seconds, whereas a muscle group with more type I fibers may only reduce to a baseline of 35% of fresh output after a period of 180 seconds.

The graph below shows an individual’s force production during a maximal voluntary contraction sustained for 3 minutes. You can see that by 135 seconds of sustained contraction this individual’s force output is reduced to their type I motor unit output potential.


The Goal of A HIT Set

We want to recruit and fatigue all the voluntarily accessible motor units and fibers of the targeted muscle group during a set. Whether this happens with a synchronous recruitment of all motor units from the outset (80% 1RM) or a sequential recruitment that builds over the set (40% 1RM) may not matter.


Fiber Types and Hypertrophy

Type I and type II fibers are both capable of hypertrophy, although the size increase in pure type I fibers is considerably less compared to type II fibers. As Grgic et al point out, type I fibers have a capacity for protein synthesis (important for hypertrophy) and favourable anabolic factors, however they also have greater protein degradation mechanisms (like autophagy) which likely reduce hypertrophy potential. They suggest that to optimize hypertrophy of type I fibers, longer time under loads (TUL) may be required. Grgic and Schoenfeld however, also point out this is theoretical as there is currently a lack of evidence to support longer TULs for improved type I hypertrophy. And a potential concern with using longer TUL’s is that the there may be a reduction in stimulus to the IIa fibers.


Testing the limits of set length and load

Research published in 2018 by Lasevicius et al, studied the effects of performing (volume-matched) sets to failure with differing loads from as little as 20% of 1RM up to 80% of 1RM. Their results showed that exercises performed with 40%, 60% and 80% of 1RM loads all stimulated similar amounts of hypertrophy. In comparison, using 20% of 1RM only stimulated about half the amount of muscle growth.


Repetitions and TUL’s for the loads used approximately averaged:

80% 1RM- 12 reps, 50 seconds

60% 1RM- 18 reps, 70 seconds

40% 1RM- 30 reps, 120 seconds

20% 1RM- 65 reps, 260 seconds

(A 2/2 cadence was used and sets were taken to MMF).


The results of this research suggest that a load that allows a TUL of 260 seconds is too light to be used to achieve MMF with, in a way that optimizes total hypertrophy. Chronically there may be a shift downward with type I/IIa hybrids converting to type I fibers, likely useful for nearly 5 minutes of load time! Perhaps less useful if you want bigger muscles. It may also be that the hypertrophy seen in the 20% of 1RM condition is largely of the type I fibers, which have less size potential.

We can tentatively infer from this research that at somewhere between 20-40% of a 1RM or somewhere between 120-260 seconds of a set taken to failure, loading becomes too light and the TUL too long for optimal hypertrophy and fiber conversion to IIa. Positively, this paper suggests that a wide range of loads (between 40-80% of 1RM), and therefore TULs (50-120 seconds), in sets taken to MMF, are effective at stimulating hypertrophy and are effective to a similar degree.

Think back now to the sustained MVC that appears to significantly fatigue all type II motor units within 90-180 seconds (individual dependent). One may infer from this that a load (even in dynamic exercise) that cannot be used to bring about MMF in <180 seconds (at most) is not an optimal load for hypertrophy across all fiber types because it may be too light to activate and fatigue a significant amount type II motor units. Hence Lasevicius et al.s research showing 40% and up of 1RM being more effective for hypertrophy than 20%.

This graph shows the same sustained maximum voluntary contraction (MVC) force output as before. See that the curve flatlines at 33 units of force suggesting a theoretical minimum in this case of 33% of a 1RM being required to activate and fatigue all motor units at MMF. Whilst 20% of a 1RM may stimulate some hypertrophy of type I fibers and perhaps type I/IIa hybrid fibers, it does not provide enough force to achieve this in an optimally effective or timely manner.

  • MVC/Isometric= 120 units of force achieved in 1.5 seconds
  • 1 Rep Max= 100 units of force achieved within 5 seconds
  • 80% of 1 RM= 80 units of force, 50 seconds to MMF
  • 60% of 1RM= 60 units of force, 70 seconds to MMF
  • 40% of 1RM load= 40 units of force, 120 seconds to MMF
  • MVC fatigued baseline= 33 units of force (27.5% of initial force), reached in 135 seconds
  • 20% of 1RM= 20 units of force, below theoretical type I force output potential


Bringing us back to Arthur Jones

All of the above brings to mind the work that Arthur Jones oversaw at MedX, some of which he shared in The Lumbar Spine, the Cervical Spine and the Knee: “Resistance should be low enough to permit at least eight full-range movements, but high enough to prevent more than twelve. Fast-twitch subjects should use a lower range of repetitions than indicated above, from six to nine repetitions. Some slow-twitch subjects will produce greater gains in strength with a range of fifteen to twenty repetitions.”

The repetition performance style recommended by Jones typically results in repetitions lasting about 6 seconds each. If we translate Jones’ recommended repetition ranges above to Time Under Loads we get the following:

  • 50-70 seconds for normal muscle fiber type distribution
  • 35-55 seconds for fast twitch (type II) dominant muscles
  • 90-120 seconds for slow twitch (type I) dominant muscles

Note that the TULs are nearly identical to those produced during the use of 40-80% of 1RM in Lasevicius et als research.


Practical Takeaways

In a resistance training model focused on hypertrophy, IIx and hybrid fibers will effectively convert to IIa fibers, whilst the proportion of type I fibers remains constant.

Type I fibers can hypertrophy, although IIa fibers get much bigger in comparisson.

To optimize fiber conversion and hypertrophy during exercise, recruit and fatigue as many motor units/fibers as possible. With dynamic continuous tension sets choose a load that you can use to bring about MMF in the target muscle group within 35-120 seconds. It may well prove to be that sets to MMF can go on as long as 180 seconds and still stimulate optimal hypertrophy for some muscle groups in some individuals.

Fisher and Steele do point out one possible downside to very long sets: the discomfort factor. Despite MMF occurring in both scenarios it is likely most individuals will experience a 120+ second set as feeling much more uncomfortable and unpleasant, compared with a 60 seconds set. Indeed very few trainees in the wider population will be prepared to tolerate a James Steele style 3-minute wall sit.

Two decades of practical experience can be distilled to this: gravitate toward a TUL for each muscle group which allows you to focus on the targeted musculature, to mentally engage it/feel it and extract its best potential performance en route to MMF. To do this, stay away from loads that you wrestle with: which feel so heavy that the weight distracts you from technique. And avoid loads which are so light that the set experientially takes forever, where your mind falters and fatigues before your muscles, or where intolerable slow-building fatigue-related sensations get in the way of you getting to MMF.

Somewhere between those two places is your sweet spot.


1. Microgravity-Induced Fiber Type Shift in Human Skeletal Muscle James R. Bagley, Kevin A. Murach, and Scott W. Trappe Human Performance Laboratory, Ball State University, Muncie, IN 47306, USA Gravitational and Space Biology Volume 26 (1) Apr 2012
2. Cambridge University Press 978-0-521-87629-2 – Disorders of Voluntary Muscle, Eighth Edition Edited by George Karpati, David Hilton-Jones, Kate Bushby and Robert C. Griggs Excerpt. Available here.
3. Inducing hypertrophic effects of type I skeletal muscle fibers: A hypothetical role of time under load in resistance training aimed at muscular hypertrophy Medical Hypotheses February 2018 DOI 10.1016/j.mehy.2018.01.012 Jozo Grgica,⁎, Jan Homolakb, Pavle Mikulicc, Javier Botellaa, Brad J. Schoenfeld
4. Jozo Grgic and Brad J. Schoenfeld Front. Physiol., 18 April 2018 | https://doi.org/10.3389/fphys. 2018.00402 Are the Hypertrophic Adaptations to High and Low-Load Resistance Training Muscle Fiber Type Specific? https://www.frontiersin.org/ articles/10.3389/fphys.2018. 00402/full
5. Thiago Lasevicius, Carlos Ugrinowitsch, Brad Jon Schoenfeld, Hamilton Roschel, Lucas Duarte Tavares, Eduardo Oliveira De Souza, Gilberto Laurentino & Valmor Tricoli (2018): Effects of different intensities of resistance training with equated volume load on muscle strength and hypertrophy, European Journal of Sport Science, DOI:10.1080/17461391.2018. 1450898. Available here.
6. Effects of strength training on muscle fiber types and size; consequences for athletes training for high‐intensity sport J. L. Andersen P. Aagaard First published: 14 September 2010. Available here.
7. Jones, A. The Lumbar Spine, the Cervical Spine and the Knee MedX Corporation 1993. Available here.
8. Current concepts of muscle and tendon adaptation to strength and conditioning Jason Brumitt, PT, PhD, ATC, CSCS Tyler Cuddeford, PT, PhD1
9. High- and Low-Load Resistance Training: Interpretation and Practical Application of Current Research Findings August 2016 Sports Medicine DOI10.1007/s40279-016-0602-1 James Fisher James Steele Dave Smith
10. Heavier and lighter load resistance training to momentary failure produce similar increases in strength with differing degrees of discomfort. Fisher JP1, Steele J1. Muscle Nerve. 2017 Oct;56(4):797-803. doi: 10.1002/mus.25537. Epub 2017 Apr 17.

6 responses on "The Long Read on Muscle Fibers: Types, Strength, Hypertrophy and Training Optimization"

  1. Hi Simon,
    I have frequently performed James Steele-style 3-minute wall sits and can attest to the deeply, deeply unpleasant nature of them! So much so, that I took to holding a couple of dumbbells thereafter, more to reduce the horrible, growing wretchedness of the TUL, rather than because I thought I couldn’t go on longer…


    • Nick,

      Have experienced the same. There comes a point in the wall sit where a load increase will become preferable to the extra seconds… exactly where that point lies may differ slightly for each individual.

  2. “Resistance training has been shown to down regulate the gene expression of type IIx fibers, converting type IIx fibers to type IIa fibers” – If resistance training down regulates Type IIx to IIa does this make you initially weaker at a 1RM range or is this offset by increase in type IIa?

    • Paul,

      It is very unlikely that fiber change as you describe has any real-world negative impact on 1RM ability. Yes the overall shift from “above” and “below” toward IIa expression would likely produce more force. Even if there was a tiny negative change, this would be outgunned by aspects such as improved neural drive and coordination when practicing 1RM.

  3. Hi Simon,

    How do you determine your muscle type makeup? Slow vs. Fast? It sounds like you should run longer times if you are heavier on Type I.

    • Hi Ed,

      The only way to accurately assess your fiber make-up is by muscle biopsy.

      The reality is there is no need to know what your fiber-type make up is in each muscle group. As long as your set lasts somewhere between 30-120 seconds and you reach muscular failure all your available fibers will have been recruited.

      Over time you may well notice a personal preference for shorter sets (30-50 seconds), moderate length sets (50-80 seconds) or longer sets (80-120 seconds) and the preference may vary between muscle groups/exercises. Part of the preferences may come from fiber type, there are other elements too which will have an impact such as tolerance to the sensations of fatigue, neurologic factors and confidence in load handling. This overall big picture perspective is more important and practical than knowing an exact fiber-type make up for each muscle group.

      You can read another blog post for further information on muscle fibers here which explains how muscle fibers can change in response to training.

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