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The Impact of Training on Muscle Fiber Types

We recently received a great question about muscle fiber types from an individual taking the HITuni personal trainer course. The student was concerned that when an individual partakes in HIT, there may be conversion of IIx fibers to IIa fibers. He pondered that if this does happen, then it might be a negative for aging muscle, as it would speed up the loss of the fastest-twitch fibers. He asked if I thought this was problematic.

This is a valuable question and gives a springboard to dive into the fascinating subject of fiber types from a slightly different perspective than my last post on the topic – The Long Read on Muscle Fibers: Types, Strength, Hypertrophy and Training Optimization. If you need a refresher on the basics of muscle fiber types that article is a useful read, especially the first few paragraphs, which will provide context for the current post.

In order to answer the above question – is the conversion of IIx fibers to IIa fibers, as a result of HIT, problematic as we age? – we’ll first take a closer look at how muscle fibers are classified… it begins with myosin.


The Science Bit

Myosin a complex protein present in every muscle fiber, is responsible for driving muscle contraction, using ATP (adenosine triphosphate) as its fuel. Within myosin, there are Myosin Heavy Chains (MHC) made up of amino acids. It is these MHCs that reveal the characteristics of each individual fiber and tell us whether they are slow-twitch or fast-twitch. This is because there are different versions or isoforms of MHC.

MHC Isoforms and Fiber Type

Firstly, there are three pure MHC isoforms found in humans- MHC I, MHC IIa and MHC IIx. These different isoforms process the fuel source ATP at different speeds:

  • MHC I processes ATP the slowest and is found in slow-twitch muscle fibers,
  • MHC IIa processes ATP faster and is found in many fast-twitch fibers,
  • MHC IIx processes ATP fastest of all and is found in few fast-twitch muscle fibers.

MHC IIx isoform fibers are very rare. Typically, only 0-2% of fibers in a given muscle will be of the pure IIx isoform. It is common for a muscle to contain no pure MHC IIx isoform fibers at all.

Pure isoforms are not the only isoform types found in humans. There are also three hybrid types too: MHC I/IIa, MHC IIa/IIx and MHC I/IIa/Ix.

All the MHC isoforms found in human muscle fibers:

  • MHC I (pure isoform)
  • MHC I/IIa (hybrid)
  • MHC IIa (pure isoform)
  • MHC IIa/IIx (hybrid)
  • MHC IIx (pure isoform) – rare
  • MHC I/IIa/IIx (hybrid) – rare

Each individual fiber in human muscle will contain one of the above isoforms. Our personal genetic make up will to a degree determine how many of our fibers express each isoform and therefore the overall proportional make up of a muscle: whether you have more slow-twitch or fast-twitch fibers.


The Sedentary Individual

As an example, a sedentary or minimally active individual may have a quadriceps muscle fiber makeup that looks like this:

  • 40% of fibers have MHC I isoform
  • 10% of fibers have MHC I/IIa isoform
  • 35% of fibers have MHC IIa isoform
  • 15% of fibers have MHC IIa/IIx isoform
  • (Note that no IIx or I/IIa/IIx fibers are present).


Plasticity and Exercise

Here’s where it gets interesting. These percentages are not set in stone. The isoform in a given fiber can change and thus the overall fiber makeup of a muscle can change too. What can cause this shift? Environmental factors- such as exercise.

Compare in the table below our sedentary individual against the isoform expression (fiber-type) in active individuals with differing training approaches:

Sedentary 40% 10% 35% 15% 0%
Average Resistance Trainer 25% 8% 60% 7% 0%
Elite Weightlifter 25% 2% 73% 0% 0%
Marathoner 65% 10% 25% 0% 0%


One of the first things to note is that active individuals have less hybrid isoforms than sedentary folks. Hybrids are likely the quickest to change in response to training, particularly the IIa/IIx hybrids. However, all isoforms including pure ones can change. It is also likely an elite weightlifter ultimately converts some pure MHC I fibers to pure MHC IIa fibers. In contrast, a marathoner will probably convert some MHC IIa fibers to MHC I fibers, in addition to converting many of their hybrids.

This points to the plasticity of muscle. There are individual genetic parameters, but environmental factors such as exercise have a strong impact on how many fast-twitch fibers and slow-twitch fibers you have.

Note that just as exercise can cause change in the fibers, so too can deconditioning. A sedentary individual who engages in resistance training for 6 months and then ceases training will see fiber-type percentages revert toward their original sedentary profile. If that individual switched from resistance training to endurance training they would see a change in fibers reflective of the different demands placed on the muscle, i.e. a move from a more fast-twitch profile to a more slow-twitch profile.

Hybrid fibers can make up 20-40% of a muscle in sedentary individuals, but usually only 10-20% in trained/active people and in highly-trained even less- look at the profile for the elite weightlifter in the table! Higher amounts of hybrid fibers are associated with inactivity and deconditioning.


The Rarity of Pure IIx

Another fascinating fact about MHC isoforms is how rare type IIx is. This can be a point of confusion for many, including personal trainers. This is because research from prior decades suggested that IIx fibers were common: making up 5-30% of a muscle’s fibers. This has more recently proven to be inaccurate as research methods have improved. It is now understood that hybrid IIa/IIx fibers were previously being misclassified as pure IIx fibers. It now seems that pure MHC IIx typically occurs in less than 0.1% of fibers. There are very rare anomalies such as sprinter Colin Jackson whose quadriceps were calculated to have 24% pure MHC IIx. To put this into perspective consider that 21 elite weightlifters who have been tested in recent research have zero pure MHC IIx fibers… between all of them! (1)

Previously it had been thought that when a person engages in resistance training, there was a conversion of their pure MHC IIx to MHC IIa. It is now understood that this conversion is MHC IIa/IIx hybrids changing to pure MHC IIa.


HIT and Sarcopenia: Resistance Train for Life

Which brings us around to answering the original question the HITuni student has about age-related muscle decline or sarcopenia. An individual experiencing sarcopenia will see a decrease in the number of all fiber-types and distinct atrophy of type II fibers. The more sedentary the individual, the greater and quicker the likely impact.

Research has now confirmed that older muscle tends to have a greater proportion of hybrid type I/IIa (28.5%) compared to younger muscle (5-10%). This is similar in effect to deconditioned muscle having a greater proportion of hybrids in comparison to trained muscle, but even more pronounced in sarcopenia.

Non-active older adults can be hit by a sarcopenia triple whammy- loss of fibers, atrophy of fibers and conversion from pure to hybrid fibers- deconditioning in extremis. The upshot is a greater proportion of slower fibers and resulting weakness.

When an older adult suffering from sarcopenia starts and continues with a resistance training intervention many of these type I/IIa hybrid fibers will convert to type IIa fibers, along with some IIa/IIx hybrids converting to IIa as well. There will also be an increase in the cross-sectional area of individual fibers as hypertrophy occurs. These shifts represent a positive adaptation in muscle tissue and importantly strength, functional ability and metabolic health. We needn’t be concerned about losing pure IIx fibers when we resistance train as we have very few to none of them in the first place. Instead we need to focus on having robust IIa expression as we age- and the key to this is resistance training.

Indeed, research has concluded that resistance training is “extremely important in elderly individuals.” (2) And in terms of appropriate exercise modality when it comes to staving off sarcopenia “the use of weight or resistance training have yielded positive results, but aerobic activities alone have little impact.” (3)


What does this all mean in practice?

Muscle has a high degree of plasticity and the characteristics of individual muscle fibers can change significantly depending on the stimuli/exercise they are exposed to.

Having a greater proportion of hybrid fibers is associated with being sedentary, deconditioning and aging.

Hybrid fibers change quickly in response to exercise. Notably resistance training will stimulate conversion to pure MHC IIa expression and endurance training will stimulate conversion to pure MHC I.

Despite what scientists thought previously, pure MHC IIx fibers are extremely rare. The supposed shift from IIx to IIa fibers in response to resistance training was mis-attributed. It is actually MHC IIa/IIx hybrids that convert to a pure IIa isoform- which is a positive adaptation.

High intensity resistance training stimulates a robust and health promoting muscle fiber profile and is perhaps the best intervention we can choose for the goal of living longer, stronger.


1. Nathan Serrano, Lauren M Colenso-Semple, Kara K Lazauskas, Jeremy W Siu, James R Bagley, Robert G Lockie, Pablo B Costa, Andrew J Galpin. Extraordinary Fast-Twitch Fiber Abundance In Elite Weightlifters. doi: 10.1101/468744
2. Justin P. Hardee, MS, James A. Carson, PhD. Understanding Sarcopenia Development- A Role for Healthy Behaviors. doi: 10.1177/1559827616674163
3. Drew L. Fighting the inevitability of ageing. nature.com/articles/d41586-018-02479-z
4. Larsson L, Degens H, Li M, Salviati L, Lee YI, Thompson W, Kirkland JL, Sandri M. Sarcopenia: Aging-Related Loss of Muscle Mass and Function. doi: 10.1152/physrev.00061.2017.
5. Wayne Scott, Jennifer E Stevens-Lapsley, Stuart A Binder-Macleod. Human Skeletal Muscle Fiber Type Classifications. doi: 10.1093/ptj/81.11.1810
6. Ildus I. Ahmetov, Olga L Vinogradova, Alun G Williams. Gene Polymorphisms and Fiber-Type Composition of Human Skeletal Muscle. doi: 10.1123/ijsnem.22.4.292


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