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Monday Article #79: Muscle fiber types: More than just slow and fast-twitch!




The Olympic Games: the place where the best of the best from each nation gathers. From the powerful weightlifters to the explosive sprinters, to the consistent marathon runners. Why is it that they all have such diverse athletic prowess?


It turns out that aside from talent and training, this assortment of capabilities may stem from the intricate composition of our skeletal muscles. Deep within these powerhouses lie molecular differences that define the typology of muscle fibers.


Muscle fiber anatomy and physiology


To understand how muscle fibers are classified into different types, as well as the different phenotypes of the muscle fibers, it is valuable that we understand the basic overview of muscle fiber anatomy and physiology.


Zooming out, a muscle is wrapped in a connective tissue, the epimysium, and consists of many fascicles wrapped in a layer of perimysium. Within the fascicles are muscle fibers (muscle cells), wrapped in a layer of endomysium. Within these muscle fibers, are myofibrils which themselves are composed of functional units known as a sarcomere (Dave, Varacallo and Shook, 2018). 


Figure 1: Structure of muscle. Image taken from eCampusOntario Pressbook.


Within each sarcomere are the thick filaments (myosin), and thin filaments (mainly actin). The heavy chains contain the myosin heads that interact with actin and allow the muscle to contract (Riddle et al., 2019). The head region of the myosin heavy chain (MHC) is also the site for the ATPase enzyme that is responsible for hydrolyzing ATP into ADP and Pi, which provides the energy for contraction (Dulyaninova and Bresnick, 2013).


Figure 2: Thin filament and thick filament of the sarcomere. Image taken from the Medical gallery of Mikael Häggström 2014.


Figure 3: Myosin structure. Image taken from “Myofilament dysfunction in diastolic heart failure” (Anahita Aboonabi and McCauley, 2023).


The exact mechanism of muscle contraction needs not be known, but the jist of it is the myosin head will bind to actin in the presence of ATP and calcium ions., forming a cross-bridge. The myosin heads will then pull the actin, release, and reattach. This process is known as cross-bridge cycling (Krans, 2010). 


Figure 4: The sliding filament theory describes the mechanism of muscle contraction. Image taken from Online Biology Notes.


How are muscle fibers classified?


In human skeletal muscles, different muscle fiber types have different isoforms (essentially types) of ATPases. These can be identified by histochemical staining, which separates the fibers based on the staining intensities, as different types of ATPases have different pH sensitivity (Staron, 1997). Advances in this technique have led to the identification of seven muscle fiber types, ranked in order from slow to fast: types I, IC, IIC, IIAC, IIA, IIAB, and IIB. Humans do not have type IIAB and type IIB, but instead have type IIAX and type IIX fibers (Pette, Peuker and Staron, 1999).


Another way that researchers define muscle types is by the different MHC isoforms. In humans, the 3 MHC isoforms that are predominant in skeletal muscles are MHCI, MHCIIa, and MHCIIx, in order from slowest to fastest (Staron, 1997). In animals like mice, an even faster MHCIIb isoform exists (Hilber et al., 1999). Generally, MHC isoforms are identified by sodium dodecyl sulfate-polyacrylamide gel electrophoretic (SDS-PAGE) separation. This technique can either be done on a piece of biopsied muscle tissue (which serves as a proxy for the overall muscle fiber type composition of a muscle) or on a single muscle fiber (which is more accurate and can identify hybrid fibers) (Roberts et al., 2012). Hybrid fibers contain more than one myosin heavy chain isoform, and of all the techniques listed, only single-fiber SDS-PAGE separation allows for the relative concentration of different MHC isoforms in a hybrid fiber (Medler, 2019).


Figure 5: Different ways of categorizing muscle fiber types. a represents homogenate SDS-PAGE, b represents single fiber SDS-PAGE, and c represents histochemical staining. Image taken from “Muscle Fiber Type Transitions with Exercise Training: Shifting Perspectives” (Plotkin et al., 2021).


A third classification scheme analyzes the enzymes present in the muscle fibers to determine their dominant metabolic pathways that are either oxidative or glycolytic. This classification technique leads to 3 fiber types: fast-twitch glycolytic (FG), fast-twitch oxidative (FOG), and slow-twitch oxidative (SO) (Pette, Peuker and Staron, 1999). Oxidative metabolism produces more energy per unit of glucose, but is much slower, while glycolytic metabolism produces less energy per unit of glucose, but is much faster (Pfeiffer, Schuster and Bonhoeffer, 2001).


Though using different techniques, the different classification schemes can be correlated. The type IC, IIC, and IIAC hybrid fibers coexpress the MHCI and MHCIIa genes, whereas the type IIAX fibers coexpress the MHCIIa and MHCIIx genes to varying degrees. The type I, IIA, and IIX pure fibers on the other hand only express MHCI, MHCIIa, and MHCIIx respectively (Staron, 1997). In general, fibers at the type I end of the continuum depend on oxidative energy metabolism, and fibers at the type IIX end of the continuum depend on glycolytic metabolism (Hämäläinen and Pette, 1995). 


Figure 6: Correlation between different classification schemes of muscle fiber types. Image taken from “Microgravity-Induced Fiber Type Shift in Human Skeletal Muscle” (Bagley, Murach and Trappe, 2012).


Figure 7: Pure vs hybrid fibers. Image taken from Renaissance Periodization.


What are the functional differences then?


In terms of capillarity and oxygen-storing capacity, SO fibers have more myoglobin and more capillaries surrounding them, FG fibers have almost no myoglobin and little amounts of capillaries around them, while FOG fibers are somewhere in the middle (Sullivan and Pittman, 1987). This is due to the different ways these fibers acquire energy. While glycolytic fibers rely on the rapid ATP generation from glycolysis, which doesn’t require oxygen, oxidative fibers take advantage of sustained ATP production from oxidative respiration (which requires oxygen) to support their long-duration activities (Hargreaves and Spriet, 2020). The capillaries function to deliver oxygen and nutrients to the working muscle, whereas myoglobin is an oxygen storage molecule. To cope with their oxidative metabolism, oxidative fibers also have more mitochondria volume (Scarpulla, 2008), as well as mitochondria that are more capable of oxidizing fatty acids compared to glycolytic fibers (Mogensen and Sahlin, 2005). Mitochondria in oxidative fibers are also more filamentous while mitochondria in glycolytic fibers are more fragmented (Mishra et al., 2015). The filamentous form minimizes metabolite distribution and maximizes energy utilization efficiency which is important for oxidative fibers (Glancy et al., 2015).


There are also differences in terms of nervous system innervation. Slow muscle fibers are innervated by neurons that have a lower threshold for activation, whereas faster muscle fibers are innervated by neurons that a have higher threshold for activation (Mantilla and Sieck, 2003). Faster muscle fibers are much more susceptible to neuromuscular transmission failure during repetitive stimulations compared with slower fibers (ie: they fatigue faster) (Johnson and Sieck, 1993).


Figure 8: Difference between slow, fast, and intermediate motor neurons. Image taken from Neupsy Key.


The size of muscle fibers is also different, with type IIX>type IIA>type I fibers in terms of size. This could be because large fibers have a smaller surface area to volume ratio, which reduces oxygen extraction efficiency, crucial in type I fibers that are mainly oxidative (van Wessel et al., 2010). Furthermore, type IIX fibers have more MHC per sarcomere than their type IIA and type I counterparts, with type I having the least MHC. A higher number of MHC allows more cross bridges to be formed, which produces higher force (Geiger et al., 2000). The force produced per cross-bridge is also lower in type I fibers than all fast fiber types, though the molecular basis for the type difference is unclear (Geiger et al., 2001). However, type IIX fibers also have the highest tension cost (ATP used per unit of force), type IIA is intermediate, while type I is lowest (Young Min Han et al., 2001).


Differences in the structure of MHC isoforms also determine their shortening velocity and ATP hydrolysis rate. MHCIIb is characterized by a relatively faster shortening velocity and higher ATP hydrolysis rate, followed by MHCIIx, MHCIIa, and MHCI lowest (Schiaffino and Reggiani, 2011).


All in all, muscle fibers exist on a spectrum, with slow muscle fibers relying more on oxidative metabolism, contracting slower, producing less force, and fatiguing less slowly, while fast muscle fibers rely more on glycolytic metabolism, contract faster, produce more force, but fatigues much faster.


Do muscle fibers change their type?


Certainly, there is plenty of evidence to ascertain that muscle fibers can change their type. 


In response to high-load resistance training, a shift from IIX and IIAX hybrids to a more pure IIA phenotype has been observed (Carroll et al., 1998). Power training, which incorporates high-velocity movements such as plyometrics, on the other hand, shows somewhat less of a loss in IIX and IIAX fibers (though still present) and a concomitant shift from type I type IIA fibers (Liu et al., 2003). Sprint training has also been shown to increase the proportion of type IIA fibers in the vastus lateralis, with a corresponding reduction in the percentage of type I fibers and type IIAX fibers (Andersen, Klitgaard and Saltin, 1994).


Endurance training also seems to shift muscle fibers from IIAX and IIA towards type I fibers, though the degree to which this occurs seems to differ according to the muscle (Luden et al., 2011). The more type I fibers dominant soleus muscle experienced a less pronounced shift compared to the less type I fibers dominant vastus lateralis muscle (Gallagher et al., 2005). Most investigations have found patterns indicating a shift toward type I fibers or a more oxidative phenotype after endurance training (Howald et al., 1985, Jansson, Sjödin and Tesch, 1978, Trappe et al., 2006).


Inactivity also seems to elicit a shift towards faster fiber types, especially type IIX muscle fibers (Vikne et al., 2020). Athletes have been using this concept to improve their performance in the short term through tapers. During the days or weeks leading up to their competition, they take planned reduced training volume days. As a result, their muscle fiber types shift more towards type IIX. This can be helpful in terms of giving athletes such as distance runners their final kick during the home stretch (Murach and Bagley, 2015).


Though it is more than certain muscle fiber can change type, we still aren’t sure to what extent it can occur. Can someone born with 50% type I fibers change it to 95%, we don’t know.


Conclusion


Muscle fiber types are not the be all end all, as has been shown that muscle fibers can increase force-generating capabilities, lactate-buffering capacity, and energy metabolism without changing their muscle fiber type (Scott, Stevens and Binder-Macleod, 2001). Many other considerations also have to be factored in making any predictions about performance such as pennation angles, biomechanics, bones, tendons, the extracellular matrix, the nervous system, and many more. In a nutshell, muscle fiber types are certainly important, but should not be taken as the sole determining factor of athletic performance.

 

Article prepared by: Jared Ong Kang Jie, R&D Director of MBIOS 2023/2024


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