Skeletal muscles are complex and highly specialized group of cells that are not isomorphic but can be categorized in a heterogeneous group with different qualities. This heterogeneity of the skeletal muscles is important because ensures different sets of functions that skeletal muscles perform in the human body. This heterogeneity also enables a significant ability of the human organism to adapt to different conditions in life (Pette and Staron 2000). In addition of this heterogeneity, skeletal muscles also posses the ability of plasticity. This means that muscle cells can evolve and adapt to different stimuli and change their characteristics (Tavi and Westerblad 2011). A number of techniques for classification of the muscle types have been attempted in the past and all of them have used different method of classification. This classification at the beginning was made solely on the basis of speed of muscle contraction and the morphologic appearance of the muscle based of which the muscles were divided to white or fast twitching and red or slow twitching muscles. This classification had anatomical correlation because the red color in the slow twitching muscles is from the high myoglobin content and developed network of capillary system and vasculature as opposed to the white muscles (Wang et al. 2012). Additionally muscles were classified based on the histological staining of the myosin ATP-ase based of which muscles are divided in two groups, type I or slow twitching and type II or fast twitching muscle fibers. Biochemical analysis is showing that these two types of muscles have differences in the ATP-ase activity and this activity is correlated with the speed of muscle contraction. The activity rate of ATP-ase found in the type II fibers is found to be 2-3 times greater compared to the type I fibers. Now this difference of ATP-ase activity correlates with the classification made by histological staining. Based on the histological staining muscle fibers were divided in further 7 classes which in order from fastest to slowest are: I, IC, IIC, IIAC, IIA, IIAB and IIB (Taylor et al. 1974).
This polymorphism of skeletal muscle fiber that I seen with heavy myosin ATP-ase staining is actually caused by different types of heavy myosin molecules in the human organism. In the human organism there are identified 10 different genes responsible for the transcription of the heavy myosin molecule, however only 3 of them are expressed which are, ordered from slowest to fastest - MHCI, MHCIIa and MHCIIx or d (depending from the author). However research has shown that same muscle fibers can actually have two different isoforms of the heavy myosin molecule and this is the main reason for the high heterogeneity of skeletal muscles. As a result of this fact it is identified that slower muscle fibers classified as types IC or IIC possess the myosin type MHCI and/or MHCIIa and on the other hand the faster muscle fiber possess the faster heavy myosin filaments MHCIIa of MHCII d/x (Pette et al. 1999) (Alan et al. 2006). Above mentioned classifications can be further complicated by the fact that the composition of muscle fibers can have different isoforms of the light myosin fibers, or they come in two forms fast and slow acting light myosin fibers. Because of this fact muscle cell with fast acting heavy myosin fibers may have a fast or slow light myosin fibers and the opposite is true also (Pette et al. 1999).
Another very important and essential classification method used for categorization of skeletal muscles is based on their biochemical characteristics or their metabolism. Based on this method muscle fibers are divided depending if they predominantly use the aerobic or oxidative metabolic pathway for energy production or the anaerobic or glycolytic pathway. Having in mind the predominant metabolic processes in the muscles using this classification muscles are divided in 3 categories: fast acting glycolytic fibers, fast acting oxidative fibers and slow acting oxidative muscle fibers (Pette and Staron 2000). It is important to note that in general there is a similarity between this classification and the classification made by histological staining of the heavy myosin fibers however this correlation is not linear in nature and there are differences. As an example the histological classification of IIB or the slowest muscle fibers does not always use solely the aerobic or anaerobic pathway for energy production (Pette et al. 1999).
This complex classification of muscle fibers is important in order to understand the different structure and class of muscle fibers that are predominantly found in extended exercises like the marathon runners or in the muscles of the sprinting athletes. The complexity of the problem is further aggravated by the plasticity of the muscle fibers and their ability to change the composition of their myosin fibers (Pette and Staron 2001). Another problem is that the polymorphism of the muscle cells varies in great percentages. As an example, the rate of maximal oxygen consumption or VO2 max in different types of muscle cells can vary up to 100-fold. The morphology of the cells that use predominantly oxidative or anaerobic metabolic processes is also different. Muscle cells that use oxidative processes are much smaller compared to cells that predominantly use the anaerobic or glycolytic process. This difference can be highly variable in different cells. The reason for this difference is the fact that oxygen needs to diffuse into the cells and this is why cells that use the oxidative processes are much smaller. On the other hand cells that use the glycolytic processes are much larger because they need the cell volume to compensate for the rapid production of degradation products of glycolysis (Wessel et al. 2010).
Based on the above review of the morphology and classification of muscle cells in general it is difficult to identify a specific muscle cell type or muscle cell fibers present in marathon versus sprint runners. The complexity of this question arises from the fact that muscle cells posses significant ability for plasticity and their structure, metabolism and morphology can vary significantly as a result of different stimuli (Pette and Staron 2001). As a general rule marathon or endurance runners need muscle cells that are able to sustain prolonged exercise at relatively smaller intensity and speed compared to the sprint runners. The sprint runners on the other hand need muscle cells that are able to produce explosive power with high intensity that lasts relative small amount of time. These differences in the performance are reflected in the muscle cell structure in these two types of athletes.
Marathon or endurance exercise is resulting in increase of the oxidative capability of the muscle cells by incising the number of mitochondria and enzymes involved in the process of oxidation and increase of the capillary circulation in the muscle. However, more importantly endurance training can also induce changes in the myofilament composition of the muscle cells. As example muscle fibers from the class MHCIIb can be replaced with MHVIIa fibers with increased concentration of the MHCIIa subtype of heavy myosin fibers. With the increase of the intensity of the exercise and prolonged stimulus of the muscle, this change can result in changes in the muscle composition. As a result of this the muscle fibers can be predominantly constructed from IIA fibers or exclusively IIA fibers (Ricoy et al. 1998). Another recorded change with endurance training that is characteristic to marathon runners is in the type I muscle fibers. Type I fibers over time have increased speed of contractility compared to the basal values and this is due to changes in the light myosin fibers predominantly (Seene et al. 2005) (Audrey et al. 2011).
Sprinting athletes on the other hand are exposed to high intensity and short duration training. Sprinting runners are found to have 30% higher concentration of MHCIIa and b fibers compared to other athletes on average. (Audrey et al. 2011). This fact can now be accurately measured by using magnetic resonance imaging spectroscopy. High intensity training characteristic for sprinters is changing the composition of the muscle fibers in a way that increase the concentration of MHCIIa and decrease the concentration. Andersen et al. (1994) found that the muscle fibers in sprinter athletes are mostly composed from MHC IIa and IIb (x or d by new classification). They also reported transformation of isoforms MHC I and MHC IIB towards isoform IIA in these athletes.
Based on the above reviews there is no clear classification of muscle fiber type’s characteristic for marathon of sprint runners. It can be concluded however that marathon runners tend to stimulate growth of muscle cells that use oxidative processes and sprint runners develop fast acting, muscle cells that use glycolysis. This is however highly dependent of the type and duration of the sporting activity.
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