Hello everyone! In the last part we looked at the basic essence of muscle fibers and several aspects that can affect these fibers. Today we will continue this topic and finish it. Enjoy reading!
Glycolytic (GMF), Intermediate (IMF), and Oxidative (OMF) Muscle Fibers
This classification of muscle fibers directly depends on energy supply processes, based on the number of mitochondria.
For example, if we examine each subtype of muscle fiber in more detail, we will have the following picture:
- OMF - Oxidative Muscle Fibers: In their energy supply processes, they actively use oxygen. The main energy supplier is fatty acids, and, of course, these MF have a lot of mitochondria.
- Next are GMF or Glycolytic Muscle Fibers: As in the first case, glucose is the main energy source, which will be broken down via anaerobic glycolysis, that is, with the participation of carbohydrates in an anaerobic (without O2) environment. As you understand, since they are glycolytic, there are not as many mitochondria in these MF as in OMF, but there are still some. In addition, glycolytic MF have a larger diameter than oxidative ones, so the former can stretch much more, and their strength will also be greater.
- The last type of MF - Intermediate: They differ from all the others in that they can, with training, turn into either GMF or OMF, depending solely on the specificity of the load on the muscles. As an energy source, they use both fatty acids and carbohydrates (glucose). Lactic acid is formed slowly in these MF, and they fatigue slowly accordingly.
Intermediate MF contain an average number of mitochondria, and this number can vary significantly depending on your training program. For example, training with an emphasis on strength exercises (turning IMF into GMF) will decrease the number of mitochondria, while training with an emphasis on overall endurance (turning IMF into OMF) will increase it. Accordingly, in the first case, carbohydrates will be actively used as an energy source via anaerobic glycolysis, and in the second, carbohydrates via aerobic glycolysis, that is, in an oxygen environment.
It is noteworthy that in beginners, white MF are glycolytic and intermediate, and red MF are oxidative. Accordingly, when working on endurance, fast MF transition from GMF to intermediate, and those, in turn, can also transition to oxidative. However, if you add strength training, as mentioned above, intermediate MF can transition to glycolytic. However, the transition of one MF to another is minimal, around 1-3%. The ratio of red/white muscle fibers is determined exclusively by our ancestors (genetics), and not by training, as many still think.
The Influence of Testosterone (Steroids)
Experiments on animals have clearly demonstrated how the use of anabolic steroids increases the number of chains of slow myosin isoforms (study by Fritzshe et al., 1994; Czesla et al., 1997). It was also reported that AAS (anabolic-androgenic steroids) intake in rodents increases MF with MyHCIIA chains and decreases MF with MyHCIIB, but at the same time, there were cases when the proportion of chains with MyHCIIA decreased in relation to MyHCIIB (study by Kelly et al., 1985; Lyons et al., 1986; Salmons, 1992). All this indicates that anabolics affect different types of muscle fibers differently.
In addition, there is data that the use of anabolic steroids has no effect on the ratio of MF containing various MyHC isoforms. For example, in one experiment, animals received intensive loading, the results of which led to an increase in the content of slow MyHCI fibers, but further steroid use did not affect the increase in myosin heavy chains (study by Boissonneault et al., 1987). Essentially, the same thing happened in a study on humans, when some experienced weightlifters took anabolic steroids and others did not. As a result, scientists found no difference in the ratio of different MyHC isoforms in the trapezius muscle (study by Kadi et al., 1999b).
The Influence of Estrogens
Scientific research (Greeves et al., 1999; Dionne et al., 2000; Meeuwsen et al., 2000) tells us that strength indicators inevitably decrease during the time when menstrual cycles end, that is, from approximately 49 to 52 years of age.
If we remove the ovaries and see what happens at the cellular level in MF, we will see a predominance of slow fibers in the heavy chain of myosin and a decrease in the fast running speed in mice (studies by Kadi et al., 2000). That is, the picture in relation to MyHC will generally look like this: MyHC I < IIA < IIX < IIB, which tells us that the removal of the ovaries shifts the ratio of MyHC isoforms towards an increase in slow isoforms and activation of the genes of slow MyHC isoforms.
After the removal of the ovaries, the mice continued to run in the experiment, after which estrogens were introduced to them. As a result, it was found that the composition of MyHC isoforms did not change. The scientists concluded that the introduction of estrogens during motor activity helps preserve red and white muscle fibers.
The Influence of Growth Hormone
According to a study by Lange et al., 2002, growth hormone stimulated an increase in MyHCIX chains in the vastus lateralis muscle of adult athletes, meaning the ratio of MyHC isoforms changed towards an increase in MyHCIX. The scientists concluded that somatotropin promoted the renewal of heavy myosin chains, as aging of these muscle groups typically leads to a decrease in MyHCIX chains.
A study by Daugaard et al., 1999 tells us that people who were deficient in somatotropic hormone had a greater proportion of MyHCIX than people who had sufficient growth hormone. The same study also stated that treating the population with growth hormone deficiency with recombinant somatotropin for six months did not cause any changes in the ratio of different MyHC isoforms.
Another study, Aroniadou-Anderjaska et al., 1996, on mice showed that the use of the hormone led to a significant thickening of fast muscle fibers in the soleus muscle, while the quantitative content of various isoforms in the MF remained at the same level. As a result, scientists came to a general agreement: additional research is needed to determine the effect of taking somatotropic hormone on the ratio of MyHC isoforms towards an increase in fast myosin isoforms.
The Influence of Thyroid Hormones (T3)
According to a study by D’Albis, Butler-Browne conducted back in 1993, thyroid hormones in skeletal muscle have a significant impact on the ratio of different MyHC myosin isoforms. Another study, Larsson, Yu, 1997, showed that the regulation of the ratio of heavy chains in the musculature of mice depends on gender and muscle type. For example, after the application of the thyroid hormone T3 on rats, both female and male individuals experienced an increase in MyHCIIA and a decrease in MyHCI in the soleus muscle, while activation of MyHCIX in the muscles was only observed in male rats.
The same study by Larsson, Yu, 1997 states that the use of triiodothyronine had no effect on the muscles of the extensor digitorum longus in male mice, while in female mice, on the contrary, significant changes occurred in the ratio of MyHCIIA and IIB isoforms.
The Influence of Thyroid Hormones
Scientists have concluded that the contractile characteristics of muscles are influenced by the hormonal background and other growth factors, which determine the properties of muscles in connection with their physiological needs. Therefore, the structure and function of certain muscle groups will directly depend not only on the ratio of hormones in the body, but also on gender and muscle type.
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