Muscle fiber splitting

 Human muscle fibers are classified by miosin heavy chain (MHC) isoforms characterized by slow to fast  contractile speeds:

Type l, or slow-twitch fibers, are in high abundance in elite endurance athletes, such as cyclist or long-distance runners.

Fast-twitch lla or llb fibers commonly appear in the body of weightlifters and sprinters.

There are also hybrid fibers, which are a mix of the anterior ones.

There is a clear ability of fibers to shift between hybrid and pure fibers as well as between fast and slow types.

When occurs muscle fiber hipertrofy it is usual that it is caused by a grow of individual muscle fibers. But sometimes there is a longitudinal "branching", "fragmenting" or "splitting" of existing muscle fibers during a robust overload. Greater muscle fiber number during hypertrophy usually occurs concomitant with signs of regeneration, and is therefore considered pathological. It is caused by high intensity exercises and it results in muscle hypertrophy of abnormal magnitude.

branching: appearance of a smaller muscle fiber from a larger one.

splitting: roughly symmetrical division that may or may not run the entire lenght of the fiber.

There are 404 proteins that show a statistically significant difference between fiber types.

It is also caused of  a synergist ablation overload in the absence of muscle stem cells.


Links:

https://skeletalmusclejournal.biomedcentral.com/articles/10.1186/s13395-021-00279-0

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6422761/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8473039/

https://journals.lww.com/acsm-essr/Fulltext/2019/04000/Muscle_Fiber_Splitting_Is_a_Physiological_Response.8.aspx

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