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It is worth mentioning that it is not yet discovered exactly how muscles grow, there is no an established opinion among scientists if it is because of new fibres or just thickening of the existing ones or splitting and thickening and how much of it is hard-wired by genes.
Nonetheless, there are four factors which are prerequisite for muscle growth. We use the word ‘factors’ since exact relationship between factors and growth is not yet established in its fullness. That is they work, it is been tested and proved, but why exactly they work it still remains unclear.
1. Increase level of anabolic hormones in blood
Among todays four factors this one is the most important because it starts the process of synthesing miofibrills in cells. Increasing level of anabolic hormones in blood may become as a result from phyisiological stress. During training session hormones come into cells, but don't go out, so the more sets you do, the more hormones you get in your cells.
Hormones affect the creation of differential protein structures in muscle fibers. It is also worth noticing that anabolic hormones are fully utilised during the process of protein synthesis.
2. Increase in free creatine level within muscle fibres
Accumulation of free creatine (Cr) in sarcoplasm of the cell (non contracting part of the fibre) is the criterium of intensified metabolism in the cell. Creatine phosphate (CP) transfers energy from mitochondria to myofibrils in Type I (oxidative, red, slow) fibres and from sarcoplasmic Adenosine TriPhosphate (ATP) to myofibrillic ATP in Type II (glycolitic, white, fast) fibres. It also supply energy into the nucleus of the cell. If a fibre is activated then the cell spends ATP as well as the nucleus and the cell needs CP to recombine Adenosine DiPhsphate (ADP) back into ATP. Besides, ATP is the only energy source in the cell. The nucleus needs energy in the form of ATP to create mRNA and ribosomes. Spent CP is released back into sarcoplasm in form of free creatine (Cr) and phosphate. However, the most important role of Cr if to provide energy for RNA synthesis triggered my hormones. The more free Cr is around the more intensive the process is. While the cell is not active it contains almost 100% of Cr in the form of CrP, metabolism and synthesis are slow. Although all organelles (subunits of the cell) are being incessantly renewed, physical stress that activates the fibre, more creatine is released into sarcoplasm in the form of free Cr. It triggers matabolic prosses and synthesis. CrP releases its energy within the nucleus and then in the form of free Cr moves to mitochondria where it is being recombined back into CrP.
3. Increase in level of hydrogen ions within myofibrils
Energy expenditure during intensive exercise causes increase in concentration of hydrogen ions (H+) in the cell. Moderate concentration of H+ ions makes cell membranes more transparent to hormones and activates cell enzymes, thus creating grounds for protein synthesis. This process does not happen in Type I (slow, red, oxydadative) fibres since the high number of mitochondria continue to consume oxygen and the cell works aerobically effectively preventing increase in concentration of free H+ ions and eliminating the fourth factor of muscle growth from establishing itself within the cell.
New myofibrils are grown within 7-15 days but the most active synthesis happends during the training session and in the first hours right after it. H+ do most of the work while training and for an hour after. Hormones help to create new RNAs and proteins for 2-3 days more but this process is much less intensive than during training, when this process is also helped by higher concentration of free creatine (Cr).
4. Amino acids stores in cells
Every protein structure, which includes muscle tissue, uses amino acids as building blocks. The amount of amino acids in the cell is accumulated gradually. The whole pool of free amino acids on the body is called amino acid pool. Hence there is no need to increase level of amino acids in blood stream during exercise. Since most intensive protein synthesis happens in the first 24 hours after the exercise you need to provide fuel for this. Protein synthesis returns to base level in 36-72 hours after exercise, according to various research.