How do muscles grow? To answer the question, we must first look at the basic structure of muscle tissue. In essence, muscle is composed of fascicles surrounded by a layer of connective tissue known as the perimysium. Each fascicle is then built from individual muscle fibres, synthesised from the contractile proteins actin and myosin and linked through another layer of connective tissue called the endomysium. These individual muscle fibres undergo microtears from external stimuli such as weightlifting. These microtears signal the activation of satellite cells to repair the muscle tissue.

Skeletal muscle satellite cells can be found on the surface of muscle fibres. They are quiescent mononucleated myogenic cells that are responsible for the regeneration of muscle fibres. These stem cells differentiate depending on the type of damage that the muscle has undergone. Most commonly, satellite cells fuse with the damaged fibrils and aid in the regeneration of the muscle fibres while also increasing the concentration of contractile proteins found within the damaged fibres. This increases muscle size and girth and, to a degree, the contractile potential as well. However, less commonly, the body undergoes a process called myogenesis, which involves the fusion and differentiation of satellite cells to form new multinucleated muscle fibres, increasing the density of the tissue and its subsequent power output. But what causes these different anabolic pathways?

The stimulation of these anabolic responses first requires stress on the muscle, as mentioned above. However, high repetitions and continuous use of the muscle result in traditional hypertrophy, widening damaged fibres, and increasing muscular endurance. Contrastingly, exercise that solicits your muscles to perform maximum power output typically results in higher rates of myogenesis, developing new muscle fibres, and increasing muscle density, allowing for a higher muscular output. After muscle stimulation, both anabolic pathways require sufficient protein metabolism in order to source essential amino acids. These amino acids allow for the synthesis of actin and myosin as well as connective tissue, allowing both anabolic pathways to be carried out. On top of the physical effect of exercise there is also a hormonal response which stimulates muscle growth and can be amplified using steroids in order to provide huge increases in anabolic rates.

Physical exercise, specifically weightlifting, stimulates the release of testosterone. Testosterone, along with other androgenic hormones such as dehydroepiandrosterone sulphate (DHEAS), dehydroepiandrosterone (DHEA), androstenedione, and androstenediol, all activate ligand-dependent androgen receptors on cell plasma membranes. The newly formed androgen complex translocates to the nucleus, where it dimerizes and binds to gene promoter regions, resulting in increased transcription rates and resultant protein synthesis, aiding in the development of muscle tissue. Finally, steroids such as additional testosterone or trenbolone increase the activation of androgen receptors, forming more AR complexes that bind to more promoter regions, overall leading to a 2-3-fold increase in muscle anabolism rate resulting in unbelievable muscle growth potential. 

The process of building muscle at the biochemical level is immensely more complex than what can be summarised in a short article. However, the general idea is that the regeneration of micro-torn muscle fibres increases muscle mass, and myogenesis which is more prominently activated through heavy lifting increases muscle density by forming new fibres. In addition to physical exercise, nutrition and hormonal balance particularly that of androgens play a big role in muscle anabolism.

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Works Cited 

Chen, C.-Y.A. and Shyu, A.-B. (1995). AU-rich elements: characterization and importance in mRNA degradation. Trends in Biochemical Sciences, 20(11), pp.465–470. doi:https://doi.org/10.1016/s0968-0004(00)89102-1.

‌Snijders, T. et al. (2015) Satellite cells in human skeletal muscle plasticity, Frontiers. Available at: https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2015.00283/full (Accessed: 21 June 2024). 

De Lisio, M. and Farup, J. (2017). The role of satellite cells in activity‐induced adaptations: breathing new life into the debate. The Journal of Physiology, [online] 595(19), pp.6225–6226. doi:https://doi.org/10.1113/JP274969.

‌National Institute on Drug Abuse. (n.d.). Anabolic Steroids and Other Appearance and Performance Enhancing Drugs (APEDs). [online] Available at: https://nida.nih.gov/research-topics/anabolic-steroids#:~:text=Anabolic%20steroids%20act%20at%20androgen.