Re: Genetics and Muscle Growth - What are the factors?
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Is there a relationship between the lokal anabolic response from mechanical stimuli and endocrine anabolic hormones, and do the latter in any way influence the magnitude of the former?
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Local factors will increase powerfully even if endocrine factors are taken out of the equation.
So for example you could have low circulating IGF levels, and still have high local IGF levels. Which is exactly what happens in gymnasts for example. because of their low levels of calories relative to caloric expenditure, they have low circulating IGF, but clearly have great local muscular growth responses. This allows us to adapt to training even under lower calorie conditions
As for part two of the question, yes circulating factors can influence local factors. However the extent of this influence is not well characterized. For example, even though exercise stimulated mechanogrowth factor big time, evidence from studies which have injected GH have found that GH enhanced the amount of MGF in the muscle. So, essentially it seems that you need the mechanical stimulation to activate it, but that systemic factors definately influence this.
In fact one model used in animal studies will transplant an old muscle into a young animals body. Once in the young animals body satellite cell activation acts just as it would have, if they were the original cells of the young animal. This clearly suggests that the surrounding environment also effects the satellite cell response
In addition, in the origional study by kim and colleagues, they found that the young individuals had higher testosterone levels than both elderly males and young females.
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If there is an adding effect of the two, which would be considered more important for muscle hypertrophy? By the way, if you for some reason would be looking to do a case study on a guy who has an abnormal ability to put on muscle mass and who seems to lack the ability to put on fat let me know.
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Great question! I would suggest from current data that at physiological levels (e.g. your normal hormone levels) that the local effect is the most dominant aspect that drives muscle tissue growth and that systemic or endocrine factors play a larger role in maintaining your overall gains from training. For this reason, people with lower hormone levels may actually need a bit more frequent stimulation to get gains, and definitely to maintain their gains after training.
In terms of their direct effect on muscle growth, I would say that the endocrine factors do play a role at physiological levels in the skeletal muscle adaptation. for example, acute growth hormone responses after exercises are correlated pretty high with muscle growth, suggesting they play a strong role, but not as strong as local factors.
Now, I do not think that the rise in endocrine substances and its effects on growth are completely additive, but may actually augment the local response. As stated earlier, it appears that GH administration, at least in the elderly increases MGF in response to training, so it could augment the local response. So for this question, it is difficult to really come to a solid conclusive answer.
Finally, at supraphysiological levels, such as is used extensively in the IFBB, clearly hormones have a big time effect directly on muscle growth.