Muscle growth is primarily regulated intrinsically, not by systemic factors like testosterone or IGF-1.

“muscle growth appears to be mostly intrinsically regulated and i think in the late 90s early 2000s what people were really focused on was systemic what they thought was systemic regulation of muscle growth right your your systemic levels of testosterone igf-1 and even like the acute responses like we know for example resistance exercise will increase testosterone you know for 30 or 60 minutes and it used to be thought well that is what's that is what's driving these responses of hypertrophy and through a few elegantly designed studies a lot of them by stu phillips we know that that's probably not the case”