I would guess its almost all because of the hormone myostatin. It's a negative growth regulator in muscle (perhaps other tissues as well). Myostatin is precisel

I would guess its almost all because of the hormone myostatin. It's a negative growth regulator in muscle (perhaps other tissues as well). Myostatin is precisely why our muscles don't grow wildly out of control, and why it is so hard to grow a significant amount of muscle. Cell growth is tightly controlled by the level of myostatin and the levels of positive growth regulators. I wonder if myostatin or an analogue could have an affect as an anti-cancer compound.