Recent work from the Phillips lab has shown that the hormonal response to resistance exercise plays little role in either the acute protein synthesis, signalling or the hypertrophy response to resistance exericse. Additionally a number of studies have brought into question the role of IGF-1 signalling in response to resistance exercise adaptations. However, these hormonal responses to resistance exercise are real and robust - so if they don't impact upon resistance exercise adaptations, what have they evolved to do?
To keep this brief (because i could go off on this ;-) my opinion is that growth hormones are critical for muscle growth through childhood and puberty and in extreme scenarios where the hormone is supraphysiological (AAS use hypogonadal males) or null (dwarfism). In the majority of adults basal and exercise-induced circulating hormone concentrations are permissive for muscle growth and this appears to be the case at the muscle level also. In a nice study, Danish researchers showed that exogenous GH administration increased the synthesis of matrix collagen but not myofibrillar protein. IGF-1 I think this is critical for SC, which in turn are required (if not directly stimulatory) for muscle hypertrophy.
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