The leading studies in this area: (Ney, 1999, Yarasheski, 1994.... Am J. App. Phys.)
In the Yarasheski study, no increase in lean muscle mass was noticed in the subjects using GH alone, but significant gains were found in subjects that supplemented with IGF and GH...add in the steroids and look out! Yarasheski studied weight trained athletes, supplementing one group with GH alone, and one group with GH and IGF. "So IGF-1 alone does not promote such effects. (Leanness and increased lean mass) Nor does GH. It appears the combination of the two most consistently lead to increased protein synthesis." Both seem to negatively downregulate the other over time, so as to lead to diminishing returns. Cycling would be in order for that reason. Also supplementing both is necessary because one or the other alone will suppress the natural production of the non-supplemented Latest study by Yarashevski - with GH alone...8 to 12% change in lean body composition. 6% increase in muscle mass.