Unfortunately rats and people are two totally different things. You can't compare them saying "this group gained muscle without working out, therefore it will be like that with everthing". First off the dosages given to these rats is often rediculous considering the size of the rat. Secondly, rats have very low overall muscle mass compared to humans, their structures are much different from that of ours.
Although you may gains a little extra muscle mass from using exogenous hGH for a long period of time, the gains you would get from it would not be worth the well over $1000 tag that would come along with it. To invoke growth you must first invoke a response, which is done by causing muscle damage, a.k.a. resistance training. If you study the entire IGF-1 axis, or read some of the posts I have posted in here before, you realize you can't just look at a substance such as IGF-1. You must look, instead, at the entire system. MGF is upregulated during resistance exercise and the concnetration of it in a non-exercised human is VERY VERY low, not enough to cause the proliferation needed for IGF-1 to do its job of differentiation (hyperplasia/hypertrophy). Therefore, you need to cause the muscle damage to upregulate the MGF, then use the LR3 IGF-1 to give the new myoblasts an identity persay and therefore cause growth. As most of you know by previous posts, MGF is greatly downregulated after a couple of days in the trained muscle and IGF-1 is upregulated due to the fact that high levels of MGF in a damaged muscle in the presence of a high level of IGF-1 will actually inhibit differentiation.