Found this over at Testosterone Nation.
by TC Luoma
The effect of steroids on muscles varies tremendously from individual to individual. It has a lot to do with the age of the patient, existing T levels (primarily existing levels of free Testosterone), exercise stimulus, nutritional factors, growth hormone, and various muscle growth factors.
However, taking amounts of Testosterone above and beyond that which a man might need for the purposes of T replacement will generally lead to additional muscle mass, even without exercising. The landmark study of 1996 by Bashin and associates found that 600 mg of
Test, given weekly over a course of several weeks, resulted in muscle mass gains that generally exceeded those of an average weight trainer who was working out regularly but who wasn't taking steroids.(3)
Even so, having high levels of T generally makes it easier to put on muscle mass than for individuals with lower T. Still, even that's uncertain because men differ on how their bodies process the stuff. Some men may have more testosterone receptors, which would probably improve responsiveness. Others might have a higher clearance rate of Testosterone, which would probably decrease responsiveness. And, another group might have very high levels of bound T, but very low levels of free T (the stuff that's biologically available for growing muscle and the rest of the stuff associated with T).