Third, GH may influence the activity of IGF-1 on an autocrine/paracrine level. Autocrine means that a hormone has an effect on the cell that produced it, paracrine means to have an effect on the “cell(s)” next to it as well. This is a completely localized effect, not dependent on the blood stream to carry things where you want them. Muscle growth from weight training is the result of IGF-1 being produced by the muscle cells themselves, not the liver. In fact, IGF-1 form the liver is genetically different from IGF-1 produced in your muscles. This information should explain why using IGF-1 systemically (from the blood stream) has been a hit and miss proposition.