IGF-1 plays a crucial role in muscle regeneration. IGF-1 stimulates both proliferation and differentiation of stem cells in an autocrine-paracrine manner, although it induces differentiation to a much greater degree. IGF-1, when injected locally, increases satellite cell activity, muscle DNA, muscle protein content, muscle weight and muscle cross sectional area. The importance of IGF-1 lies in the fact that all of its apparent functions act to induce muscle growth with or without overload although it really shines as a growth promoter when combined with physical loading of the muscle. IGF-1 also acts as an endocrine growth factor having an anabolic effect on distant tissues once released into the blood stream by the liver. IGF-1 possesses the insulin-like property of inhibiting degradation, but in addition can stimulate protein synthesis. The insulin-like effects are probably due to the similarity of the signaling pathways between insulin and IGF-1 following ligand binding at the receptors. The ability of IGF-I to stimulate protein synthesis resembles the action of GH, which was shown in separate studies on volunteers to stimulate protein synthesis without affecting protein degradation. Although it is often believed that the effects of GH are mediated through IGF-1, this cannot be the case entirely. First, the effects of the two hormones are different, in that GH does not change protein degradation. Second, the effect of GH is observed with little or no change in systemic IGF-1 concentrations. Age related muscle loss has been prevented with GH injections, however it is believed that this is accomplished through IGF-1.