Testosterone Propionate often referred to as just “prop” test-p or “test prop” is a version of testosterone that promotes nitrogen retention in the muscle (6), and the more nitrogen the muscles hold the more protein the muscles store, and the bigger the muscles get. Testosterone Propionate can also increase the levels of another anabolic hormone, IGF-1, in muscle tissue. IGF-1 is alone highly anabolic and can promote muscle growth . It is responsible for much of the anabolic activity of Growth Hormone (GH). IGF-1 is also one of the few hormones positively correlated with both muscle cell hyperplasia and hyperphasia (this means it both creates more muscle fibers as well as bigger fibers).