The well-defined musculature of the male starts to develop at puberty when the previously inactive testes start to develop the hormone testosterone. This course through the body producing major physiologycal and psychological changes. Male animals undergo exactly the same transformation; we know this from the effects of castrating pets such as cats.
The growth of bigger muscles is just one of the changes at puberty; this is the anabalic action of testosterone. Alongside this change there are so-called androgenic alterations; for instance, the growth of facial hair and lengthening of the vocal cords which leads to the voice “breaking”. A further change is that the ends of the growing bones (the epiphyses) start to “seal”, thus stopping further lengthening. None of these change have anything to do with sex characteristics. However, sperm production and greater development of the penis do concern sexual function and are called primary sex characteristics.
Now, the powerful anabolic properties of testosterone at puberty have been known for over half a century, and it is this effect that chemistry have tried to copy, and has attracted the attention of the athletes in power events. Modern anabolic steroids effect muscle growth far more than testosterone does.Why go to expense and effort of producing more powerful drugs if testosterone could be used instead? The answer is two. For it to work, testosterone has to be given by injection. When taken orally it goes to the liver quickly and is broken down before it reaches muscles.
Because injections are tire someone for both receiver and doctor, other anabolic steroids that can be taken by mouth are preferred to testosterone. Also, the hormone, which is strongly androgenic, can produce in females irreversible male characteristics -they do not disappear when the drugs is stopped.