Resistance training works by causing microscopic damage or tears to the muscle cells, which in turn are quickly repaired by the body to help the muscles regenerate and grow stronger. The breakdown of the muscle fiber is called "catabolism," and the repair and re-growth of the muscle tissue is called "anabolism." You're probably familiar with the term anabolic when used with steroids . Anabolic means to grow, and that's exactly what happens after you break down the muscle fibers with resistance exercise. In fact, many biological processes of growth in the body require some breakdown, or catabolism, prior to re-growth. For instance, bones must be broken down first before calcium and other growth factors repair the bone and make it stronger. With muscles, testosterone, insulin-like growth factor, growth hormone, protein, and other nutrients rush to the muscle after a resistance-exercise session to help repair the muscles to make them stronger. Importantly, your muscles heal and grow when you aren't working out, and so that's why it's necessary to leave time between workouts for recovery.
Transdermal patches (adhesive patches placed on the skin) may also be used to deliver a steady dose through the skin and into the bloodstream. Testosterone-containing creams and gels that are applied daily to the skin are also available, but absorption is inefficient (roughly 10%, varying between individuals) and these treatments tend to be more expensive. Individuals who are especially physically active and/or bathe often may not be good candidates, since the medication can be washed off and may take up to six hours to be fully absorbed. There is also the risk that an intimate partner or child may come in contact with the application site and inadvertently dose himself or herself; children and women are highly sensitive to testosterone and can suffer unintended masculinization and health effects, even from small doses. Injection is the most common method used by individuals administering AAS for non-medical purposes.