The relationship between testosterone and muscle growth is well-known to virtually everyone who has ever set foot in a gym or a physical education class. Indeed, the presence of anabolic steroids---synthetic hormones that mimic testosterone and boost muscle growth---in professional sports has only further informed people of this relationship. It's common knowledge that the more testosterone you have, the more muscle you will likely add throughout the course of a training program. But how does this principle operate in the body of the athlete? What effects does testosterone exert in the body and what biological factors does it effect to increase muscle growth?
Testosterone is a member of a group of hormones known as androgens, which are responsible for starting and directing the growth and development of male sexual traits during puberty, though they also play a role in the growth and development of females. In adults androgen levels are related to muscle mass, sex drive and aggressiveness. Testosterone, in particular, plays a key role in regulating muscle mass and the body's response to exercise. However before this role can be understood we must explore how the body builds muscle in the first place.
Resistance exercise and other strenuous physical activities trigger muscle growth by causing small amounts of trauma to the muscles themselves. In response to this stimulus, the body adapts by repairing the muscular tissue and increasing its strength and size by adding new protein strands to muscle fibers.This process is called protein synthesis, and it is modulated by a variety of hormones called growth factors, of which testosterone is one of the most important.
Testosterone directly effects muscle growth by binding to receptors on the surface of muscle cells and amplifying the biochemical signals in muscle tissue that result in protein synthesis. Testosterone also increases levels of another growth factor, called growth hormone, that the body releases in response to exercise. Like testosterone, growth hormone increases protein synthesis and can result in increased muscle growth.
By boosting protein synthesis in skeletal muscle, testosterone increases both the rate and extent to which muscles adapt to exercise. By amplifying the release of other growth factors such as growth hormone, testosterone also facilitates the process of protein synthesis in general. The result of all of these functions is an overall increase in muscular size, strength and recovery from exercise.
Though the role that testosterone plays is important and significant, it is only one of many factors that controls muscle growth. Other growth factors include insulin, insulin-like growth factor 1, hepatocyte growth factor, fibroblast growth factor and growth hormone. Outside of an athlete's body chemistry, his nutrition, quality of sleep, training experience, discipline and quality of training plan all play important roles in muscle growth.