Testosterone is an androgen (male sex hormone) that promotes the development of secondary male characteristics during puberty.  These include characteristics as a deeper voice, facial hair and an increase in muscle mass. Testosterone stimulates, directly (through testosterone receptors on muscle fibers), and indirectly, (through stimulating Growth Hormone production in the pituitary), hypertrophy. The increase in muscle mass caused by testosterone increases muscle strength. However this is not the only reason muscle strength increases. Testosterone can also bind with receptors on neurons (nerve cells), where it influences the production of neurotransmitter and the structure of nerve cells. Testosterone increases strength not only by increasing muscle mass but by improving muscle activation as well.

Testosterone is primarily produced in the testicles in men, and in the ovaries and adrenal glands in women. Men produce 15-20 times more testosterone than women, and the effect of different training protocols is greater on testosterone levels in men as well. In women, testosterone levels are hardly influenced by strength training or other forms of exercise, which partly explains the lower potential in women for increasing muscle mass.

In addition, an artificial form of testosterone (androgenic anabolic steroids) is used to stimulate muscle strength, hypertrophy and contraction speed, to increase performance in sports and athletic events. Most athletic organizations have banned the use of androgenic anabolic steroids and athletes who are caught using them risk suspension. Moreover, the use of androgenic anabolic steroids increases risk of developing cardiovascular disease and several forms of cancer.

References:

– Baechle, T.R., Earle, W.R. (2008). Endocrine Responses to Resistance Exercise. Essentials of Strength Training and Conditioning. Third Edition. USA. Human Kinetics. -Wilmore, J.H., Costill, D.L., Kenney, W.L. (2008). Ergogenic Aids and Sport. Physiology of Sport and Exercise Fourth Edition. USA Human Kinetics.