[Exercise and the male reproductive system]

In contrast to the effect of exercise training on the menstrual cycle in female athletes, the effect of physical activity on the male reproductive system is described far less extensively in the scientific literature. The male reproductive system consists of the hypothalamic-pituitary unit and the testes. The testes are responsible for the production of sperm and androgens, mainly testosterone. Androgens are responsible for the development of secondary male characteristics, muscle and bone growth, production of red blood cells, sex drive and other behavioral aspects. The effect of physical activity on the male reproductive axis depends on the intensity and duration of the activity, the fitness level of the individual, and his nutritional status. A single bout of short and intensive aerobic and anaerobic exercise usually increases serum testosterone level. Prolonged moderate to intense exercise (> 2 hours) leads to an initial increase followed by a decline to or below the baseline levels. Exercise training frequently results in a decrease of serum testosterone, and may rarely be associated with reduced libido, sperm production and fertility. In addition, the reduced testosterone levels may attenuate the exercise-associated muscle hypertrophy, reduce the repair of muscle damage, reduce post exercise muscle rehabilitation and may play an important role in the development of over-training syndrome. Other consequences include decreased bone density and a possible effect on mood and behavior. Surprisingly, even in the medically well-supervised elite athletes, changes in testosterone levels and their effects on performance and health are rarely evaluated.