Physical activity is good for overall health and wellbeing, including reproductive health - but too much exercise has been proven to be detrimental to a man’s fertility.
Firstly, strenuous activities can cause heat to build up around the testicles. Prolonged or regular exposure to higher temperatures can override the body's ability to keep the testes cool. Even an increase in temperature of only a few degrees, can cause infertility.
Too much exercise can also cause hormonal imbalance. Thorough exercise can produce high levels of adrenal hormones in a man's body, which can lower testosterone. Besides affecting sperm production, low testosterone levels cause decrease in libido and sex drive.
A clinical study showed evidence that chronic endurance training can result in mild degrees of reproductive system abnormalities in men. Published in the Medicine & Science in Sports & Exercise in February 1996, the study found significant reduction of testosterone altered pituitary release of luteinizing hormone and prolactin, and altered sperm characteristics. The main topics addressed in the article were, acute and chronic exercise effects, consequences of hormonal changes associated with endurance training, and the potential mechanisms for the reproductive system changes detected.
Hackney, Anthony C. “Medicine & Science in Sports & Exercise: The male reproductive system and endurance exercise” , Volume 28 - Issue 2 - pp 180-189, February 1996.
Rima Dada, Narmada P. Gupta, Kiran Kucheria. “Spermatogenic arrest in men with testicular hyperthermia”, Department of Anatomy, All India Institute of Medical Sciences, New Delhi, India.