Previous research has indicated that participation in competitive athletics increases the risk for reproductive problems in men (e.g., reduced testosterone levels and abnormal sperm morphology), particularly in long-distance runners and cyclists. Few studies, however, have focused on the effects of recreational exercise performed at a moderate-intensity level on the reproductive function of men in the general population. A recent study published in the December 1, 2010 issue of Fertility and Sterility found that men who biked for at least 5 hours per week were more likely to have lower sperm concentrations and less active sperm than their non-exercising counterparts; however, overall, the data indicated that there is no apparent relationship between physical activity and semen quality.

Researchers from Boston University investigated 2, 200 male subjects, average age 36 years, attending infertility clinics in the greater Boston area during 1993-2003.  At the start of the study, each subject answered a questionnaire that inquired about personal medical history, lifestyle choices and behaviors, and physical activity level. Based on the answers to the questions regarding exercise participation, the men were grouped according to exercise mode and exercise frequency. In order to determine semen quality, each participant was required to provide at least one semen sample which was assessed for ejaculate volume, sperm concentration, sperm motility, sperm morphology, and total active sperm.

The researchers found that neither regular physical activity (an average of 4 hours per week) nor exercise intensity (moderate versus vigorous) had an effect on the semen parameters assessed during the study.  When the effects of exercise mode were examined, biking was associated with lower sperm concentrations and fewer total active sperm levels for all age and body mass index categories. The investigators suggest that mechanical trauma (e.g., scrotal compression from the bike seat) and prolonged, elevated core scrotal temperature may be factors; however, they note that further research is needed for confirmation.

Reference
Fertility and Sterility; December 1, 2010; "Physical Activity and Semen Quality Among Men Attending an Infertility Clinic"; Lauren A Wise et al.,.