In the present study, there was little evidence for an overall effect of regular exercise on sperm concentration, volume, sperm motility, TMS, or morphology. However, in the subgroup of men who reported bicycling as their primary form of exercise, bicycling at levels of ≥5 hours per week was associated with low sperm concentration and TMS. These findings, from a study of moderate exercisers, generally agree with previous studies that have shown deleterious effects of bicycling on semen parameters among competitive cyclists (14
The few studies that have evaluated the relation between exercise and semen quality have been confined to one or two types of activity, or have examined endurance or high-intensity athletic training only, making it difficult to compare results. Our null results for jogging and running do not support the few studies that have shown significant alterations in sperm count (5
), concentration (4
), motility (4
), and morphology (4
) among endurance-trained runners (4
), high-mileage runners (5
), and high-intensity long-term treadmill runners (11
). However, the runners in our study had more moderate levels of exercise, potentially limiting statistical power to detect a true effect. A previous study found that “high-mileage” runners (mean: 108 km/wk) experienced significant alterations in semen quality relative to sedentary men (5
), but not runners of “moderate mileage” (mean: 54 km/wk), similar to what another study found among three groups of athletes differing in exercise intensity and frequency (12
). Therefore, another explanation is that running at the levels observed in our study may not influence semen quality.
Study limitations include self-reported exercise data and the evaluation of activity for current exercisers only. For example, lifetime exercisers who eliminated their activity prior to completing the baseline questionnaire would have been misclassified as non-exercisers. Furthermore, in the analyses of exercise subtypes, there was no information on the actual intensity of the activity. For instance, two men who reported bicycling ≥5 hours/week could have had vastly different intensities of bicycling (e.g., leisurely cycling for transportation versus competitive cycling). Such errors in classification would generally dilute our effect estimates.
Although exclusions at baseline were similar according to several risk factors for infertility, men reporting a history of male factor infertility were more likely to be excluded. Couples whose IVF treatment included donor sperm were ineligible for enrollment, precluding analysis of their data. While such exclusions may have reduced variability in semen quality, it is unlikely that these exclusions were a significant source of bias because our findings persisted among men with and without a history of male factor infertility. Finally, our results did not differ markedly by the number of semen samples provided over follow-up, suggesting that results did not depend on success with IVF treatment.
Study strengths include the large sample size and the collection of multiple cycles of data over a 10-year period. Because men reported their exercise and covariate data before the provision of a semen sample, reporting errors are not likely to depend on the measurement of semen quality. Thus, our effect estimates comparing extreme categories of exercise are probably conservative. We controlled for a wide range of potential confounders such as age, BMI, smoking, and history of infections. Given all outcome data were abstracted from medical records, we had nearly complete information on the study endpoints of interest.
A link between reduced sperm concentration and bicycling—the one class of exercise to show an effect—has some biologic plausibility. Bicycling has been linked to genitourinary problems (15
), including nerve entrapment syndromes (50–91% of cyclists), erectile dysfunction (13–24%), and other less common symptoms (priapism, penile thrombosis, hematuria, torsion of spermatic cord, perineal nodular induration, and prostatitis) (16
). Two studies among long-distance competitive cyclists reported changes in semen parameters including a lower percentage of sperm with normal morphology (17
), a higher proportion of morphologically abnormal tapered forms (17
), and reduced sperm motility (14
). Moreover, small changes in total and free testosterone have been observed in male athletes (3
), potentially affecting spermatogenesis. It remains unclear as to whether the changes associated with bicycling are due to mechanical trauma (i.e., caused by compression of scrotum on bicycle saddle), to a prolonged increase in core scrotal temperature (i.e., related to exercise itself or wearing of constrictive clothing), or some other factor (16
). A report among moderate bicyclers showed no correlation between cycling duration and scrotal temperature (25
Because men who attend infertility clinics differ from the general population of reproductive-aged men, caution should be used when interpreting our findings. If those attending infertility clinics differ with respect to exercise levels and semen quality, our results may not extend to a larger population of men. However, it is unlikely that the exercise levels of men in our study differed materially from those of the general population. Moreover, because associations were uniform across levels of age and BMI, our findings, if real, may extend to other groups of men with similar characteristics.
In summary, in this study of moderate exercisers, no association was observed between regular exercise and semen parameters overall, but bicycling at levels of ≥5 hours per week was associated with reduced sperm concentration and TMS. Our findings warrant confirmation in larger studies of moderate bicyclers and men from the general population.