In most cases, exercise appears to have no relationship to either the quality or quantity of sperm, according to US researchers.
But Dr Lauren Wise at Boston University and colleagues found one exception: Men who biked at least five hours per week had fewer and less active sperm than men who didn't exercise.
Previous research has suggested competitive athletes may have issues with their sperm. The new study, however, looked at the relationship between sperm health and exercise in 2200 average men attending fertility clinics.
Each of them provided semen samples and answered questions about their general health and physical activity.
After adjustment for use of multivitamins, body weight, blood pressure, choice of underwear, and other variables, the authors found that men who exercised regularly - even vigorously - were no more likely to have problems with the quality or quantity of their sperm than men who never exercised, according to their 1 December online report in Fertility and Sterility.
However, when Dr Wise and her team looked at specific types of exercise, they saw that men who said they spent at least five hours per week biking were twice as likely to have both a low sperm count and relatively poor sperm motility.
Among men who did not get regular exercise, 23% had low sperm counts - but so did more than 31% of those who biked at least five hours per week. And nearly 40% of frequent bikers had low numbers of sperm with good motility, versus only 27% of men who didn't exercise.
Biking linked to poor semen quality
Research in competitive athletes has linked biking to genital or urinary problems and poor semen quality, Dr Wise said.
"However, we were uncertain whether we would find an association among a sample of men engaged in more moderate levels of physical activity," she told. It's possible that trauma or temperature increases in the scrotum may explain the relationship between biking and semen health, she added.
It's also too early to say that regular biking caused the sperm problems, Dr Wise cautioned. "More studies are needed to replicate our findings before they can be considered causal."
For instance, it's possible the men included in the study may not be representative of the general population, since they were all attending a fertility clinic and therefore are more likely to have problems with their sperm, she said. It is difficult to say how the results might change when looking at men who haven't been to a fertility clinic.(Reuters Health/ November 2010)