Compared to non-cyclists, mountain bikers register less than half the sperm count and less sperm movement, researchers said.
"We were surprised that the motility (spontaneous movement) of the sperm and sperm count were reduced in the mountain bikers and only the mountain bikers," said Dr. Ferdinand Frauscher, head of the department of uroradiology at University Hospital, Innsbruck, Austria.
"Based on our findings, we believe that extreme mountain biking results in semen alteration, which may have an impact on fertility."
In Frauscher's study, ultrasound imaging of the scrotums of 40 mountain bikers -- most of whom showed up at their doctors' offices because they found suspicious lumps in their scrotums -- found the cyclists had more tiny calcifications than did a group of similar men who were not mountain bikers.
Nearly 90 percent of the cyclists -- 35 out of 40 -- demonstrated abnormalities in their scrotums compared with 26 percent -- 9 out of 35 -- of the non-cyclists. Mountain-biking men had more cysts and blood vessel abnormalities than the non-mountain bikers.
The cyclists were included in the study if they spent more than two hours a day on their bicycles and rode their mountain bikes more than 3,000 miles a year.
Frauscher noted in North America more than 10 million people pursue the sport of mountain biking and that number continues to increase.
In the study, presented at the annual meeting of the Radiological Society of North America, Frauscher said the sperm counts of non-cyclists average about 47 million per milliliter, while the sperm counts in mountain bikers was 20 million per milliter. About 51 percent of the non-cyclists' sperm showed normal movement compared to 29 percent of the sperm in the mountain bikers.
To demonstrate, Frauscher showed a film of microscopic images that depicted scores of non-cyclists' sperm rapidly moving across the slide. In contract, the film of mountain-bikers' sperm showed a lone sperm inching across the frame.
"This may be the first study to show a well-document correlation between scrotal injury and decrease sperm counts and motility," said Dr. Barry Goldberg, professor of radiology at Thomas Jefferson University in Philadelphia.
"Certainly it is the only case I have seen in the radiological literature. We do know that lower sperm counts and sperm motility reduce the ability to conceive," he added.
"The exact causes for the decreased sperm motility are unclear," Frauscher said. "We believe that repeated mechanical trauma to the testicles results in some degree of vascular damage, and may thereby cause a reduction in sperm motility."
Frauscher reported his patients between age 20 and 30 tended to say they were not worried about their fertility, but were concerned instead about the lumps they were feeling because testicular cancer is the most frequent cancer among men of that age group.
None of the patients had cancer but a higher percentage of the mountain bikers had hardened objects in the scrotum that might be related to cancer development. Two of the patients had elevated cancer markers in their blood, and underwent biopsy.
"Microtrauma that is caused by mountain biking causes the body to go through its healing process," said Goldberg. "And when there is healing going on there is always the chance that one of those cells will inappropriately continue to grow, that is, become a cancer."
Frauscher suggested that patients who engage in mountain biking should invest in more expensive and somewhat heavier vehicles that include saddle and pedal suspension systems, use padded saddles and padded pants. Goldberg concurred, saying these were reasonable attempts to reduce the impact on the scrotum and to reduce injuries.
Frauscher added that three patients who were approaching 40 did visit his office because of concerns about fertility. He and colleagues are investigating whether using impact suspension equipment reduces the risk to the cyclists' fertility. He said larger calcifications and larger cysts could be surgically removed and surgery also could resolve other related malformations.
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