A team led by Diana Vaamonde of the University of Cordoba Medical School in Spain looked at triathletes, athletes who run, swim and cycle, to further a probe into a long-suspected link between male infertility and long-distance biking.
They looked at semen provided by 15 healthy Spanish triathletes whose training routines were known in detail.
The more time the triathletes spent in the saddle, the worse their sperm morphology, or appearance, looked.
"While all triathletes had less than 10 percent of normal-looking sperm, the men with less than four percent, at which percentage they would generally be considered to have significant fertility problems, were systematically covering over 300 kilometres (180 miles) per week on their bicycles," Vaamonde said.
No such link was seen for running and swimming.
The study was released in Amsterdam at the annual conference of the European Society of Human Reproduction and Embryology (ESHRE).
The association between endurance cycling and male infertility first surfaced half a dozen years ago, in research among mountain bikers.
The suspected causes include irritation and compression of the testes against the saddle and localised heat caused by tight clothing. Sperm production is affected by excess heat.
But Vaamonde said there could be additional reasons, including rogue oxygen molecules that are boosted by local stress and which damage cell structures.
"Protective measures for these sportsmen need to be developed," said Vaamonde said in a press release.
"Depending on the mechanism leading to the creation of abnormal sperm, these could include giving antioxidants and modifying training regimes to allow for recovery. Or we could take preventive measures by freezing athletes' sperm before they start high-intensity training."