By Emma Wilkinson
BBC News health reporter in Amsterdam
Friction in the saddle may be a factor
Professional cyclists should consider freezing their sperm before embarking on their careers, say researchers.
They found sperm quality drops dramatically with rigorous training.
The Spanish study of top triathletes found those who cover more than 186 miles (300km) a week on their bikes have less than 4% normal looking sperm.
At such levels, men would have "significant fertility problems", the European Society of Human Reproduction and Embryology heard.
However, a UK expert said the average man cycling to work would be unlikely to suffer fertility problems because of their time in the saddle.
Study leader Professor Diana Vaamonde, from University of Cordoba Medical School in Spain said other studies had shown very high levels of exercise affected fertility in both men and women.
In the latest study, 15 triathletes with an average age of 33, were asked not to have sex for three days before giving a sperm sample.
When the results were compared with their training routines, only cycling - not swimming or running - was linked with sperm quality.
All of the men - who had been training for an average of nine times a week for eight years - had less than 10% of normal looking sperm, compared with the 15-20% seen in the most fertile men.
In those who managed more than 186 miles a week on their bikes, the proportion of sperm that was the correct size and shape had fallen to 4%, the point at which men may struggle to conceive without fertility treatment.
Heat from wearing tight clothing, friction of the testes against the saddle and stresses on the body from the sheer amounts of energy needed to do such rigorous exercise, could all contribute to poor sperm quality, said Professor Vaamonde.
The team are doing further research work in how cycling may effect metabolic processes in the body which lead to the development of abnormal sperm.
She added it was unclear whether sperm quality would improve if men retired from the sport but that after years of wear and tear this was unlikely.
"Something which could be done would be to have their sperm frozen but when they start training they do not realise what damage can be done to their sperm."
Ways of protecting cyclists against fertility problems should also be researched she said.
"Depending on the mechanism leading to creation of abnormal sperm, these could include giving antioxidants and modifying training regimes to all for recovery."
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said there had been a lot of interest in cycling and male fertility but results had been mixed.
"It is important to stress that even if the association between cycling and poor sperm morphology is correct, men training for triathlons are spending much more time in the saddle than the average social cycler or someone who might cycle to and from work."
He added that 40 years ago cycling was much more common but there is no evidence men then were less fertile.