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 said。
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。
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。
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."
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.