A recent study showed that rigorous and extended training on a bicycle can lead to a dramatic risk of infertility.
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.
