Men who spend more time riding a bicycle could be pedalling their fertility away, suggests a new study.
The research showed that those riding more than 180 miles a week had fewer than four per cent normal sperm. It means their chance of fatherhood is extremely low.
The research was presented at the 25th annual conference of the European Society of Human Reproduction and Embryology.
Professor Diana Vaamonde, from the University of Cordoba Medical School, Cordoba, Spain, said that the triathletes who did the most cycling training had the worst sperm morphology.
Vaamonde and team had previously shown that both high exercise intensity and high exercise volume may be detrimental to sperm quality. They decided to take a more profound look at the sportsmen who seemed to show the greatest alteration - the triathletes - and assess the correlation between the volume of training in each activity and sperm quality.
Of the three modalities, only cycling, the activity for which triathletes undertake the most training, showed a clear correlation with sperm quality.
The more cycling training the sportsmen undertook, both in time and kilometers, the worse their sperm quality became.
The design of this particular study did not allow the scientists to isolate a single factor responsible for this problem, but Vaamonde believes that it is likely to be mainly due to either the irritation and compression caused by friction of the testes against the saddle, or the localised heat produced by wearing tight clothing.
However, she also believes that reactive oxygen species - small molecules that are a natural by-product of oxygen metabolism and which react to stress by increasing to such an extent that they can damage cell structures - and energetic imbalances may play an important role in the alterations in sperm that the team observed.
"The fact that this effect is greater in triathletes than in other sports practices seems to indicate that it is something to do with the volume of training that they need to undertake to achieve and maintain a high level of fitness," said Vaamonde.
"We believe that the same effect would be observed in any athletes undertaking a similar amount of cycling training," she added.
To reach the conclusion, the team studied the semen values of 15 healthy Spanish triathletes, with an average age of 33. They were competing at both national and international level.
Their training routines were analyzed in detail, and particular note taken of the time they spent cycling each week. Sperm was taken after three days of sexual abstinence and analysed for their shape and form.
"We found a statistically adverse correlation between sperm morphology and the volume of cycling training undertaken per week," said Vaamonde.
"While all triathletes had less than 10 percent of normal-looking sperm, the men with less than 4 percent - at which percentage they would generally be considered to have significant fertility problems - were systematically covering over 300km per week on their bicycles," she added.