Male endurance cyclists may be at risk of significant changes in the structure if not the quality of their sperm, according to a new study from South Africa.

Previous studies have suggested that regular endurance training may compromise testicular function. And this may be a particular risk for cyclists, who are exposed, additionally, to mechanical compression and irritation of the testes while performing their sport.

The researchers compared semen samples from 10 long-distance competitive cyclists with those of 10 sedentary controls. The cyclists, who were non-professional, had trained for more than 40 minutes a day on at least three days per week for at least three months before the study began, and none of the subjects had any history of reproductive disorders or were using medication that could have altered sperm production in any way.

No significant difference in volume of samples, motility, viability or sperm count were observed between the two groups. However, the cyclists had a significantly lower proportion of spermatozoa with a normal morphology (structure) than the controls and produced a high percentage of tapered forms of abnormal spermatozoa.

This is contrary to the findings of a previous study of cyclists, which reported no significant alterations in normal morphology. The researchers provide two possible explanations for this apparent contradiction:

1. The cyclists in the previous study were professionals, whose superior cycling techniques may have minimised the mechanical stress associated with the cycling action;
2. The previous study was carried out in the lower ambient temperatures of Europe. The hotter climes of Africa probably resulted in higher scrotal temperatures, which might have had a negative impact on sperm production.

The researchers comment: ‘During training, cyclists are exposed to periodic increases in body temperature and reduction in testosterone levels due to prolonged bouts of exercise. In addition, cyclists are exposed to mechanical compression to the testis, epididymis and vas deferens because of the mode of exercise.

‘These mechanisms could act synergistically in some endurance-trained cyclists and may diminish reproductive function and result in potential problems with fertility.’ Nevertheless, as they point out, although alterations in semen morphology occurred in the cyclists, these remained within the normal range for adults.

‘The morphological alterations may therefore not warrant fertility concerns at this stage,’ they conclude. ‘However, our subjects were still young and as they grow older the alterations may become clinically significant, particularly if the exercise is continued.’

Int J Sports Med 2004;25:247-251

Isabel Walker