Men who spend five hours a week or more cycling have lower sperm counts and semen quality than those who undertake most other forms of exercise, not to mention those who lead sedentary lifestyles, according to new research.

The study was led by Lauren Wise of Boston University, with the findings published in the journal, Fertility and Sterility, and followed previous research that suggested that cyclists were more likely than other competitive athletes to have poor quality semen and genital and urinary issues.

"However, we were uncertain whether we would find an association among a sample of men engaged in more moderate levels of physical activity," Dr Wise told Reuters Health.

She added that it was as yet unknown whether cycling itself actually caused the issues with their sperm.

The study sought to establish the relationship between exercise and the health of men’s sperm among 2,200 responents who went to fertility clinics.

Each gave a sample of semen as well as responding to questions regarding their health and the kinds of exercise they undertake.

The authors of the report found that once they had adjusted the data for variable factors such as weight, the type of underwear worn, blood pressure and use of vitamins, their was no greater propensity to have sperm problems among those who undertook rigorous exercise compared to those who did no physical activity.

But once the data were analysed by type of exercise, it was discovered that those who spent five or more hours cycling per week showed double the rate of having a low sperm count, and also demonstrated poor sperm mobility.

Nearly one in three (31%) of men who cycled five hours or more each week had a low sperm count, compared to less than a quarter (23%) of those who did no regular exercise.

Regular bike riders were also much more likely to show poor sperm mobility at 40% versus 27% of those who undertook no physical exercise.

Wise said that the finding might be explained by trauma or an increase in temperature in the scrotal area making bike riders more exposed to the risk of a lower sperm count.

However, she added that since the sample comprised attendees at a fertility clinic, the study might not reflect the population at large, since the men may already have been encountering problems.

"More studies are needed to replicate our findings before they can be considered causal," she concluded.