But a study has found that, for men, forsaking the car or train for pedal power could have an undesirable effect. Men who cycle are, in medical jargon, more likely to experience erectile dysfunction.

Articles in next month's Journal of Sexual Medicine report a direct link between cycling and erection problems. Studies of cyclists, including bicycling policemen in the US, have found that their erections are not only less frequent, but tend to last for shorter periods.

The elongated shape of bicycle seats could be responsible. Several new studies published in the journal have found evidence that, as a cyclist pedals, blood flow and oxygen to the genital area is cut off.

Dr Steven Schrader, a research biologist, warns that "the relationships between bicycle riding using a saddle and erectile dysfunction are now based on a high level of evidence".

Dr Schrader's study of bicycling policemen found that "night-time erections were of poorer quality in biking police officers compared to non-biking officers". Furthermore, "night-time erection quality decreased as seat pressure increased".

The research concludes that the long noses of bicycle saddles favoured by men are largely to blame.