Constant saddle vibrations and shocks from the rough terrain mean that 96 percent of mountain bikers examined in a study were found to be suffering from scrotal abnormalities.
This is similar to the scare stories of 2-3 years ago which led to scrotum-docs appearing on mainstream US chatshows. Sales of split-cheek saddles such as the Specialized Body Geometry ones rocketed after the US research findings were publicised.
Its a moot point whether such findings put blokes off cycling in the first place (the US study said cycling made you impotent!) or whether extra sales of padded skin shorts and comfy saddles feed through instead.
The Lancet report studied 45 mountain bikers and 31 medical students who did not cycle.
Researchers from University Hospital in Innsbruck, Austria, found that 96 percent of the mountain bikers were suffering from scrotal abnormalities, compared with 16 percent of the medical students.
Half the mountain-bikers reported tenderness and discomfort when they were physically examined.
Three times as many cyclists as students were diagnosed with spermatoceles (sperm-containing cysts), the researchers found.
Report author Dr Ferdinand Frauscher said: "We assume that the abnormalities of scrotal contents detected in our study are essentially a saddle-related problem, resulting from a higher rate if microtrauma to the scrotal contents than previously assumed.
"The mechanical component responsible for these changes can be reduced by improving the padding of the seat or shorts, adjusting the saddle angle to either horizontal or upward in front, using an ergonomically designed saddle and by taking frequent rests during each ride.
"Furthermore, new shock-absorbent saddles and full suspension bikes might help reduce saddle vibration and microtraumatisation so that mountain biking remains a relatively safe and healthy sport."
A spokesman for the British Cycling Federation said: "We would support everything this study says about making sure all cyclists get a bike and equipment that fits them, wear padded shorts and see their GP if they experience any problems."