A raft of new studies suggest that cyclists, particularly men, should be careful which bicycle seats they choose.
The studies add to earlier evidence that traditional bicycle saddles, the kind with a narrow rear and pointy nose, play a role in sexual impotence.
Some saddle designs are more damaging than others, scientists say. But even so-called ergonomic seats, to protect the sex organs, can be harmful, the research finds. The dozen or so studies, from peer-reviewed journals, are summarized in three articles in September's Journal of Sexual Medicine.
In a bluntly worded editorial with the articles, Dr. Steven Schrader, a reproductive health expert who studies cycling at the National Institute for Occupational Safety and Health, said he believed that it was no longer a question of ''whether or not bicycle riding on a saddle causes erectile dysfunction.''
Instead, he said in an interview, ''The question is, What are we going to do about it?''
The studies, by researchers at Boston University and in Italy, found that the more a person rides, the greater the risk of impotence or loss of libido. And researchers in Austria have found that many mountain bikers experience saddle-related trauma that leads to small calcified masses inside the scrotum.
This does not mean that people should stop cycling, Dr. Schrader said. And those who ride bikes rarely or for short periods need not worry.
But riders who spend many hours on a bike each week should be concerned, he said. And he suggested that the bicycle industry design safer saddles and stop trivializing the risks of the existing seats.
A spokesman for the industry said it was aware of the issue and added that ''new designs are coming out.''
''Most people are not riding long enough to damage themselves permanently,'' said the spokesman, Marc Sani, publisher of Bicycle Retailer and Industry News. ''But a consumer's first line of defense, for their enthusiasm as well as sexual prowess, is to go to a bicycle retailer and get fitted properly on the bike.''
Researchers have estimated that 5 percent of men who ride bikes intensively have developed severe to moderate erectile dysfunction as a result. But some experts believe that the numbers may be much higher because many men are too embarrassed to talk about it or fail to associate cycling with their problems in the bedroom.
The link between bicycle saddles and impotence first received public attention in 1997 when a Boston urologist, Dr. Irwin Goldstein, who had studied the problem, asserted that ''there are only two kinds of male cyclists -- those who are impotent and those who will be impotent.''
Cyclists became angry and defensive, he said, adding: ''They said cycling is healthy and could not possibly hurt you. Sure you can get numb. But impotent? No way.''
The bicycle industry listened, said Joshua Cohen, a physical therapist in Chapel Hill, N.C., and the author of ''Finding the Perfect Bicycle Seat.'' Manufacturers designed dozens of new saddles with cut outs, splits in the back and thick gel padding to relieve pressure on tender body parts.
Scientists also stepped up their research. Since 2000, a dozen studies have been carried out using sophisticated tools to see exactly what happens when vulnerable human anatomy meets the bicycle saddle.
The area in question is the perineum, between the external genitals and the anus. ''When you sit on a chair you never put weight on the perineum,'' Dr. Schrader said. ''But when you sit on a bike, you increase pressure on the perineum'' sevenfold.
In men, a sheath in the perineum, called Alcock's canal, contains an artery and a nerve that supply the penis with blood and sensation. The canal runs along the side of a bone, Dr. Goldstein said, and when a cyclist sits hard on a narrow saddle, the artery and the nerve are compressed. Over time, a reduction of blood flow can mean that there is not enough pressure to achieve full erection.
In women, Dr. Goldstein said, the same arteries and nerves engorge the clitoris during sexual intercourse. Women cyclists have not been studied as much, he added, but they probably suffer the same injuries.
Researchers are using a variety of methods to study the compression caused by different saddles. One method involves draping a special pad with 900 pressure sensors over the saddle. The distribution of the rider's weight is then registered on a computer. In another technique, sensors are placed on the rider's penis to measure oxygen flowing through arteries beneath the skin. Blood flow is detected by other sensors that send a ''swoosh'' sound to a Doppler machine.
The research shows that when riders sit on a classic saddle with a teardrop shape and a long nose, a quarter of their body weight rests on the nose, putting pressure on the perineum. The amount of oxygen reaching the penis typically falls 70 percent to 80 percent in three minutes. ''A guy can sit on a saddle and have his penis oxygen levels drop 100 percent but he doesn't know it,'' Mr. Cohen said. ''After half an hour he goes numb.''
Dr. Goldstein added, ''Numbness is your body telling you something is wrong.''
Today's ergonomic saddles have splits in the back or holes in the center to relieve pressure on the perineum. But this may make matters worse: the ergonomic saddles have smaller surface areas, so the rider's weight presses harder on less saddle, Dr. Schrader said. The perineum may not escape injury because its arteries run laterally and they are not directly over the cutouts. The arteries can come under more pressure when they come into contact with the cutouts' edges.
Thick gels on saddles can also increase pressure to the perineum, the studies found, because the material can migrate and form clumps in all the wrong places.
Just as many smokers do not get lung cancer, many cyclists will never develop impotence from bicycle seats, the scientists said. What makes one person more vulnerable than another is not known. Body weight seems to matter: heavier riders exert more pressure on saddles. Variations in anatomy may also make a difference.
Dr. Goldstein said he often saw patients who were stunned to learn that riding a bicycle led to their impotence. One middle-aged man rode in a special cycling event to honor a friend and has been impotent since. A 28-year-old who came in for testing, Dr Goldstein said, showed the penile blood flow of a 60-year-old. A college student who had competed in rough cycling sports was unable to achieve an erection until microvascular surgery restored penile blood flow.
''We make kids wear helmets and knee pads,'' Dr. Goldstein said. ''But no one thinks about protecting the crotch.''
The safest seats and saddles, experts say, force the rider to sit back firmly on the sit bones so the perineum is protected.
Dr. Schrader advocates saddles that do not have noses. After finding that traditional saddles reduced the quality of nighttime erections in young policemen who patrol on bicycles, he has persuaded scores of officers in several cities to use noseless seats and is now studying the officers' sexual function over six months.
Nunzio Lamaestra, a 46-year-old police officer in San Antonio, said he appreciated his noseless bicycle saddle.
''You get used to riding without the nose,'' he said. ''I can do everything, including ride with no hands.''