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.