For men, the health benefits of bicycling may involve a troublesome trade-off. While riding a bicycle burns calories and improves cardiovascular fitness, too many hours on a bicycle saddle can compress the artery and vital nerves leading to the penis.

The result? A risk of numbness, pain, and erectile dysfunction.

A male cyclist can place a significant percentage of his weight on his perineum, an area between the scrotum and the anus where the nerves and arteries to the penis pass. This pressure -- and a narrow saddle seat -- can injure the arteries and nerves.

"The earliest warning sign is numbness or tingling," says Irwin Goldstein, MD, director of San Diego Sexual Medicine.

Even a young man may lose the ability to achieve an erection, says Goldstein, who pioneered an operation that restores blood flow and sexual potency in 65%-75% of cases.

How much riding does it take to put a man at risk? The Massachusetts Male Aging Study found that the risk was highest among men who cycled more than three hours a week.