Research suggests that riding a bike for long periods can cause temporary erectile difficulties, explains "What to Do about Erectile Dysfunction," a new report from Harvard Medical School. The risk appears highest among men who cycle more than three hours a week.

Sitting on a bicycle for a long time puts pressure on the perineum, the area between the genitals and anus. This pressure can harm nerves and temporarily impede blood flow, causing tingling or numbness in the penis and, eventually, erectile dysfunction. However, one study showed that while a conventional bicycle caused a dramatic (though temporary) drop in oxygen supply to the penis, a recumbent bicycle did not.

If you don't want to switch to a recumbent bike, these precautions may help you prevent cycling-related erectile problems:

-- Wear padded biking shorts.

-- Raise the handlebars so that you're sitting relatively upright. This shifts pressure from the perineum to the buttocks.

-- Use a wide, well-padded or gel-filled seat instead a narrow seat, which places more pressure on the perineum.

-- Position the seat to reduce pressure: Make sure it is not so high that your legs are fully extended at the bottom of your pedal stroke, and don't tilt the seat up.

-- Change your position and take breaks during long rides.

-- If you feel tingling or numbness in the penis, stop riding for a week or two.

"What to Do about Erectile Dysfunction" is a 32-page report edited by Michael Philip O'Leary, M.D., Ph.D., associate professor at Harvard Medical School. This report explains:

-- the anatomy and physiology of normal sexual function and erectile problems

-- the causes of erectile dysfunction, including a self-assessment questionnaire

-- the range of treatments available (including alternatives to Viagra and similar drugs)

-- the role of counseling

-- how to include your partner in discussing and treating erectile dysfunction.