Millions of women could be risking arthritis in their knees by wearing high-heeled shoes, doctors have warned.

Osteoarthritis of the knee is twice as common in women as it is in men–and the researchers suggested in an article in The Lancet medical journal that high heels might be the reason why.

Researchers asked 20 healthy women with an average age of 36 to walk along a special platform in bare feet and then in shoes with two-and-a-half inch heels. Sensors under the platform, and cameras, recorded the movements of the women’s ankle and knee joints.

The results showed that when the women were walking in high heels there was greater strain between the kneecap and thigh bone than when they walked with bare feet.

Walking in high heels increased the normal torque forces at the knee by an average of 23%, which imposed a greater stretching force through the lateral knee ligaments.

Ultimately, Dr. Kerrigan would like her research to lead to improved shoe design and answer specific questions such as “How high is too high?” or “What about wearing heels only for special occasions?” Says Dr. Kerrigan, who, incidentally, never wears heels: “As for wearing high heels, probably the more you wear heels, the more likely you’ll develop arthritis. My advice is don’t wear heels.”