(Health.com) -- You may want to think twice before strapping on those sky-high Manolos.
A new study shows that regularly wearing high heels can cause muscle and tendon changes in your legs -- to the point where wearing flats or flip-flops can be painful.
Wearing two-inch heels (or higher) five or more days a week shrinks a woman's calf muscle fibers by 13 percent, on average. It also thickens her Achilles tendon -- which attaches the calf muscle to the heel -- by 22 percent, according to the study, which was published Thursday in the Journal of Experimental Biology.
These changes alter the resting position of the ankle, causing the foot to point down more than normal. For some habitual high-heel wearers, this can make switching from stilettos to flats a shock, says the lead author of the study, Robert Csapo, a doctoral candidate at the University of Vienna's Centre of Sport Sciences and University Sports, in Austria.
"Whenever women regularly wearing high heels stand or walk in flat shoes or barefoot, the calf muscles and tendons are placed at relatively longer length," says Csapo. "This stretches and increases the tension in the muscles and tendons, [causing] discomfort."
Fortunately, only die-hard fashionistas appear to be at risk. Discomfort "will primarily occur in women wearing almost exclusively high-heeled shoes," says Csapo. In the study, the women who experienced pain wore heels for an average of about 60 hours a week.
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Although some women may not want to be seen without their Jimmy Choos or Christian Louboutins, the study suggests that it's healthy to mix up your footwear.
"We often like to be very stylish," says Dr. Carol Ewing Garber, Ph.D, an exercise physiologist and associate professor of movement sciences at Columbia University, in New York. "But it's probably wise to wear different kinds of shoes. Moderation is a good idea."
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Csapo and his colleagues used MRI scans to compare the muscles and tendons of 11 high-heel wearers with those of nine women who primarily wore flats.
Their findings don't necessarily apply to all high-heel wearers, however. The study was small, and the researchers didn't take other personal habits, such as exercise, into account. "One group engaging more in sports, affecting the strength of the calf muscles, might have influenced our results," Csapo says.
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The muscle and tendon changes described in the study are just the latest high-heel hazards to be identified.
High heels have been shown to contribute to a range of ailments, including ankle sprains, bunions, hammertoes, knee problems, and low back pain. Super-high heels -- like the pair of 12-inch Alexander McQueens worn by Lady Gaga in her "Bad Romance" music video -- put so much extra strain on legs and feet that they can even cause ankle ligaments to snap or bones in the feet to break.
To keep their legs healthy and prevent pain when switching from heels to flats, stiletto-lovers should stretch frequently to keep their muscles and tendons limber and strong, Garber says.
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She recommends the following exercises, which can be done just about anywhere:
• While sitting, loop a resistance band around the top of your foot (a belt or towel works too). Gently pull back on the band while pushing against it with your toes.
• Stand on a step, facing up. Keep your toes on the step and slowly lower your heels until you feel your calf muscles stretching.
• Slowly make a circle with your toes, alternately flexing and pointing your foot.
• Try to pick things up with your toes. (This will help strengthen your feet.)
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Although they are no guarantee, exercises such as these will help minimize the side effects of high heels and may even prevent injury, Garber says.
"Say you twist your ankle a little bit," she says. "The Achilles tendon being stiffer and the calf muscle being shortened might make a woman more susceptible to injury."
Copyright Health Magazine 2011