Wearing high heels can shrink calf muscle and can result in pain and possible injuries when switching to flat shoes, according to a study published in the Journal of Experimental Biology.

Researchers discovered that continued wear of high heels (five days a week or more) caused the Achilles tendon to stiffen by 22 percent on average in women between 20 to 50 years. Habitual high heel wearers’ calf muscle fibers shrank roughly 13 percent.

Using magnetic resonance imaging scans, the researchers examined the leg musculature of women who wore mainly high heels and women who wore flats.

The changes to calf muscle can make switching from heels to flats a shock, says Robert Csapo, the lead author of the study.

Researchers recommended women wear heels moderately and alternate between heels and flats instead of wearing heels most of the days. They said stretching exercises after wearing heels can prevent injuries such as muscle tears or pulls.