Women who wear high heels for long periods of time should not switch to flats, a scientist warned Friday.

Scientists at Manchester Metropolitan University, in northern England, found continued wear of high heels caused the Achilles’ tendon to thicken and stiffen permanently, so calf muscles were forced to stretch painfully when switching to flat shoes.

In the study, published in the Journal of Experimental Biology, two groups of women aged between 20 to 50 years were tested. One group regularly wore high heels of two inches or more for two or more years, while the other group had always wore flats.

The high heel wearers, who complained of discomfort when they switched to flats, were found to have muscle fibers 13 percent shorter than the women in flats.

“If you wear high heels for two years or more, five times a week, muscle fibers will become shorter and tendons will thicken and stiffen,” Professor Marco Narici, who led the research, told NewsCore.

“This has an immediate effect in restricting ankle movement and impacts your ability to walk fast and run — both in heels and flats.”

Narici said he would recommend women wear heels and flats on alternate days, but said he found most women who wear heels prefer to wear them at least five days a week.

The study said heel lovers should try stretching exercises at the end of each day's wear to avoid soreness.