According to a 2010 study published in the journal Nature, runners who wear shoes tend to strike the ground with the heels of their feet first. This gait, called a heel-strike, generates a force up to three times the body’s weight, which can lead to injuries such as Achilles tendinitis and stress fractures. In contrast, barefoot runners land on the balls of their feet, generating less impact when their foot strikes the ground.

"We’ve oversupported our feet [in running shoes] to the point that our foot doesn’t have to do what it’s designed to do," explains Irene S. Davis, PhD, PT, professor of physical medicine and rehabilitation at Harvard Medical School and director of the Spaulding National Running Center. "When you support a muscle, it doesn’t have to work as hard; when it doesn’t have to work as hard, it gets weak."

Davis believes your body instinctively knows how to adjust when you shed your shoes or run in "barefoot shoes," ultra-lightweight shoes designed to mimic barefoot running. Barefoot runners shorten their strides, reducing the impact on their lower bodies, and automatically flex their knees, hips, and ankles for a softer landing on hard surfaces, Davis says.

Ditching your shoes means the muscles in your calves and feet will have to work harder to accommodate to a different foot strike and shorter stride; it takes time for new barefoot runners to build up those muscles.

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