Clogs, the iconic footwear of the Netherlands, were the shoes of choice for Dutch laborers of centuries past. The wooden slip-ons were sturdy, cheap and—when stuffed with straw—cozy and warm. But as Jane Sims of the London Free Press reports, a recent study of the skeletons of 19th-century Dutch farmers suggests that clogs caused a type of rare bone damage among people who wore the rigid shoes regularly.

Back in 2011, a team of archaeologists began excavating 500 skeletons from a church cemetery in Middenbeemster, a small dairy-farming village near Amsterdam. According to CBC News, the excavation was prompted by a planned expansion of the church into cemetery grounds. Archaeologists weren’t looking for anomalies among the remains, but upon examination, they realized that many of the skeletons displayed strange bone chips in the feet.

“They’re like craters in the bones, at the joints, as if chunks of bone have just been chiseled away,” said Andrea Waters-Rist, an associate professor of anthropology at Western University in Ontario and one of the authors of the study, according to Sims. “We didn’t need a microscope to see them, they were that obvious.”

In a study published in the International Journal of Paleopathology, researchers ascribe the deformities to Osteochondritis Dissecans (OD), a rare condition that sees bone beneath the cartilage of a joint die due to lack of blood flow. Less than one percent of the general population is afflicted with this condition—which usually affects the elbow—but 13 percent of the 132 skeletons studied by researchers had foot lesions consistent with OD. The skeletons belonged to a roughly equal number of men and women, who were 18 years or older at the time of their death.

Waters-Rist told the CBC that OD is caused by “repetitive micro traumas”—and in the case of the Dutch farmers, these traumas were likely caused by a combination of hard labor and hard shoes. The farmers would have used their clogs, or klompen, to stomp, kick and dig as they worked. But because the soles of clogs are so stiff, they may have exacerbated the physical stresses associated with farm work.

“The sole is very hard and inflexible, which constrains the entire foot and we think because the footwear wasn’t good at absorbing any kind of shock, it was transferring into the foot and into the foot bones,” Waters Rist said in a Western University statement.

Both men and women, she added, would have been toiling on the farm every day. But “even just working in and around the house, that alone would not be enough to see what we were seeing,” Waters-Rist said. “What the other agent was, then, was the klompen.”

In cases where the OD was minimal, farmers may not have experienced any symptoms. But even if the farmers did feel pain, “they couldn’t afford to rest that part of the body and take time off to put their feet up,” Waters-Rist noted.

The study is part of a broader research initiative into the Middenbeemster remains. Archaeologists hope that wear-and-tear patterns of the bones can shed new light on the lives of the Dutch working class during the pre-industrial era.

“We don’t have nearly as good of an understanding of what rural citizens were doing in the 1800s and the post-Medieval period as much as we do in urban centres,” Water-Rist said, according to the Western University statement. “[S]o we can figure out a lot by looking at the bones.”