You couldn’t be blamed for hearing the word “Pilates” and thinking about super-fit starlets and medieval-looking machines like the Reformer. But the popular fitness system didn’t begin in a boardroom or a gym. In fact, Pilates has its roots in a World War I internment camp on a British island.

After World War I broke out, the British government feared that German men between 17 and 42 years of age would become German soldiers if they were deported. So they were locked up in camps all over Britain, including, starting in 1914, at a camp in a village on the Isle of Man known as Knockaloe. Knockaloe would eventually host over 23,000 internees, becoming the British Isles’ largest internment camp—so big that it required its own railroad.

But things weren’t so great at the overcrowded Knockaloe, where inmates began to succumb to the pressure of their ostracism and imprisonment. The camp inspector dubbed their behavior—which we now know was depression—barbed-wire disease.” 

One of the inmates at Knockaloe was a German boxer, athlete, and all-around health nut who was working for a British circus when he was imprisoned. He was strongly influenced by Germany’s physical culture movement, which advocated exercise as a way to strengthen the body and connect the individual to others. His name was Joseph Pilates, and he found unexpected purpose in the camps.

Pilates had been a sickly child, but he managed to rehabilitate himself through exercise and conditioning. So when he saw the condition of his fellow inmates, many of whom were bedridden and hospitalized, it sparked an idea. He began to teach his fellow internees to work out.

As he watched the progress of his bedridden countrymen, Pilates started to wonder if he could apply the fluid stretching movements he had observed in animals to humans who were incapacitated. He took straps, bunk bed springs, and other parts and began to experiment with crude homemade fitness machines that let people work out even when they were in bed. The machine would eventually be adapted into what is now called the Pilates Cadillac, a bunk-like apparatus with springs, bars, and a bed-like surface.

In 1918, influenza swept through Great Britain and the camp. None of Pilates's “trainees” died from the disease—an accomplishment he attributed to what he was starting to think of as his method. By the time Pilates was released later that year, he was passionate about his new technique. He began to teach it to Germans, contributing to a new movement called “Lebensreform” (life reform) that encouraged a return to nature and respect for the body.

But Pilates’ return to Germany didn’t last long. Though he later claimed that he escaped Germany due to pressure to teach his methods to the army, others note that he went to the United States in part to look into patenting one of his fitness devices. In America, he found a rabid audience for his new fitness method, which he first called “Contrology” and later renamed the Pilates Technique.

Almost a century later, Pilates is seen as the province of yoga pants-wearing yuppies and green-juice-guzzling fitness freaks. It’s come a long way from the bunks of a British internment camp—Pilates is now big business, generating nearly $7 billion of revenue in 2012 alone.