Watch and learn the favorite exercise routine of early 20th century Europeans.

Mark Anderson, the author of Kafka's Clothes and a professor of German and comparative literature at Columbia, argues that the author was likely drawn to Müller's unabashed commentaries on the crush of modern life. ("The town office type is often a sad phenomenon … prematurely bent, with shoulders and hips awry from his dislocating position on the office-stool, pale, with pimply face.") The system also promised benefits for artists and writers ("geniuses who have no thoughts for their bodies"), and Kafka appears to have shared Müller's pleasure in the naked body. (Kafka himself did a short stint at a nudist summer colony and often sunbathed nude at home—at least until his tuberculosis caught up with him.)

Most of all "for someone like Kafka who was allergic to nationalist group identity and belonging, the advantage of Müller is that he can do it alone at home—it's his system," Anderson elaborated on the phone recently, noting Kafka had likely been aware of the (not terribly welcoming) Czech Sokol nationalist gymnastics clubs. He continued:

It's divorced from ideological context and that's a great advantage, but, nonetheless, he still feels he is one of these neurotic, overintellectualized, urban Jews that need some beefing up. He feels too tall, too thin, too weak. His nerves are shot. He … very strongly believes that becoming a writer means becoming disciplined and toughening up his physical organism. He does a series of things to create a new identity. The gymnastics is one part of it.

Anderson sees Kafka's body-identity struggle, and embracing of Müller, as loosely connected to the movement to create "muscle Jews." It's a reference to Max Nordau, a contemporary of Theodore Herzl, who prodded Jews at the turn of the last century to toughen up and cast off the pallor of the city, the study room, and the literary salon. At the 1898 World Zionist Congress, Nordau issued a call to arms to his brethren: "Let's go! Pull your courage together! Do something! Work for yourself and make a place for your people under the sun! Don't rest until you have convinced the indifferent and downright hostile world that your people have a right to live and enjoy life just like other peoples."

Jews across Europe began working out. Hundreds of athletic clubs were formed, some of which—like Hakoah Vienna, which developed winning rowers, swimmers, water polo players, and a champion soccer team of the same name—were successful by any measure. It was in this heady context that my grandfather, a lifelong Zionist, learned his Müller as a 12-year-old in the 1920s, alongside other Jewish Austrian kids. Some of these other young athletes were also early Zionists, developing their bodies to work on kibbutzim in Palestine; some were merely seeking access to a burgeoning culture of German and Austrian athleticism. It's not clear that Jews flocked to Müller more than any other gymnastics regime, but being a part of a secular gymnastics movement was a means of conveying one's national as well as ethnic identity; ironically, given how the some gymnastics clubs would feed into Nazism in a few years, in Germany such Jewish athletic clubs were also seen as a means of being "German."

Müller took great pride in his system's popularity. In My System for Ladies, he notes that his first book sold "over a million copies" (later estimates pegged it at 2 million), and devotees included Crown Princess Sophia of Greece. Eventually he settled in London, dropped his umlaut, and opened the Muller Institute at 45 Dover Street, where he welcomed guests like the Prince of Wales. British soldiers were told to practice his exercises in the trenches of World War I. He gave hundreds of lectures on hygiene and exercise throughout the 1910s and early 1920s across Europe. A radio program, helmed by a devotee, broadcast his system from the mid-1920s until the mid-1950s.

By the end of World War I, Müller had begun to think the system was a kind of El Dorado that would keep the devoted alive well past the age of 100.

It was not, as the tubercular Kafka soon discovered. Müller himself died in 1938. In memoriam, the town of Nykobing, Denmark, erected a statue of him, a bronze casting of his body as it was in the height of his physical ability. Nude but for a loincloth, he is towelling himself off briskly—an essential part of the system. (The skin is an organ!)

Müller and his system faded into obscurity, some historians say, because he turned to the occult at the end of his life, alienating would-be followers by drifting a great distance from the corporeal world he'd long cultivated. In truth, Müller the man was less important to my family than the books themselves, which my grandfather, Karl, collected. Karl taught my father "to Müller," as the Germans put it, at age 5. My father, now a physician and in his 60s, believes that Müller's emphasis on developing the core is exactly right; half of his exercises are now part of his standard back-pain recommendations for patients.

For me, seeing my father lace his fingers together and swoop his entire upper body first in one direction, and then the other, is something like a visual madeleine. I can see myself, age 7 or 8, in my parents' old bedroom, watching my father hasten to finish his Müller, pushing his feet under the bed to hold them steady as he does sit-up after sit-up. Not much has changed, really, though the child watching now is my own.