It’s that time of year again, when many Americans head to the gym in an attempt to keep a New Year’s resolution to exercise more. About 11% of gym memberships are purchased in January, more than any other month in the year, according to a recent survey by The International Health, Racquet and Sportsclub Association, and a recent YouGov poll found that, of the nearly 30% of respondents who said they are making New Year’s resolutions, 50% said their top resolution was to exercise.

While New Year’s resolutions in general are thought to date back to the ancient Babylonians, the New Year’s resolution to get in shape is part of a much more recent trend.

Most Americans didn’t see exercise as necessary until the mid-20th century, says Natalia Mehlman Petrzela, a historian and professor at The New School who is writing a book about the history of American fitness culture. Previously, food scarcity meant overeating wasn’t a problem for many people, and it didn’t make sense to try to burn off the few calories they could consume; besides, it was generally only the most well-off Americans who could afford extra exercise equipment. But industrialization changed the nature of work and food production. Fewer farmers in the field doing physical labor and more office desk jobs, later joined by the growth of automobile culture and suburban sprawl and the popularity of television, all led to more couch potatoes.

“You don’t see exercise culture taking off in the U.S. until after World War II, when there’s more widespread affluence,” says Petrzela.

And the change didn’t go unnoticed. After a 1955 report showed that 57.9% of American kids failed one or more of six physical fitness tests — versus about eight percent of European kids — President Dwight D. Eisenhower issued an Executive Order creating the President’s Council on Youth Fitness on July 16, 1956.