When fitness guru Jack LaLanne opened a gym in Oakland, Calif., in the 1930s, he had to hire a blacksmith to build the equipment: fitness machines did not yet exist. As Natalia Mehlman Petrzela writes in “Fit Nation,” in the late 19th and early 20th centuries, exercising, except in the context of organized sports, was “marginal, and even suspicious,” and being fat was taken by many as “a positive sign of affluence.” Being lean or muscular was likely the result of manual labor, hardly something to which the middle classes aspired. Weightlifting was for the circus or the effeminate; ladies didn’t perspire, much less sweat. “Fit Nation” is the story of how all that changed.

There were always outliers: bodybuilders and diet advocates like Bernarr Macfadden (publisher of Physical Culture magazine), exercise proponents like Charles Atlas (a former “97-pound weakling” whose mail-order regimen promised “everlasting health and strength” in 15 minutes a day). But they were viewed from a distance; it would be the postwar prosperity of the 1950s that shifted the public attitude to fitness, and created a consumer base with the time and money to pursue it. From the White House, John F. Kennedy promoted a glamorous outdoor athleticism, while pioneers like LaLanne brought fitness into the home via television. No longer a niche pursuit, fitness grew to be a standard part of a middle-class lifestyle (even if many of us, it must be said, observe it in the breach). Between sometime in the 1960s and the mid-1980s, working out went from eccentric interest to “social imperative.” Today, we all know we should be exercising, especially when we see someone else out running.

Ms. Petrzela, who teaches exercise classes when she’s not lecturing on cultural history at the New School, offers an informative overview of this transformation. In 1961, she tells us, 24 percent of American adults were regular exercisers, but “that number jumped to 50 percent by 1968, and 59 percent in 1984.” Those dramatic rises were driven by inventive fitness styles, catering to different consumers: Jazzercise for the housewife; Nautilus gyms for the yuppie; jogging for pretty well anyone.

Other cultural and political trends converged. Jogging, for instance, got a boost thanks to an increase in urban marathons and fun runs from the 1970s onward. Ms. Petrzela suggests that these races became popular because they were cheap for cities to host and helped to promote “an image of fiscal and personal health that offset depictions of urban crime and decay.” Local governments are happy to promote fitness as a social good and community-building activity.