anuary — the month when our thoughts turn from eating to exercise. Gym rats might loathe the crowds of New Year’s resolvers, but they can take comfort in the fact that most newbies rarely last more than a few months.

Seasonal fluctuations of the gym-going public aside, the idea of fitness has thoroughly conquered America. There are more than 50 million gym memberships in the country, up 22 percent from a decade ago. All told, gyms are a $27 billion business in the United States.

The history of the gym touches on many areas of life, culture, and society that, at first glance, seem to have very little to do with the gym, exercise, or physical fitness. Indeed, throughout its history, the gym has been a place for the artistic and the political, the enlightened and the reactionary.

How the human body is portrayed, cared for, and displayed at any given time is determined by attitudes toward sexuality and gender, and the prevailing moral code. And these have fluctuated tremendously over time. The ancient Greeks, who gave us Narcissus, competitive sport, and the first gyms, worshiped the naked male form. Physical beauty was a sign of divine favor. The Romans, surprisingly, were much more suspicious of the body and its pleasures. Up until the Middle Ages, the body was reviled as sinful and disappeared from view. The nude staged a comeback during the Renaissance, when artists combined pagan imagery with Christian themes to produce such masterworks of Renaissance kitsch as Botticelli’s “Birth of Venus’’ and Michelangelo’s “David.’’

Today, we have gradually returned to something akin to the classical Greeks’ unselfconsciousness about seminudity in public, making gyms not just economically viable but socially necessary. Indeed, researchers in the United Kingdom found that half of those they surveyed said they only went to the gym to check out the opposite sex or meet with friends.

Aside from the mechanized apparatuses, modern gyms would probably not be all that surprising to those who created the idea. The reassuringly masculine, Anglo-Saxon monosyllable “gym’’ is a contraction of the Greek word “gymnasion,’’ which translates to the much more deviant “school for naked exercise.’’ But contrary to our modern notions, a Greek gymnasion more closely resembled an outdoor athletic field than an indoor gym.

The idea of a place dedicated to exercise was resurrected a millennium later, though not by elite warriors who reclaimed them as a place to train for war but by Renaissance physicians who recommended the ancient “Gymnastic Art’’ to their aristocratic patrons in the vain hope that it might mitigate the damage caused by the dietary excess then in favor among the rich and powerful.

If 16th-century art glorified the naked human form as an abstraction, and 16th-century medicine recognized the health benefits of regular exercise, the idea that humans could change their body shape was still very far in the future. It would require a radical shift in attitudes in many areas of society and belief. In short, before man could transform his body in the gym, he needed to change the way he thought about embodiment.

That shift began with the Enlightenment, a philosophical movement that emerged in 17th-century Europe, but whose crowning achievement lay across the Atlantic in the foundation of the United States. The entirety of the Enlightenment project could be summed up by seven words culled from the Declaration of Independence: “Life, Liberty and the Pursuit of Happiness’’ — the foundations of the edifice of civil rights we take for granted in the West, but which also stress the personal by giving equal billing to Happiness alongside Life and Liberty. The Western individual, who, besides being the possessor of civil rights, was also a physical being, whose body — at least for white men — was no longer a spiritual abomination or a state chattel but something to be cared for, maintained, improved, and even, in strictly circumscribed ways, enjoyed.

It’s little surprise that Enlightenment gave rise to many improvements in physical education, especially in the school setting. Yet the genesis of the modern gym wasn’t philosophical speculation or educational reform but war.

In 1806, France’s self-made emperor Napoleon Bonaparte crushed Prussia, until then considered to be Europe’s military superpower. Faced with the humiliation of his country, schoolteacher Friedrich Jahn reasoned that if the crack professional armies of Germany’s divided monarchies had failed to beat France’s conscript army, a united Germany would prevail if it had its own citizen army. Blaming Prussia’s defeat on the physical degeneracy of the Germanic race, Jahn created in 1811 the first institution devoted to exercise for its own sake since antiquity, which he called a turnplatz, or exercise field. It was open-air venue for turnen (the basis for the sport of artistic gymnastics), equipped with parallel and high bars, rings, vaulting horses, climbing masts, ladders, and additional facilities for running and other track and field sports. Those who exercised were called turners.

Although a civic enterprise that would later develop into state-sponsored physical education classes for school students and conscripts, the turnplatz was initially considered a threat to the established order because it was open to all, regardless of social class, and because the turners advocated the unification of Germany. After Napoleon’s defeat in 1815, the conservative German monarchies quickly moved to ban turnen. Two of Jahn’s students emigrated to the United States in the 1820s to escape persecution at home and set up the first collegiate gymnasium in the United States at Harvard in 1826. But the import was not popular with the student body and the gym was soon forced to close. A second introduction in the late 1840s was more successful, and by the outbreak of the Civil War, there were 150 turnvereine (turnen clubs) with an estimated 10,000 turners, recruited primarily from German immigrant communities.

The turners were both physically and politically active. They were some of the earliest supporters of Abraham Lincoln and a group of them formed the new president’s body guard during his inauguration.

It was a time of conflicting attitudes about fitness. The wild men of the frontier were ultra-masculine and no doubt extremely fit, but the physical ideals of the civilized East oscillated between the emaciated, effete “exquisites’’ — wasp-waisted young men more concerned with the cut of their clothes than the size of their biceps — and the double-chinned, corpulent businessman, whose wealth was advertised by an expansive waistline.

The turnplatz attracted young radicals who saw communal exercise as a means of reforming society, but the first commercial gyms that began to appear in Europe in the mid-19th century introduced a now-familiar business model: an institution open to all, regardless of social class, gender, or age, tailored to meet the needs of the individual, as long as he or she had the means to pay. What had happened between 1820 and 1850 to account for this exercise revolution? Primarily, the rapid industrialization, democratization, and urbanization of Western Europe, with France and Great Britain at the industrial and political forefront. The Enlightenment citizen who had won Life and Liberty had become the affluent, leisured Victorian consumer engaged in the Pursuit of Happiness.

One of the world’s first commercial gyms was Hippolyte Triat’s Grand Gymnase, which opened in Paris in 1849. Although called a gymnasium, its closest modern equivalent would be a large studio specializing in group exercise classes. In 1897, Eugen Sandow created the world’s first chain of branded gyms, the Institutes of Physical Culture, in the United Kingdom. With their personal trainers and racks of free weights, Sandow’s institutes catered to both men and women who had chosen strength training as their preferred method of physical self-improvement.

Both Triat and Sandow started their careers as Vaudeville strongmen — naturally gifted athletes, who titillated audiences with displays of their near naked bodies and wowed them with feats of strength. Once established as performers, they realized that they could use their fame to meet the growing demand from Europe’s middle class who regarded physical fitness, once the preserve of the idle rich, both as a natural right and beneficial pastime. In this, Sandow was greatly assisted by two new technologies: still photography that enabled him to disseminate the “Grecian ideal’’ that he embodied, and mass-production, which provided his gyms and mail-order businesses with cheap, standardized equipment.

Sandow was particularly influential in the United States, which he toured twice in the 1890s, bringing his strongman act to American stages and promoting physical fitness through progressive weight training. His counterpart, Bernarr Macfadden, though he did not open a gym, launched the career of a man who would become synonymous with health and fitness in the United States, Angelo Siciliano, better known under his strongman stage name Charles Atlas.

By the end of the 19th century, the gym had gained a foothold in American life. The elite trained in collegiate gyms and exclusive private clubs, such as the New York Athletic Club, founded in 1868, while the working man went to the YMCA, which promoted a combination of religion and physical fitness known as “Muscular Christianity.’’ Several ingredients of the contemporary gym were now in place: Fitness enthusiast George Baker Winship is credited with the invention of the adjustable dumbbell, and other ingenious inventors on both sides of the Atlantic designed weight-training machines not unlike today’s multigyms.

But in order to become firmly established, the gym needed to become a mainstream fitness activity on par with other major sports and leisure pastimes, giving it a critical mass of participants who would make it an attractive proposition for entrepreneurs. What transformed weight training in a gym into a mass-participation activity in the mid-20th century was a cross-fertilization between the Californian culture of hedonism, bodybuilding competitions, and Hollywood.

These three elements came together when, in 1934, the city of Santa Monica opened a new seaside amenity that quickly became known as “Muscle Beach’’ because it drew gymnasts and physique athletes to its open-air facilities, and to the gyms that quickly opened in the city to cater to them. Among the Muscle Beach regulars was Abbye “Pudgy’’ Stockton, the “Barbelle of Santa Monica,’’ who would go on to open a women’s gym in Los Angles, and Steve Reeves, who cashed in on his extraordinary physique and success as a Mr. America and Mr. Universe to launch his Hollywood career as the star of sword-and-sandal epics. It was a collaboration that would come into full bloom with the career of Arnold Schwarzenegger.

The association of the hypermuscular superhero with the gym created an image of an all-male, heterosexual preserve, perhaps most memorably brought to life in the 1977 docudrama “Pumping Iron’’ that followed Schwarzenegger to victory in the 1975 Mr. Olympia contest. But what was happening in the real world was quite different. In the 1940s and ’50s, three gym entrepreneurs, Vic Tanny, Jack LaLanne, and Bob Delmonteque, created the first fitness club chains where men and women could train together in clean, comfortable surroundings. Although financially unsuccessful, Tanny et al. established a business model that persuaded casino operator Bally to invest heavily in the fitness industry. With the creation of Bally Total Fitness, the American gym had gone corporate.

During the 1970s, going to the gym still meant weight training, which appealed to men who were interested in building muscle, but did not cater to the majority of women who had different fitness goals. What brought them into the gym in their millions was the “Aerobics Revolution,’’ which combined the scientific discoveries of Dr. Kenneth Cooper with the Hollywood glamour and marketing genius of Jane Fonda, who popularized the aerobics dance class with a series of bestselling books and videos during the 1980s. Post-Fonda, the gym as we know it today had been born, with its tripartite offering of weights, cardio, and group exercise classes.

Starting as the preserve of the rich and religious during the 19th century, the American gym became the preserve of the committed bodybuilder in the mid-20th century. By the late-20th century it had gone corporate, catering equally to men and women, gay and straight, young and old. Today working out at the gym has become an indispensable part of the American consumer lifestyle, as well as a business model that has been exported to the rest of the world.

GYM, Page K4-5

Eric Chaline is a journalist based in London. He’s the author of “The Temple of Perfection: A History of the Gym.’’