Welcome to Fashion History Lesson, in which we dive deep into the origin and evolution of the fashion industry's most influential and omnipresent businesses, icons, trends and more.
The whole world, it seems, has gone "athleisure." We're surrounded by Lululemon yoga pants, Yeezy sneakers, Nike sports bras and Outdoor Voices exercise dresses. Experts say it's a $300 billion dollar international industry and only expected to grow in the coming years as it appears that no one is immune. As Lisa Armstrong of Harper's Bazaar wrote in the September 2016 issue, "Even fashion's die-hard romantics, bohos, and glamour junkies have bought into this athleisure moment." 
But why? Part of it's due to science. Designers these days have access to materials that wash easily and wick away sweat, making them comfortable and practical for everyday use. This is big news in a society that values fashion that can be worn for different occasions and in various locations. Today's designers have taken the trend and run with it, increasingly starting athleisure lines of their own (Stella McCartney with Adidas, Tory Burch started Tory Sport) or including athleisure in their regular collection. Celebrities, too, are dashing to cash in, partnering with brands to bring their own versions of athleisure to consumers. (Kate Hudson established Fabletics, Beyoncé gave us Ivy Park, Rihanna collaborated on Fenty Puma, to name a few.)
But there's more at play than science and fabrics. Changing needs and wants are at play, too, especially among women.
What is athleisure?
The word athleisure combines two seemingly contradictory terms, "athletic" and "leisure." As early as 1979, the term was used in an issue of Nation's Business to describe garments and shoes designed for those who want to appear athletic. Almost two decades later in 1997, Women's Wear Daily defined athleisure footwear as "non-performance styles such as canvas or suede."  The key idea in these early definitions of athleisure is that these styles are made to resemble exercise wear, not primarily serve as workout garments. This differs from today's interpretation of the term, which Merriam-Webster added to the dictionary in 2016 as "casual clothing designed to be worn both for exercising and for general use." Athleisure was no longer just emulating the look of exercise clothes, but actually attempting to serve dual purposes.
From "sportswear" to "athleisure"
Athleisure as we know it today evolved from a century-long history of American sportswear into its own unique trend that capitalizes on both our society's emphasis on comfort and the technology that allowed for advanced synthetic fibers. A hundred years ago, people wore clothes that were dictated by the event they were attending, but modern-day athleisure is for people who want a one-size-fits-all approach to their wardrobe: What can take me from running errands to work to the gym? Basically, American fashion became sports-crazed (or at least obsessed with appearing sporty) and as it developed, the fashion industry responded to consumers' demands. 
Sportswear of the late 19th and 20th centuries referred to the interchangeable separates worn by both players and spectators.  They were made in response to a more active lifestyle led by women, who were riding bicycles, playing tennis and dancing in nightclubs. This kind of clothing was often made of knits or jersey and exposed arms and legs for freedom of movement. One notable sportswear designer was Parisian Jean Patou, who is credited with creating the tennis skirt, first worn by tennis superstar Suzanne Lenglen to Wimbledon in 1921. The sporty dress, cardigan and stockings that she wore on and off the court helped popularize sportswear in Europe. In 1926, Vogue reported, "Three-quarters of daytime fashions offered in Paris are of the sports type. Simple, practical, and youthful, they constitute an influence that is more and more felt outside the realm of action sports in dress for general daytime and resort wear and for travel."
While other Parisian designers, like Gabrielle "Coco" Chanel, became known for their sportswear designs, they never deviated from the tenants of couture construction that emphasized quality. Instead, it is American designers who came to exemplify the ease and simplicity of sportswear. Designers like Claire McCardell and Bonnie Cashin helped the American fashion industry emerge from the shadow of French couture to promote a more casual approach to dressing. Their designs emphasized the practicality and democratization of fashion. It's also worth noting that many of the most successful and well-known sportswear designers were women, possibly because they understood the needs of their contemporaries and injected their own values into their designs. In the 1940s, McCardell exemplified the American sportswear aesthetic. Her bathing suits, day dresses, playsuits and other signatures were characterized by functionality, soft construction and utilitarian fabrics, all features of today's athleisure. Similarly, Cashin promoted practicality in her clothes through layering separates, large pockets and utilitarian closures. Much later, the 1980s saw Donna Karan's emphasis on interchangeable, simple wardrobes when she introduced her influential "Seven Easy Pieces."
By the latter end of the twentieth century, most of what was worn would be considered sportswear. But fashion changing because of the influence of more casual dress was hardly new by this point. In fact, it's fashion's nature to take what used to be considered scandalously casual and turn it into the latest trend.  For example, the man's business suit of today originated as the sack coat, the most casual cut of coat that was meant for lounging in intimate settings and never meant to be formal dress.  In a manner typical of the fashion system, the idea of clothes made for sporting activities has been appropriated into fashion that is casual but stylish enough for people to consider wearing it both to the gym and running errands.
As society grew to accept sportswear separates as the norm, a new term was needed to differentiate this new generation of multipurpose fashion from its predecessor. That's where athleisure comes in. A 1989 issue of Vogue discusses the prolific use of fitness fashion as casualwear when it claimed that the result of designer and sports brands designing "body-hugging actionwear" was "a whole new category of 'athleisure' dressing that's not just for exercising."  In 1994, the same title declared, "The fitness revolution not only wrought better-looking, higher-performance workout wear, but it also sent sports gear out of the gym, off the hiking trails, into the street, and, recently, onto the runway." 
What really sets today's athleisure apart from its history is technology. The athleisure that we know of today developed because of advanced textile science that allowed for the advent of improved performance fabrics. Lycra was invented in 1958 , allowing designers to utilize a fabric's inherent stretch. In 1986, Women's Wear Daily reported that a new Du Pont Dacron fabric would be released for use in the athleisure market, describing it as "extremely soft, dry and comfortable."  More recently, the textile Gore-tex® has allowed for clothes to be both waterproof and breathable without even looking like they were designed for sport. This experimentation with textiles continues to this day and propels changes in athleisure design. Within the past year, textile scientists have developed a fabric that can actually respond to your body's temperature. It's this marriage between innovation and design that mirrors athleisure's own mixing of function and fashion.
The future of athleisure
Athleisure sales in 2018 and 2019 didn't climb as aggressively as they did in the previous 5 years, but that doesn't mean that the market is dead. Athleisure sales continue to increase, though the days of double-digit growth may be over. According to Vogue Business, brands are hoping to attract new customers by focusing on different cuts that are more acceptable in the workplace. The hope is to appeal to an older generation by offering looser styles that can be a middle ground between gym clothes and business wear. And while the market may be saturated with stretchy pants, it seems the greatest opportunity for growth is in shoes. Forbes claims that sneakers are set to overtake dress shoes as the largest footwear category in the U.S. through 2021. If last year's ugly-sneaker trend is any indication, it isn't performance shoes that are in demand. Instead, the market seems to be shifting back to a desire for active-inspired clothing rather than actual performance gear. It's about comfort and versatility, down to the shoes.
As the market continues to grow, both larger companies and smaller newcomers will continue to expand their athleisure offerings. Brands like Girlfriend Collective, Wone, and Onzie are seeing success at the same time as Lululemon, Athleta, and Adidas by focusing on athleisure's connection to society's larger emphasis on health and wellness. (In the case of Girlfriend Collective, they are also innovating to offer a more sustainable option by using recycled water bottles to make their products.) What it all seems to mean that athleisure is here to stay.
Sources not linked:
 Lisa Armstrong, “The Best of What’s New: The Key Pieces to Invest in Now,” Harper’s Bazaar, September 2016, 327.
 “Sports Talk: Pricy Footwear Rises,” Women’s Wear Daily, December 18, 1997, 11.
 Ligaya Salazar, ed. Fashion V Sport (London: V&A Publishing, 2008), 18-19.
 Rebecca Arnold, The American Look: Sportswear, Fashion and the Image of Women in 1930s and 1940s New York (New York: I.B. Tauris, 2008), 15.
 Catherine Smith, Women in Pants: Manly Maidens, Cowgirls and and Other Renegades (New York: Harry N. Abrams Books, 2003), 18.
 Christopher Breward, The Suit: Form, Function and Style (New York: Reaktion Books), 52-53.
 Dorothy Schefer, “Second Skin,” Vogue, September 1, 1989, 252.
 Katherine Betts, “Fashion Pumps Up,” Vogue, January 1, 1994, 84.
 R.W. Moncrieff, Man Made Fibers (New York: John Wiley & Sons, Inc., 1970), 440.
 Marvin Klapper, “Fabrics: New Du Pont Dacron to be Unveiled for Knits,” Women’s Wear Daily, July 15, 1986, 6.