The sweatpant and flannel gym wear is credited to Emile Camuset, the founder of Le Coq Sportif. The original grey jersey pants were created for athletes in the 1920s to keep muscles warm and comfortable and offered stretch for ease of movement.
By the late 1930s, the French dubbed the sweatsuit "the Sunday uniform." In 1926, Russell Athletic had the idea to replace the wool jersey with soft, comfortable cotton. About a decade later, Champion created the Reverse Weave sweatshirt, turning the fabric's grain 90 degrees, thus eliminating vertical shrinkage.
The combination of sweatpants and top became a mainstream look through the 60s and 70s. It reached iconic status in the 1979 blockbuster Rocky II as Stallone sprinted through the streets of Philadelphia to the equally iconic soundtrack.