There’s no doubt that American designers are responsible for introducing the concept of sportswear to the rest of the fashion world.  Back in the post war 1940’s, when our economy was booming and people had more time for leisure activities, a whole new demand for "casual wear" developed. 

Here’s a quote from a 1955 Time magazine article on the new "American look".
"Salads & Mambos. Making clothes with the American Look is no simple
trick. U.S. women, says President Hector Escobosa of San Francisco’s
I. Magnin, "don’t want their sports clothes to look like overalls, but
they want them to act like overalls." While Claire McCardell and other
top designers lead the way, the U.S. fashion industry is now busy
turning out garments to keep up with the fast modern pace—dresses that
are as at home in the front seat of a station wagon as in the back seat
of a Rolls, as comfortable in the vestibule of a motel as in the lobby
of the Waldorf, as fitting for work in the office as for cocktails and
dinner with the boss. Most of all, they must be practical. Sports
clothes must swing as easily on the laundry line as on the golf course,
and evening clothes must be designed as much for tossing a salad as
treading a mambo."

Of course, being the playground of the stars and a resort destination,  California became the center for this fashionable  casual style.  Think of Cole, Lanz, Koret, Louella Barbarino,  Addie Masters and also Pat Premo.

Another quote from Time.
"St. Louis, Chicago and Philadelphia are all important garmentmaking
centers. Around Dallas, some 70 firms are turning out $40 million worth
of women’s clothes a year and selling 35% of their output outside the
Southwest. In California, where designers were once willing to try
anything ("crazy pants" in wild harlequin designs and 6-ft.-round straw
hats) just to get talked about, fashion has come of age. Now 1,200
women’s-apparel manufacturers, including such leaders as Pat Premo,
Rudi Gernreich and Georgia Kay, are grossing $350 million a year, and
selling 60% to 75% of their wares east of the Rockies."

Pat Premo orignally worked for Peggy Hunt in the 1930’s, then started her own line in the 1940’s.  She is known for her  of interesting design details,  unusual fabrics and generous use of horizontal stripes. She believed horizontal stripes were more flattering to a woman’s figure.

Here’s a 1954 ad for one of her beautiful dresses.

Courtesy of Amazing Adornments , where there is an amazing collection of vintage fashion ads!

Here are some more examples of her fabulously feminine fashions. The first three are offered on my website.

A brilliant tropical print dress.

A blue gingham check with zig zag stripes.

Floral sateen sheath dress with cut out back.

This lovely Pat Premo sundress is offered at Tasty Vintage .

For more information on the mid-century California apparel industry, Vintage Clothesline has a very informative article.