Sportswear: an American specialty

American excellence in sportswear didn't come about through sheer happenstance. Unlike European couture, which has always been rooted in ornamental styles, fashion grew up in this country with an eye on practicality.

Our indigenous attire was born of necessity, dating from the spare utilitarianism of pious early settlers on to the prairie schooner days of sunbonnets, broomstick skirts, and frontier jackets. In time, functionalism was mated with fashion, which resulted in the superior sportswear we have today.

The process developed gradually. Logical thinkers like Claire McCardell came along to question, and to change, existing modes of dress.

McCardell (who was probably the most American of United States designers) has told us in her memoirs how her observation of ''a shivering female in ice blue satin at a ski resort'' caused her to head for her workroom and create a sweaterlike wool dinner dress.

That was in the late 1930s. It wasn't until World War II separated us from the dominance of Paris high fashion that pioneers like her were encouraged to work in the relaxed American idiom.

During the 1940s and early '50s, McCardell gave us clothes to wear to New England clambakes, stretchy leotards that served as underlayers for a variety of skirts and pants, and weekend wardrobes composed of separate interchangeable pieces, among other inventions. As like-minded creators followed her lead, American sportswear was on its way.

Today our supremacy in low-key, easy looks is still based on a pragmatic attitude that differs sharply from the mannered formality that is characteristic of much of French and Italian dress.

As Halston puts it, ''We have the 'foot on the ground' approach. We offer common sense.'' In Ralph Lauren's view: ''Americans want clothes for living; Europeans wear fashion for fashion's sake.''

Our more energetic way of life has, of course, also contributed to the chic but casual mode that goes by the generic term of ''sportswear.'' Beginning with Amelia Jenks Bloomer's bicycling costume in the 19th century, clothes for active sports have successively made their mark in the annals of US fashion.

We have seen streetwise interpretations of baseball jackets, dancers' warm-up gear, tennis sweaters, and riding pants emerge. Since the start of the fitness boom, styles influenced by joggers' outfits have turned up in panne velvet and in suede. In the hands of Norma Kamali, innovative, brightly dyed clothes made of good old sweatshirt jersey have become day-to-day uniforms for young sophisticates.

To give credit where credit is due, it's true that most of the classic forms originated overseas. We borrowed the blazer from the English public schoolboy and the twin set from his mother. We took the cardigan from Britain's Earl of Cardigan and the balmacaan topcoat from the region of Scotland which gave it its name. Glenurqhart plaids, houndstooth checks, and Highland tartans are among the numerous other appropriations.

In Americanizing these traditional standbys, US designers have altered them appreciably. Sometimes the change is slightly whimsical - using the plaids or checks in great enlargements of the conventional size, for instance. At other times there's a startling departure from accepted color schemes. Often the garment is given its due importance by means of an amplified, more contemporary cut.

But whatever their contributions may be, the Americans' way with sportswear is acknowledged everywhere as tops. Even Charles, Prince of Wales, wears a Ralph Lauren rugby shirt on the polo field.