The American spirit

Sportswear - our national specialty - is having a banner year. Its relaxed mood pervades fall fashion. In every area of dress, there's the insouciant mood associated with the all-American sportive look. An air of offhand ease prevails - in sweaters, pants, roomy jackets, flared skirts, and comfortably oversized capes and greatcoats - for day and evening.

Only six months ago, designers were proposing a tight, body-hugging silhouette. That is now past history. It did not take long for the realization to sink in that overly constricted clothing has no validity in today's world.

Dressing well, in a polished manner, does, on the other hand, suit the requirements of the modern woman. So, while designers have loosened shapes and abandoned the idea of the superfit, they've kept the finished aura of sophistication that went with it. This season's editions of standard sportswear forms are of the urbane rather than the carefree-casual or rugged variety.

All the familiar classics are represented in force. Yet the clothes collector who cherishes her old blazer is apt to find it somewhat dated and on the skimpy side. The lines of the latest blazer are generous. It is also likely to be knitted instead of being of tweed or velvet.

Other traditional favorites differ from the norm, too. The cardigan has grown several inches and ends well below hip length, which practically puts it in the sweater-coat class.

Gray flannel trousers, which haven't been around for a time, are back and are among this year's basic essentials. Straight-leg styles with pleat tops are tailored with customized precision, but sometimes feature unconventional touches. The Calvin Klein gray flannels look longer and leaner, thanks to a tuxedo band of black braiding down the side of the leg. What's more surprising is the way this year's flannels can be worn. They not only work as expected - with a cabled Shetland, for instance. They also go out to dinner with a sequined cashmere and a colorful silk trenchcoat.

That sort of concord between informality and luxury is a mark of the season. Anything that looks too pretentious has been put on the shelf. The extravagant ball gown overblown with ruffles and ballooning sleeves has reverted to its period-piece status. The top choices for upscale dressing in the evening are sweaters with pants or slim skirts, long columnar sheath dresses in exquisite materials, or extremely short dance dresses with frothy little skirts and bare-shouldered tops.

But the main change this fall is the difference in scale. Checks and plaids are larger than life. The most striking - in black and white - are Prince of Wales and houndstooth tweeds. They are used extensively (for both Klein labels, Calvin and Anne, among others) with one pattern sometimes being played off against the other.

As to overall dimensions, they have assuredly been amplified, although not to an overpowering degree. There's a fair amount of yardage, but it isn't the old uncontrolled ''big look'' kind that swamped the wearer.

Shapes, which are well defined, descend from accentuated shoulders. They may be squared (on suit jackets especially), but are more often rounded like arcs and show little sign of padding.

The long, extra-sized coat that was worn by fashion-conscious males last winter (a vogue started by Perry Ellis, whose men's collection garnered several awards for the year), is now high chic for women. Skirts may be midcalf or at the knee, but what goes over them is much longer: an all-enveloping steamer coat , balmacaan, or greatcoat modeled after the naval officer's, for instance.

Breadth at the top is also achieved by means of raglan or dolman sleeves, which slide easily over suit jackets, many of which have deep armholes, too. Alternative outerwear includes light wool ponchos and capes. Ralph Lauren's are cashmere and cost a mint. The gray wool cape from the new Halston III collection at J. C. Penney is an affordable $68.

The fundamental colors are somber: gray, black, and smoky autumn tones. These are livened up with shots of clear red, cobalt blue, and jade as well as by the new wildlife-patterned separates and accessories. Strong doses of color - such as a vivid raspberry wool three-quarter coat with a black skirt - have plenty of impact in a generally dark picture.

What to buy for work should present no problem; the array of suits, blouses, skirts, and dresses is extensive. An especially good choice is the tailored coat dress, which exists in many colors and fabrics at all price levels. Suits with cropped jackets and softly gathered skirts range from dressy with velvet collars to color-flecked tweeds. Mannish numbers of the type once advocated by self-proclaimed dress-for-success experts are in the minority. Few women, it seems, need that kind of support now.

When it comes to the year's most experimental fashions - the strange, controversial clothes by the new wave of Japanese designers - the question ''Would I wear it to the office?'' is not an issue.

Interesting as these voluminously layered, stark-black, or earth-toned garments may be, with their holes, slashes, and bunched-up gatherings, the majority of women will view them as curiosities. Given time, the Japanese influence may seep through to the mainstream of fashion. But for the moment, elegant American-style sportswear has the field to itself.