Saturday afternoons at the American University track, where I have been going around in therapeutic circles for the past 10 years, are times of pastoral quiet. Only snow thwarts us runners, who prefer the word unthwartable to compulsive. Last Saturday we were disrupted -- noisily, happily and memorably. The university's soccer team, which for decades has been playing unnoticed on the field inside the track, now had 5,200 people watching.

The AU Eagles had reached the semifinals of the National Collegiate Athletic Association soccer tournament. They were playing Hartwick College of Oneonta, N.Y., a school feared as "a soccer power." On Saturday, with America's TV screens filled with the athletic refuse that is college football, the game of the week, and maybe the year, was being played on the AU field. It had everything that sports should have: tension, flair, grace and daring.

The Eagles won. Seventy-seven minutes into the 90-minute game, an open AU player headed a pass to a teammate who, cerebral himself in front of the goalie box, flicked a head shot into the net. That was the game-winner, a single shot that sent the fans into mayhem and the team on its way to the NCAA finals. The championship game is against UCLA tonight in Seattle.

The high moment at the AU field is currently matched by the ascendancy of soccer as a fast-growing participatory sport. The NCAA reports that 10 years ago it had 421 college teams in three divisions. The number is now up to 549, with more than 14,000 athletes on the soccer teams. According to the U.S. Soccer Federation, one million players were registered on teams in 1978. This year, the number is 2.5 million. This includes women's, men's and children's leagues, 400,000 YMCA players, 4,000 Special Olympians and 500,000 military personnel.

The rise in playground and recreational soccer means that the soccer players do it for kicks. The appeal of the sport is that you can begin early. Few team sports are as suited to childhood growth and development. Soccer is a running free-style game centered around the natural roll of a ball on the ground. The other three main team sports are not like that. Baseball is physically limited: more standing in the field and sitting on the bench than running. It is rule- complicated. Basketball for children means lifting a heavy ball and trying to throw it overhead to too-high a hoop. Football is forbidding with its equipment and irrational with its violence. Soccer is to football what a craft store is to a body-and-fender shop: finesse vs. banging.

That leaves soccer as the ideal team sport for the young. Parents cherish it for their children because it fulfills the great either/or of surviving Saturdays and Sundays: either we wear them out or they wear us out. Soccer is an exhaustion, not a mere game. Ninety minutes of running everywhere over a 125-by 75-yard rectangle of the neighborhood increases the parents' chances of hearing the most exhilarating words on Saturday night: "Mom and Dad, I'm going to bed early. I'm bushed." The only more beatific words are: "But before I go up, I'll do the dishes."

One who understands children, or at least the college kind, is Pete Mehlert, the soccer coach at American University. Every college athletic department should have someone like him: an improviser, a friend of students who truly enjoys their company and a believer that desire counts more than talent. Mehlert, 37, is the kind of coach players will hoist to their shoulders and carry off the field after the victory. In this case, it helps that Mehlert is light and small: 130 pounds and 5 feet 5 inches. He is weighty in other ways: his 14 years as a soccer coach, a record of about twice as many wins as losses, and an awareness that soccer is a lifetime sport.

My exchanges with Mehlert over the years have been mostly howya-doins at the track when the soccer teams were doing well and the runners were doing nine- minute miles. The other morning in his office, Mehlert was looking forward to a victory in Seattle. Win or lose, he said, being in the NCAA finals is "another educational experience in life for my players. They will never forget it."

Nor will Mehlert. Collegiate soccer is a low budget, high-quality sport. Much of the media ignore it. Only ESPN cable television and Channel 26 (WETA) will carry tonight's game (10 p.m.). UCLA's reaching the finals by winning last Sunday merited only a page-22 wire-service story in the Monday Los Angeles Times sports section. Heavy breathing goes to football, yawns for soccer.

The players don't mind. Their eyes are on the ball, not the publicity.