SPRING SUNDAYS in Arlington find a motley assortment of self-declared "softball senior citizens" taking to practice fields for hours of misjudging flies, overthrowing first base, missing grounders and swinging in the breeze.

Laughs are served up as frequently as high-arc pitches, ranging from the Sigh Young award given pitcher Ellen Jenkins for throwing behind batters to the record held by shortfielder Judy Brabender for th e most complete circles turned under one pop fly.

They are among the stars of the Cavaliers, one of 12 teams in the expanding Arlington recreation league for women.

But the slow-pitch league that started out four years on a light note has become somewhat serious business. What else can explain grown women pulling on long underwear in February and tromping through sodden fields to seize a water-logged ball; performing ritual oilings throughout the year on a fielder's glove they have stuffed and trussed; swooning over a Valentine's present of the latest plastic spiked shoe, or registering in the sports store before a bridal shower?

The inspiration for this aberrant behavior is varied. For players like Corky Howell and Faye Mench, it's a return to the backyard playing lots when the biggest decisions were which kid to pick for your team and wheather a ball that hits the tree in center fields is an automatic home run.

For others, probably the majority, it's indulging in a sport that eluded them as youngsters - a game they were never encouraged or allowed to play seriously.

League rules specify that players must be 17 or older. The Cavaliers have heavy representation from the "or olders." The average age is around 30, with a range of 13-39. The players like to think of themselves as experienced.

In contrast, several others teams rely on an extensive farm club system to produce young players each year. These recruits have come up through county pigtail and ponytail programs and have spent several years on high school teams.

When old meets young on the field, the game takes on the dimensions of a battle between the generations. Seasoned veterans have been known to take offense when their younger counterparts offer to help them down the first-base line.And youthful batters seldom appreciate the aging catcher's warning to get home at a reasonable hour.

Close of the business day finds the clandestine softball stars shedding assumed identities as counselors, florists, graduate students, house painters, teachers, lobbyists, researchers, school crossing guards. The minute they pull on their oversized tee shirts with the team name emblazoned on the front they are transformed into a band of softball fanatics.

The enthusiastic novice is the player posing the particular challenge to the coach. How would you answer the base runner who didn't advance on a ground ball and thendemands: "Why can't I stay here if I want?" And for anyone who claims that softball is a rational game, there is the ultimate challenge of explaining the infield fly rule to players who have just mastered the concept of "infield."

Fundamental skills are difficult to teach a beginnerof advanced years. It's rough when the glove can't quite reach the ground on an infield dribbler, when the catcher takes 2 1/2 innings to get out of her crouch and when the outfielder's basket catch is more of a sieve.

The coach's great challenges is to find reward in small things: the infielder who knows which base to throw, throw to, even though the ball never quite makes it; a well-hit foul; the fielder who keeps her eye on the ball from the moment it's hit to the time it plops on the dirt beside her.

There is more to the game, however, than onfield playing. A lot of effort goes into such previously unconsidered skills as devising original, if not always believable, excuses. Some of the most popular are:

Calling someone else's name for a fly ball.

Inspecting one's glove thoroughly after missing the grounder to determine the exact location of the gaping hole.

Removing the splinter in one's finger as quickly and as agonizingly as possible after striking out (do use and aluminum bat).

Squinting into the sun/lights after misjudging a fly ball.

Intimating that the hit-run sign was on after getting doubled up on a fly.

Another large part of the game is creating side-line cheers. "Easy out; no batter, no batter; pitcher can't pitch" are elementary efforts. "We want a pitcher, not a glass of water" shows more flair. But this group's attitude toward the game is best portrayed by Sally Wade's command as she rounded third base: "Move out of the way - this model doesn't come with brakes.

"An integral part of the May-though-July schedule are postgame beer parties which feature a replay of the contest's entire scenario, with embelishments.

And then there are the groupies. Friends room-mates, husbands, children, dogs, and scouts (the cookie-selling variety) constitute a loyal following. Families frequently become as involved as the participants, as evidenced in a 4-year-old's reply to her teacher's question about her mother's occupation. "She plays second base."

The faithful return season after season, despite pulled muscles, sore arms, long practices, last-inning defeats, crushing losses, tension-filled innings, and the frustrations of more than momentary mental lapses. Why? Simple - for the friendship of kindred spirits and the undiluted love of the game.