When the triathlon first reared its iron head back in 1978, the idea was to separate the men from the boys - not to mention the women. But in just a few short years women have made such a splash on the swim-bike-run scene that they compete regularly these days both in separate, females-only versions and in open events involving both sexes.
A year ago the world's first ''women-only'' triathlon at San Jose, Calif., drew some 900 entries, 70 percent of them first-timers. This year thousands more put themselves to the test in a number of such gruelling competitions scattered throughout the country. And it seems clear that the trend is toward even more triathlons - both mixed and all-female - in the future.
Triathlons are not for the weak of will. For example, the 1984 ''Women's Challenge'' in San Jose, won by Joanne Ernst of Palo Alto, Calif., included a one kilometer swim, a 30 kilometer bike ride, and 10 kilometer run.
And that's just a ''short course!'' The famed Ironman Triathlon, held in Hawaii each fall and coming up again this year on Oct. 6, features a 2.4 mile ocean swim, a 112 mile bike ride, and then a full 26 mile, 385 yard marathon.
Over 250,000 people participated in 1,000 triathlons throughout the world in 1983, says Harald Johnson, editorial director of Triathlon Magazine. And as the 1984 season heads toward its close, Johnson estimates that the number may reach half a million this year. Sixteen percent will be women, he says.
''There is more social acceptance of being athletes.'' says Sally Edwards, a top triathlete and the author of three triathlon books. She foresees, in fact, that by the year 2000, half of all triathletes will be women.
Back in 1978 when the triathlon was born, however, women were conspicuously absent. Fifteen men showed up to run the first Ironman, dreamed up by Hawaii Navy Commander John Collins. The next year, Bostonian Lyn Lemaire broke the sex barrier, placing fifth among 12 finishers. And in 1982, when ABC's ''Wide World of Sports'' televised an utterly exhausted Julie Moss crawling across the finish line, the triathlon came of age for endurance buffs of both sexes. (Moss, by the way, has continued to be a top competitor, and took first place in her most recent effort this summer at Wellesley, Mass.).
For all but the top contenders, the triathlon is a sport in which the also-rans are as important as the winners, so long as they cross the finish line. That appeals to a lot of women, says 22-year-old Jennifer Hinshaw, a top triathlete from Saratoga, Calif., who finished the San Jose Women's Challenge in fourth place. ''To finish a triathlon is a major accomplishment,'' she said. ''Triathletes don't have to be super great athletes.''
What they have to be is determined. Though most triathletes finish their events with legs like Jell-O, they start them off with wills of steel. Twenty-one-year-old Renee Arst dived into 56-degree water without a wetsuit for the first leg of a triathlon in Northern California last November. She finished third.
Risks of the race aside, most triathletes freely admit the hardest part is the training, which requires discipline and, above all, time. Hinshaw, who finished ninth in the 1982 Ironman, took a year's leave from UCLA so she could train 6 to 8 hours a day for the 1983 season. Edwards, a strong enough marathoner that she qualified for the US Olympic Trials, trains 20 to 25 hours a week, come rain or shine.
But all that time is money. ''The sport is constantly in danger of becoming a rich person's sport,'' says Tim Calahan, spokesman for the Bay Area Triathlon Club. Not all triathletes can afford to pursue the sport without financial assistance.
Arst, a physical education major at University of California, Berkeley, who teaches aerobics three nights a week, feels she has the potential to be in the top 10. But to really compete, she needs a sponsor to pick up travel expenses and pay her to train. As it is now, juggling training with job and schoolwork is tough. ''I always tell myself that triathlons will still be there,'' says Arst, who finished this year's Women's Challenge in 24th place. ''But sometimes I worry that others may pass me by.''
Demographically, triathletes are a unique group. Median household income is $ 34,000 and over half have either a college or a post-grad degree. And though most female triathletes are young - median age is 23 - a growing number of older women have taken the plunge.
Janie Paone, who began training for her first triathlon at age 39, now has a regular training regimen that includes a six mile run each morning, a mile swim at noon, and a cross-country bike ride whenever possible.
''If women are already into sports and into swimming, they're probably into triathlons,'' she says.
The question is, what makes Janie - and the thousands of women like her - run? Top-ranked triathletes can expect to make some money, athough not much: Linda Buchanan, top money-winner for 1983, made $12,000. There is also the lure of product endorsements and personal appearances for the stars. But the most the majority of women triathletes can hope for is a finisher's T-shirt at the end of the race.
For some, the sport is a way of life. ''It brings discipline into my life,'' says Beth Davis, a 26-year old East Coast triathlete who relocated to Southern California to work for Triathlon Magazine. ''It keeps me balanced.'' For others, training is a way to even out athletic development.
But for Paone, who crossed the finish line of her first triathlon in a flatbed truck after her bike broke down, the biggest reward is the exhilaration of doing something she never thought she could. The proudest moment of her life came four hours and 46 minutes after the opening gun of a San Francisco triathlon - her second attempt - when she was met by her beaming husband and children at the finish line. ''It's all up here,'' says Paone, tapping the side of her head. ''What it takes is someone telling you that you can do it.
''Most women really want to do something like this,'' she continues. ''They just don't think they can.''