NOT TOO long ago, Americans dismissed pain as unseemly. Our heroes rarely flinched under the most grueling torture, barely blanched at the most gaping wound. Humphrey Bogart, John Wayne, Clark Gable were masters of that slight smile which told us all we needed to know about pain.

Now Tom Selleck, Burt Reynolds, Harrison Ford and even Sylvester Stallone have a practiced repertoire of grimaces, grunts and groans. They make no bones about suffering. Their visible, audible pain is the stuff of stardom.

From guerrillas in garlands of grenades to chanteuses in chains and spikes, our heroes and antiheroes have taken to pain with enthusiasm. As Christian vigilantes or Jewish Defense League commandos, they wear their weapons with pride. Like high fashion these days, they betray a reverence for bruises, welts, lashes and slashes.

Fashions in sex have followed sexy fashions away from languorous seduction toward a blatant savagery. Pain, once a perversion, is now an aphrodisiac. Sadism has entered the pages of Penthouse and New Look as a form of erotic art; hostile sex spices almost every potboiler from the novels of Harold Robbins to "The Story of O" and "Looking for Mr. Goodbar." Pain is sensual, provocative, exciting.

It is also entertaining. More than ever, sports commentators insist on the bone-crushing tackles, the violent checks, the thundering collisions in football, hockey and basketball. Wrestling, that ultimate cartoon of physical pain, has been resurrected as genuine entertainment. The enthusiasm, the humor and the suspense of television wrestling comes, of course, from the illegal punches, the wildly mean choreography and the unfair double-teaming which goes on behind and beneath the sourpuss referee. It is that playful extra measure of cruelty which makes our day.

Early in the morning, many of us wake to train for marathons and triathlons, or the next 10K run. We "go for the burn" with Jane Fonda or push at gleaming silver machines until it is time for our natural-bran cereal with plenty of roughage. Pain is something we work toward and welcome. Each new pain is the harbinger of new stretch, new power, new integrity. Pain makes us better people.

Perhaps we should restore corporal punishment as a means of personal growth. So, at least, argue some members of the Christian right in defense of the instructive thwacking of children at home and in school. So argues Graeme R. Newman, associate dean of the School of Criminal Justice at the Nelson A. Rockefeller College of Public Affairs and Policy.

In his book "Just and Painful," Newman recommends that we establish a graded system of electroshocks for most convicted criminals. Substituting corporal punishment for short prison sentences, he claims, would allow for a far more humane and far less costly criminal justice system.

Electroshock itself has come into therapeutic favor once again, arousing stunned protests against it and stubbornly scientific arguments in its favor. After primal-scream therapy and Rolfing, we should have been prepared for the resurgence of induced convulsions as a technique for healing. Pain is good for us. It heals. It gives us back our youth.

After all, we were born to pain, we must all work through the trauma of birth. Birth pains are not to be sidearmed by the infant or muffled by the mother. Prospective parents take classes to appreciate the rhythms of a natural (and naturally painful) childbirth. Those women nowadays who because of medical emergency must be shot up with anaesthetics feel cheated. Giving birth without pain seems inauthentic.

This is not to say that Americans believe that all pain is good or healthy. No one speaks on behalf of the pain of bone cancer or third-degree burns. Even such malignant pain, however, has acquired a newly reputable standing. If we do not like it, we have begun to respect it, admit it and do something about it.

Where before the cramps and headaches associated with some women's periods were socially ignored and medically disparaged, we have now the Pre-Menstrual Syndrome. Where before the victims of backaches and rheumatism were known as whiners and slackers, now we have more than 300 pain clinics to attend to them. Where before pain was a diffuse and ambiguous symptom, we now consider it a disease in itself, and there is a new profession -- algology -- to go with the disease.

The American Association of Algology, founded in 1983, is compeer to the International Association for the Study of Pain (1974) and the American Pain Society (1977). Algologists believe that the lasting pains from lower-back disorders, migraines, headaches and arthritis afflict as many as a third of all Americans and cost the nation $60 billion a year in lost productivity. Dr. Steven F. Brena in 1978 called chronic pain "America's hidden epidemic."

The legal awareness of pain has been sharpened as well. Juries in workmen's compensation and product liability cases tend to award more money to plaintiffs for their pain than for their initial injuries. The courts have become more concerned with and more sensitive to divorce appeals based upon mental and physical cruelty. There are refuges for battered women and legal battles to restore their rights.

The avant garde has adopted the ordeal as "hardship art": a man carrying a cross from Los Angeles to San Diego; Tehching Hsieh living alone in a barred cell for a year without radio, television, reading materials or talk (1978-79); Hsieh and performance artist Linda Montano binding themselves together at the waist by an eight-foot rope from July 4, 1983 to July 4, 1984, pledging never to touch one another. When released, Montano confirmed art-as-ordeal: "What this piece has done, for me, is to exhibit what I need to purify."

The medieval trial-by-ordeal, which placed people in physical jeopardy to determine their innocence, their purity and their virtue, has been taken up again in this new age. Trial by red-hot iron, for instance, has been translated into today's popular talent for walking on hot coals to test one's psychological wholeness and spiritual powers.

This is dangerous stuff. The ordeal tends toward the egocentric (my pain cannot ennoble you) and the reliance on fiat (my endurance entitles me to lay down the law for you). It may be healthy to acknowledge rather than to repress pain, but we must keep in mind that all torturers desire to convince their victims that they themselves are responsible for their own pain, and that their ordeal is their own doing.

The more we come to believe that pain makes us better people, the more we are likely to accede to such a brutal logic and the more we will look to pain for the answers that should be forthcoming without it.