Ever since some curious or indignant hominid stood up on hind legs a few million years ago, human beings have had a back problem.

"Our back was designed to be horizontal, with us on all fours," says Steven Herron, director of occupational health for the National Safety Council.

The human back -- 33 vertebrae stacked in a double S-curve -- is a remarkably sturdy and versatile column, but it's hardly built to withstand abuse from the upright posture and sedentary life style of modern man and woman.

A bad back is the second most common ailment in the United States, after headaches. Back injuries are the biggest source of worker's compensation costs to American industry, and they're as likely to occur in office buildings as in foundries and factories.

"You could bend over to pick up a paper clip," Herron says, "and put a thousand-pound-per-square-inch force on your lower vertebrae."

Eight of 10 Americans suffer a back problem sometime in their lives. Only the common cold keeps more people out of work; only sore throats and coughs send more people to medical waiting rooms.

What Time magazine once called the "gross national backache" is the leading cause of restricted physical activity for Americans under 45. Every year back trouble causes some 93 million lost workdays, 18 million visits to the doctor, $14 billion in medical costs and absenteeism.

"Back pain is the common bump in the road of life," says Dr. Stanley Bigos, an orthopedist at the University of Washington School of Medicine, who heads one of the largest studies of industrial back injuries ever undertaken.

"If you're an attorney, it interferes with your tennis or your golf game. If you're a hod carrier, you can't work. But it's the same thing."

Bigos' study, funded by the National Institute of Occupational Safety and Health (NIOSH) and Boeing, is monitoring more than 3,000 aerospace workers at Boeing's assembly plant in Everett, Wash., for five years, in hopes of pinpointing the risk factors for back injury. The study is expected to be completed by 1987.

"Most people just don't realize how vulnerable their backs are," says Dr. Alicia Hastings, a specialist in physical medicine at Howard University Hospital. "They subject those small muscles of the back to repetitive insults."

The most common type of backache is a strain, or insult, of those back muscles, often caused by what doctors call "weekend athlete syndrome." A person who is literally and figuratively out of shape, for example, tries to make up for years of sedentary living in a single weekend.

"Suddenly," says Hastings, "they decide they're going to take up jogging or move pianos or play 18 holes of golf."

By Monday morning, if not sooner, that weekend athlete is hobbled by a severe pain in the lower back.

"When you're out of shape," says Dr. Michael Dennis, chief of neurosurgery at the Washington Hospital Center, "if you start doing exercises your body's not used to, the muscles will rebel."

Warning to gardeners: Weekend athlete syndrome is especially common at this time of year in people who decide to throw off winter and till an entire garden in one spring Saturday afternoon.

Another kind of backache includes searing pain that shoots down the hips and thighs, caused by pressure on the sciatic nerve. That pain may indicate a more serious problem: an injury to the infamous "disk."

The spinal cord and its nerve bundles are protected by the spinal column, a long curve of bony vertebrae surrounded by muscles and ligaments. Between the vertebrae are round, rubbery cushions called disks, which act as shock absorbers and give the back its flexibility.

When the back is stressed, the jelly-like material in the center of the disk sometimes bulges outward and presses against the nerve roots. The result is a herniated or "slipped" disk -- and intense pain, sometimes combined with tingling, numbness and muscle spasms.

The least effort -- brushing your teeth or lifting a skillet off the stove -- becomes excruciating, if not impossible. A cough can be a terrifying experience.

"It's very demoralizing, because you think you're never going to get well," says Dr. Peter Kenmore, chief of the division of orthopedics at Georgetown University Medical Center. "It's frightening. You can't bend down. You can't tie your shoes.

"You feel nature has betrayed you."

The good news, Kenmore says, is that backaches usually go away after a few days, with rest.

"Within a couple of days, no matter what you do, most back pain goes away," he says. That's why the best approach to a sore back begins with what doctors call "conservative" treatment: rest, non-narcotic pain relievers or anti-inflammatory medicines (such as aspirin) and application of heat.

Rest usually allows the offending disk to shrink back to normal size, relieving the painful pressure on the nerve. But if the pain persists, it's probably time to consult a doctor.

In a small percentage of cases, surgery may be necessary because the herniated core never returns to its normal size and position inside the disk. Getting it back inside, says neurosurgeon Dennis, would be like "trying to stuff toothpaste back into a tube."

The two most common back operations for disk problems are:

Laminectomy, or disk removal. About 200,000 patients a year have surgery to remove a disk or disk fragment. Disk removal is sometimes followed by spinal fusion, in which bone chips from the pelvis are inserted as a bridge between adjoining vertebrae. They fuse into a permanent bond, making that section of the spine rigid. Chemonucleolysis, or disk injection. An enzyme called chymopapain, an extract of the papaya plant that is also used in meat tenderizers, is injected into the herniated disk. Chymopapain dissolves the gelatinous core of the disk. Nearly 100,000 Americans have undergone chymopapain disk injection since it was approved by the Food and Drug Administration in 1982. It is less expensive than disk removal (about $4,000, compared with about $10,000) and has a shorter recuperation. The operation remains controversial, however, because of rare but serious complications, including nerve damage and potentially fatal allergic reactions.

Neither operation is a sure-fire cure for back pain. Both have a failure rate approaching 25 percent. No patient should undergo either disk removal or chymopapain injection without careful diagnosis -- and a second opinion. Immediate surgery is warranted only in rare cases where a sudden loss of bowel or bladder control indicates a tumor or nerve damage.

"Of all the people with backaches, less than 1 percent should have surgery," orthopedist Kenmore says.

Back pain is a frustration to doctors as well as patients, because it has innumerable causes: anything from arthritis to tuberculosis, from a sagging mattress to a tumor. Diagnosis and treatment often amount to educated guesses.

For example, take the case of a person who gets a backache after lifting something.

"That's the most common problem," says orthopedist Bigos. "But what we don't know is which comes first, the chicken or the egg. Did the lifting cause the back problem, or did the back problem come first," only to be aggravated by the lifting.

Anything that tenses the back can force the muscles into a clench, which only tires them further, causing knots of pain or an excruciating state of steady contraction known as a spasm.

"The muscles of the back don't know the difference between lifting a 50-pound bag of concrete on the weekend and tensing up in the office because the boss is yelling at you and you've got to meet a contract deadline," neurosurgeon Dennis says.

"Everything has to be individualized," says Dr. Alfred Pavot, chairman of rehabilitation medicine at Greater Southeast Community Hospital. "Every backache is a little different."

A successful individualized treatment program, says Anne Kuntavanish, chief occupational therapist at Greater Southeast, must take into consideration every aspect of a patient's life from job routine to sexual activity.

"A back problem affects the whole family and the whole working environment," Kuntavanish says.

And then there's what Kenmore calls "the emotional factor," which he says comes into play in about 10 percent of cases of acute back pain.

"If one day your boss is riding you and your wife is mad at you," he says, "it is not socially acceptable to say, 'The hell with the world, I'm staying home. I'm going to stay right here in the warmth of my bed.' It is socially acceptable to say, 'I threw my back out.' "

The key to a healthy back is its support system -- the muscles of the back, abdomen and legs -- which act like guy wires and help keep the bones aligned and distribute the huge forces that tend to concentrate on the lower vertebrae.

The way to minimize the risk is to avoid excess weight and keep the muscles, particularly the abdominal muscles, strong.

"If you exercise properly and lift things properly, you could probably eliminate 75 percent of disk problems in the United States," Dennis says.

Even sitting down, especially in a slouch, puts a strain on the lower back, compressing the shock-absorbing disks inside the vertebrae. Sitting strains the lower back much more than walking does.

Sometimes back patients even stand, rather than sit, in the waiting room, Kenmore says, because sitting hurts so much.

The typical back pain patient, experts say, is between 35 and 45, with a sedentary job and life style and a history of come-and-go back trouble that suddenly has worsened. Usually, the patient is at least slightly overweight.

Ten pounds of extra weight on the abdomen puts an extra strain of 100 pounds on the disks of the lower back by exaggerating the normal curve of the spine.

"Think of a 40-year-old man with a beer belly or a woman who's eight months pregnant," Dennis says. "Those are classic examples of mechanical back pain." The extra weight is not borne evenly across the back, but concentrates on the lower back, causing a swayback posture.

Back pain is often treated by unconventional therapies: everything from acupuncture to gravity boots and yoga. Every treatment has its proponents and its critics.

By far the most popular alternative treatments are osteopathic and chiropractic manipulations or "adjustments," applying pressure by hand to move a misaligned bone back into place. There are about 26,000 chiropractors and about 21,500 osteopaths practicing in the United States.

But do such treatments work?

A team from the University of Texas Health Science Center in 1983 reviewed 59 studies of back therapies, including 14 studies of spinal manipulation. Almost all of the studies were flawed, the team reported, but the two best studies of manipulation suggested that it could provide some immediate benefit but "no long-term benefits."

"They can provide symptomatic relief," says neurosurgeon Dennis, who occasionally refers back patients to a chiropractor for therapy, "but it's like taking an aspirin. It doesn't result in any long-term improvement if you don't correct the underlying causes, which usually have to do with life style."

Sorting out what works from what doesn't is especially tricky, notes orthopedist Bigos, when only one in eight back conditions can be definitively diagnosed and most patients get well on their own with rest.

"If you gave me 100 people with back problems, even herniated disks, I'll treat them with hot camel dung behind each ear for six weeks, and I'll have a 70 to 80 percent success rate," Bigos says.

He compares it to the "hot tub effect: It makes you feel better whether you have a bad back or not, but if you do, it doesn't make your back better and it doesn't help you in the future."

"The problem with chiropractic," Dennis says, "is the theory that all disorders are due to spinal maladjustment. The chiropractor may not be attuned to other diseases that mimic back strain, such as cancer of the prostate or cervix."

In such cases, manipulation of the spine can delay medical treatment and worsen the condition.

Most experts offer this advice: Try conservative therapies first, such as rest, weight loss and regular exercise; use surgery only as a last resort; seek medical attention for any severe pain that doesn't go away with rest. And be wary of any practitioner, traditional or untraditional, who tells you there's a quick cure for backache.

"The real experts in back problems," says Bigos, are the ones who know what they don't know. If you've got answers to everything, you're in real trouble in this business. We can't even define the problem in 88 percent of the cases.

"The three most underused words in medicine are, 'I don't know.' "