Bad backs. Sometimes it seems that everybody has one.

Sen. Ted Kennedy has a back problem. So does Elizabeth Taylor. So do Wisconsin Rep. Robert Kastenmeier and Kentucky Rep. Tim Lee Carter, himself a doctor.

So do former Redskin cornerback Pat Fischer, who retired prior to this season, and Bullet guard Phil Chenier, who missed most of last season and recently underwent surgery. The reason: badly ruptured disks.

So does slugger Reggie Jackson, who was hospitalized with back spasms just before the last days of the baseball season, when he played in pain to help lead the New York Yankees to a pennant.

Twenty million Americans have serious back troubles, just counting those who will seek treatment this year. Two hundred thousand of them will undergo surgey.

Many millions more - half the entire United States population, by one estimate - have either suffered a back problem at one time or someday will.

The number of Americans with ailing backs, it seems certain, is growing, and doing so strangely, because of two completely opposite American trends.

One trend is inactivity, which some say became epidemic starting with the invention of the horseless carriage. Inactivity breeds muscle weakness and that breeds troubled backs.

The other, more recent trend is toward activity, not mainly regular and moderate activity as part of daily living, but tense, frantic and, worst of all, sporadic activity. Jogging. Running. Skiing. Fiercely competitive handball. And Sunday tennis with the American twist serve.

All these and more generally are pursued without adequate physical conditioning or preexercise warmup. Tense, weak muscles subjected to sudden activity produce physical troubles, including hurt backs.

Bad backs have many other causes. They range from outright injuries like fractures, strains and sprains - there are 400,000 such disabling back injuries each year in industry alone - to condition like aging and pregnancy.

The so-called "normal processes" of aging can affect anyone from the 35-year-old father who wrestles with his son and then can't stand up, to the older woman with osteoporosis, or of calcium from her bones.

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Osteoarthritis ("grandpa's rheumatism") and rheumatoid arthritis both can cause back agonies. So can undetected cancer or tuberculosis.

So can the changes and strains of pregnancy, child-bearing and child-rearing, especially when combined with overweak and unprepared abdominal muscles.

So can diseases of the heart, lungs, stomach, kidney, gall bladder, colon or prostate gland, or the ovaries and fallopian tubes. So can a mechanically unstable spine, a short leg, or other malformations.

So can anyone's excess weight, poor posture or a sofe mattress. And so can the intense emotional tension and stress of modern life.

Little wonder, then, that the bad back has variously been called "one of the curses of modern man," "the special pain of the late 20th century," the "epidemic plague" of our time and the ulcer's replacement as "the badge of high-pressure living."

For one reason or another, says Dr. Larry Horowitz - Sen. Kennedy's chief health aide, thus one of the best-known physicians on Capitol Hill - "It's one of the commonest complaints here by far. People are always asking me about medical problems, but I get more inquiries about this one than anything else."

"There are so many peole with back problems," says Rep. Kastenmeir. "It's sort of like asking, 'So what else is new?'"

The questions asked by the usual bad-back sufferer - not the victim of some underlying disease or unusual cause, just the sufferer from plain back pain or strain - are: "What happened? Why me?"

What happened, first of all, is that five million years ago some ape stood up. This meant that his weight was no longer spread, more or less evenly, along the whole spine, but instead became unevenly concentrated. This guaranteed that as the straight ape, or straight ape's descendants, aged or over-did, there would be trouble.

The human spine, late 20th-century version, has nonetheless become a marvelously balanced set of bones set one atop the other, something like children's blocks. Each of these blocks, or vertebra, rests on the one below at a seemingly precarious angle to no matter how well-conditioned, be-make of the entire stack a gentle, double-curved "S."

The flexible "S" shape ordinarily weight centers. And this crucial shape is maintained by a complex set of guy wires: the strong muscles, tendons and ligaments that join the vertebrae one to the other and help them withstand the huge forces that pull on them when you do something so simple, say, as bend 60 degrees to snatch an object off a low table.

If you weigh 180 pounds, the muscle force needed to keep you from topling over as you do this - the overall force needed to keep your spine steady - will measure 450 pounds. If you pick up a 50-pound object from the table, one key vertebra - "L5," the fifth lumbar vertebra at the particularly vulner-maintains body balance and distributes the burden of the body's uneven able base of the spine - must withstand a phenomenal 850-pound pull.

Everyday pressures like these, repeated again and again along the entire spinal stack, prompt one of the nation's leading back doctors, Dr. Lawrence Friedman, to say, "It may seem like a miracle that we can stand up, an even greater one that we can lift and carry any weights at all."

The main problem today is that people pile more pressures still on that hard-working spine. They neglect it, abuse it and subject it to an over-whelming combination of physical and emotional tension.

When spinal-column victims are professional athletes - the Phil Cheniers, Pat Fischers and Darryl Stingleys - the abuses are the endless twists, turns, leaps and parring shocks of unrelenting, hardball competition and ceaseless demands on bodies that, gin to lose the flexibility of youth. Stingley, the New England Patriot wide receiver, is in an Illinois hospital largely paralyzed because of a headon collision with an Oakland Raider.

Far more often, of course, sufferes are just the Sunday athletes or parlor quarterbacks, the pencil-Pushing, auto-driving, sitting, stay-at-home, TV-watching sedentary bulk of civilized humanity. The main trouble is that they are are weak.

In the mid-1940S, Drs. Hans Kraus, Barbara Stimson, Sonja Weber and colleagues examined 5,0000 consecutive patients with back pain. At two major New York City institutions - Columbia-Presbyterian Hospital and the New York University Institute for Physical Medicine and Rehabilitation - they completed the most thorough study ever made of the causes of back trouble.

They found that:

In only one case in 10 was there any physical problem or defect or disease causing the pain.

In eight cases in 10 - 81 percent to be precise - the basic cause was muscle weakness or tension. Or both.

The result, in Kraus' words, might sometimes occur high in the spinal column: "horrible stiff neck." It might sometimes be "stabbing pain between the shoulder blades," in the mid-back. And "worst of all," and most common of all, it might be low-back pain.

Why so much muscle weakness? In Kraus' phrase, "underexercise," which he calls "the most serious threat to the health of Americans." This is still true today despite the encouraging new concern of some, at least, with physical fitness.

The spine depends on its guy wires - the abdominal, hip, thigh, buttock and back muscles - to support it. Several years ago, Dr. Hardin Jones of the University of California showed that when muscles are underexercised, they require progressively less blood and other nourishment. Their tiny capillary blood vessels begin shriveling. By age 35, Jones found, the average man's blood flow is 60 percent less than during his teens and the process continues. And unused muscles will shrink or atrophy.

Ad nervious irritation to your sedentary life," summed up Dr. Kraus, "and the back muscles will get tense and stiff. Finally, when they are unable to stand the strain, they will rebel by going into spasm."

"Spasm" means involuntary contraction, tightness, tension.

"Muscle tension is a normal biological response to emotional tension," said Dr. Friedman and Lawrence Galton in their book, "Freedom from Backaches." "A chronically tensed muscle may lose its stretch and become shortened."

What we have then in the modern, upright, uptight man are not only weak muscles but taut ones. This is a prescription for trouble.

Tuesday: Treatment