Jane Fonda owes a debt of gratitude to Dr. Kenneth W. Cooper.

So, for that matter, do Cher, Cindy Crawford, Susan Powter, Nike, Reebok and countless other entities that have gained fame and a fortune courtesy of the aerobic fitness craze.

Cooper is the father of aerobics. And this month he's celebrating the 25th anniversary of the book he wrote, titled, of course, "Aerobics," and the brand of endurance exercise it described.

Aerobics started out as little more than a word, a term Cooper coined to describe a series of 31 endurance exercises outlined in one of the chapters in his book. He added an "s" to the word aerobic, which means to live in oxygen. To his surprise and initial disappointment, the book's publisher decided that aerobics should become the title of the book.

"I disagreed," he says. "I thought that was a terrible title. I thought no one would remember it. Boy was I mistaken." In fact, he admits now, when he saw how the word was catching on, he unsuccessfully tried to get a copyright for it.

"I couldn't get control of it because it was too generic, too close to the word `aerobic,' " he says. But there are no hard feelings - after all, he's more than accomplished what he set out to do.

Dedicated to aerobics

In the quarter-century since "Aerobics" was published and earned a spot on the New York Times bestseller list (Bantam Books has since brought it out in paperback), Cooper has devoted his career to studying the concept of fitness through aerobic exercise.

In 1970 he established the now-renowned Cooper Aerobics Center, a facility on a 30-acre site in Dallas. The center - actually several centers - includes a health club, professional fitness training, a business-consultation operation, and a fitness and medical research laboratory.

Still, even Cooper sometimes finds himself incredulous at the popularity of aerobics.

His original plan was to become an ophthalmologist, but that fell by the wayside when, during medical school, he put himself on a physical fitness regimen and was impressed by how much it improved his health. So instead, he entered Harvard, completing a doctorate in exercise physiology, and writing his book. He served a stint with the U.S. Air Force, where he was in charge of astronaut fitness, before he opened the Cooper Aerobics Center, then a two-room office in Dallas.

"I figured if I could sell a copy of my book to my mom and dad I'd be lucky," he says.

What he didn't figure on was that aerobics would turn into a career for himself, and practically a way of life for millions of other people.

Try to find a health club or even adult community center that doesn't offer an array of aerobics classes - low-impact, high-impact, fat-burning, step. There's aerobic dance, aerobic walking, even boxing aerobics.

Cooper's legacy

But the Cooper legacy extends outside the U.S. In 1986, the word aerobics became part of the official English language when the Oxford English Dictionary added it to the vocabulary. In Brazil, running is called "Coopering;" in Hungary, the "Coopertesz" is the national fitness test.

Still, it hasn't always been a smooth run on a treadmill for Cooper. His concept of aerobic exercise was heavily criticized in the early days by physicians who said streets would be "littered" with dead joggers. In fact, a study in last week's New England Journal of Medicine found that strenuous exercise triggers up to 60,000 U.S. heart attacks a year.

Now his exercise philosophy is more moderate: "If you run more than 15 miles a week, you're overdoing it."

That doesn't mean Cooper has slowed down.

"I have not missed a day of work because of illness since 1956," he boasts.