OLIVIA SIEMENS, in a shiny blue Speedo swimsuit and with goggles pinching her forehead, climbed out of the pool and draped a towel over her shoulders. Before she walked (stiffly) toward the women? locker room at the Koret Center at the University of San Francisco, she complained: ?wimming is just about the only exercise I can do these days. I? a fashion victim ?the fashion of aerobics.?/p>

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Hiroko Masuike for The New York Times

Frederick Schjang, above, has osteoarthritis; he taught aerobics for years.

Mrs. Siemens, 54, a jewelry designer in Berkeley, Calif., said she often took as many as six aerobics classes a week in the ?0s when, she said, ?erobics was new and everywhere and my friends and I all did it with a vengeance.?Sipping a drink from Jamba Juice, she added: ? was jogging in place with Jane Fonda. I did my jumping jacks and high knee lifts with Richard Simmons. I twirled my arms and punched the sky while hopping on one foot to the music of Olivia Newton-John. It was supposedly all about staying in shape, but look at me: I can hardly walk.?/p>

Mrs. Siemens isn? the only possible casualty of the early aerobics craze that took millions of Americans to group exercise classes for the first time. The hordes came, believing that nonstop jumping, kicking and running in place to (bad) throbbing music was the ideal way to raise one? pulse.

? was on the concrete floors in bad tennis shoes jumping with everyone else,?said Jay Blahnik, a spokesman for the IDEA Health & Fitness Association, a trade group. Mr. Blahnik, 38, now teaches rowing, running and cycling in Orange County, but he spent a dozen years leading aerobics classes. ? lot of people doing aerobics back then can no longer do any jumping whatsoever,?he said. ?hey have problems with their backs, feet and hips.?/p>

Some of the damage is severe. ?t? not uncommon for us to see acute and overuse injuries from high-impact aerobics,?said Dr. Jordan Metzl, a sports medicine specialist at the Hospital for Special Surgery in New York. ?t? part of the reason that aerobics classes are on the wane.?

Indeed, if current trends continue, aerobics will be as rare as, for example, those vibrating belts that were supposed to jiggle away fatty hips and gravity boots that were supposed to ?what was it they were supposed to do? For now, the popularity of aerobics is sharply down from when it was ?he mainstay of fitness in America,?said Mike May, a spokesman for the Sporting Goods Manufacturers Association.

It? why you may have noticed ?if you have shown up at your gym attired in your best leg warmers with a sweatshirt off one shoulder ?the lack of aerobics classes on the menu. Fewer than half of the 300 gyms and health clubs recently surveyed by IDEA offered aerobics classes, a number that is ?ontinuing to decline,?according to the summation of the report.

At its peak in the mid-?0s, an estimated 17 million to 20 million did aerobics, Mr. May said. But only five million did in 2005, according to a report by the sporting goods association. ?e expect the 2006 numbers to be significantly lower,?Mr. May said. ?erobics are increasingly out of favor.?

The legacy of injuries is one reason. Many of the original instructors like Mr. Blahnik won? teach aerobics ?because they can?. ?hose hardest hit by all those aerobics were often the teachers, because they were pushing harder than anyone else and doing the classes a dozen times a week,?Dr. Metzl said. ?ur bodies just weren? meant to withstand all that pounding.?/p>

Another reason for the decline of aerobics is that fitness has become more sophisticated, so some classes are hybrids that work the body and the mind; others offer calorie burn while minimizing wear and tear; still others alternate hard bursts with easy intervals.

?uch of the decline in aerobics has come as a result of new and innovative classes and techniques,?said Kathie Davis, the executive director of IDEA.

Dr. Kenneth Cooper, a former Air Force flight surgeon, first coined the term in the title of his 1968 book. But by ?erobics,?Dr. Cooper meant cardiovascular exercise. The name was later appropriated by the aerobics movement popularized by Jane Fonda in the late 1970s and early 1980s.