With high blood pressure and a family history of heart problems, Max Rosenfeld decided to start swimming in 1980, the year he reached 62 and retired as principal of Clark Elementary School in the District.

"I saw all these people out walking after a stroke or heart attack and I thought, 'Why wait until it hits?' " he recalls. "I went very hesitantly. One of the big problems is just getting out there.

"You have all kinds of reservations -- you'r going to be slow and people will laugh at you. At first I was puffing and panting and wheezing and splashing and I could barely swim to the other side."

But Rosenfeld persevered, swimming three times a week. Over several years, he read fitness books and monitored his heart rate, making sure it beat no more than 152 times per minute during exercise -- a maximum workout heart rate he calculated using the standard formula of "220 minus your age."

Four days after his 68th birthday last year he swam a mile -- 72 lengths of the pool. His exercise heart rate went up to 160 beats per minute -- just slightly above his maximum heart rate of 152.

Although he is exhilarated by his accomplishment, Rosenfeld still has questions about exercise and the older body.

"At my age, being able to swim three-fourths of a mile three times a week, does that put me in a different category than the guy who walks three times a week -- who doesn't work as hard as I do?" he wonders. "Will I kill myself going up to 100 percent of my (maximum) heart rate?"

Like Rosenfeld, many people past the mid-century point who want to be fit have unanswered questions. The current fitness boom tends to focus on exercisers under 30 with pulse rates under 50, leaving the older exerciser feeling left out.

"I used to do 'Lilia's Yoga' on TV, but they replaced it with 'The Body Electric' -- a fast paced aerobics with flashing lights and girls in leotards," says Jack Hughes, 65, Legislative Chair of the Alexandria Council on Aging.

"I think the hyping of exercise by the media with pictures of beautiful girls on machines and jumping around may put older people at a disadvantage. You feel like your joints have to be able to take all that pounding."

As a result of this focus on youthful fitness, the "fit after 50" set is often faced with a dilemma. Should they err on the side of over-enthusiasm and plunge into a fitness program, ignoring stress tests, heart rate and percentage of body weight in fat? Or should they take an ultra-conservative route and walk slowly -- unable to forget that Jim Fixx was just over 50 when he died while running?

Most important, what can the growing numbers of 50-plus population expect to gain from exercise and to lose with aging?

What they should not believe, says Dr. Edward L. Schneider, deputy director of the National Institute on Aging, is the popular myth that exercise extends life.

There is "no relation between a history of athletic competition and longevity" in humans, Schneider says. While "studies in animals show a fairly consistent life-prolonging effect of a regular exercise regimen if initiated in early life . . . athletic competition over a couple of decades may not be sufficient to influence longevity."

At the 1985 International Congress of Gerontology, Kentucky gerontologist, Dr. Ernst Jokl also dismissed the life-extension-through-exercise theory.

"The question whether sport exerts an influence upon longevity can be answered briefly and unequivocally," he noted. "Sport exerts no such influence. Neither long-distance running nor sprinting nor weight lifting are capable of prolonging life."

But although exercise won't increase the length of life, many experts agree that it can improve its quality -- partly by slowing some of the effects of aging.

"Notwithstanding the fact that exercise is unable to extend the life span," Jokl says, "it is capable of modifying the aging process."

Both Jokl and Schneider point to evidence showing that exercise has what Schneider calls "beneficial effects on several aging processes, as well as certain age-related disorders, such as cardiovascular disease."

Exercise also can slow the age-related decline in the body's ability to metabolize glucose and its tendency to lose bone.

Dr. Andrew Goldberg, chief of gerontology clinical research at Johns Hopkins University's Longitudinal Studies center in Baltimore, says he has seen exercise help reverse disease processes in older people.

"Chronic renal disease, fro example, has symptoms of anemia, hypertension and glucose intolerance," Goldberg says, "At the end of an exercise program, the subjects still have renal disease, but other parts of the syndrome complex were gone. Many of their problems were't due to the disease, they were due to lack of exercise."

Notes Sharon Plowman, professor of health education at Northern Illinois University: "Much of what we call aging may actually be the result of disease or detraining . . .

"Part of what we call aging, perhaps as much as 50 percent, is actually a correlate of a sedentary life style rather than just some inherent biological clock incessantly ticking away."

In sorting out the differences between age-related decline and sedentary-related decline, some of the most significant findings come from a study concluded last year by Dr. Edward Lakatta, chief of the laboratory of cardiovascular sciences at the NIA Gerontology Research Center.

"What we once thought was that you could only live so long because your heart only maintained its function so long," says Lakatta. "We thought decline was a natural thing and would say, 'Oh, well, that's aging.' "

But Lakatta discovered that "when it is free of disease, the heart of an older person pumps about as well as that of a younger adult. In people who are found to have a decline, something else besides aging per se must be looked for."

At Johns Hopkins, Dr. Goldberg's ongoing investigation of 40 elderly male runners -- all lifelong athletes and many over 70 -- suggests that an extra level of fitness provides a significant boost in many measures of health.

"They have incredible risk factor profiles," says Goldberg. "Compared to their sedentary peers, these athletes have greater aerobic capacity, higher HDLs (a protective form of cholesterol) . . . greater insulin sensitivity, greater glucose tolerance and lower percent body fat. These are all positive factors that are going to help in the aging process."

Exercise, most experts now agree, is helpful to people of all ages and physical conditions -- if they are careful to consult a physician and receive proper instruction. The question for older people is no longer whether to exercise, but how much and how often, what exercises and what precautions.

The "how much" question is tricky, says Dr. T. Franklin Williams, director of the National Institute on Aging. "We don't have precise information. Some exercise is definitely good. A half-hour brisk walk is good for virtually every one except for those with advanced heart disease."

In a much-cited long-term study of 17,000 Harvard alumni, Stanford University's Dr. Ralph Paffenbarger found that the lowest overall death rates were associated with running four to six hours a week (an expenditure of 2,500 to 3,500 calories assuming running is the person's only exercise). Among people who exercise more or less than this, the death rate from all causes rises slightly, including deaths from heart attacks.

Paffengarger found that alumni who climbed five flights of steps a day had a 25 percent lower risk of heart attack than a comparable sendentary population.

Exercise physiologist Jim Hagberg of Washington University in St. Louis has studied the amount of exercise needed to produce positive effects in the elderly. Seven males and four females over 60 exercised for six months at low intensity. The participants were instructed to walk unsupervised for 20 to 30 minutes three times a week, being sure to get their heart rate up to 120 beats per minute. in reality, they walked five times a week for 27 minutes and the average heart rate was 107, which was below the goal but nevertheless increased their maximum lung capacity by 12 percent.

The high-intensity part of the program consisted of supervised stretching, walking and biking three times a week, getting the heart rate to 160 beats per minute. This resulted in an additional 17 percent maximum lung capacity.

Habberg reaffirmed what researchers had postulated more than 20 years ago -- sustained intense exercise at higher heart rates is an important component of a beneficial workout.

Older exercisers, in particular, need to balance their fitness program with all three types of activity -- aerobics, strengthening and flexibility.

"When you just do weight lifting (for strengthening) or calisthenics (for flexibility), and you don't put a good aerobic program with it, it's like giving a car a new coat of paint when what it really needs is an engine overhaul," says Millie Cooper, 50, coauthor with her husband, Dr. Kenneth Cooper, of "Aerobics for Women."

But giving your body -- especially an older one -- aerobic attention without equal attention to other fitness factors, like strength and flexibility, may cancel out the potential positive effects of exercise.

"The myth is that they have to work on their heart rate," says Asta O'Donnell, 45, an Alexandria exercise therapist who has many students aged 50 to 75 who are "target heart rate" fanatics. They concentrate on getting their heart rate to a certain level, but forget their ligaments, tendons and muscles. The result is often injury.

"People can hardly move and they want to work on their heart rate," she says. "Stretching comes first, then you go into an aerobic program -- jumping rope, walking, swimming."

Many of O'Donnell's patients are referred by doctors because, she says, "they didn't know what else to do with them." She focuses on helping these people solve the pain associated with knotted shoulders, soft stomachs and aching backs by careful attention to slowly -- "just a little bit more" -- stretching the supporting muscles.

Activity, particularly weight-bearing exercise like walking and running, can help the body fight bone loss -- estimated at 1 percent a week in the spine and the heel if you did nothing but stay in bed.

"Exercise is very significant in helping to prevent osteoporosis," says Dr. Everett L. Smith, director of the University of Wisconsin's biogerontology laboratory. "We've found that you can design a program to prevent bone loss the same way you can prevent muscle loss. Bone is very responsive to exercise.

"Overall, the studies are showing that through exercise we can decrease bone loss by 80 percent in middle-aged postmenopausal women. Evidently you have to have some kind of weight-bearing (activity) in order to maintain the spine. I would recommend a 30 to 40 minute walking exercise for women over 35 three times a week."

As in exercise for the muscles and heart, exercise to strengthen bones ideally should be varied, too. In Smith's study of 120 sedentary women, ages 25 to 65, exercise physiologists and dance instructors taught different dances, so that over a two-month period the participants never did the same thing twice.

"We think variety is better," says smith. "There's a stress on the joints and bones and more chance of injury if you do the same thing."

In addition, variety often adds spice to a workout. And enjoyment -- the sheer physical pleasure of movement -- is one of the greatest benefits for exercisers of all ages.