Swimming, say the arthritis experts, may be the best kind of exercise for arthritis.

The buoyancy of the water protects the joints.

Swimming is a smooth exercise as opposed to jerky ones like jogging or jumping or squash.

And swimming exercises muscles in opposition to each other--evenly, gently.

Swimming protects arthritic joints against "freezing," that is, becoming immobile, and it keeps muscles from contracting into the characteristic rheumatoid arthritis (RA) deformities. But even though this has been known for some years, the use of swimming in arthritis management has been dependent on the availability of a properly heated pool--a little warmer for the arthritics than for long-distance swimmer Diana Nyad, for example. (That has generally meant limiting swim-for-your-joints to rich arthritics with heated pools and others who happen to live in Sun Belt states.)

Susan Klein, who was an obstetrics and critical-care nurse--and sometime "jock"--came up against this problem in 1978 when, at age 25, she was diagnosed with rheumatoid arthritis.

One of some 6 1/2 million young people between the ages of 18 and 45 who are afflicted with RA, Klein had to give up her strenuous career--she is now a labor coach for expectant parents. She was told by her doctor that "swimming was the best thing I could do," after "I found that all of a sudden all my athletic outlets had to be halted." She had been an avid tennis player and all-around sports enthusiast.

She investigated resources for swimming and found "really no aquatic programs that were appropriate. There were none, even for senior citizens with osteoarthritis much less for young adults with RA. So I set in my heels to set up a program . . ."

Klein interested the Metropolitan Washington Chapter of the Arthritis Foundation, which in turn interested its own medical board of rheumatologists, arthritis specialists and therapists. They designed a program and recently invited swimming instructors from all over the Washington area to a training session in leading exercise classes for arthritics.

Some 40 instructors from private and public recreation centers attended the session and several have already instituted programs. Klein says she also has had queries from groups as far apart as Maine and Texas expressing interest in expanding the aquatics program.

Barbara Patrissi, the Washington-area program director for the Arthritis Foundation, a nurse who participated in devising the program, says that its recreation and socialization aspects are as important as the exercises and the improved self-image they give.

"We don't guarantee improving or even maintaining joint mobility," she says, "although there does seem to be more ability to move the joints freely, and this should help with pain."

It's never really been researched correctly, but as more chapters adopt similar programs, she says, "We could statistically measure improvements of muscles and mobility."

So far, the association has published guidelines for instructors. Guidelines for arthritic swimmers themselves are being prepared now.

"The most important thing to remember," says Patrissi, is that "arthritics should never push against pain. On the other hand, you should go as close as you can to the point of pain. In this way people often find that they will increase their tolerance level, and after a while they may be able to go further.

"Each person needs to take the primary responsibility, knowing where the pain level is. You must move to that point to get something out of the exercises, but you have to know to stop when it hurts."

Programs for arthritics will be "slow movement. We're not into aerobics," says Patrissi. "This is not exercise for the heart and lungs, although some may get to that point. But this is slow, steady, smooth movement."

The guidelines will also include recommendations that people participate in classes--both so that instructors can teach them the correct way to do exercises and for the sociability.

Says Susan Klein: "You always think you're the only one who has it, and it affects every aspect of your life. This gets people together who have something in common."

Klein, who is spearheading the upper Montgomery County programs, developed a support group out of the swimming classes. The group meets in her house and, she says, "We've found a good exchange of ideas on how to cope with everyday life. Currently we are concerned with arthritics with young children Klein has a 3-year-old , and one member of the class asked me how I tie my daughter's shoes. To us something like that is a major challenge."

The swimming programs are geared to all sorts of chronic diseases involving muscles, joints or cartilage in all age groups, including children with muscular dystrophy or juvenile arthritis.

For information about programs now operating in the Washington area, phone the Metropolitan Washington Chapter of the Arthritis Foundation, 331-7395, or write to: 2424 Pennsylvania Ave. NW, Room 105, Washington D.C. 20037.