The fitness gospel preached by a pro from the past, combined with a method of conquering muscle pain. Prudden, the Jane Fonda of the 1960's exercise boom, has surfaced again at 72, this time directing her message with the same old breezy zeal to an audience she calls the last ""naturally fit Americans,"" i.e., those who grew up before children were driven everywhere and sat out their leisure in front of TV sets. Prudden's concern is more than fitness, however--it's promoting myotherapy. This is her name for a method she developed to cope with muscular pain, which afflicts so many of the aging, herself included: finding the ""trigger points"" that send muscles into spasm, then applying strong pressure, using knuckles, elbows or a gadget she calls a ""bodo."" There are trained Prudden Myotherapists, but the book teaches people to treat themselves or each other. Pain charts, diagrams, photographs and text explain the process. According to Prudden, 95% of all pain can disappear when myotherapy is combined with corrective exercise. There's something for everybody in the pick-and-choose fitness program. The exercises are vintage Prudden--for every problem and all parts of the body, even some to do in chairs and in bed. Yes to fitness for all ages. But a big question mark for do-it-yourself pain diagnosis and treatment.