WHO WOULD HAVE GUESSED, a few years ago, that out of a vast expanding waist land of fast food fanatics and television couch jockeys would emerge a new breed: Fitness Man and Woman? With a penchant for painful workouts that would have made the Puritans proud, an army of Americans arose, apparently eager to pay any price to achieve lung power, taut muscles, rockhard stomachs and arms stong enough to stop two speeding locomotive from colliding.

Now there's some evidence, judging from the demand for books and classes based on the methods of F.M. Alexander or Dr. Moshe Feldenkrais, that the jog-jump-do-sit-ups-till-your-eyes-water-and-your-back-muscles-say-howdy school is finally in disfavor. We are slowly discovering a refreshing fitness truth: Poise, freedom and grace are not achieved through pain and mindless movements.

Myths about muscles and fitness are falling by the wayside. Who really believes that total fitness is possible by exercising only a few "killer" minutes a week? That exercise must hurt to help? That yoga or aerobics or isometrics alone is the "perfect" exercise? (Yoga offers no aerobic benefits, as was outlined by Dr. Allan Ryan at the 1980 President's Council for Fitness and Sports White House Symposium; aerobics fails to increase flexibility -- true lengthening of the muscles; isometrics builds strength in isolated muscles only and can impede cardiovascular flow.) Who is still wedded to the notion that we are stuck forever with flat feet, bowed legs, bad back, dowager's hump? (We're not. Movement change can produce lasting body shape change.) Does anyone now not realize that you can't spot exercise one part of the body without the body compensating in another part? (Doing 50 sit-ups will not help your "stomach" if your posture is poor to begin with.) We know that hard muscles are not the same thing as toned muscles, that looking fit doesn't always mean being fit; that double leg lifts, straight-leg sit-ups and forceful bouncing stretches hurt because they are harmful to our muscles and ligaments.

In the midst of sometime conflicting advice from fitness gurus we have come to recognize a responsibility to find out for ourselves what is or isn't fit for our own unique body.

In 1888, F.M. Alexander was a young, promising Australian stage actor until his voice began to crack during performances. In time, speech failed him completely. Doctors could find neither cause nor cure. Desperate, Alexander undertook his own investigation, beginning by observing himself in a mirror . . . how he moved, how he carried his head and body. He came to the conclusion that by creating constant tension around his vocal chords, through a series of unconscious, faulty movements of his head and neck, he was actually blocking his own speech. By adjusting his movements he was able to cure his voice problem. Alexander never returned to the stage. Instead he devoted his life to developing a principle based on his discovery of the relationship between head, neck and torso. He called this relationship, "primary control" and it became the cornerstone of the Alexander Technique. Aldous Huxley, John Dewey, George Bernard Shaw and others credited the technique with prolonging their lives and in Huxley's case ridding him of an illness-plagued existence.

Wilfred Barlow's The Alexander Technique (Warner, $10), Sarah Baker's The Revolutionary Way To Use Your Body For Total Energy -- The Alexander Technique (Bantam, $3.95), and Michael Gelb's Body Learning -- An Introduction To The Alexander Technique (Delilah/Putnam, $12.95) are limited somewhat in their attempt to convey Alexander's deceptively simply message: "There are certain ways of using your body which are better than certain other ways; that when you reject these better ways of using your body, your functioning will begin to suffer in some important respects; that it is useful to assess other people by the way they use themselves."

Barlow, editor the Alexander Journal, uses a series of X-rays and time-motion photographs to illustrate how a lifetime of misuse distorts the muscles, bones and tissues, leaving us in a constant state of contraction, tension and pervading disease. Baker is the only one of the three who thinks the technique can be learned and practiced from a book; Barlow and Gelb feel this is impossible unless one experiences it first with a licensed teacher.

What is the Alexander Technique exactly? Dr. F. Jones, former director of the Tufts Institute for Psychological Research calls it "a means for changing stereotyped response patterns by the inhibition of certain postural sets and a method for expanding consciousness." Gelb prefers the definition of Leo Stein (Gertrude's brother): "It is a method for keeping your eye on the ball applied to life."

The body work created and developed by Dr. Moshe Feldenkrais is also grounded in movement as the main means to increase sensory awareness and renew health. Feldenkrais, an Israeli therapist and physicist, has taught his technique to teachers, individuals and groups around the world. In Awareness Through Movement (Harper & Row, $9.95) he contends that a lifetime of moving incorrectly results in a body drawn together trying to protect itself from the difficulty. Light and easy movements are good ones; any unnecessary efforts accompanying an action creates stress that can shorten the spine. The 12 lessons in Awareness are gentle and mild, designed to improve movement ability. The point: it is better to direct our will power to improving ability rather than attempting sheer effort for its own sake. Actions should be carried out easily giving us an understanding of how the body works.

The Body Reveals -- An Illustrated Guide to the Psychology of the Body by Ron Kurtz and Hector Prestera, M.D. (Harper & Row, $6.95) and The Protean Body -- A Rolfer's View of Human Flexibility by Don Johnson (Harper & Row, $5.95) share goals similar to those of Alexander and Feldenkrais but with an emphasis on quite a different method. Structural Integration (Rolfing) as developed by Ida Rolf is done on the body by a Rolfing practitioner. Using his fingers, knuckles and elbows, the practitioner stretches muscle bundles "that have become stuck together," stretches and moves the fascial tissue which surrounds the muscle. If that sounds painful, you're right; it is. Rolfers claim dramatic results from this controversial approach to fitness -- both emotional and physical. The goal: to realign the structure of the body and integrate the myo-fascial system. Rolfing is not a method one can practice at home!

Fourteen Days To A Wellness Lifestyle by Dr. Donald Ardell (Whatever Publishing Inc, $6.95) succeeds, despite an off-putting title, in focusing our attention on what fitness/wellness really involves. Simple basic questions that might prove tough: Are you aware of your own resting pulse rate? Your target heart rate for optimal aerobic conditioning? Your heart recovery rate, vital capacity or percent of your body fat relative to lean muscle? Can you recall an instance in the past week when you used deep breathing or progressive muscle control? Like Alexander and Fedlenkrais, he avers you do not have to grow a new body, just reshape the old one.

In The Joy of Feeling Fit (Avon, $4.95) Nicholas Kounovsky makes the point that one reason the average person knows so little about how he moves is that he "seems" to function well without supervision; he calls for help only when something is obviously amiss. By the time you feel those aches and pains, says Kounovsky, you are already in trouble. Kounovsky, a former gymnast and fitness instructor and a favorite of the jet set, offers a basic program with clear exercise instruction. Warnings of the limitations of relying on just one activity -- jogging, yoga, weight lifting, ballet, isometrics, sports, gymnastics -- are worth heeding.

Orthotherapy (Evans, $8.95) by Arthur Michele M.D. gives a red alert to the buzz phrase, "growing pains." Such pains, says Michele, are nature's earliest signals of postural distress. His conclusion, based on 35 years of medical practice, clinical research and teaching, is that pain is not normal, but your body's cry for help.

Aerobic Dancing (Rawson, Wade, $9.95) by Jack Sorenson, and Jazz-Ercise (Bantam, $2.50) by Judi Missett and Dona Meilach are heart-pumping books. Some exercises recommended are great for cardiovascular fitness but are not substitutes for a total fitness program. If you are not familiar with music-oriented approaches, you may find the dance steps as outlined by Sorenson and Missett confusing.Tape recording the instructions against a background of suggested music may make the books more effective. Morehouse offers good advice: "You don't have to kill yourself to be a winner," and "extra effort" may hold you back. He urges "imaging" -- forming in the mind's eye the physical success you are trying to achieve. He also warns about the peril of skipping either the warming-up process (making muscles malleable and therefore stretchable without tearing musle fibers from tendons) or the cool-down (exercises to soothe those reflexes that say "that's enough").

Bodyworks-The Kids' Guide to Food and Physical Fitness by Carol Burshad and Deobrah Bernik (Random House, $5.95) is full of fun fitness facts. Would you guess that dog food is the most nutritious on a list of chocolate pudding, cola, chicken noodle soup and pop rocks? Or, that, according to the American Seating Company, American bottoms have expanded nearly two inches in the last 30 years? Graphics are designed to keep kids' interest perked, but an unfortunate cover art depicts poorly executed back bends -- a mistake for young bodies that could lead to disc problems or arthiritis of the spine.

Women and their specific fitness needs are discussed in Total Sexual Fitness Ofr Women (Rawson, Wade, $14.95) by Kathryn Lance with Maria Agardy; SuperSex (Dell, $2.95) by Mary Ann Crenshaw and Nicholas Kounovsky; Post-Mastectomy Exercies (Bobbs-Merrill, $9.95) by Carol Walter with Lenore Miller. Total Fitness and SuperSex focus on exercises to tighten the pelvic muscles for "better sex." The point should be made that when the whole body is alive and fit "super sex" need not be confined to the pelvic area. Crenshaw adopts a chatty tone sometimes bordering on the ludicrous -- she warns readers not to do backflips off the bed -- but it makes for lively reading and Kounovsky's exercises are basic ones. Moving Free addresses problems of how to heal physically and mentally after a mastectomy. A series of helpful questions and answers after each exercise lets the individual judge her own response and progress.

Jane Fonda's Workout Book (Simon and Schuster, $15.95) is a striking presentation with attractive models (one of them is Fonda's step-mother, Shirlee) but bad advice. She gives high marks to an exercise class that made her so sore that "for the next three days getting into a car was an ordeal." Going for "the burn" when you exercise, as Fonda reommends, is harmful. Basic movement precautions should not be ignored: bouncing can rip muscles; stretching should not be done until after warm up; and, unless you know how, forgo "the plough" -- a yoga move in which the neck can be seriously injured. Unless one is long-muscled (as is Fonda) many of these exercises will increase muscle bulk, a result that may not be desirable. Good advice does surface: Committ yourself to an exercise class, go regularly and don't compare your progress to anyone else's.

Diana Nyad's Basic Training for Women (Harmony/Crown, $13.95) is detailed with exact directions and photos for carrying out the exercises.However, she also touts exercises that produce "sorness in muscles, tendons, ligaments and joints" -- all aspects of "good pain," says Nyad. Many of these exercises are designed to build strength far beyond the need of anyone, man or woman, not in training for combat readiness. You may decide that violent exercising and "super fitness" is not what you're after.

All these books do agree on one thing: It is never too late to get started on an intelligent fitness program.Long, lithe muscles, a lighter, balanced body, grace and naturalness -- no overdoing, no underdoing -- can begin with your next basic movement. Then we may all reach the goal of fitness expressed by Nicholas Kounovsky, who said he wanted to "die young -- as late as possible."