WHEN CALIFORNIA health club owner Harold Zinkin patented his "chest weight lifting machine" in 1960, he never dreamed that in less than a decade he would sell his company and rights to his machine for $500 million. Nor did he foresee the impact his invention, now known as the Universal Gym Machine, would have on the booming fitness craze of the 1970s.

"I wish we had never sold it," he said. "I feel as if I gave my only child up for adoption."

Zinkin's machine is in 60 per cent of California high schools and colleges and in weightlifting rooms around the world. And the machine has brought attention and controversy to muscle physiology and athletics.

In an increasingly competitive physical fitness market, the Universal Gym Co. and Nautilus Sports Industries of Florida are each claiming "the right way to build strength." Both spend thousands a year on research and endorsements. At the heart of the matter are two different types of muscle fiber - slow twitch and fast twitch - and on these fibers hangs the weight of the gym machine world.

Nautilus believes development of slow-twitch muscle fibers is the high road to athletic achievement; Universal backs fast-twitch - the muscles that athletes actually use on the field.

Medical science is as divided as the competing companies.

A little history:

"The market for a complete system of resistance exercise was terribly limited in the early '60s," said Zinkin, who won an AAU weightlifting title in 1945. "What we did was take the balancing factor out of weightlifting. This made it easier and safer."

Universal machines, which can accommodate up to 16 athletes at once, are structures of tubular bars strung with cables and pulleys attached to block units of weight in the machine. Because the weights are a part of the machine, changing the amount of desired weight means simply placing a peg in a different hole.

But the major innovation is the idea of variable resistance - a leverage-induced restraint based on the weight being lifted relative to the leverage of the weightlifter.

The idea for that wrinkle Zinkin credits in part to his friend and business rival, Arthur Jones of Nautilus. What Jones set out to do was build a machine to exercise entire muscles, not just parts of them. A barbell exercises only about 6 per cent of a muscle.

"It was obvious to me that something was wrong with the way we were exercising," said Jones, 53, who comes from a family of eight physicians. "The problem was in the tool, not the human body."

With the use of chains, cams and sprockets, Jones built a machine that gave both direct and rotary resistance to the athlete who trained slowly - "to train the muscle properly in its weakest position." Claiming that his machine used up to 92 per cent of a muscle, Jones marketed it in 1970.

Universal was then 13 years old, but the machines had not incorporated the variable resistance idea. Zinkin, who now serves as a consultant to Universal, said his company began computer testing to figure how to get variable resistance so there was no inertia or excess weight caused by the cams, sprockets and chains. Their answer came in 1974 when they marketed a machine with "dynamic variable resistance," allowing athletes to train fast and explosively, the way they perform on fields and courts.

But rival Jones calls fast and explosive training poppycock.

"To train explozively is criminal malpractice and you can quote me on that," said Jones.

"That's because you can't train fast on a Nautilus machine," said Dr. Gideon Ariel, director of research at the COmputerized Biomechanical Analysis Inc., a firm in Amherst, Mass., that analyzes human movement through cinematography and computers."If you work fast on the Nautilus machine . . . the weights will do the work for you."

Which philosophy is correct - to train dynamically because you play dynamically or to train slowly because it is more thorough? That is a question both companies answer with "ours." But the controversy is really within the human muscle itself, involving the scrutiny of the fast-twitch (white) and slow-twitch (red) fibers.

Dr. James Counsilman, the Indiana University swimming coach who guided Mark Spitz, contends that fast-twitch fiber development is necessary for fast athletic activity and slow-twitch development is necessary for slower athletic activity.

Dr. John P. Kalas, chief of pathology at West Volusia County Hospital in Florida, classifies training fast as when the repetitions exceed the muscle's ability to contract. He backs Nautilus' deliberate repetitious. In January's Athletic Journal Kalas wrote:

"Yanking muscles does not build strength. Instead, it exposes the joints, muscles, and connective tissue to danger. Since acceleration increases force and force is the primary cause of injury, slow, deliberate exercise style (about one to two seconds on the lifting portion of the movement and slightly longer on the lowering portion) is best for supplemental training."

Ariel, who has a Ph.D in exercise science and postdoctorate training in computer science, supports the dynamic training theory.

"In athletics you are utilizing multiple joints and you are using the muscle in a ballistic manner," he said. "So an athlete who throws the shot is using his fast-twitch muscles ballistically and should train the same way." Ariel also suggested that by training dynamically an athlete may convert slow-twitch fibers ot fast-twitch fibers.

Many authorities feel that neither the companies nor the doctors can claim a correct way of training. One of the most respected men in the field, Dr. Reggie Edgerton of UCLA's kinesiology department has spent the last eight years studying the slow-twitch/fast-twitch muscle phenomenon and insists that any claim to the "correct way" is premature.

"It is ridiculous for any company to claim that their training reacts to a certain type of fiber," said Edgerton. "They simply do not have this information. The arguments are academic, and it's probably not an either/or situation anyway."

For now the battle is being waged with theories. The older company, Universal, seems to be the leader, doing over $6.5 million worth of business last year. The company sells three different multistaion machines, priced from $1,725 ot $3,950. The Universal gym has the advantage of being in one frame and weighing relatively little.

Each Nautilus exercise is performed on a different machine, so the complete collection of machines requires a large area. Nautilus sells six different leg machines, 12 torso machines, three neck machines and five arm machines, priced form $800 to $3,000. But Nautilus president Jones, a vigorous salesman, says his sales are up 100 per cent over last year.

Now Cybex Co., a medical research firm in Bay Town, N.Y., is selling isokinetic rehabilitation machines which use hydraulics instead of weights. Cybex claims its machine is the one of the future. The Cybex 11, in which the speed of the machine is fixed and the resistance is variable, allows an athlete to read his progress in foot pounds from a recorder printout. According to Andy Glass, Cybex marketing manager, the machine is the only one that can develop the fast-twitch fibers effectively.

"We are most interested in absolute maximum performance training," said Glass. "The Cybex can be set at such a high speed that we feel it is the most perfectlt variable resistance machine in the world."

Cybex, too, is growing. Sales were up 33 per cent this year even though the machines generally are more expensive ($10,800) and conceptually more difficult to use.

Regardless of who is winning the gym machine race, the bottom line is that more people are working out today than ever before.

"These machines make lifting weights much easier now," said Warren Miller, president of Fitness Corp., a Maryland-based gym machine outlet. "Even unathletically inclined people are working out now. The interest in physical fitness is at a fever pitch."

What's ahead, according to Ariel, sounds a bit like a gymnastic 1984. He said his research company is working on machines to be operated by computer to design exercises according to an athlete's particular movements, no matter what the sport.