"Pumping Iron" is an amusing, buoyant documentary about competitive body building, dominated by the humorous though awesomely proportioned star presence of champion of champions Arnold Schwarzenegger as he trains and disarms the competition prior to defending the title of Mr. Olympia for the fifth time.

A diverting spin-off from the photojournalistic book of 1974, the film hsa been narrated by Charles Gaines, the original author, and directed by George Butler, the original photographer, in association with Robert Fiore, a New York-based cinematographer whose previous credits include "Greetings," "Gimme Shelter" and "Janis."

Before writing about the sport and its devotees in detail in "Pumping Iron," Gaines had incorporated body building into the plot of his novel "Stay Hungry." Although cast in a second role, more or less playing himself, Arnold Schwarzenegger proved the most appealing figure in the movie version of "Stay Hungry," released last summer. Now the movie version of "Pumping Iron" confirms that appeal, which may be connected with shifting popular tastes in screen heroes.

Schwarzenegger is the first personality since Bruce Lee who might become a unique and credible physical star, idolized in particular by kids but enjoyed and admired by a vast crosssection of the public. In its own way his physical self-possession seems as remarkable as Lee's, or Fred Astaire's.

Like them, Schwarzenegger can make unusual physical attributes appear to be the most natural thing in the world. He doesn't disarm only his competitors. He carries that phenomenal physique so nonchalantly that one can't help feeling charmed and reassured.

This is an extraordinary feat of personality projection, comparable to the apparent effortlessness with which Lee and Astaire asserted their physical dexterity and grace on the screen. Schwarzenegger is so casual that one might almost believe everyone is built the way he is, an obvious absurdity, especially to all of us in rotten shape who ought to feel his very presence as a constant rebuke.

Engagin as it is, "Pumping Iron" would probably fizzle out if Schwarzenegger were not at or near the center of attention. Butler observes a number of body-building champs - Ken Waller, Mike Katz, Lou Ferrigno, Franco Columbu; Ed Corney - and they come across as amusing or likable types, but you can't deny star quality, and it's the extra dimension of his personality that makes Schwarzenegger the rightful Numero Uno in this upper echelon of musclemen.

Sunning himself on a Pretoria beach a few days before the 1975 Mr. Olympia contest, Schwarzenegger is teased by a friend about the perils of complacency: "The king of the hill can only go down, Arnold.

Schwarzenegger rolls over and replies, "or stay up."

"Yeah, yeah," the friend acknowledges "That's the other possibility. But the wolf on top of the hill is never as hungry as the one trying to fight his way to the top."

"Maybe he's not as hungary," Schwarzenegger says, "but when he wants the food, it's right there."

Schwarzenegger invariably takes command of the repartee, no matter who has started it. Over lunch with his chief competitor, Lou Ferrigno, and Ferrigno's mother and fahter, Schwarzenegger deftly executes the psych-out job he has promised to perform a few scenes earlier. He begins by pretending to be the victim of the Ferrignos' psych-out, an opening gambit quickly followed up by casual remarks that Lou will have a lot to be proud of even if he doesn't win Mr. Olympia and that his own unprecedented streak of championships never ceased to amaze him. "I got so carried away," he confesses, "that I called my mother this morning and told her I'd already won."

Schwarzenegger's confidence is so great that he even indulges little revelations that might tend to make him look ruthless.To illustrate how single-minded a competitor may become while in training, he says that he declined to break training to attend his father's funeral in Austria. It's as if he knew his fascination would be enhanced by risking some shock or disapproval from his public.

It's difficult to foresee what direction Schwarzenegger's movie career might take. One could imagine Schwarzenegger as Superman if his accent were slightly more subdued and Clark Kent were less of a straight arrow. He would be wasted on conventional muscleman heroics of the sort that made Steve Reeves a hit for a while. One would need to devise a format that accommodated Schwarzenegger's obvious intelligence and wittiness. It might be fun if he and Woody Allen could be brought together on some pretext, perhaps as devastating ladies' men, illustrating utterly contrasting styles of sexiness.

"Pumping Iron" is the sort of entertainment that can mean equally valid things to different segments of the audience. People who choose to respond to it as a comic view of athletic types will be justified, but so will people who consider it inspirational, a demonstration of what dedication and application in any calling may do for you.

The emergence of Arnold Schwarzenegger as a potential film star seems to coincide with a rediscovery of the virtues of being in shape. Sylvester Stallone could win the Academy Award for pumping up, physically and emotionally, for the title role in "Rocky," whose roadwork sequences are paced to a theme song that Michael Small's delightful title song for "Pumping Iron" almost seems to be parodying.

Robert De Niro is working out intensively in preparation for the role of Jake LaMotta. Sam Elliott was in conspicuously good condition in "Lifeguard." So was Dustin Hoffman, old meek Benjamin and sickly Ratso himself, in "Marathon Man." In fact, Hoffman's physique was one of the few wholesome aspects of that unsavory production. The moment may have passed, fortunately, when Timothy Bottoms or David Bowie loomed as the Typical Leading Man of the '70s.