From the outside, it is unprepossessing -- a dingy, aging, rundown building. But inside can be found at least part of the secret of the Soviet Union's extraordinary success in sports.

The building houses the Soviet Institute of Physical Education and Culture, and it has existed in Moscow since 1918. Today, it is the largest of 25 such institutes in the Soviet Union, with 6,000 students, and it is here that the Soviets do their research, train their coaches and begin the search for their best athletes.

"This is where we teach our teachers how to teach and what to teach," said Sergei Popov, the vice director of the institute. "Many top Soviet athletes come here when they are through with their careers to learn how to be coaches and teachers."

From this place, the Soviets get their continuity. One generation teaches another. Research goes on constantly. The professors who work here are as much subject to the publish-or-perish principle as U.S. college professors are. Last year, according to Popov, they produced more than 1,000 research papers.

The place is huge. There actually are several buildings in the complex, each connected. There are classrooms, research laboratories, a museum and a lot of practice facilities.

Students come here -- if they are accepted -- at 17 after they have completed the usual 10 years of Soviet schooling. They spend the next four years studying sports from every possible angle: medicine, research, psychology, coaching, new techniques. Many continue after four years, doing up to three years of postgraduate work. Usually, that period is devoted to a specialty -- a sport, a specific field, an area of research.

There are children here, too. Soviet children can pick their sports early, and some come here and to other institutes to train. But the majority, if they remain past 17, end up as coaches or teachers.

"It is very difficult to train for athletics and to attend classes full-time," Popov said. "If someone is a great athlete, he will usually leave here and may return when his career is finished as an athlete."

One athlete who still is training and taking courses is Vladimir Salnikov, the great swimmer who holds world records in all the long distance freestyle events. Salnikov, 26, is a second-year student at the institute and almost undoubtedly will coach when his swimming days are over.

The concept, on a general level, is not that different from the U.S. system in the major sports: the best athletes are identified at an early age, and they are turned over to coaches who can bring out the best in them. Here, however, there is no smoke screen, no fake student-athletes. If one is an athlete, athletics is what one devotes time to.

What's more, the Soviets consider all sports important. They study ways to make a bobsledder more effective as much as they study methods of attacking a zone press. That helps explain why there are few sports in which the Soviets are weak.

The time, money and manpower devoted to all sports is staggering. The institute here has a 1986 operating budget, according to Popov, of about $8 million. It also has a capital budget of about $50 million. And that is just here in Moscow. Another institute, in Leningrad, is almost as large, and there are other smaller ones throughout the country.

This day, as Popov led several Americans on a tour of the institute, music could be heard coming from a gymnasium. Inside, two dozen girls, each 17, were training. "Candidates for the school," Popov said. "Each of them is already a Soviet master of sport. Some will continue as athletes. Others will become coaches."

Down the hall, a pickup basketball game was going on among several students. Clearly, these were future coaches. Their waistlines were already beginning to look a bit coach-like.

But most of the young athletes walking the halls and working out on the indoor track did not look this way at all. They had the same look as the athletes who have been beating Americans throughout the Goodwill Games.

The young athletes who come here do so as early as the age of 8. They train here at night for two years and, at the end of that period, the best ones are selected for a three-year specialization program in their sport. After that, if they are good enough, they go somewhere for rigorous, full-time training. Until that point, they still go to school full-time, taking a regular curriculum.

"At that stage of their lives, we want them to be students, not athletes," Popov said.

Only in the past 10 years has the United States attempted to organize its athletes, to have its national teams train together, to try to get the corporate sponsorship that makes it possible for a post-college athlete to continue in his sport.

Which system is better? Both have advantages. The Soviets have been working at perfecting their methods for almost 70 years. But after a tour of these facilities, it is more than plain that sports is far more than a game here. It is an integral part of Soviet society, fully supported by the government and growing every day.

As he wished his visitors goodbye this afternoon, Popov said with a smile, "I wish you all good luck and good health. It is only on the field of sport that we wish to do battle with you."