IN CHINA, sports and physical fitness are an integral part of national policy, I was reminded again and again on a recent tour. The doctrine is that besides improving the nation's health, sports increase productivity and strengthen national security simply by producing a more alert, fit citizenry.

But there are no full time athletes in China because athletes are workers too. The tought of anyone being paid to play a sport seems inconceivable to the Chinese.

Sports play a role in China's foreign policy. The stated purpose of participating in international competition, the Chinese say, is to build friendship with other nations.

"We would rather be friends of the world than champions of the world," our interpreter told us repeatedly. And "firendship first, competition second" is a phrase one hears constantly in China.

"Winning does not last long," one official told us. "You see the results on the scoreboard and then they are gone. But friendship strikes the roots in the hearts of people."

But Chinese athletes to do compete very hard.

"Sportsmanship, courage, tenacity and fighting spirit are stressed in competition," the official said, "but then exchanges between the sides take place for the sake of learning and friendship."

In track, for example, the winner of a race would share his or her knowledge of technique and methods of conditioning with the other runners. The aim is to raise the level of the group as a whole as well as to strengthen the friendship and unity within it.

These exchanges are insured by sports critics consisting of committees of workers, soldiers and peasants who attend matches to judge the sportsmanship, skills and attitudes of play of the participants.

They discuss their findings with the coaches, players, and referees to bring any "incorrect" behavior or thinking into line.

Sports did not escape the now-famous "cultural revolution" of the mid-1960s. The late Chairman Mao felt that incorrect thinking was creeping into the sports field as well as the rest of Chinese society; sports had become monopolized by a relatively small minority of the population and too much emphasis was being given to winning medals and championships.

Sports heroes were even emerging.

Mao put an end to this "revisionist" thinking in 1966. Schools were closed and students took to the streets criticizing and purging the "bourgeois elements."

Three years later, sports had been geared again for mass participation, health, friendship and "socialist construction."

The emphasis given sports in China is underlined in the fact that the Ministry of Physical Culture and Sport is directly under the Communist Party Central Committee, the main ruling body. This would be comparable to our having a cabinet-level "Department of Sports."

Under the Ministry are seven institutes of physical culture where teachers, coaches, and administrators are trained. These institutes, which resemble mammoth sports cities, are spread throughout the country.

The institute in Peking, for example, is as large as the University of Maryland campus at College Park. It has an outdoor swimming pool, indoor dirt track, and indoor basketball and all-purpose areas. Outdoors are 30 basketball courts, 15 volleyball courts and soccer fields everywhere.

Supplementing these institutes are 2,700 "spare time sports schools," out of which come the best young athletes in China. These schools, of which there are 27 in Peking alone, are vaguely comparable to our YMCAs and YWCAs.

In them, students 7 to 17 years old train two to three hours a day at their specialties after their regular school sessions.

We visited one of these schools in Peking and found it a beehive of sports activity involving as many girls as boys.

"Women," goes the saying in China, "hold up half the sky." And they seem to occupy half the courts in sports. We watched part of a girls' basketball game at the school and were surprised at the high level of skill and teamwork. In another part of the indoor facility, about 30 youngsters were vigorously going through badminton drills.

Outside, in the track and field area, we watched several young boys practicing the high jump, most using the Fosbury Flop technique. At the school's stadium nearby, a soccer game was in progress.

We were told that national championships are held in 28 different sports, nearly all of which are played in the United States.

The two conspicuous omissions are golf and boxing. Golf is regarded as a sport of the rich and requires too much valuable land. Boxing is considered brutal.

In the school that we toured we were told that 450 youngsters attended, trained under the supervision of 50 coaches. Students are selected on the basis of athletic ability, academic performance and, not least of all, their "correct political consciousness."

It doesn't take higher math to calculate that the number of Chinese athletes receiving intensive, supervised training is huge. With a population approaching 900 million, the athletic potential of China is mind-boggling.

The final rung of the Chinese sports ladder is the factory/school/commune level of participation. Here exercises are stressed over sports, although volleyball, basketball and table tennis are popular among workers and students.

Factory workers take regular breaks from their jobs to do exercises specifically geared to use muscles not used on the job. Students take regular breaks from their classes for group exercises, including eye exercises that the Chinese believe help sight over the long run.

An early-rising visitor in China finds the streets and parks filled with groups doing exercises. Some vigorously follow a cadence while others do "tai chi," the ancient art of slow motion shadow boxing especially favored by older people.

There is a good chance that China will participate in the 1980 Olympic Games in Moscow. If so, this will be its first appearance in full scale international competition since withdrawing from the Olympics in 1958 over the Olympic Committee's ruling in favor of a "Two Chinas" policy bringing Taiwan in.

China is mostly an unknown quantity in international sports competition, although Chinese are known to be strong in table tennis (they are world champions), gymnastics, volleyball, basketball (yes, there are tall Chinese), wrestling, soccer and distance running.