Bonnie Matthews routinely watches children straining, red-faced, to complete just one chin-up. A physical education teacher at Cameron Elementary School in Alexandria, Matthews reluctantly reduced the 600-yard run to 300 yards last year because so many of her students couldn't run the longer distance. And she fears for the health of some youngsters who are so overweight that they have difficulty completing even the simplest calisthenics.

Matthews, who has taught gym in Fairfax County for 18 years, says she is "continually frustrated" in trying to achieve her "major goal -- to keep kids aware of the importance of fitness even when they are not with me in P.E. class."

Ironically, at a time when adult fitness in the United States is increasing, youth fitness is declining.

Concern about children's poor physical condition surfaced last fall when the Department of Health and Human Service's National Children and Youth Fitness Study (NCYFS) concluded that American youngsters are fatter and less fit than they should be. The two-year study, funded by the Public Health Service, surveyed 8,800 students in grades 5 through 12. It tested fitness and exercise habits and also put students through rigorous tests designed to measure overall health and fitness rather than athletic ability.

The result: About half of American youths aren't getting enough exercise to develop healthy hearts and lungs.

"This study should serve as a warning," HHS secretary Margaret Heckler said in issuing the report. "It shows that America's school children are not achieving the lifetime fitness skills required to promote good health."

More bad news came from an even larger survey released one week later. The Amateur Athletic Union said two out of three American youths failed to pass basic tests of physical fitness. Only 36 percent of the 4 million students tested by the AAU met the group's standards for performing bent-knee push-ups, high jumps, standing long jumps, endurance runs and sprints. Similar AAU reports from 1979 to 1982 showed that 42 percent of the children were able to pass the tests.

This fitness decline results from eliminating compulsory physical education after the eighth grade, contends AAU survey coordinator Wynn F. Updike, associate dean of Indiana University's School of Health, Physical Education and Recreation.

Others blame youngsters' poor fitness scores on the amount of time they spend watching television -- an average of about 25 hours a week, compared with 13 hours of physical exercise.

Some health experts also fault a growing dependency on motorized transportation, the popularity of passive activities such as video games, computers and poor eating habits -- particularly over-indulgence in junk foods.

But whatever the cause, the result is clear. In tests of five areas of fitness -- body fatness, flexibility, abdominal strength, upper body strength and cardio-respiratory endurance -- American children showed either no improvement or a decline in performance compared with similar studies in the 1960s and 1970s.

The NCYFS tests included caliper measurements of triceps (rear portion of the upper arm) and subscapular (shoulder blades) skinfolds, sit-and-reach stretching, bent-knee sit-ups, chin-ups, and a one-mile walk/run. Performance in each activity was generally weak, and several findings were particularly distressing to fitness experts:

*"American young people have become fatter since the 1960s," with average skinfold sums, determined by measuring the amount of skin a caliper can grasp from the triceps and shoulder blades, "two to three millimeters thicker" than in a similar study 20 years ago.

*"Present school physical education programs are not geared to [the] goal" of teaching students fitness-related activities. Instead, they emphasize skills for competitive sports.

*Young people on the average spend "slightly under 13 hours per week in sports, active games and exercises." They spend three or four times that amount of time watching television and playing home video games.

*Approximately half of the nation's youths do not vigorously exercise large muscle groups for 20 minutes or longer three times a week -- the minimum standard as outlined in the study. Activities considered vigorous include basketball, bicycling, jogging and jumping rope.

*"An unhealthy precedent may be set for lifelong patterns of inconsistent participation in vigorous physical activity," the study warns, "because of the failure of many young people to perform up to even minimal exercise standards."

While youth fitness experts stop short of portraying young people today as a wheezing mass ready to collapse from lack of exercise, the study has sparked interest in how to shape up America's kids. Experts target several areas that need work.

"The weakest activity among young people is in the area of sustained cardiovascular activity," says Mel Williams, director of the Human Performance Laboratory at Old Dominion University. "Kids are not getting their heart rate up enough to burn calories. Kids don't look at it from a disease standpoint. To them, disease is a long way down the road . . .

"But there is, in general, this tendency for kids to put on weight. We see higher levels of fatness at younger ages."

Physical education teacher Bonnie Matthews acknowledges that she sees "more overweight students than I'd like to see." But she says children's fitness problems are "most severe in upper body strength and running 600 yards. Long-distance runs are very difficult."

In general, "kids are not able to integrate fitness and wellness into their life style," says Joan Pfeiffer, fitness director for the American Alliance for Health, Physical Education, Recreation and Dance. Instead, there is a trend toward sedentary lives -- supplemented by snacks -- as television, video games and motorized transportation replace outdoor play, walking and bicycle riding as daily activities among youths.

Dr. Ash Hayes, executive director of the President's Council on Physical Fitness and Sports, says: "My opinion is that 25 years ago we had a bell curve -- some kids very active, some not at all, most in between. It seems that now we may actually have a larger number at the vigorous end, but at the same time a larger number at the less vigorous end. That also reduces the number in the middle. They are the people we must zero in on."

Zeroing in is proving difficult because it means changing the life style of the younger generation. Professionals appear frustrated when they speak of the impact of television and automobiles.

"Students who live less than half a mile from school are driven to and from school every day," says physical education teacher Matthews. "Everyone tends to drive today when we could be walking or riding bicycles."

Youth sports programs do not seem to be picking up the slack, fitness experts say. First of all, says the Hayes of President's Council, "fewer kids are active [in sports] in their free time than might be expected," because the programs too often emphasize organization and winning rather than participation.

Youth sports frequently are not beneficial to the child's fitness, says Marie Sterne, a health and physical education curriculum specialist for Fairfax County Public Schools. She says some games, like tee-ball (generally played by children under 9), in which players are stationary for long periods of time if they can't hit a baseball off of a tee, provide little exercise. Even soccer or football, Sterne says, provides limited exercise if a participant is limited to only a few minutes of playing time.

"Negative experiences in sports affect a kid's outlook on physical fitness," Sterne says. "In too many cases, adults force adult values on kids' games. Anyone can be a coach; you can get some really perverse characters. They can get 6-year-olds to the point where they don't want to play anymore. Kids burn out on sports rather than becoming interested in developing some lifetime sports."

Youth sports should be an end in themselves, not a means to getting fit, Matthews says. "You should get fit to play a sport," she says, "not play a sport to get fit."

However, even in physical education classes, states the NCYFS report, youngsters "have little opportunity for effective participation . . . physical educators continue to rely heavily on competitive sports and other activities that students can not readily continue throughout adulthood."

Locally as well as nationally, P.E. teachers complain that fitness development is limited by overcrowded classes, insufficient time with students, inadequate equipment and a negative attitude toward gym classes as students grow older. Indeed, the NCYFS shows a consistent decline in P.E. enrollment. At grade 5, when P.E. is required, nearly 98 percent of all students are enrolled; by grade 12, when the class is elective, enrollment is about 52 percent.

Matthews sees her elementary school students in three 30-minute sessions each week, which is typical for area schools. "It's not enough time to have an impact on fitness," says Matthews. "My major goal is to give students an awareness and knowledge so even when they're not with me, they're aware of fitness goals."

At the high school level, many students attend gym classes five days a week, but "competitive sports are still the bread and butter of the [high school] program," according to the NCYFS.

The problem with emphasizing competitive sports, according to P.E. teacher Sterne, is that they tend to promote "motor skills which are conducive to team sports rather than fitness."

The youth fitness dilemma has prompted action among professional organizations. The American Alliance for Health, Physical Education, Recreation and Dance has developed a monthly classroom magazine called Fitting In, which was piloted in several local elementary schools from January through June to "overwhelmingly favorable response from students and teachers," according to the Alliance's Pfeifer. The June issue included an article on triathlon training, a guide to summer sports, a story called "Listening to Your Body," which urged paying attention to various danger signals that may occur when exercising, and short articles on sunburn and vegetarianism. With each issue teachers receive a resource sheet suggesting ways to build on the stories.

At Cameron Elementary, classroom teachers used Fitting In as reading and discussion material, says Matthews, who in turn targeted her P.E. class work to go along with it.

"It was very valuable because it gave fitness an added emphasis, since it was discussed in the regular classroom," Matthews said. "For example, kids normally resist flexibility type activities. But there was an article on flexibility in Fitting In that they had read in class. Then they came to me and I talked about it and did a related activity. They were much more receptive to it."

Another resource available to schools this fall is Fitnessgram -- a fitness report card developed by the President's Council with funding from Campbell Soup Co. The Fitnessgram compares, at different points in the school year, an individual student's performance in the six tests prescribed by the council, showing how the child compares to his or her peers and suggesting activities to improve specific weaknesses.

Both Fitting In and the Fitnessgram are geared toward encouraging youngsters to "form good habits so they can integrate fitness and wellness into their life style," Pfeiffer says. "We're just trying to get kids to think more about what they're doing."

Sterne emphasizes the need for "professionals to motivate kids" and notes the success Lake Braddock Secondary School in Fairfax had this year in awarding a T-shirt to students who earned a "Presidential" rating (85th percentile) on all six fitness tests. "That's an incentive to perform well," she says. "We need more of that."

Adults caught up in their own fitness boom must make more of an effort to pass along their knowledge and enthusiasm, experts say, if youth fitness statistics are to be turned around.

"Poor nutrition, steady diet of snack foods like chips, sweets and sodas, plus a lack of physical activities leads to obesity," Matthews says. "Those habits come from the home."