WASHINGTON (AP) _ Grownups could prevent many youth sports injuries just by being less demanding of the players, fitness experts say.

Too much focus on how to play the game, and too little on whether kids' bodies are ready for it, causes the problems, according to research by the American College of Sports Medicine.

Coaches, parents and doctors should concentrate on improving a player's flexibility, strength and fitness, and on assessing whether the athlete has the right body type to do well in the sport, the report said.

Up to half of these injuries are preventable, according to the report, whose authors all are doctors. But they also note that there is scant research to support the figure, and concede the estimate is based mostly on clinical experiences.

''We 'guesstimated' that 50 percent are due to training errors,'' said Dr. Lyle J. Micheli, director of sports medicine at Children's Hospital in Boston.

However, there are plenty of injuries to keep doctors busy, said Dr. Jack T. Andrish, another author.

''Certainly a good 40 percent of the injuries we see in the sports clinic in kids are overuse injuries,'' said Andrish, a pediatric orthopedic surgeon at the Cleveland Clinic, and another author.

Adult-organized sports are a typical cause, Andrish said.

''You don't get overuse injuries from kids playing in the playground,'' he said. ''You get them when you start practicing in a structured environment where there are rules like 'You will practice an hour a day.'''

Training makes muscles stronger, provided the muscle is given time to repair the small tears it suffered in training.

Overtraining - too many repetitions of the same act, such as pitching a baseball - does not allow this time for recovery.

''These microscopic overuse injuries may cause nagging, chronic pain or predispose the individual to sustaining a much more severe injury,'' the report said.

Children can be the best judge of when they have been working too hard, the researchers said.

''Kids have a very good ability on their own - better than adults - to sense when it's safe for them to do something,'' Andrish said.

Adults can help young athletes avoid injury by making sure they are flexible and strong. Flexibility exercises before practice can prevent strains and sprains, Micheli said.

Strength training can build youngsters' abilities to compete safely, especially if they are out of shape, the report said.

Strength and flexibility work should start a month or two before the beginning of the sport's season, and increase no more than 10 percent a week in difficulty, the report said.

But this needn't force kids onto the field even sooner; a coach could, for instance, mail pre-season training regimens to players, Micheli said.

The report also calls for pre-participation physicals, now typically used for school-related sports, to be expanded to all sports. The exams should look for deficits in strength and flexibility, with an eye toward correcting them before the youth starts the sport, it said.

Athletes should be guided toward an activity suitable for their body type, the report said. This is far from an exact science, but doctors can spot some potential problems, such as a lack of adequate hip rotation in ballet dancers, Micheli said.

Overall, the position paper on preventing sports injuries makes important points, but may be too alarmist, said Dr. Greg Landry, a member of the American Academy of Pediatrics' committee on sports medicine.

Parents should not let the fear of injury keep their children away from the playing field, said Landry, an associate professor of pediatrics at the University of Wisconsin Medical School in Madison.

''We should be doing things to encourage children to be more active,'' Landry said.

The chair of the panel that drafted the position statement agrees. Considering the huge number of youngsters in sports, these injuries are relatively rare, said Dr. Angela D. Smith, head of pediatric orthopedics at Mount Sinai Medical Center in Cleveland.

''They fill our offices, but when you go into the population at large, it's not that common,'' Smith said.

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