(Bloomberg) — Allowing children to pitch for long stretches during baseball and softball games may cause painful shoulder damage, University of Kentucky researchers said.

Heavy pitching, often associated with children playing year- round or in multiple sports leagues, can interfere with proper shoulder-tissue growth, the scientists said in a statement today. Boys, who undergo rapid physical changes during puberty and are known to participate in extreme sports more often that girls, are especially prone to injury. For the most part, doctors found that changes in upper-arm bone and soft tissue in the shoulders of young players can protect them from injury and allow for better pitching speed. Excessive activity can push young shoulders beyond their threshold for healthy adaptation to the exercise.

"Throwing is fine as long as it's in moderation and the parents and child use common sense," lead researcher Scott Mair said in the statement. "A 10-year-old pitcher shouldn't be throwing through pain to win a Little League game, for example."

Mair, a professor of surgery in the university's department of orthopedic surgery in Lexington, and colleagues studied 32 male baseball players, aged 13 to 21, for six years to observe shoulder strength and range of motion. The scientists also observed changes in the growth plate, the developing tissue near the end of the long bones in children and adolescents, using X- ray images.

Little League International, the 68-year-old nonprofit group representing children's baseball leagues on six continents, had already imposed pitching limits, according to the statement. Multiple cases of young pitchers experiencing arm and shoulder problems prompted the move. The rules, imposed in August at the group's World Series championship games in South Williamsport, Pennsylvania, require that a child throwing more than 20 pitches in a day needs to rest one day before pitching again, according to the statement. Children throwing 85 pitches in a day must rest at least three days.

Previously, the rules allowed pitchers age 12 and younger to throw as many as six innings a week and that many in one game. More research is needed to determine how the youngsters' growth plates fully respond to throwing, Mair said.

Any future discoveries may contribute to more injury- prevention strategies and to determining optimal pitch count and rest days, he said. The researchers studied boys exclusively because maturation differences between the sexes would have made the results less cohesive, he said. "I don't know of much research on underhand softball, but I would expect it pretty soon," Mair said of the baseball derivative played mostly by girls starting in their Little League through college years. "They often throw a lot more innings than boys."

For now, youth coaches and parents can refer to the American Orthopaedic Society for Sports Medicine's two-year-old guide covering injury-prevention strategies and well as suggested pitch count limits, he said.