The largest study of its kind concludes that children shouldn’t spend more hours per week than years of their age playing one sport.

Kids are taught that hard work pays off, but too often when they’re training to make the majors, overdoing it leads to lasting injuries.

Researchers from Loyola University Medical Center followed 1,206 young athletes who received treatment for injuries and discovered that those who played a single sport for more hours a week than years they were old—such as a 10-year-old who played 11 or more hours of soccer each week—were 70 percent more likely to experience serious overuse injuries.

Among the findings, the authors reported that athletes ages 8 to 18 were more likely to experience injury if they spent double the time playing organized sports as they spent in free play or playing pick-up games.

Also, the athletes who suffered major injuries spent an average of 21 hours being physical each week, including in organized sports, at the gym, and in free play. Athletes who weren’t injured during the study period were only active for about 17 and a half hours a week.

Researchers say their study confirms that specializing in one sport—without cross-training or recreational play—can increase a young athlete’s overall risk of getting hurt.

“We should be cautious about intense specialization in one sport before and during adolescence,” Dr. Neeru Jayanthi, a Loyola University Medical Center sports medicine physician, said in a press release. “Among the recommendations we can make, based on our findings, is that young athletes should not spend more hours per week in organized sports than their ages.”

In the largest study of its kind, researchers followed each young athlete for three years. During that time, the athletes suffered 859 injures, 564 of which were attributed to overuse. Of those injuries, 139 were considered serious, such as stress fractures of the back or limbs, elbow ligament injuries, and injuries to cartilage and underlying bone.

Serious overuse injuries can have a dramatic impact on young athletes, forcing them to ride the bench for one to six months or longer.

The researchers offered the following easy-does-it tips to prevent serious sports injuries in children:

  • Do not spend more hours per week than your age playing one sport, because younger children are less able to tolerate physical stress.
  • Do not spend more than twice as much time playing organized sports as you spend in unorganized play.
  • Do not specialize in one sport before late adolescence.
  • Take a break from playing competitive sports for one to three months each year.
  • Take at least one day off per week from training.

Researchers say they intend to next test their hypothesis that many of the injuries they reported can be prevented.

Jayanthi presented his findings Friday at the American Medical Society for Sports Medicine’s annual meeting in San Diego, California.