By Lisa Rapaport

(Reuters Health) - As soccer has soared in popularity in recent decades, concussion rates for youth players have also surged, a U.S. study suggests.

Researchers examined data on high school soccer players from 2005 to 2014 and found non-concussion injury rates declined for boys and were little changed for girls. But concussions increased in both male and female players.

The significant rise in concussion rates "could be mainly due to a better recognition of concussion by medical and coaching staff," study leader Dr. Morteza Khodaee, a sports medicine researcher at the University of Colorado School of Medicine, said in an email.

The research team looked at injuries per minute of athletic exposure (AE), which includes both practices and competitions, for U.S. high school athletes.

Overall, there were 6,154 injuries during 2.98 million athletic exposures, for an injury rate of 2.06 per 1,000 AEs, the study found.

That included about 1.8 million soccer injuries among girls and 1.5 million among boys.

Girls were 27 percent more likely to sustain soccer injuries than boys, the authors reported online December 28 in the British Journal of Sports Medicine.

Injuries were 42 percent more common in competitions than during practice.

"The majority of injuries during competitions occurred during the second half indicating a potential accumulated effect of fatigue," the authors reported.

"It is well known that the risk of injury is higher in competition compared with practice," Khodaee said. "This is most likely due to more intense, full contact and potentially riskier play that occurs in competition."

Still, while injury rates were significantly higher in competition, more than one third of all injuries occurred in practice.

About 43 percent of injuries overall happened when athletes collided with another player.

Midfielders sustained the most injuries, accounting for 38 percent of cases for boys and 37 percent among girls.

The most common diagnoses were ligament sprains, accounting for about 30 percent of injuries.

Concussions accounted for about 18 percent of injuries, followed by muscle strains at 16 percent.

Injuries that forced players to stop participating most often involved ligament sprains and fractures, but concussions made up almost 11 percent of these cases.

Overall, the concussion rate during the study period was about 0.36 per 1,000 AEs. Among girls, it rose from about 0.4 per 1,000 AEs at the start of the study to about 0.6 per 1,000 AEs at the end. For boys, the rate rose from about 0.2 to 0.45 per 1,000 AEs over the course of the study.

Concussions accounted for about 17 percent of injuries among boys and 19 percent of injuries among girls.

In about 21 percent of concussion cases, symptoms resolved within one day. But recovery took more than one week in 29 percent of concussions.

Most concussions required athletes to miss between one and three weeks of soccer. Athletes were medically disqualified for an entire season in 3.5 percent of concussion cases, the study found.

One limitation of the study is that not all schools in the U.S. contributed data for analysis, the authors note. Only injuries reported to an athletic trainer were included.

Researchers also lacked data on how many minutes individual players participated in sport, requiring them to instead calculate injury rates based on the total minutes of play for teams. This means injury rates in the study don't necessary reflect how much time athletes spend on the field.

"This was only an epidemiologic study to calculate the risk of injuries and (find) any differences in sex, position of players, and mechanism of injuries," Khodaee said.

Br J Sports Med 2016.