A recent study released by the American Academy of Orthopaedic Surgeons found that female athletes are 12.1 percent more likely to sustain a concussion than male athletes playing the same sport.

Study author Wellington Hsu, professor of orthopedic surgery at Northwestern University, told The Washington Post, “We were surprised at how the incidence of concussions particularly in girls over the past five years has increased. And we found that sports that weren’t typically linked to concussion are actually quite risky.”

Researchers from Northwestern and Wake Forest University tracked concussion data in sports relative to total injuries from 2005 to 2015 using the injury surveillance system High School Reporting Information Online.

The study looks at data from boys’ football, soccer, basketball, wrestling and baseball in comparison to girls’ soccer, basketball, volleyball and softball.

Researchers were surprised to find that football, notoriously linked to head injuries, was only fourth on the list of concussions as a percentage of total injuries. It came in behind girls’ soccer, girls’ volleyball and girls’ basketball.

In girls’ soccer in particular, study authors noted an increase in concussion statistics. “It remains unclear why boys’ soccer players do not appear to have the same risk as girls,” they wrote.

Geoff Manley, chief of neurosurgery at Zuckerberg San Francisco General Hospital and co-director of the Brain and Spinal Injury Center at the University of California-San Francisco, said, “We’ve seen a lot of data come out of women’s soccer that shows the women may very well be playing harder than the men. These are tremendous athletes with incredible skill who play really hard,” he said. “And there is no protection.”

With regards to protection, the authors suggested that the first line of defense against concussion is muscle mass. “The neck muscles of girls just aren’t as developed as boys are,” the study said, “so if girls experience an impact, it makes sense they might be affected by it more than boys if they don’t have the muscles to cushion that impact.”

Secondly, the study suggests that adequate protective equipment is less readily available for female athletes, even as the emphasis on physical play in girls’ sports continues to grow.

Courtney Cameron is Editorial Assistant of Athletic Business.