In the first study, researchers in the Netherlands said that 39 percent of soccer players in their study had impaired planning ability, while 27 percent had impaired memory. Concussions and repeated blows to the head from soccer balls were the probable reason for lower test scores, they concluded.
The other study, which focused on U.S. college football players, found that those with learning disabilities suffered added problems from concussions suffered on the field.
"Although cognitive impairment appears to be mild, it presents a medical and public health concern with 200 million Federation International Football Association (FIFA) registered soccer players worldwide," said the soccer report from St. Anna Hospital in Geldrop.
"Accordingly, methods for surveillance and prevention should be developed and adopted to maximize safety," it added.
Both reports were published in this week's Journal of the American Medical Association.
The Dutch researchers said they compared the results of tests on 33 amateur soccer players with those from 27 middle-distance runners and swimmers. The tests measured various brain functions.
Thirty-nine percent of the soccer players showed impaired performance on tests that measured planning abilities compared to 13 percent in the other group of athletes. On memory tests, 27 percent of the soccer players showed impairment compared to 7 percent of the swimmers and runners.
"These findings suggest that participation in amateur soccer may be associated with mild chronic traumatic brain injury, as evidenced by impairment in cognitive functioning based on tests of memory and planning," the report said.
Of the soccer players studied, 27 percent had suffered one concussion during their playing career and 23 percent reported a history of two to five concussions. The median number of balls headed in a match was 8.5 among those studied.
In the football study, researchers at Henry Ford Health System in Detroit looked at nearly 400 football players from Michigan State, the University of Florida, the University of Pittsburgh and the University of Utah. More than a third had suffered one concussion playing and one in five had suffered two or more.
In addition 53 of the athletes were diagnosed as having learning disabilities.
"Players with a history of two or more concussions and (learning disability) performed significantly worse on tests of executive functioning...and speed of information processing...relative to players with one or no prior concussions," the study said.
The authors said that suggests there is "an additive effect of learning disability and multiple episodes of concussion on lowered functioning."
he report said athletes with learning disability often have a tough time in school, "especially within the context of excessive academic and athletic demands. Our data suggest that experiencing two or more prior concussions is associated with an attenuation (weakening) of cognitive skills, which, when combined with the deficits associated with (learning disability) leads to even further compromised functioning."
In a third sports injury article in the same publication, researchers at Med Sports Systems in Iowa City, Iowa, estimated that U.S. high school athletes suffered 62,816 cases of mild traumatic brain injury each year, with football accounting for 63 percent of them. Wrestlers account for 10 percent of those injuries, while boys and girls playing soccer account for 12 percent.