Heading a soft(ish) soccer ball would seem fairly benign compared with crashing head-on into a 350-pound hulk in football, a sport clearly linked with the brain disease CTE (chronic traumatic encephalopathy—a degenerative disease common in athletes).

But when scientists at the U.K.’s University of Stirling launched 20 machine-fired soccer balls at players’ heads to simulate corner kicks, the players showed a 41–67 percent loss of memory function.

You see, it's not just the brain-rattling, you're-seeing-stars hits that requires a trip to the doctor—and not just NFLers who need to be worried. It's actually really easy to suffer a concussion. Part of a rec league, prone to accidents, or always getting into random bouts of flag football with your buddies? Familiarize yourself with the 6 most common signs you've suffered a concussion.

Because of the 3.8 million concussions that occur in the U.S. each year during competitive and recreational activities, as many as 50 percent go unreported, according to research published in the British Journal of Sports Medicine. Be smart; get your head checked.