Thud. Clunk. The sounds heard across athletic fields when helmets and bodies collide make coaches, parents, fans—and especially the athletes themselves—wince. Nearly 4 million sports-related concussions occur in the United States each year, according to the Centers for Disease Control and Prevention. Football, hockey and lacrosse are sports that clearly contribute to concussion numbers, but there are many other sports, such as soccer and basketball, that also carry concussion