Concussions are an issue in American football and other high contact sports. Players can suffer a concussion from a blow to the head. They also can develop one from a hit to the body that causes the head to move back and forth. This can create chemical changes in the brain and sometimes damage brain cells. In the United States, concussion rates are higher among high school students playing football than among those playing other sports. But a new study suggests these brain injuries are becoming less common among high school football players in training before games. It also found that repeat concussions are happening less often in all sports. The study comes at a time when Americans are increasingly concerned about the dangers of brain damage and brain diseases such as CTE.