HARD knocks to the head are a constant concern in contact sports — and not just in football or boxing, where recent attention has focused. Millions of girls and boys play hockey, soccer, lacrosse and other sports where blows to the head from collisions and falls are part of the game, even in youth leagues and on high school teams.

Head injuries can come from a single jarring impact during a game, or from a series of smaller jolts. But in the midst of play, many blows aren’t necessarily easy to spot by coaches, physicians or parents in attendance.

A crop of new lightweight devices that athletes can wear on the field may help people on sidelines keep better track of hits to players’ heads during games and practice sessions. The devices, packed with sensors and microprocessors, register a blow to a player’s skull and immediately signal the news by blinking brightly, or by sending a wireless alert.

Athletes can wear the devices pressed tightly to their heads, held in place by a headband within a beanie, for example, or even by an adhesive patch and Velcro.