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Reebok’s skull-impact monitor, to be worn under a helmet, lights up after a blow to the head.

What is it?

The Reebok Checklight markets itself as a conversation starter: It's a $150 device that fits inside a skullcap with an LED indicator hanging down the back of the neck (below where a typical sports helmet stops) and is designed to light up if you've taken a blow to the head strong enough to possibly cause injury. Reebok has been "seeding" free devices in some youth sports leagues in the United States, to get parents used to the idea (they are planning on similar programs with Canadian hockey soon). Green means go, no impacts measured; amber signals a moderate smack, a warning sign; red, in old hockey parlance, means you've had your bell rung – it's time to go chat with the trainer.

How does it work?

This device measures impacts to the head with a triaxial accelerometer that tracks directional linear G-forces (how much energy hit the centre mass of your skull) and a gyroscope to measure the direction of the hit. A Massachusetts wearable-technology company, MC10, invented it and crunches the data recorded by those sensors with its own proprietary algorithm to create a "threshold" it calls the Head Injury Criteria. Isaiah Kacyvenski, a former NFL linebacker and head of MC10's sports segment, says the Checklight calculates the energy delivered to the head, the location (it divides the head into nine impact zones) and duration of impact. It lights up when a hit crosses a threshold that MC10 developed after 15,000 laboratory drop tests and seven rounds of field tests with eight sports (including lacrosse, hockey and football) across all age groups.

So does it indicate concussion?

It tells you no such thing. Reebok says Checklight is not designed to "indicate, prevent, diagnose or treat concussions." Reebok, which has been sued in the past for using the word "concussion" to market its gear, admits that the amber light measures a pretty broad range of impacts, and even overlaps somewhat with what sets off a red. According to Paul Litchfield, Reebok's head of advanced concepts, the Checklight is about taking the guesswork out of whether you took a hard hit. "It's about getting the athlete onto the pathway to assessment," he said. You may or may not be injured, but if the Checklight goes on, parents and coaches are at least supposed to make sure you are monitored and perhaps get medical advice.

What do the experts say?

The ones we spoke to worry it could create a false sense of security. "I'm not keen on sensors," says Patrick Bishop, professor emeritus at the University of Waterloo and a member of its Centre for Applied Health Research. The issue is twofold for Bishop: He is skeptical that this combo of sensors and software can accurately measure rotational acceleration, the critical metric in brain injury because the direction of an impact is vital in understanding severity (when Sidney Crosby got crushed by Matt Cooke, his head snapped up and to the side, a movement linked by researchers to longer recovery times). Second, even if the sensors work, how can a bench-tested algorithm account for all the necessary extrapolations about varying body sizes, different sexes and even skull sizes, all of which create variables in whether rotational acceleration will cause a brain injury. "No two people are the same," he says.

Forget the head, get to the root?

Physician and concussion researcher Dr. Paul Echlin of Burlington, Ont., goes even further. "You are not getting at the basic cultural issue," he says, which is that this does nothing to take hits to the head out of junior sports, and may encourage people to think that as long as they are monitored, hits to the head aren't so bad. "They want to do everything but change the games. Half of the people in my clinic are children who can't go to school. … Show me the peer-reviewed science, then we can talk." Reebok says it can't reveal the algorithm, that the research is proprietary.

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