In the United States' National Football League, SUV-sized men are paid astronomical sums to delight stadium crowds with their ability to run down and demolish the opposing teams' ball carriers. The automotive analogy is quite apt. When they collide, the forces that thickly muscled behemoths such as the San Diego Chargers' Shawne ”Lights Out” Merriman exert on each other regularly exceed 100 times the force of gravity--the kind of jarring that passengers experience in a car crash. The result is roughly 230 000 concussions among professional, college, and youth football players each year.

Concern is growing over the long-term effects of skull-rattling tackles where a brain injury occurs, but the signs--including headache, nausea, and short-term memory loss--are difficult for coaches and trainers to spot; the injuries are unlikely to be reported by players because of the gladiator mentality that makes them keen to shake off any injury and get back into the game.

In the absence of hard medical data for assessing the severity of a player's head injury, coaches and trainers have to wrestle with tough-to-answer questions: When should a player sit out the remainder of a game? The remainder of the season? It's still a judgment call. But now a device installed in a player's helmet, which measures each blow to the head and reports the force of the impact, could make it a simple question of physics. The device would provide, for the first time, a data set large enough for researchers to understand when a concussion is likely to have occurred, according to how hard a hit was and to which part of the helmet. Armed with that knowledge and real-time data from the helmets, sideline staff will know when a player should be brought off the field for neurological tests.

The Head Impact Telemetry (HIT) System developed by Simbex, of Lebanon, N.H., combines six strategically positioned MEMS accelerometers, a temperature sensor, a wireless transceiver, nonvolatile onboard memory, and a nickel-metal-hydride battery pack in a halo that fits inside a helmet. The package allows the spring-mounted accelerometers to sit right up against a player's head, so the movement of the skull is measured instead of the movement of the helmet itself. It adds just 170 grams to a 1- to 2-kilogram helmet's weight and does not significantly alter its fit.

The system automatically generates a data report when any single sensor detects an acceleration that exceeds 10 gravities. The report includes 12 milliseconds of data from before the system is triggered and 28 ms following the instant of impact. A controller connected to a laptop on the sidelines receives this information wirelessly.

An early version of the HIT system was introduced during the 2003 football season: four football players from Virginia Polytechnic Institute and State University, in Blacksburg, Va., were monitored through 35 practices and 10 games. Researchers recorded roughly 3300 head hits and found that, on average, players endured 50 impacts strong enough to trigger the system during the course of a single game. The average acceleration caused by those hits was 40 g's per blow, the same level of impact delivered by the gloved fist of a professional boxer. At least twice a game, the players took shots to the head with forces on the level of a car crash. The data reports include parameters such as the Gadd Severity Index (GSI), a method developed by automobile crash researchers for describing just how jarring a blow someone has received. A human head can withstand GSI values as high as 1000 without serious injury; the blows endured by the Virginia Tech players ranged from 1 to 1599.

Simbex, working with researchers at engineering and medical schools at Virginia Tech and Brown University, in Providence, R.I., improved the communication system, allowing as many as 64 players to be monitored simultaneously with a single controller on the sideline. In 2004, nearly 500 players at five colleges--including Virginia Tech; the University of North Carolina, Chapel Hill; and the University of Oklahoma, Norman--wore the device in their helmets throughout the season. Simbex says 15 schools will take to the field this fall with the HIT System, capturing a snapshot of the forces at play in each tackle.

With a large data set from on-field collisions, ”we may be able to develop predictive algorithms, using a player's impact history, to remove players before they get seriously injured,” says Simbex's director of engineering, Jeffrey J. Chu. Duke University neurologist Joel C. Morgenlander, who recently joined the NFL's brain injury committee, thinks this is a good idea. ”The medical decision has to be separated from the heat of the moment,” he says.