BALTIMORE — Once cheered as the sound of a good hit, the loud crack heard on the gridiron when two football helmets collide is more often greeted these days by gasps, as fans recognize the potentially season-ending injuries such jarring impacts can cause.
Driven by concern for athletes’ concussions, Naval Academy researchers have developed a relatively simple model for understanding how brain injuries occur in helmet-to-helmet hits. With further refinement, the model might help design more protective headgear for football players, according to Murray Korman, a physics professor at the Annapolis military college who directed the study.
“The sensational cracking sound that’s made during a hit, that tells you that collision took place very quickly,” Korman explained, “which means the energy, the forces that are transferred are very large.”
Instead of using human subjects, Korman and one of his midshipman students, Duncan Miller, borrowed a pair of helmets from the academy’s football program and stuffed one with materials meant to simulate a player’s head and the brain cushioned by fluid inside the skull.
Miller, 22, a native of Winston-Salem, N.C., said the project was his idea, prompted by the death from brain trauma of a close friend’s brother who was playing football in high school.
“It started getting me thinking: How could I do a project about head-to-head collisions related to physics?” Miller said.
But the research resonated with Miller’s professor as well. Korman said he remembered from his own high school football experience how his coach would smack players’ helmeted heads with a ringed hand whenever he was displeased with their performance in practice.
“You’ll hear a very sharp, annoying sound, a snap,” said Korman, who presented Miller and his work recently at the annual conference of the Acoustical Society of America.
Miller graduated from the academy in the spring. Now an ensign, he said in a telephone interview that he is undergoing pilot training in Pensacola, Fla., at the Naval Aviation Schools Command.
Korman said they originally intended to conduct their experiment by strapping the helmets on mannequin heads that they would then customize for their research. But the mannequins were slow to come, he said, so they improvised.
For a player’s skull, they used a thin polycarbonate hoop, about six inches in diameter and three inches wide, that would vibrate when hit sharply. They then stuffed the hoop with porous plastic foam, similar to the material covering stereo speakers, to replicate the fluid inside a skull which helps cushion the brain from shocks. Finally, they fashioned a brass cylinder weighing about what a brain does to place inside the foam.
To measure the impact of the collisions, the researchers attached accelerometers — tiny motion sensors about the size of two breath mints stacked together — to the helmet receiving the hit, to the hoop and the brass weight. They then suspended the helmets on clothesline from the ceiling of Korman’s laboratory and proceeded to swing one into the other. Oscilloscopes hooked up to the accelerometers displayed visually the reverberations triggered by the hits.
“We learned that a helmet itself rings or vibrates very much like a bell or a tuning fork,” Korman said. The hoop, or skull, vibrated at a slightly slower rate, and the metal slug simulating the brain vibrated even more slowly and for a very short period of time.
They were studying the physics behind concussions rather than the anatomy of brain injuries, Korman said. But from the varying vibrations measured, he said they could effectively see “the sloshing motion” of a player’s brain as it collides with the inside of the skull after a big hit to his helmet.
“The whole point was to use very simple materials to understand the process as a model,” Korman said.
“With our rough model,” Miller said, “we could definitely see the correlation with the impact of the collision.”
Crude as the model was, Korman said, it suggested to him that brain injuries might be lessened by adding some kind of padding or impact-absorbing material to the outside of helmets, not just the inside. He noted that car bumpers these days are made of plastic and foam that crumples and breaks up when it hits a tree or another vehicle, absorbing some of the force of the collision and sparing the vehicle’s occupants from being thrown around as much inside.
“I think down the road some kind of helmet designed like a car bumper to absorb shock is the way to go,” Korman said.