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Athletes take a shot to the head. It is hard to know exactly what will happen to their brains. NPR's Jon Hamilton spent some time with a scientist who is trying to understand why certain head impacts are worse than others.

JON HAMILTON, BYLINE: Philip Bayly's interest in brain injuries began with a question about soccer players. Bayly's a mechanical engineer at Washington University in St. Louis. And in the 2000s, he met with some doctors who treat injured athletes.

PHILIP BAYLY: They said, well, we've got some kids who have concussions. And they want to know if they can go back to play, and we don't know what's happening to their head when they're heading a soccer ball.

HAMILTON: Was it a big impact or a small one? The doctors thought Bayly might have the answer.

BAYLY: I said that's really interesting. I play soccer, and my kids play soccer. And I don't know what's happening when you head a soccer ball, either. But I know how we can find out.

HAMILTON: Bayly brought some soccer players into the lab to find out how much acceleration their heads experienced. The answer - 15 to 20 times the force of gravity. Bayly says that's a moderate impact.

BAYLY: Jump up and down, you're maybe feeling 4 or 5 Gs when you hit the ground. When you play football - you have a hard collision with someone else, it's maybe 50 to 100 Gs.

HAMILTON: But Bayly realized these numbers didn't mean much unless he knew how much of this force was reaching a person's brain. So he spent more than a decade trying to figure that out. It's an effort that has involved jiggling a lot of living human brains. Charlotte Guertler, a graduate student in Bayly's lab, shows one way to do that.

CHARLOTTE GUERTLER: So your head is resting on this. And so now if I turn it on, it'll make it vibrate again.

(SOUNDBITE OF MACHINE VIBRATING)

GUERTLER: Can you feel it vibrating? Yeah, that's what vibrates the back of your head, and that's how we see the waves inside your head.

HAMILTON: A special type of MRI shows how moving a person's head causes their brain to deform slightly. And Bayly says the research has revealed something about head impacts he didn't expect.

BAYLY: And so what we saw surprisingly was that the brain wasn't, you know, colliding and bouncing against the walls of the skull. But it was pulling away from points of attachment.

HAMILTON: These points of attachment anchor the brain to the skull, and Bayly says they usually act like the shock absorbers on a car.

BAYLY: Your brain is much better protected and suspended than it would be if it was just, you know, rattling around inside your skull. But like any suspension system, it can fail.

HAMILTON: Especially with certain types of impacts. One is a blow that causes a person's head to rotate. Another is an impact to the back of the head. This can actually tear the attachment points at the front of the brain and cause dangerous bleeding. Bayly says the brain's suspension system probably works pretty well when someone heads a soccer ball. What's still not clear, he says, is whether that's enough to prevent damage when players head the ball over and over again as they often do in practice.

BAYLY: So understanding the mechanical behavior of the brain at relatively low accelerations is more important than we ever thought.

HAMILTON: Jon Hamilton, NPR News.