It was a question about soccer that got Philip Bayly interested in brain injuries.
Bayly, a mechanical engineer at Washington University in St. Louis, was approached by several doctors who wanted advice about some young soccer players they were treating.
"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,' " Bayly recalls.
Does a header have a big effect or a small one? The doctors thought Bayly might have the answer.
"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,' " Bayly told them. "But I know how we can find out."
So in the early 2000s, Bayly brought soccer players into his lab to figure out precisely how much acceleration their heads experienced when they headed balls hurled at them by a machine.
The answer was 15 to 20 times the force of gravity, a relatively minor impact.
"Jump up and down you're feeling maybe 4 or 5 G's when you hit the ground," Bayly says. "When you play football, you have a hard collision with someone else, it's maybe 50 to 100 G's."
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 the next decade trying to figure that out.
It's an effort that has involved jiggling and jarring a lot of living human brains.
Bayly's lab has become expert at using MRI techniques developed to study beating hearts to see how the brain changes shape when a person's head moves.
In one experiment, volunteers rapidly turned their heads. In another, they placed their head in a cradle and pulled a string that allowed the cradle to drop about an inch.
More recently, the lab has been using a technique called magnetic resonance elastography along with a device that vibrates the skull. Charlotte Guertler, a graduate student in Bayly's lab, demonstrates how it works.
"So your head is resting on this," she says, pointing to a foam cradle. "That's what vibrates the back of your head and that's how we see the waves inside your head."
The waves are much gentler than the motion caused by heading a soccer ball. But the technique has allowed Bayly's team to get a detailed look at how forces applied to a person's head are transferred to the brain inside.
And in 2017, the team published a study in the Journal of Biomechanical Engineering that challenged the way scientists had been thinking about head impacts.
"People have built mental models of what's going on inside your head," Bayly says. "They think it's like a rubber ball bouncing around inside your skull or a ball floating in fluid."
But the MRI images of a vibrating brain suggested a much more sophisticated system.
"What we saw, surprisingly, was that the brain wasn't colliding and bouncing against the walls of the skull, but it was pulling away from points of attachment," Bayly says.
These points of attachment are part of the membranes that separate the brain from the skull. And they usually act like the suspension system in a car, absorbing impacts and smoothing out bumps, Bayly says.
"Your brain is much better protected and suspended than it would be if it were just rattling around inside your skull," he says. The study found that this system could reduce the vibrations by 90 percent.
"But like any suspension system, it can fail," Bayly says. That can happen when an impact is simply too powerful for the system to absorb. And it is more likely with certain types of impacts.
One is a blow like a roundhouse punch to the chin, which causes a person's head to rotate violently. This can damage delicate fibers that connect the brain and skull.
Another dangerous impact is one to the back of the head, which can happen when athletes fall backward and their head slams into the ground. This can actually tear attachment points called bridging veins at the front of the brain and cause dangerous bleeding.
Scientists can't replicate those impacts in the lab, though, because it wouldn't be safe for human participants in a study. "We're getting a picture of your brain under sort of normal operating conditions as opposed to injury conditions and that's really the weakness of our approach," Bayly says.
Even so, the sort of research Bayly is doing is helping doctors understand why certain impacts are especially dangerous to the brain, says Dr. Mark Halstead, who directs the sports concussion clinic at St. Louis Children's Hospital but isn't involved in Bayly's research.
"We don't have the ability to take the brain out of the skull and look at it in a living person," he says. "So we're always trying to figure out what happens in the brain through an imaging technique or physics."
And Bayly thinks he is a lot closer to answering the question that got him into research about head injury, the one about heading a soccer ball. The research so far suggests that the brain's suspension system probably works pretty well to dampen a relatively mild impact like that, Bayly says.
What's still not clear, he says, is whether that's enough to prevent long-term damage in players who head the ball hundreds or even thousands of times in games and practice.