Oct. 29, 2024 — You see it in movies, TV shows, and cartoons — a character shaking off the cobwebs after a blow to the head.
But in real life, rapidly shaking your head like that after a hit could indicate a serious brain injury.
Athletes who made the motion after an impact had a concussion 72% of the time, according to a new study. In football players, it coincided with concussion 92% of the time.
The study's lead author, Chris Nowinski, PhD, was not surprised.
As a former college football player and professional wrestler, he’s seen video of himself shaking his head after an impact.
"It's something I've always recognized as a sign of concussion," said Nowinski, co-founder and CEO of the Concussion Legacy Foundation.
Two years ago, Miami Dolphins quarterback Tua Tagovailoa shook his head after a hit but wasn’t removed from the game. The incident — considered an undiagnosed concussion — led to a “lightbulb moment” for Nowinski.
If head shaking had been recognized as a potential warning sign, then Tagovailoa might have been benched and evaluated instead of continuing to play and having a second concussion days later. But it’s not on sports leagues’ lists of warning signs, and Nowinski couldn't find it in any medical literature, either.
"I realized it had never been studied and never named, and if we did that, maybe it could be used to protect athletes," Nowinski said.
As for the name, the researchers dubbed the movement spontaneous headshake after a kinematic event, or SHAAKE.
They surveyed 347 current and former athletes, including men and women across different sports. All were between ages 18-29 — players in an era marked by increasing awareness and education around concussions, Nowinski said.
Participants watched video clips of SHAAKEs and answered questions about their playing careers. Among the 69% who reported experiencing a SHAAKE, 93% had SHAAKE with a concussion at least once.
Self-reported SHAAKEs could provide a more accurate picture of the behavior than video alone, Nowinski said. That’s because SHAAKEs don’t always happen right after an impact, occurring instead on the bench or in the locker room, in moments that aren’t recorded.
"It's often minutes later when you're still not right,” Nowinski said.
Why Do People Shake Their Heads After a Big Hit?
Researchers don’t fully understand why SHAAKE happens because the motion hasn’t been studied. But they have theories.
Senior study author Daniel Daneshvar, MD, PhD, chief of brain injury rehabilitation at Harvard Medical School, compares it to scratching an itch, another mysterious compulsion that’s not fully understood.
Both impulses seem to combine voluntary and involuntary action. “In the same way you could stop yourself from scratching an itch, you can stop yourself from doing this,” Daneshvar said. But “there's something about this particular head shake that people feel compelled to do.”
Asked why they shook their heads, some study participants said they felt confused and needed to “jumpstart” their brain. Others were trying to adjust to a change in their spatial perception.
Further investigation may link SHAAKE to a certain type of concussion. “We’re increasingly realizing that not every concussion is the same,” Daneshvar said. A concussion that affects balance might need to be managed differently than one that causes headaches.
“SHAAKE might indicate something about the type of concussion that a person has,” Daneshvar said, “and might inform the most effective treatments.”
Why This Matters
A concussion can cause a range of less-visible symptoms, like headaches, nausea, and ringing in the ears. But athletes often hide or dismiss them so they can stay in the game.
“The studies are very clear that athletes are withholding the majority of their concussion symptoms from medical staff,” Nowinski said.
Separate research by Eric Nauman, PhD, a professor of biomedical engineering at the University of Cincinnati, found that athletes and training staff hid many “non-obvious symptoms” of concussion, from a metallic taste in the mouth to sensitivity to light. While at Purdue University, Nauman and his colleagues also showed that “even without a diagnosed concussion, the brain changes a lot, almost a terrifying amount, throughout the course of a football season,” he said.
When it comes to chronic traumatic encephalopathy (CTE), “all of those head impacts matter,” said Daneshvar.
That makes observable signs like SHAAKE all the more important to enforce.
Despite widespread knowledge of ataxia (a loss of muscle control that can affect balance), it was only added to the National Football League’s list of concussion signs after Tagovailoa’s injury.
“That's crazy, because every MD knows that that's a sign of a concussion,” Nauman said. “Head shaking is another form of ataxia. Anything that is obvious like that absolutely has to be documented and measured.”
Since the study was published last week, Nowinski has been hearing from athletic trainers. Some have kept an eye out for the gesture for years.
While it’s unclear if SHAAKE will be added to any official lists of concussion signs, the researchers are encouraging teams to look for it.
“It could help diagnose athletes with concussions, and potentially fundamentally affect their games, their seasons, their athletic careers, their lives,” Daneshvar said.
