A jab between the eyes can send a fighter’s head moving straight back—and that direct motion may matter more for concussion risk than the twisting that scientists have long focused on. In a Stanford-led study, researchers found that straight-line head movement, measured as linear acceleration, predicted sports concussions better than rotational acceleration, which measures turning motion.
The finding challenges nearly 85 years of thinking about how blows injure the brain. In the United States, about 3.8 million sports-related concussions happen each year. Better evidence about which forces are dangerous could help guide helmet design and safety rules, though the study’s results need confirmation from further research.
The older view grew from work by Scottish physicist A.H.S. Holbourn during the World War II era. He argued that a turning head could stretch and strain the brain as different parts moved at different speeds. This type of internal strain is called shearing. Linear motion was thought to be less harmful because it did not cause the same kind of shearing. Some animal studies later appeared to support this idea, and helmet ratings commonly came to focus on reducing rotational acceleration.
But measuring head movement in people has been difficult. Helmet sensors can shift because a helmet moves on the head, making it hard to record the head’s motion directly. Stanford bioengineer David Camarillo’s lab has spent more than a decade helping develop motion-sensing mouthguards, which can track head movement more closely.
The new study examined mouthguard data from 203 male and female athletes across seven sports, including lacrosse, mixed martial arts and football. It compared 47 impacts that led to concussion with 3,804 impacts that did not. Researchers used video and reports from medical staff, along with later video reviews for signs such as disorientation or trouble with balance, to determine which impacts caused concussion.
The team expected the results to support Holbourn’s theory. Instead, lead author Jessica Towns, a graduate student in bioengineering, said linear head motion was more predictive than rotational motion. The study’s mathematical model also found that peak linear acceleration—the highest level of straight-line acceleration during an impact—was the single most predictive factor in whether an impact caused concussion. Towns said the level of linear acceleration linked to danger was lower than people had previously thought.
The results point to a possible shift in how sports safety is studied. If other research confirms the findings, helmet makers and safety officials may need to give greater attention to impacts that drive the head directly, not only those that twist it. Examples include a stick striking the side of a lacrosse player’s head, a blow to a fighter’s face, or a football player hitting the ground after a fall. The study does not settle which designs or rules would prevent injuries, but it offers a broader set of human impact data to help researchers ask that question. Better measurements may bring safety advice closer to the forces athletes actually experience.