A concussion study from Stanford Medicine has challenged 85 years of thinking about what makes a blow to the head dangerous. The research, published on 9 November in the Annals of Biomedical Engineering, found that straight line motion of the head is more predictive of concussion than the turning motion that scientists have blamed since the 1940s. The team analysed data from motion sensing mouthguards worn by 203 male and female athletes in seven sports, from lacrosse and mixed martial arts to American football. The concussion study findings could change how helmets are designed and how safety rules are written.
In the concussion study, linear acceleration, the measure of straight line motion, tends to dominate when an impact is directed to the head’s centre of mass. In mixed martial arts, that could be a jab between the eyes. In lacrosse, it could be a stick striking the side of the head. In football, it could be a collision to the top of the head when the head is lowered, or a fall backward with the head hitting the ground. The researchers found that peak linear acceleration was the single most predictive factor in whether an impact would lead to concussion.
Concussion study data: 47 injuries in 3,851 impacts
The concussion study used the largest and most varied set of mouthguard data collected to date. It covered 47 impacts that led to concussions and 3,804 impacts that did not, for 3,851 impacts in total. Every impact was recorded on video, and clinical diagnoses were confirmed by medical staff on site or by later video review of symptoms such as balance problems and disorientation. The researchers built a mathematical model from the data and tested which measure of head motion best predicted whether an impact caused a concussion.
The concussion study’s headline number is stark. Head impacts with a linear acceleration of 100 g, or 100 times the acceleration of gravity, carry a 50 percent risk of concussion. Lead author Jessica Towns, a graduate student in bioengineering at Stanford, said the threshold for dangerous impacts is lower than people had previously thought. Towns said the threshold “is lower than people had previously thought in terms of the levels that are unsafe.” The work was funded by the National Institutes of Health, with researchers from SoftShox, Monash University, Deakin University, Duke University and Alfred Health contributing.
Concussion study overturns a theory from the 1940s
The concussion study overturns a theory that dates to the Second World War. The prevailing view of head injury biomechanics traces to work by the Scottish physicist A.H.S. Holbourn, who argued from mechanical principles that rotational motion was the main driver of concussion. David Camarillo, associate professor of bioengineering at Stanford and senior author of the study, said Holbourn’s framework was built on mechanical reasoning rather than on biological observation. The new findings turn nearly 85 years of that thinking on its head, though the authors said more studies are needed to confirm the result.
The concussion study authors were careful about what the findings do not mean. Rotational motion is still harmful, and nearly all real world head collisions produce a mix of linear and rotational movement. The study found only that linear motion was more predictive of concussion than rotation within the data collected. Camarillo said the findings suggest safety guidelines for helmets, from football to cycling, could be underestimating the risk of linear acceleration. Concussions are among the most common sporting injuries, with an estimated 3.8 million sports related concussions every year in the United States alone.
The concussion study points to helmets and rules that pay more attention to straight line impacts. Helmet tests have long weighted rotational forces heavily, following Holbourn’s theory, and the new data suggest that balance may need to shift. ABC News reported the findings on Monday, with its medical correspondent Dr. Darien Sutton explaining the study on air. The Stanford team said the results, if confirmed by more studies, could lead to better protective equipment and treatments for athletes in every sport where head impacts are part of the game.