Soccer is the most popular team sport in the world, boasting more than 200 million players who avidly chase, kick and "head" a one-pound pressurized ball in the hope of scoring that coveted GOAL! But a team of researchers from Washington University in St. Louis is now trying to determine whether that last technique might have some unwelcome consequences.
Emergency medicine physicians Rosanne Naunheim, M.D., and Lawrence Lewis, M. D., together with neurosurgeon Carl Lauryssen, M. D., and BJC Hospital's Human Performance Lab engineer John Standeven, Ph. D., have enlisted the help of mechanical engineering professors Guy Genin, Ph. D., and Philip Bayly, Ph. D., to study the "impact" of soccer heading on the brain. One recent study, determining specific forces and accelerations on the head, has just been presented at the 2002 Engineering in Medicine and Biology Society Conference in Houston, Texas. A second study, testing commercially marketed "safety headbands," has been accepted for publication in a forthcoming issue of Academic Emergency Medicine.
The group's concern is twofold. Some recent studies have indicated a short-term cognitive decline in players immediately after heading. At the same time, physicians remain uncertain about the risks of long-term cognitive degeneration, a condition likened to the dementia pugilistica that afflicts boxers.
"What got us interested in soccer heading is that it's a really good example of mild head impact," said Bayly. "If you say to a typical person, 'OK, I'm going to throw a one-pound object at you from 50 yards away, why don't you get hit in the head with it?' most of us would decline."
The chief danger is that the act of soccer heading involves both linear and angular accelerations — changes in velocity over time. But where linear accelerations happen in a straight line, angular accelerations involve rotation.
"We know for a fact that rotation of the brain has been shown to cause the most injury," said Naunheim, pointing to an animal study conducted in the 1960's at the National Institutes of Health, which demonstrated that if head rotation was limited by use of a cervical collar, subjects could sustain significantly greater impacts without receiving concussions. "So, if you prevent rotation you decrease the likelihood of injury."
"Unfortunately, even impacts that are fairly straight-on-center can result in angular acceleration in certain parts of the brain because of the way the brain is tethered to certain parts of the skull," Lewis added.
Though the exact physiological events accompanying head impact are still unknown, scientists and physicians hypothesize that, as the head undergoes rotation, the brain experiences a shear force–that is, a tangentially applied force causing deformation.
"As the skull rotates, the brain twists or shears as it catches up," Naunheim explains. "Think of a grape in a bowl of Jello; as you shake the bowl, the most significant shear occurs at the interface of the grape and the Jello [and eventually causes] either the grape to tear free, or the Jello to tear near the interface. Similarly in the brain, the junctions of gray and white matter–the areas where blood vessels penetrate and where the hemispheres come in contact with the brainstem–are areas where shear is most intense."
Naunheim and Lewis have studied head injuries since the early 1990's when they were colleagues in the Emergency Medicine Division at St. Louis University Hospital. They explain that what makes soccer a unique sport in terms of injuries is that, in addition to accidental head impacts from hitting the ground, other players or, say, the goal-post, heading is an accepted part of the game. What they did not know–since a quantitative assessment had never been made–was what a person's head undergoes physically when it strikes a soccer ball.
Working with the medical school team, Genin and Bayly, the mechanical engineering professors, devised a series of experiments to determine the exact forces and accelerations on the head. Four adult male test subjects between the ages of 25-36–all of whom had played soccer in high school (including Bayly himself)–headed soccer balls ejected from a machine at 20 and 26 mph. Each of the test subjects wore a headpiece (designed by Washington University mechanical engineering student Jeremy Neubauer) equipped with three "accelerometers," small devices that directly measure accelerations in the x, y and z directions.
What the researchers discovered were linear accelerations in the realm of 15 to 20 G's , or about 200 meters per second squared, and angular accelerations of 1,000 to 2, 000 radians (one radian is about 60 degrees, or one-sixth of a revolution) per second squared. These are values well below established thresholds for acute injury. Still, what is not known is whether repeated below-threshold impacts could result in cumulative brain damage. And some research suggests that players who define themselves as "headers" do suffer from short-term cognitive effects.
"There's a Dutch study in Journal of the American Medical Association in 1999 that compares runners, swimmers and soccer players," said Genin. "Soccer players who head the ball and are more likely to receive concussions indeed appear to show cognitive deficits when compared to swimmers and runners with similar training regimens."
The group's next study involved testing a new product on the market, "soccer head protection" headbands, to determine whether or not they actually reduce either linear or angular acceleration. Trials took place at BJC's Human Performance Lab in St. Louis, where engineer John Standeven arranged magnesium headforms (dummies made of material that approximates human head stiffness) equipped with the headbands. Results were fairly surprising–the headbands appeared to provide little benefit during heading and, at lower ball speeds, actually increased accelerations.
"The headbands would be very good if you wanted to reduce the impact forces between a bowling ball and your head," said Genin. "But with a soccer ball, headbands don't do that much."
So, pending further research, what are the group's recommendations?
"I agree with the American Academy of Pediatrics that children under 12 should not head the ball," concluded Naunheim, explaining that children's heads have a lower mass relative to the ball, and that their large heads (relative to their bodies) make them more unstable after heading. "As for older players, I think the risk will eventually be shown to be greatest for those with previous concussions."