American Scientist

When I was a high school physics teacher, I once asked my class to design devices that would help an egg survive a 30-foot fall. The students brainstormed some creative and funny solutions. One pair surrounded their egg with foam packing peanuts inside the cavity of a rubber chicken. Another encased their egg in chocolate-chip cookie dough. These solutions were all variations on the same theme: Keeping the eggs from breaking requires reducing the peak contact force applied to the eggshells during impact with the ground.

A similar problem of preventing breakage was considered in the early 20th century, but the goal then was to avert cracking of the human skull. Large numbers of American college football players were seriously hurting each other, causing fractured skulls, concussions, and other head trauma. Football was a very different game in the early 1900s than it is today, with frequent pile-ups and scrambles for the ball punctuated by players kicking opponents in the gut or the head. At least 45 players died between 1900 and 1905. In October 1905 the president’s son, Teddy Roosevelt, Jr., made headlines when he was treated for a cut to his head suffered during football practice at Harvard.

Football injury and death rates fell soon afterward, but the solution involved modifying behavior more than applying technology. Piles of kicking and gouging players were minimized by new rules providing for the forward pass, the “neutral zone” at the line of scrimmage, and an increase from 5 to 10 in the number of yards required for a first down. Football helmets began to appear around this time, but those early helmets were little more than leather caps that probably did little to attenuate contact forces during impacts to the head. More substantial protection for players’ skulls arrived in the 1920s and 1930s in the form of helmets of hardened leather with wool padding on the inside.

With recent advances in understanding the consequences of concussions, especially those that occur repeatedly, another head injury crisis has arisen in football as well as in other sports. The Centers for Disease Control and Prevention reported in 2011 that emergency room visits by sports and recreation participants that were related to traumatic brain injury increased by 62 percent between 2001 and 2009. Cyclists accounted for the greatest number of such visits, with football players second, and large numbers from other sports, such as soccer and basketball. The recent jump is probably attributable to a combination of greater injury incidence, higher sports participation rates, and increased awareness of the dangers of head injuries, including stories in the media about the devastating effects of chronic traumatic encephalopathy. This disease is the progressive degeneration of the brain that follows multiple head traumas and leads to changes in mood, cognition, and behavior, but it does not appear to be prevented by modern helmets. Today’s football and cycling helmets are made from more advanced materials and fit the head more closely, but they are essentially similar to earlier designs, with hard outer shells of polycarbonate plastic alloy and internal foam padding, sometimes with additional air cushioning.

Recent research shows why so many sports injuries keep occurring and points to some possible solutions. Protecting the brain from concussion and chronic traumatic encephalopathy requires more than just increasing the amount of helmet padding. It necessitates an understanding of how brain injury differs from the cracking of an egg dropped on the floor, and how helmets influence the behavior of those who wear them.

Mechanics of Brain Injury

For an impact force to be applied directly to the brain, it would have to first breach the thick, protective skull. Injury to the brain does not require breaking through the skull, however. Damage can occur as the brain collides with the hard skull, like a toy bouncing against the insides of a gift box being shaken by a child. Inertial forces such as these, created when a stationary object is moved suddenly, depend on the mass of the object. That variable explains why ants may escape a fall of a few feet uninjured whereas humans might not.

The considerable mass of the adult brain (about 3 pounds or 1.36 kilograms) means that large forces are applied to it when the skull changes velocity suddenly following a collision, as when a football player is struck in a helmet-to-helmet collision. At the same time, cerebrospinal fluid within the skull cushions that movement, somewhat ameliorating the effect. David Hodgson, James Shippen, and Robert Sunderland of the University of Birmingham proposed a simple model to illustrate how inertial forces might injure the brain despite the surrounding fluid. They described an egg suspended by two threads glued to its shell inside a jam jar filled with water. If the jar is shaken back and forth, water pressure prevents the egg from accelerating nearly as quickly as the jar. The egg remains intact for all but the most violent linear accelerations of the jar along a straight line, showing how effective the body’s natural protections can be.

A strange thing happens when the jar is suddenly rotated, however. The water inside can do nothing to turn the egg along with the jar, so the egg remains stationary. Water’s low viscosity prevents it from transmitting such shear (or sliding) stress; this is the reason floors are slippery when wet with water but not when covered with honey, which has much higher viscosity. The threads connecting the egg to the lid of the jar twist around one another and either break or tear away a piece of the egg’s shell. These threads are like blood vessels and nerve axons connecting to the rest of the body at the base of the brain. The brain itself, made mostly of water, is resistant to direct pressure but is delicate in response to the shearing forces that arise during sudden rotation. Twisting of the brain, which gives rise to large shearing forces, seems to damage axons more than direct pressure does.

Making things more confusing, researchers have not yet been able to identify a single factor that determines the threshold for concussion—the kind of information that would make it possible to modify behaviors and design helmets to keep that factor in a safe range. Large linear and rotational accelerations seem to play a role in concussion, but no one variable alone seems to explain the risk. Concussions sometimes occur following a large linear acceleration, for example, but at other times they do not; sometimes concussion results when the linear acceleration of the head is relatively low.

There is no ethical way to perform controlled experiments to determine human concussion risk, but we now have detailed measurements of real concussions. The Head Impact Telemetry System (HITS), marketed by New Hampshire-based Simbex (a subsidiary of the helmet manufacturer Riddell), wirelessly transmits readings from six accelerometers placed inside a standard football helmet to a computer on the sideline. HITS allows researchers to monitor linear and angular accelerations during real-world impacts, as well as the locations where the impacts occur. The system is currently used by nine colleges in the United States, although no professional U.S. sports team has yet adopted it to monitor head accelerations in their players.

Richard Greenwald and his colleagues from Simbex and Brown University used HITS data in a 2008 study of 449 high school and college football players. Nearly 300,000 impacts occurred while helmet accelerations were being monitored; 17 of the impacts produced a concussion. Those concussions were generally characterized by high linear and rotational accelerations. Nearly half of the concussions took place when the impact occurred at the front of the head, with the remainder distributed among the top, sides, and back of the head. Based on those data, Greenwald’s group devised a new concussion measure that combined peak helmet accelerations along with two existing measures of impact severity, the Gadd Severity Index and the Head Injury Criteria. Both of these indexes are based on the duration of the accelerations (the brain is known to be able to tolerate lower accelerations for longer durations without injury). The proposed multifactorial measure was more predictive of concussion than guessing, suggesting that the causes of concussion are complex and cannot be identified by a single biomechanical variable. Despite these findings linking impact mechanics to concussion, the authors cautioned that HITS should not be considered as a tool for diagnosing brain injuries. There was no way, for example, to determine whether unreported minor brain injuries with potentially dangerous cumulative effects resulted from any of the impacts recorded during the study.

Protection for the Brain

In Greenwald’s study, the highest helmet accelerations recorded using HITS were about 100 g, or 100 times the acceleration due to Earth’s gravity, clearly a dangerous level of acceleration for the human head. The heads of woodpeckers, however, undergo accelerations of 1,200 g as they hammer their bills on trees, and the birds show no signs of concussion or brain damage. (Other birds are known to be momentarily stunned when their flight is interrupted by crashing into windows, so it doesn’t seem that birds in general are immune to head injuries.) Researchers have tried to solve the mystery of woodpeckers’ resistance to concussion in hopes that it might prevent conditions in humans such as detachment of the retina and shaken baby syndrome, or lead to biologically inspired helmets.

In 2006, Ivan Schwab of the University of California at Davis and the late Phillip May of University of California at Los Angeles received a satirical Ig Nobel Prize in Ornithology for their work explaining “why woodpeckers don’t get headaches.” The current intense interest in head injury epidemiology makes their study seem strangely cast as a source of humor. Schwab and May pointed out that woodpeckers experience the equivalent of 20 face-first crashes into a brick wall every second. One of the unique protective mechanisms the researchers described is related to the woodpecker’s hyoid bone system, which lies beneath the mouth in the neck. Unlike that of other bird species, the bone in woodpeckers forms a kind of straplike harness encased in muscle. The hyoid begins in the upper beak, encircles the cranium, and ends in a sharp, barbed tongue that points forward. When the upper portion of the beak strikes the surface of a tree, the hyoid absorbs energy from the impact as well as tightens into a sling that cradles the skull like a seat belt. Helmets featuring active restraint systems inspired by the woodpecker hyoid could theoretically help the cyclists and football players of the future avoid brain injury and chronic traumatic encephalopathy. A team of engineers at Mississippi State University led by Mark Horstemeyer has begun this work by creating computer simulations of different shapes subject to impact loading. In a conference paper presented in 2013, they reported that tapered spiral shapes inspired by the woodpecker hyoid and the ram’s horn were especially effective at reducing the impulse transmitted.

For now, though, every available helmet design is still a hard plastic outer shell lined with some form of padding. A helmet concept called the Multi- Directional Impact Protection System (MIPS), developed by Swedish engineer Peter Halldin and his neurosurgeon partner Hans von Holst, shows a possible way forward. Halldin and von Holst set out to minimize the rotational acceleration of the skull during impacts by permitting sliding to occur between the helmet and the head. An inner shell is attached to the helmet with joints that only move under certain angles of impact. During a direct hit to the head, a MIPS helmet behaves much like a conventional helmet, but when a glancing blow lands at an angle to the helmet surface, the helmet will rotate while the head initially does not. MIPS essentially takes the jar with the egg inside and floats it in a larger water-filled jar. Rotate the outer jar (the helmet), and the inner jar and the egg (skull and brain) stay in place.

Bench testing and computer modeling performed by Halldin and von Holst’s company suggests that the new design is effective at reducing rotational accelerations for such impacts, but it remains unclear whether protecting the brain from very high rotational accelerations is the same as protecting the brain from injury. Large-scale studies using instrumented helmets are the best indicators of what causes concussion, and evidence from those investigations has not identified rotational acceleration alone as the major determinant of injury. Nevertheless, MIPS technology is now commercialized by several helmet companies.

Risk Compensation

In addition to the physical and physiological factors, the researchers trying to cut down on sports injuries are battling human psychology. Late in the 2010 football season, Pittsburgh Steelers wide receiver Hines Ward was knocked out of a game when he suffered a concussion after being struck helmet-to-helmet by an opposing New England Patriots player. Both players probably would have been much worse off if not for their helmets, but that is not how Ward saw it. Appearing on a radio show two years later, Ward said, “If you want to prevent concussions, take the helmet off: Play old-school football with the leather helmets, no facemask. When you put a helmet on you’re going to use it as a weapon.”

Ward’s suggestion that the danger of concussion derives from risky behavior that is encouraged by helmet use is an expression of the Peltzman effect. In 1975, University of Chicago economist Sam Peltzman noted that drivers seemed to take greater risks behind the wheel when safety regulations are put into effect. The result is that the benefits that might be expected from mandatory seat belt laws or antilock brakes never fully materialize. Whether those safety benefits are completely canceled out by human risk compensation is the subject of much controversy.

Risk compensation appears to be especially pronounced among downhill skiers who wear helmets. Jasper Shealy and his colleagues at the Rochester Institute of Technology performed a comprehensive analysis of ski fatalities in the United States over a 14-year period and found that helmets were worn by 39 percent of skiers suffering fatal accidents. This rate of helmet use exceeded the 33 percent noted for the general skiing population, suggesting that helmet wearers were disproportionately represented among those who died. Helmet use did seem to affect cause of death, shifting it from head injuries for skiers without helmets to chest injuries for helmet wearers. In related work, Lana Ružic and Anton Tudor of the University of Zagreb administered questionnaires to determine risk tolerance in a group of 710 skiers and found four groups that independently stood out for their willingness to engage in dangerous behavior on the slopes: young people, men, experienced skiers, and helmet wearers. However, only male helmet users were open to greater risk-taking; for female skiers, helmet use made no difference.

Even if your behavior does not become riskier when you wear a helmet, you may be in greater danger due to changes in the behavior of those around you. Ian Walker of the University of Bath demonstrated this possibility in an experiment in which he rode his bicycle for a total of 320 kilometers while a video camera recorded how much clearance he was given by the drivers who passed him. For about half of the 2,355 times he was overtaken by cars he was wearing a helmet, and for the rest he was bare-headed. Sometimes Walker wore a long wig to give the passing drivers the impression that he was female. When he wore a helmet, drivers gave him about 5 centimeters less room as they passed (he was struck by cars twice during the experiment, and he was wearing a helmet on both occasions). On the other hand, drivers gave him 14 centimeters greater cushion when they thought he was a woman.

Whatever the implications of this kind of Peltzman effect by proxy, studies comparing injured and uninjured cyclists show that wearing a helmet is clearly associated with reducing the risk of injury. Nevertheless, some cyclists are concerned that, in addition to preventing injuries, helmet use may prevent something else—cycling. When mandatory bicycle helmet laws are enforced, people seem to cycle much less often. Because cycling has well-established health benefits, deciding on the merits of such laws may mean choosing between head injuries and diseases related to inactivity.

Perhaps it is because the brain is both valuable and vulnerable that we not only take special steps to safeguard it, but also change our actions in response to having our heads protected (enforcing helmet laws may make the activity less appealing and hence people are less likely to participate) and seeing protection applied to the heads of others (as players assume helmets will keep opponents safe). Developing effective strategies for reducing the incidence of head injury will require a better understanding of the intersection between biomechanics and behavior. In football, it may be that a behavior-based solution to the problem of head injury is already underway, without anyone having organized it. A Wall Street Journal/NBC News poll showed that 40 percent of U.S. adults would discourage their child from playing football because of concerns about concussions, and even President Barack Obama, an avid sports fan, recently said that if he had a son he would not want him to play football. Many have voiced concerns that football may go the way of boxing and become a sport without substantial youth participation due to its violence.

Although work continues on understanding how concussions occur and developing innovative and biologically inspired helmet designs, presently we do not have a technological solution for reducing injury rates. Engineering a better helmet, however, might win only half the battle. More effective helmets may be bulkier, more expensive, or may look unfashionable, any of which might make it less likely that people will actually wear them, but similar concerns have been overcome in the past. For example, professional hockey players first wore helmets in the 1930s, but large numbers of players did not wear them until the 1970s after the death of a player in 1968 from an on-ice head injury. Today helmets are required equipment in the National Hockey League. NASCAR racing has become much safer after the use of a Head and Neck Support (HANS) system became mandatory after the 2001 death of driver Dale Earnhardt, Sr. Unfortunately it often takes tragedy to motivate the widespread adoption of new technology designed to prevent injuries and save lives.