Projectiles, shockwaves, and other impacts have very different effects on the brain. But while the science of injury prevention is complicated, helmets are standard protection

The consequences of traumatic brain injury (TBI) are well-known: Many of those who survive an impact to the brain face short- and long-term memory loss, decreased motor skills, and a host of crippling disabilities. What's not as easily understood is the science behind these wounds—and whether helmets, the go-to safeguard against TBI, have what it takes to mitigate or prevent many of these injuries.

In this article, we look at some of the research connecting brain injuries and helmets, explaining how tactical helmets aim to spare military personnel, police officers, and others from the life-altering consequences of a severe blow to the head.

We also cover the limitations of modern equipment—and where helmet research and design need to go in the future.

A quick review of brain injury types

Brain injuries, and the physics underlying them, are involved. And although helmets worn in combat share broad similarities with those worn in athletics or other professional applications, each is somewhat tailored to specific threats in that field. 

Traumatic brain injuries broadly fall into one of two categories: open ("penetrating") or closed. In an open TBI, a foreign object breaks, fractures, or enters the skull. A closed TBI leaves the skull intact and is usually characterized by diffuse, rather than concentrated, damage. Expanding those categories, some of the most common injury types include:

• Concussions, where the head and brain move rapidly back and forth, chemically or mechanically altering cells and tissues.
• Diffuse axonal injuries, which damage the links between the brain's white and grey matter.
• Contusions, or bruises.
• Chronic traumatic encephalopathy (CTE), a degenerative brain disease linked to multiple head traumas.
• Skull fractures, which range from fine cracks to broken or crushed bones. Depending on the location and severity, these injuries may damage brain tissue, lead to a life-threatening loss of spinal fluid, or simply heal on their own.
• Subdural hematoma, a potentially life-threatening condition where blood released by damaged veins compresses brain tissue.

Figures from the U.S. Department of Defense (DoD) show that hundreds of thousands of service members have experienced a traumatic brain injury in the past 20 years, chiefly closed TBIs. As the graphic below shows, most injuries fall into the "mild" category, which includes concussions with only a short-term loss of consciousness and memory.

By comparison, TBIs involving skull fractures are rare: "penetrating" injuries make up just over 1% of the total.

Over the past two decades, TBIs in the Army, Marines, Air Force, and Navy have been overwhelmingly “mild” or “moderate.” Nevertheless, the lasting effects of these wounds can still be quite serious. Source: Health.mil

But as we'll get into shortly, “mild” injuries do not necessarily have mild effects.

The basic mechanics of helmets

Every collision, from a whizzing bullet to a head-on-pavement scenario, transfers energy from one object to another. Heavier, faster-moving objects with a low surface area are more likely to penetrate the materials they collide with, while things that weigh less and move more slowly are more likely to crush, bruise, or deform.

But in every situation, the energy of that collision goes somewhere—and, in the case of head injuries, it goes into the victim's skull, brain, or surrounding tissue. As the head stops, the brain continues its motion, causing it to crash “into [the] skull displacing cerebral spinal fluid.” This “stresses brain tissue both by compression and shear” forces. Compression is increased pressure on brain tissue, while a shear injury (also known as a diffuse axonal injury) stems from increased “rotational velocity” that can “distort and rupture axons, blood vessels and major fibre tracts.” 

The skull (and helmets) is designed to protect us. But these injuries can still happen when the “kinetic energy is dissipated in [the] brain rather than in [a] helmet or skull.” Nevertheless, helmets do create a significant extra layer of separation between the wearer and those forces.

While the obvious function of a helmet is to protect the wearer against penetrating injuries and skull fractures, guarding against TBIs requires accounting for some unique physics involving kinetic energy. It’s accomplished by reducing the head’s acceleration after an impact and lowering the amount of energy that makes its way to the brain. This is done by stopping an extreme rotation of the head and cushioning it.

Thus, each of the individual components of a helmet plays a role in protecting wearers from this kinetic energy and TBI—at minimum, keeping a bad impact from having more severe results:

• The retention system—typically a strap—stops the helmet from rolling off, ensuring that the shell and liner can distribute and absorb an impact's force as intended.
• The outer shell spreads the force of an impact over a larger area, reducing the energy felt in any one place, while also protecting against penetration. Thermoplastic shells cost less; composite types, like those made of carbon or glass, are typically tougher and lighter.
• The energy-absorbing liner serves as a cushion that reduces the force on the wearer's head—and with it, the sudden change in speed (acceleration) that would otherwise jostle the brain and damage the neck.

Tactical helmets, like Hard Head Veterans' Ballistic Helmet ATE® HHV, combine impact-absorbing layers with a tough outer shell to reduce the severity of many brain injuries.

A helmet spreads an impact over a larger area and a longer period of time, making collisions less severe and sudden. A core benefit is that it prevents particular hits from both penetrating and gives the wearer's head more time—a matter of crucial milliseconds—to safely come to a complete stop.

That said, all helmets—ballistic, football, motorcycle, and other types of headgear—have their limitations. A hard enough impact will send enough energy to diffuse itself in the brain, while modern helmets don’t adequately protect against many lower-velocity hits. And there are inevitable trade-offs in helmet design that make a piece of equipment better or worse for different impact velocities.

An introduction to tactical helmets

Combat-related brain injuries vary—and, as such, so does the protective headgear designed to prevent them. Bump helmets are designed, first and foremost, to withstand intense collisions. At a minimum, this headgear should meet Army purchasing description AR/PD 10-02, which requires that advanced combat helmets (ACHs) endure blunt impacts from multiple directions without causing the wearer's head to accelerate too quickly.

Like bump helmets, ballistic helmets protect against impact threats—but they also feature a tough outer shell that resists bullet penetration. While there's no such thing as an entirely "bulletproof" helmet, this equipment is laboratory-tested and classified according to the types of rounds it’s designed to stop.

Core standards for all types of ballistic-resistant materials are issued by the National Institute of Justice (NIJ), which evaluates performance against:

• .22 Long-Rifle and .38 Special rounds (NIJ Type I)
• .357 Magnum and 9 mm rounds at lower velocities—roughly 1,250 and 1,090 feet per second, respectively (NIJ Type II-A)
• .357 Magnum and 9 mm rounds at higher velocities—about 1,395 and 1,175 feet per second, respectively (NIJ Type II)
• .44 Magnum and submachine-gun-fired 9mm rounds at velocities of roughly 1,400 feet per second (NIJ Type III-A)
• High-powered rifle fire: 7.62-mm full metal jacketed ammo with a velocity of roughly 2,850 feet per second (NIJ Type III)
• .30 Caliber armor-piercing rifle fire at speeds of roughly 2,850 feet per second (NIJ Type IV)

Modern helmets generally cap out at Type III-A to balance protection with realistic weight. However, additional rifle-rated armor plates can be added to up the defense to Type III.

Hard Head Veterans' Ballistic Helmet ATE® HHV offers NIJ Level IIIA protection—enough to stop many .44 Magnum and submachine-gun-fired 9mm rounds.

Each of these NIJ-type ratings also protects against lesser threats, including some types of 12-gauge and handgun rounds. Further, the more advanced levels build on the protection offered by their lesser counterparts: Type II-A protects against the threats considered Type I, Type II against Type II-A and Type I threats, and so on.

That said, NIJ is not the end-all-be-all standard for ballistic or helmet protection. A series of subsequent helmet standards, including minimum specs for impact, fragmentation, and blunt impact resistance, have been developed as part of major helmet design and manufacturing contracts, such as the U.S. Army’s Advanced Combat Helmet (ACH). Individual manufacturers test to varying standards to create a helmet that offers adequate protection against a range of threats.

For a more in-depth look at the different standards and the tests designed to meet them, read this blog.

The intuitive conclusion is that both tactical helmets—ballistic and bump—reduce the likelihood of many open and closed TBIs. And while that's true, the relationship between their design and brain injury prevention is slightly more complicated.

Links between impact, brain injury, and helmet testing

Ballistic testing

Ballistic helmets used by members of the armed forces have long been evaluated in accordance with NIJ and individual helmet contract standards. During these tests, a helmet is placed on a model head, called a "headform," and fired upon from a series of angles; a clay section inside the headform measures the depth, shape, and location of impacts that deform or penetrate the shell.

The goal is to determine that the helmet successfully resists penetration by specific rounds—depending on the helmet type—and has an acceptable level of backface deformation (BFD).

Here's how the science of BFD works, as explained by Hard Head Veterans' own Sgt. Pecker:

Penetration leads to open and possibly fatal injuries—and if the shell deforms too readily, the impact can also cause skull fractures or severe closed-head trauma. While there's no scientific basis for a particular BFD limit, helmets manufactured to meet exacting standards can keep bullets from penetrating the skull. And when they do, wearers tend to report only minor injuries.

In short: the less backface deformation, the better.

Blunt impact testing

Blunt impact standards set acceptable limits for impact acceleration. These evaluate how helmets slow or spread the force of a collision and are intended to reduce brain injuries from falls, parachute drops, vehicle crashes, and more.

While there is a wide range of international blunt-impact standards, the US Army has based theirs largely on the US Department of Transportation Laboratory Test Procedure for Motorcycle Helmets (TP-218-06), which involves:

• Placing a rigid headform in the helmet
• Dropping the helmet and headform on an anvil at 10 feet per second
• Repeating drop tests on the crown, left and right sides, front and back, and left and right nape of the helmet
• Ensuring that the peak acceleration—the maximum speed—of the headform is less than 150g (g-force) or less

As with BFD, it's difficult to determine a "safe" cap on peak acceleration. Researchers at the University of Michigan have suggested that young football players may experience concussions at around 90 or 100 g-force. A study from the Association for the Advancement of Automotive Medicine—based on more than 27,000 recorded head impacts—suggests that a peak acceleration of 165g carries a one in 10 risk of mild traumatic brain injuries.

But in the end, peak acceleration is a measurement of how much force is transferred from an object to the wearer's head—the less, the better.

And while BFD and penetration testing are key in preventing TBI caused by shrapnel or small arms fire, impact assessments are the go-to measurement of a helmet's effectiveness against other threats to military and law enforcement personnel. Again, this harkens back to how the retention system, shell, and energy-absorbing liner work together to dissipate energy and slow down an impact to protect the brain.

Unfortunately, there are limitations to how much modern helmets can protect wearers against certain injuries, along with various trade-offs in design.

The limitations of the research and modern helmets that protect the brain from kinetic energy and TBI

Professional sports and the wars in Iraq and Afghanistan have generated intense focus on brain injuries over the past two decades. But as research progresses, diagnostics and our understanding of these wounds still have a lot of questions to answer.

Modern ballistic helmets and body armor have saved many, many lives and prevented more serious injuries, helping many people survive events that would have been fatal in previous wars. But this has left behind more individuals who also suffer poorly understood injuries that nevertheless have profound effects.

Explosions, for example, have unique impacts. They generate over-pressurization blast waves that can cause high-frequency stress waves and low-frequency shear waves, along with “blast winds” that “can exceed hurricane-force winds." Further, blasts have far less linear acceleration than impacts, along with a range of other characteristics that create a unique physics problem for helmet designers.

And diagnosing brain injuries—specifically, those judged as “mild”—can be difficult, as various imaging scans can’t easily detect the subtle damage done in less-severe injuries.

Nevertheless, these “less-severe” and under-diagnosed injuries can have severe consequences. Experiencing numerous “mild concussions” in a short period can result in chronic traumatic encephalopathy (CTE, aka “repetitive head injury syndrome”) with serious long-term, degenerative effects. This is one reason why suffering numerous low-level impacts have been judged by some researchers as the “most serious TBI issue” while being the “least understood.”

While there are various helmet testing standards for impacts, there is currently no standard for protecting against the chronic traumatic encephalopathy often caused by repeated, low-level hits.

The little green blob in the bottom-left-hand corner above shows the peak acceleration vs. time characteristics of blast simulations with injuries, whereas the smaller grey blog to the right shows NFL concussion data. These areas and their corresponding data are far removed from “the majority of biomechanical data” in the top-left corner commonly used to assess modern helmets. Source: “Closed TBI and its Protection: A Physics Perspective” (University of Rochester)

In addition, there are various trade-offs inherent to the design of helmets that relate to impacts. Professor Eric Blackman of the University of Rochester analyzed data and found that modern helmets' cushioning is better-suited to preventing skull fractures than closed TBIs. For example, while harder helmet pads protect “better at higher impact velocities,” softer pads are superior when it comes to “lower impact velocities,” the repetitive, sub-concussive impacts that can lead to CTE.

More research and advanced designs are needed to enhance helmet protection against a broader range of threats. But Blackman concluded that the best “immediate strategy” for impacts is to use a larger-size helmet and double the thickness of the padding. He notes that wearing a helmet without pads, such as older versions that use webbing, is “worse than [wearing] no helmet.”

Further, Blackman argues that helmet design needs to focus on stopping rotational energy and suggests that future headgear might "need [a] decoupled shell" that diverts that energy away from the wearer's head and brain.

The bottom line: current helmets effectively protect wearers from many TBI causes, including penetrating wounds and hard impacts. But advances are needed to develop equipment that stops more threats.

The future of combat helmets

Writing for the Australian Journal of Military and Veterans' Health, psychiatrist and veteran Dr. Duncan Wallace noted that:

"Combat helmets were designed primarily to protect wearers from blunt force trauma—from shrapnel, projectiles, and objects such as earth and rocks. However, the wars in Iraq and Afghanistan, with their frequent exposure to blast injury and subsequent traumatic brain injury have focused new demands on helmet design… At present, no existing helmet is able to fully protect against all threats faced on the battlefield."

Nevertheless, the science is evolving to meet new challenges, from the risks of explosives to the milder brain injuries suffered by those who—thanks to protective gear—survive. In the end, today’s tactical helmets do an unparalleled job of transforming many potentially fatal scenarios involving gunfire, falls, and vehicle collisions into non-fatal incidents.

Hard Head Veterans is proud to offer helmets that meet and exceed the current safety standards discussed here, including bump and ballistic models.

Our Ballistic Helmet ATE® keeps blunt impacts well below recommended levels—with a measured peak acceleration of less than 75g in 14 collisions—and protects against .44 Magnum and submachine-gun-fired 9mm rounds (NIJ Type III-A). And our gear has passed a range of rigorous tests, including those shown in this video:

And we will continue to provide the best headgear as new designs become available by rigorously testing our helmets to modern safety standards.

Still curious about combat-related injuries and the science of helmets? Read more about:

• New materials that make ballistic helmets lighter and safer
• The nuts and bolts of ballistic tests
• Factors you should consider before buying a bullet-resistant helmet

Hard Head Veterans stays on top of helmet research to ensure we provide the best protection possible. Read more blog posts, and be sure to check out our gear, including a selection of the best tactical helmets and essential helmet accessories.