You can still get a concussion while wearing a helmet. A concussion is a mild traumatic brain injury (mTBI) that occurs when the head is suddenly moved, causing the brain to rapidly accelerate and decelerate inside the skull. This violent motion leads to functional disturbances in the brain cells, even without visible structural damage. Helmets are extremely effective safety devices, significantly reducing the risk of severe trauma. However, they are not designed to eliminate the specific forces that result in a concussion, meaning the injury remains a possibility even with the best head protection.
The Critical Distinction Between Skull Protection and Concussion Prevention
The primary engineering purpose of a helmet is to prevent catastrophic head injuries, which it achieves with remarkable success. Helmets are designed to manage linear impacts—a direct blow straight to the head—which can cause skull fractures and dangerous internal bleeding like subdural or epidural hematomas. The hard outer shell spreads the force of an impact over a larger surface area, preventing a concentrated blow from breaking the skull. The inner foam layer, often made of expanded polystyrene (EPS), absorbs energy by crushing and deforming. This process increases the time it takes for the head to decelerate, reducing the peak force transmitted to the skull and brain surface. By mitigating these direct, linear forces, helmets protect against injuries that commonly result in permanent disability or death.
How Concussions Occur Even with Head Protection
A concussion is typically caused by a different type of force than the one helmets are optimized to manage. While helmets effectively mitigate linear forces, they are less successful at stopping rotational forces, which involve angular momentum or twisting of the head. Rotational forces are often generated by oblique or glancing blows to the head, or even by impacts to the body that cause the head to whip suddenly.
The brain, suspended in cerebrospinal fluid, has a texture similar to soft gel and is highly sensitive to this twisting motion. When the head rotates rapidly, the brain lags behind the skull, causing different layers of tissue to move relative to one another. This relative movement creates shear strain, which stretches and deforms the axons, the long cable-like extensions of neurons. This stretching of neural tissue, known as diffuse axonal injury, is the primary physical event underlying a concussion.
Current helmet designs struggle to completely decouple the head from the rotational forces of an impact. The outer shell may absorb some linear energy, but the angular acceleration is still largely transferred through the helmet and into the head, causing the brain to rotate inside. Because concussions can be caused by rotation at levels of force far below what is required to fracture the skull, the helmet’s ability to protect the skull does not translate to full concussion prevention.
Factors Influencing Concussion Risk While Wearing a Helmet
The risk of sustaining a concussion while wearing a helmet is significantly influenced by variables beyond the helmet’s design limitations. Proper fit is one of the most important factors, as a loose helmet allows the head to move inside the shell before the padding engages, increasing both linear and rotational acceleration. Studies have shown that an improperly fitted helmet can be a risk factor for concussions with more severe symptoms and longer recovery times.
The specific type of helmet is also relevant, as different sports have gear designed to protect against distinct mechanisms of injury. For example, a hockey helmet is built to withstand high-velocity, low-mass impacts, while an American football helmet is designed for repeated, lower-velocity, high-mass impacts. These variations mean that a helmet optimized for one scenario may offer less protection in another.
Impact velocity and the mechanics of the collision also play a role, as higher-speed or unexpected impacts give the neck muscles less time to brace the head. Proper technique in contact sports, such as avoiding leading with the head or helmet in a collision, is a behavioral factor that can dramatically reduce the transmission of force to the brain. Ultimately, a helmet is a piece of safety equipment that reduces risk, but it must be paired with correct usage and an understanding of its biomechanical limits.