A concussion is a traumatic brain injury (TBI) that temporarily alters normal brain function. It results from a sudden blow or jolt to the head or body, causing the brain to accelerate and decelerate rapidly within the skull. Scientists quantify the physical forces involved in these injuries to predict concussion risk. This risk is measured by acceleration forces imparted to the head, expressed in units of “G” or G-force.
Understanding G-Force and Brain Acceleration
G-force, or gravitational force equivalent, is a measure of acceleration expressed in multiples of the acceleration due to gravity on Earth. One G represents the force of gravity; thus, a 50G impact means the head experienced an acceleration 50 times greater than gravity. In the context of head injury, G-force measures the severity of the change in velocity experienced by the brain mass during an impact.
Head impacts generate two primary types of acceleration. The first is linear acceleration, a straight-line force that causes the brain to move directly forward, backward, or sideways within the skull. Linear forces are often associated with focal injuries or skull fractures. The second, and generally more damaging, is rotational acceleration.
Rotational acceleration involves a twisting or glancing blow that causes the head to spin. This creates shearing and stretching forces on the brain’s internal structures. Because the brain’s white and gray matter have different densities, they move at different speeds during rotation. This differential movement stretches and tears the delicate axons, a mechanism strongly implicated in concussions and diffuse axonal injury. Rotational forces are considered a more significant predictor of concussive injury than linear forces alone.
The Concussion Threshold Range
Research indicates there is no single, definitive G-force number that guarantees a concussion; instead, a range exists where the risk is high. For linear acceleration, studies often cite a threshold range for adult athletes between approximately 70G and 120G. This range is derived from analyzing impacts in high-contact sports and controlled crash test simulations.
Rotational acceleration thresholds are typically expressed in radians per second squared (rad/s²), with concussions in adult athletes often occurring between 4,500 and 6,000 rad/s². However, researchers have recorded concussions from impacts as low as 60G, while other players have sustained hits exceeding 169G without a diagnosed injury. This variability highlights why a specific, absolute threshold is elusive.
Historically, the Head Injury Criterion (HIC) was a standard measure used to assess the risk of severe head injury. HIC primarily focuses on linear acceleration, making it less effective at predicting concussions, which are often caused by complex rotational forces. Modern research emphasizes that the injury results from the brain’s response to the force, not just the magnitude of the impact itself.
Factors Influencing Injury Severity
Whether an impact results in injury is complicated by several biological and physical variables. The location of the impact on the head significantly changes how the force is transmitted to the brain. For example, a glancing blow to the side of the head generates a higher degree of rotational acceleration, which is more damaging, than a direct, radial impact to the front.
The duration of the force application, known as the impulse duration, is also a significant factor. A very high G-force impact lasting only a few milliseconds may be less injurious than a slightly lower G-force impact sustained over a longer time period. This temporal characteristic affects the total energy transferred to the brain tissue.
Individual biomechanics contribute to injury susceptibility, including age and the strength of the neck musculature. Stronger neck muscles may help dampen the acceleration and deceleration of the head. Younger athletes, such as children and adolescents, appear to have a lower linear acceleration threshold for concussion, with some studies suggesting a range of 32G to 92G.
Measuring Impact Forces in the Real World
Data used to establish G-force thresholds are collected through specialized technology that measures head kinematics. The primary tools are accelerometers and gyroscopes embedded into wearable devices, such as helmets and mouthguards. These sensors record the linear and rotational acceleration forces experienced by the head in real time.
Instrumented mouthguards are increasingly favored because they are fixed to the upper teeth, positioning the sensors closer to the head’s center of gravity. This provides a more accurate measurement of actual head acceleration. Helmet-mounted sensors, in contrast, can sometimes overestimate acceleration due to the helmet’s movement relative to the skull.
Laboratory simulations using anthropomorphic test devices, or crash test dummies, are also employed to establish injury criteria under controlled conditions. Data from these studies, alongside on-field measurements, are fed into complex computer models of the human brain. Researchers use these models to predict the resulting tissue strain and refine injury risk models.