Biomechanics of Sport Concussion: Quest for the Elusive Injury Threshold

Kevin M. Guskiewicz; Jason P. Mihalik

Disclosures

Exerc Sport Sci Rev. 2011;39(1):4-11. 

In This Article

The Biomechanics of Concussion and Impact to the Brain

The biomechanics of TBI remains an area elusive to many researchers. Investigators in this area face a number of challenges in trying to understand head injury impact mechanics. Current ethics standards have made the use of primate and other mammalian animal models very difficult to pursue; animal basic research in this area has been limited to the rat and small mammals in recent years. Second, the use of postmortem cadavers does not allow researchers the ability to study impact mechanics in the context of everyday activities, including sport participation and work. The lack of muscle tonus and decreased volumes of cerebrospinal fluid further make it difficult to replicate an in vivo sample in the context of this area of study.

In the context of mTBI, the term "impact" typically denotes an injurious blow that makes direct contact with the head. An indirect impact, on the other hand, refers to an impact that sets the head in motion without directly striking it. Examples of impacts range from helmet-to-helmet collisions, striking an opponent's head with a stick, or being struck in the head by a projectile used in the sport (e.g., soccer ball, hockey puck, etc.). Indirect impacts are most commonly caused by tackling or body checking and are the result of abruptly stopping an opponent's body from traveling in the direction in which it was headed. To relate this notion in laypersons' terms, it is similar to the effect experienced by passengers when a car quickly accelerates or stops. Direct and indirect impacts are traditionally linear (translational) or angular (rotational) in nature. In real-world activities, there are usually some combinations of both linear and angular accelerations associated with direct and indirect impacts. Many factors are thought to play a role in the body's ability to dissipate head impact forces, including individual differences in cerebrospinal fluid levels and function, vulnerability to brain tissue injury, relative musculoskeletal strengths and weaknesses, and the anticipation of an oncoming direct or indirect impact. Thus, the relative contributions of angular and linear accelerations are not clearly understood with respect to mTBI.

Another underlying question in this area of research is why does every direct or indirect impact not result in an injurious episode? If the head does not move after a collision, the kinetic energy transferred by the blow should theoretically be transmitted elsewhere, leaving the athlete otherwise unharmed. It was this principle that did not allow researchers before Denny-Brown and Russell to more fully understand the phenomenon of mTBI.[4] However, even when the head does not move, kinetic energy can still be transferred through the skull, resulting in internal, potentially injurious, deformations. Furthermore, cerebrospinal fluid protects the brain within the cranium. As a result, some direct and indirect impacts do not exceed a threshold needed to drive the brain to impact the inside walls of the cranium and cause transient lesions and subsequent mTBI. When an athlete experiences a rotational mechanism, it is thought that rotation of the cerebrum about the brainstem produces shearing and tensile strains. Because activities in the midbrain and upper brainstem are responsible for alertness and responsiveness, rotational mechanisms of TBI are believed to more likely result in loss of consciousness than predominantly linear types of impacts.[25] Regardless of the type, attribute, or severity of a particular impact, the end result is as follows: the mass of the head has become too large for the body to overcome the acceleration or deceleration forces that have sent it in motion.

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