Whereas the GSI was a good tool for estimating short-duration impacts (ie, focal brain injuries), it was not as good at estimating longer-duration injuries that are more indicative of diffuse brain injury. Building on the WSTC, additional impact-severity measures, including the Gadd Severity Index 36 (GSI) and Head Injury Criteria 37 (HIC), were developed to study moderate to severe TBI. 35 The concept behind the WSTC is that humans can tolerate larger acceleration magnitudes for shorter periods and smaller accelerations for longer periods. 34 The objective of the WSTC was to inform protective material development by understanding the risk of skull fracture in moderate and severe TBI. In addition to simple magnitudes, impact-severity measures quantify injury tolerance, and the original work in car impacts yielded the Wayne State Tolerance Curve (WSTC). Therefore, in some instances, lowering resultant LA magnitude lowers resultant AA. 28 Given that few impacts are solely aligned with the head's center of gravity, most impacts comprise both linear and rotational components. Alternatively, force vectors in line with the center of gravity create greater resultant LA. Vectors farther away from the center of gravity create greater resultant AA relative to resultant LA. 26, 27 The relative magnitude of resultant LA and AA depends on the distance between the force vector and the center of gravity of the head. Resultant LA and AA are closely correlated with each other. Similarly, resultant AA is the vector sum of the 3-dimensional AAs of the skull resulting from an impact and is measured in units of radians per second squared. It is measured in gravitational units ( g), which is equal to the acceleration due to gravity (approximately 9.81 m/s 2). Resultant LA is the vector sum magnitude of the 3-dimensional LAs of the skull resulting from an impact. Acceleration represents the rate of change in velocity, and in this review, we report resultant linear accelerations (LAs) and resultant angular accelerations (AAs) of the head. Instead, skull acceleration is measured as a correlate to the pressure and strain responses of brain tissues. It is impossible to directly measure the tissue-level response of the brain to impact in vivo. 25 If the pressure gradients or strains exceed the tolerable limits of the brain tissue, injury occurs. 24 These accelerations result in transient pressure gradients and strain fields within the soft tissue of the brain. Impacts to the head cause a combined linear and angular acceleration of the skull. 23 If the kinematics of concussive injury are elucidated, clinicians may be able to more rapidly identify the concussed athlete and advise protective-equipment improvements or rule changes to mitigate concussion risk. Head-impact biomechanics have been investigated to determine the kinematic signature of a concussion. 22 Whereas these measures assess the clinical symptoms of concussion, no measure can identify the concussed athlete while on the field or be used as a preventive tool. Researchers have investigated more objective measures for concussion diagnosis, including balance testing, 20 neuropsychological testing, 21 and advanced imaging. 17– 19 Therefore, objective and quantitative diagnostic tools that are more sensitive and specific to concussive injury are needed. 12– 14 However, given underreporting, 5, 7 transient symptoms, 15 delayed onset of symptoms, 16 and the few concussions occurring with loss of consciousness, concussions are often difficult to detect and diagnose. It does not store any personal data.Removing injured athletes from participation close to the time of injury reduces the risk of secondary injury when they are vulnerable to the cumulative effect of concussions.
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