American-style football participation is associated with high risks to a spectrum of sports-related brain injury involving acute reactions and chronic manifestations. Traditional methods of identifying injury have proven ineffective at protecting athletes and mitigating risk as they rely on the presence and recognition of inconsistent symptom expression. This is, in part, due to the lack of an objective measure of quantifying exposure. Brain trauma profiling was defined to capture a spectrum of exposure by incorporating the primary characteristics that associate with risk of neurological injury. This profile includes strain magnitude associated with impact, frequency at which impacts are experienced, time interval between impacts, over the duration of exposure. Trauma profiling methods differentiated player field position in professional American-style football where three unique trauma profiles were identified based on similarities among the characteristics of trauma. Regional strain from common head impacts showed that distribution was independent of field position regardless of variation in impact conditions. Rather, brain regions vulnerable to strains were dictated by the frequency and magnitude that govern the position profile. The extent of tissue volume involved in common head impacts was field position dependent. Skill positions tended to experience impacts involving greater tissue volumes reaching deeper white matter structures, but were infrequent. Impacts common to line positions typically involved less brain tissue of predominately superficial cortical gray matter, but were experienced at high frequency counts. The primary findings from this research show that brain trauma profiling may be used as an objective measurement tool to define exposure. The results indicate that exposure is not uniform and that brain trauma and injury risk can be described using unique combinations of these characteristics. Regional areas vulnerable to strain are dictated by the frequency and magnitude of impact and therefore in order to effectively protect against brain injury, both characteristics need to be managed. Lastly, this research demonstrates that either few impacts involving high brain volume or frequent impacts with little brain volume involvement may both result in brain dysfunction. Brain trauma profiling methods has broad application in future research. This measurement tool will be useful in identifying how injury occurs in various sports, military units, and particularly important for vulnerable populations and the developing brain. This knowledge is instrumental in establishing risk prevention strategies and public health policies for specific environments.

Head impacts can result in various levels of brain trauma, from mild to severe, and often result in long lasting effects on human brain function (McAllister & McCrea, 2017; Sollmann et al., 2018). Over the past two decades alone the National Hockey League (NHL) has made several rule changes to the game (Marek, 2015; National Hockey League Official Rules 2010–11, 2010; National Hockey League Official Rules 2011–12, 2011; National Hockey League Official Rules 2014-15, 2014). Frequency and magnitude are needed to examine brain trauma as examining brain trauma solely on magnitude does not capture a full brain trauma profile or the long-term consequences of repetitive brain strain; higher frequencies at lower magnitudes of strain may result in long-term neurologic complications. The purpose of this study was to compare frequency of head impacts and frequency-magnitude of brain strain between the 2003-04 and 2016-17 seasons of North American professional ice hockey. Videos of head impact events from twenty 2003-04 and twenty 2016-17 regular season NHL games were analyzed. Head impact conditions were characterized by events type, inbound velocity, location and elevation, and reconstructed using physical and finite element model methods. Overall frequency of head impacts was similar between the two seasons. Head-to-glass had the highest frequency for event type in both seasons. Mann-Whitney U tests found there was a significant decrease in glove-to-head impact events in the 2016-17 season compared to the 2003-04 (U=111, p=0.009). There was also a significant decrease in the frequency of fight events in 2003-04 during regulation time when compared to 2016-17 (U=86, p<0.001). A significant increase in the frequency of head impacts within the low MPS level was found in the 2016-17 season compared to 2003-04 (U=130, p=0.050). Given the popularity of ice hockey nationally, continentally, and globally, the results of this study provide a better understanding of frequency of head impacts and magnitude of brain strain, allowing stakeholders to make informed decisions involving repetitive brain strain during the game and give insight in the effectiveness of rules involving head contact. Future studies should consider including the effect of rule changes on overtime and pre- and post-season game play compared to in-season games.

Despite the alarming incidence rate and potential for debilitating outcomes of sports-related concussion, the underlying mechanisms of injury remain to be expounded. Since as early as 1950, researchers have aimed to characterize head impact biomechanics through in-lab and in-game investigations. The ever-growing body of literature within this area has supported the inherent connection between head kinematics during impact and injury outcomes. Even so, traditional metrics of peak acceleration, time window, and HIC have outlived their potential. More sophisticated analysis techniques are required to advance the understanding of concussive vs subconcussive impacts. The work presented within this thesis was motivated by the exploration of advanced approaches to 1) experimental theory and design of impact reconstructions and 2) characterization of kinematic profiles for model building. These two areas of investigation resulted in the presentation of refined, systematic approaches to head impact analysis that should begin to replace outdated standards and metrics.

Impact induced injury to the human head is a major cause of death and disability; this has driven considerable research in this field. Despite this, the methods by which the brain is damaged following non-penetrative (blunt) impact, where the skull remains intact, are not well understood. The mechanisms which give rise to brain trauma as a result of blunt head impact are frequently explored using indirect methods, such as finite element simulation. Finite element models are often created manually, but the complex anatomy of the head and its internal structures makes the manual creation of a model with a high level of geometric accuracy intractable. Generally, approximate models are created, thereby introducing large simplifications and user subjectivity. Previous work purports that blunt head impacts of short duration give rise to large dynamic transients of both positive and negative pressure in the brain. Here, three finite element models of the human head, of increasing biofidelity, were employed to investigate this phenomenon. A novel approach to generating finite element models of arbitrary complexity directly from three-dimensional image data was exploited in the development of these models, and eventually a highly realistic model of the whole head and neck was constructed and validated against a widely used experimental benchmark. The head models were subjected to a variety of simulated impacts, ranging from comparatively long duration to very short duration collisions. The dynamic intracranial pressure response, characterised by large transients of both positive and negative pressure in the brain, was observed following short duration impacts in all three of the models used in this study. The dynamic intracranial response was also recorded following short duration impacts of high energy, involving large impact forces, which were deemed to be realistic representations of actual impact scenarios. With the aid of an approximate analytical solution, analysis of the simulations revealed that the dynamic response is caused by localised skull deflection, which induces flexural waves in the skull. The implications of these magnified pressures are discussed, with particular regard to the potential for intracranial cavitation.

The current focus of research efforts in the area of the biomechanics of traumatic brain injury is the development of numerical (finite element) models of the human head. A validated numerical model of the human head may lead to better head injury criteria than those used currently in crashworthiness studies. A critical step in constructing a validated finite element model of the head is determining the mechanical threshold, should it exist, for various types of injury to brain tissue. This thesis describes a biomechanical study of axonal injury in the anaesthetised sheep. The study used the measurements of the mechanics of an impact to the living sheep, and a finite element model of the sheep skull and brain, to investigate the mechanics of the resulting axonal injury. Sheep were subjected to an impact to the left lateral region of the skull and were allowed to survive for four hours after the impact. The experiments were designed specifically with the numerical model in mind; sufficient data were collected to allow the mechanics of the impact to be faithfully reproduced in the numerical model. The axonal injury was identified using immunohistological methods and the injury was mapped and quantified. Axonal injury was produced consistently in all animals. Commonly injured regions included the sub-cortical and deep white matter, the hippocampi and the margins of the lateral ventricles. The degree of injury was closely related to the peak impact force and to kinematic measurements, particularly the peak change in linear and angular velocity. There was significantly more injury in animals receiving fractures. A three-dimensional finite element model of the sheep skull and brain was constructed to simulate the dynamics of the brain during the impact. The model was used to investigate different regimes of material properties and boundary conditions, in an effort to produce a realistic model of the skull and brain. Model validation was attempted by comparing pressure measurements in the experiment with those calculated by the model. The distribution of axonal injury was then compared with the output of the finite element model. The finite element model was able to account for approximately thirty per cent of the variation in the distribution and extent of axonal injury, using von Mises stress as the predictive variable. Logistic regression techniques were used to construct sets of curves which related the extent of injury, to the predictions of the finite element model, on a regional basis. The amount of observable axonal injury in the brains of the sheep was clearly related to the severity of the impact, and was related to the predictions of a finite element model of the impact. Future improvements to the fidelity of the finite element model may improve the degree to which the model can explain the variation in injury throughout the brain of the animal and variations between animals. This thesis presents results, and a methodological framework, that may be used to further our understanding of the limits of human endurance, in the tolerance of the brain to head impact. All experiments reported herein conformed with the Australian Code of Practice for the Care and Use of Animals for Scientific Purposes. / Thesis (Ph.D.)--Mechanical Engineering, 2000.

Head injuries are a major health concern for sport participants as 90% of emergency department visits for sport-related brain injuries are concussion related (Canadian Institute for Health Information, 2016). Recently, reports have shown a higher incidence of sport-related concussion in female athletes compared to males (Agel et al., 2007). Few studies have described the events by which concussions occur in women’s hockey (Delaney et al., 2014, Brainard et al., 2012; Wilcox et al., 2014), however a biomechanical analysis of the risk of concussion has not yet been conducted. Therefore, the purpose of this study was to identify the riskiest concussive events in elite women’s hockey and characterize these events through reconstructions to identify the associated levels of peak linear and angular acceleration and strain from finite element analysis. 44 head impact events were gathered from elite women’s hockey game video and analyzed for impact event, location and velocity. In total, 27 distinct events based on impact event, location and velocity were reconstructed using a hybrid III headform and various testing setups to obtain dynamic response and brain tissue response. A three-way Multivariate Analysis of Variance (MANOVA) was conducted to determine the influence of event, location and velocity. The results of this study show that head-to-ice impacts resulted in significantly higher responses compared to shoulder-to-head collisions and head-to boards impacts however, shoulder and boards impacts were more frequent. All events produced responses comparable to proposed concussion threshold values (Zhang et al., 2004). This research demonstrates the importance of considering the event, the impact characteristics, the magnitude of response, and the frequency of these impacts when attempting to capture the short and long term risks of brain trauma in women’s hockey.

Head injury is common in many different sports and elsewhere, and is often associated with differentdifficulties. One major problem is to identify and value the injury or the severity. Sometimes there is no sign of head injury, but a serious neck distortion has occurred, causing similar symptoms as head injuries e.g. concussion or mild TBI (traumatic brain injury). This study investigated whether direct and indirect measurements of head kinematics, combined with machine learning and 3D visualization can be used to identify head injury and value the injury. Injury statistics have found that many severe head injuries are caused by oblique impacts. An oblique impact will give rise to both linear and rotational kinematics. Since the human brain is very sensitive to rotational kinematics, many violent rotations of the head can results in large shear strains in the brain. This is when white matter and white matter connections are disrupted in the brain from acceleration and deceleration, or rotational acceleration kinematics which in turn will cause traumatic brain injuries as e.g. diffuse axonal injury (DAI). Lately there has been many studies in this field using different types of new technologies, but the most prevalent is the rise of wearable sensors that have become smaller, faster and more energy efficient where they have been integrated into mouthguards and inertial measurement units (IMUs) the size of a sim-card that measures and reports a body's specific force. It has been shown that a 6-axis IMU (3-axis rotational- and 3-axis acceleration measurements) may improve head injury prediction but more data is needed to confirm with existing head injury criterions and new criterions needs to be developed, that considers directional sensitivity. Today, IMUs are typically used in self-driving cars, aircrafts, spacecrafts, satellites etc. As of today, more and more studies have evaluated and utilized IMUs in new uncharted fields have shown promises, especially in sports, and in the neuroscience and medical field. This study proposed a method to 3D visualize head kinematics during the event of a possible head injury to indirectly identify and value the injury, by medical professionals, as well as, a direct method to identify and also value the severity of head injury with machine learning. An erroneous data collection process of reconstructed head impacts and non-head impacts have been recorded using an open-source 9-axis IMU sensor and a proprietary 6-axis IMU sensor. To value the head injury or the severity, existing head injury criterions as the Abbreviated Injury Scale (AIS), Head Injury Criterion (HIC), Head Impact Power (HIP), Severity Index (SI) and Generalized Acceleration Model for Brain Injury Threshold (GAMBIT) have been introduced. To detect head impact including the severity and non-head impact, a Random Forests (RF) classifier and Support Vector Machine (SVM) classifiers with linear- and radial basis function have been proposed, the prediction results have been promising.

In Sweden fall-related injuries among the elderly lead to over a thousand deaths and close to 300.0000 hospital visitations annually. Fall related injuries can in many cases lead to serious head injuries along with other fractures. The elderly are more prone to fall and sustain an injury because of conditions such as osteoporosis and weak muscles. Researchers at KTH from the Division of Neuronic Engineering have developed a special floor that absorbs energy from an impact which can reduce the severity of injury when a person falls. There has been research done on the floor focusing on hip fractures which have shown good results. The floor has been set up in an elderly home in Stureby, Stockholm, Sweden, however further testing is needed to fully develop the floor to its maximum capacity. One goal of this master thesis was to investigate certain obstacles with the floor which arose during testing in Stureby. One of the challenges was that the floor had greater rolling resistance since it is softer than a normal concrete floor. The experiment was done by using a dynamo meter to measure the force needed to move a test wagon with a fixed weight and different sizes of wheels. Another goal of this thesis was to investigate head impacts on the KTH floor. That was done in a helmet testing lab at Mips helmet company. There a dummy head was dropped from various heights and the acceleration was measured. The 1st principle strain of the brain was calculated from the collected data. Other companies have created similar floors so it was a part of the thesis to compare the KTH floor with its competitors. The results from the rolling resistance showed that by increasing the width and diameter of the wheels there was less force needed to move the test wagon. Further tests are needed to investigate the problem regarding the indentation issues seen on the floor set up in Stureby. Some research has already begun with different types of linoleum and glue. In the head impact studies, the KTH floor showed the best results of all tested floors when dropped from 60 cm. However, when dropped from 100 cm a competitor floor showed better results in the resultant translational acceleration.

Despite evidence indicating negative consequences of repetitive head impacts (RHIs) on the brain, the long-term effects remain largely unknown. Contact sports, such as football, expose players to multiple collisions. Professional sports players have undergone thousands of concussive and sub-concussive RHIs over their careers. In this study we used structural 3T MRI to evaluate cortical thickness of 86 former NFL players (mean age ± SD = 54.9 ± 7.9 years old) and 24 former professional non-contact sport athletes as controls (mean age ± SD =57.2 ± 6.9 years old). Cortical thickness was compared between groups using FreeSurfer. The NFL players displayed decreased cortical thickness in the right temporal lobe and fusiform gyrus (cluster-wise p-value=0.0003, 90% CI=0.0001-0.0005) and the left pre- and postcentral gyrus (cluster-wise p-value=0.0096, 90% CI=0.0084-0.0109). When looking only at NFL subjects impaired in measurements of mood and behavior (n=36) compared to controls, NFL players displayed a similar but more extensive cluster of decreased cortical thickness in the right temporal lobe and fusiform gyrus (cluster-wise p-value=0.0001, 90% CI=0.0000-0.0002) and in the left supramarginal gyrus and pre- and postcentral gyrus, (cluster-wise p-value=0.0002, 90% CI=0.0000-0.0004). Reduced cortical thickness in NFL players is suggestive of the long-term effects of RHIs. Still, future studies are necessary for examining the time-course of damage and the implications of regional cortical thinning.

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