The primary goal of the American football helmet has been protection of players against skull fractures and other traumatic brain injuries (TBI) [Cantu 2003, Benson 2009]. TBI can result from short, high magnitude linear impact events typical of when the head impacts a hard surface [Gilcrhist 2003, Doorly 2007]. The modern helmet, which has evolved and become well designed to mitigate TBI injuries, does not offer sufficient protection against injury such as concussion, and the incident rate remains high in sport [Broglio 2009, Rowson 2012]. Researchers speculate rotation of the head leads to shear strain on the brain tissue, which may be the underlying mechanism of injury leading to concussive type injuries [Gennarelli 1971, Ommaya 1974, Gennarelli 1982, Prange 2002, Gilcrhist 2003, Aare 2003, Zhang 2004, Takhounts 2008, Greenwald 2008, Meaney 2011]. This has led researchers to investigate new liner materials and technologies to improve helmet performance and include concussive injury risk protection by attempting to address rotational acceleration of the brain [Mills 2003, Benson 2009, Caserta 2011, Caccese 2013]. To improve current football helmet designs, technology must be shown to reduce the motion of the brain, resulting in lower magnitudes of dynamic response thus reducing maximum principal strain and the corresponding risk of injury [Margulies 1992, Zhang 2004, Mills 2003, Kleiven 2007, Yoganandan 2008 Caserta 2011, McAllister 2012, Caccese 2013, Post 2013, Fowler 2015, Post 2015a/b]. Recent research has studied the use of decoupling liner systems in addition to the existing liner technology, to address resultant rotational acceleration. However, none of this previous work has evaluated the results in terms of the relationship between brain motion, tissue strain, and injury risk reduction. This thesis hypothesises the use of decoupling strategies to reduce the dominant coordinate component of acceleration in order to decrease maximum principal strain values. The dominant component of acceleration, defined as the coordinate component with the highest contribution to the resultant acceleration for each impact, is a targetable design parameter for helmet innovation. The objective of this thesis was to demonstrate the effect liner strategies to reduce the dominant component of rotational acceleration to decrease maximum principal strain in American football helmets.

Probabilistic finite element analysis was used to determine whether there is a statistically significant relationship between maximum principal strain orientations and orthotropic material stiffness orientations in a primate cranium during mastication. We first sought to validate our cranium finite element model by sampling in-vivo strain and in-vivo muscle activation data during specimen mastication. A comparison of in vivo and finite element predicted (i.e. in silico) strains was performed to establish the realism of the FEM model. To the best of our knowledge, this thesis presents the world’s only complete in-vivo coupled with in-vitro validation data set of a primate cranium FEM. Our results indicate that a validated FEM of a Cebus apella cranium was achieved. Giving collaborating anthropologists, biologists, and engineers the confidence that these models have sufficient accuracy to address the research questions pertaining to cranial structure morphology. Probabilistic finite element analysis design was then utilized to determine the dependence of maximum principal strain orientations on material stiffness orientations in particular craniofacial regions during mastication. It was discovered that the maximum principal strain orientations are more dependent on loading conditions and/or the shape of and location in the cranium rather than the material stiffness orientation of a particular region. It was also uncovered that the material stiffness orientations are not developed in a way that is optimal for feeding biomechanics from the perspective of minimization of total elastic strain energy. Results from this research will provide insights into the co-evolution of bone morphology and material properties in the facial skeleton.

Head injuries in American football is a serious issue regarding player health which is highly affected by velocity and its direction. Impact location can affect the severity of the head injury in both helmet-to-helmet impacts and helmet-to-ground impacts hence the understanding of concussive outcome from velocities and impact locations must be improved. In this thesis a video analysis resulted in simulation of five helmet-to-helmet impacts and two helmet-to-ground impacts, where velocity in each impact also was approximated with the method of least squares to avoid extreme values. The average velocity in helmet-to-helmet impacts was 5.1728 m/s for tackler player and 4.4766 m/s for tackled player and in helmet-to-ground impacts it instead was 6.1975 m/s. With the regression method an average velocity of 4.3982 m/s for tackler player and 5.3854 m/s for the tackled player in helmet-to-helmet impacts and 5.874 m/s in helmet-to-ground impacts. The simulations were performed with LS-DYNA and examined in LS-PrePost where head kinematics and the strain of brain tissue or more specific the maximum principal strain (MPS) was of interest. Further the MPS was scaled to its 95th percentile which determined the concussive likelihood for each impact scenario. The highest concussive outcome for an impact scenario was 100% and the lowest was 15%. The head kinematics of interest was linear acceleration, angular acceleration and angular velocity which in high risk for concussive outcome wasn't dominated by a single head kinematic. Impacts locations in helmet-to-helmet impacts didn't show any connection between impact location and high concussive risk. In helmet-to-ground impacts a connection between impact location at the back of the head and high concussive risk was observed. / Huvudskador inom Amerikansk fotboll är ett återkommande problem när det gäller spelarnas hälsa. Hastigheten och vart tacklingen träffar är starkt kopplat till hur allvarlig en huvudskada kan bli i både hjälm-mot-hjälm tacklingarn och hjälm-mot-mark. För att förhindra huvudskador måste förståelsen om kinematiken och vart tacklingen träffar förbättras.  I detta examensarbete gjordes en videoanalys vilket resulterade i fem hjälm-mot-hjälm simuleringar och två hjälm-mot-mark. Hastigheten approximerades också genom att använda uppskattnings metoden minsta kvadratmetoden. Medel-värdet av hastigheterna från videoanalysen blev 5.1728 m/s för spelaren som utförde tacklingen och 4.4766 m/s för spelaren som blev tacklad i hjälm-mot-hjälm tacklingar. I hjälm-mot-mark blev det istället ett medelvärde på 6.1975 m/s. Med uppskattnings metoden blev hastigheterna istället 4.3982 m/s för den tacklande spelaren och 5.3854 m/s för den tacklade spelaren i hjälm-mot-hjälm tacklingar. För hjälm-mot-mark blev medelvärdet av hastigheten 5.874 m/s med uppskattnings metoden. Simuleringarna av tacklings fallen gjordes med LS-DYNA och analyserades i LS-PrePost där huvudets kinematik och töjningen av hjärnvävnad är av intresse. Töjningen mättes av maximum principal strain (MPS) och den 95:e percentilen av MPS för att bestämma risken för hjärnskakning där den största risken för hjärnskakning var 100% och den minsta 15%. För huvudets kinematik var det linjär acceleration, vinkelacceleration och vinkelhastighet som var av intresse. Det fanns ingen koppling mellan endast en av kinematikerna och hög risk för hjärnskakning. Gällande vart tacklingen träffar fanns det ingen koppling mellan vart den träffar och hög risk för hjärnskakning i hjälm-mot-hjälm tacklingar. För hjälm-mot-mark tacklingar fanns det ett samband mellan att bakre delen av huvudet träffar marken och hög risk för hjärnskakning.