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  • 1.
    Alvarez, Victor
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Fahlstedt, Madelen
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Halldin, Peter
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Kleiven, Svein
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Importance of neck muscle tonus in head kinematics during pedestrian accidents2013In: 2013 IRCOBI Conference Proceedings - International Research Council on the Biomechanics of Injury, 2013, p. 747-761Conference paper (Refereed)
    Abstract [en]

    Unprotected pedestrians are an exposed group in the rural traffic and the most vulnerable human body region is the head which is the source of many fatal injuries. This study was performed to gain a better understanding of the influence that the neck muscle tonus has on head kinematics during pedestrian accidents. This was done using a detailed whole body FE model and a detailed FE vehicle model. To determine the influence of the muscle tonus a series of simulations were performed where the vehicle speed, pedestrian posture and muscle tonus were varied. Since the human reaction time for muscle activation is in the order of the collision time, the pedestrian was assumed to be prepared for the oncoming vehicle in order to augment the possible influence of muscle tonus. From the simulations performed, kinematic data such as head rotations, trajectory and velocities were extracted for the whole collision event, as well as velocity and accelerations at head impact. These results show that muscle tonus can influence the head rotation during a vehicle collision and therefore alter the head impact orientation. The level of influence on head rotation was in general lower than when altering the struck leg forward and backward, but in the same order of magnitude for some cases. The influence on head accelerations was higher due to muscle tonus than posture in all cases.

  • 2.
    Alvarez, Victor
    et al.
    KTH.
    Halldin, Peter
    KTH.
    Kleiven, Svein
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Biomedical Engineering and Health Systems, Neuronic Engineering.
    The Influence of Neck Muscle Tonus and Posture on Brain Tissue Strain in Pedestrian Head Impacts2014In: 58th SAE Stapp Car Crash Conference, STAPP 2014, Vol. 58Article in journal (Refereed)
    Abstract [en]

    Pedestrians are one of the least protected groups in urban traffic and frequently suffer fatal head injuries. An important boundary condition for the head is the cervical spine, and it has previously been demonstrated that neck muscle activation is important for head kinematics during inertial loading. It has also been shown in a recent numerical study that a tensed neck musculature also has some influence on head kinematics during a pedestrian impact situation. The aim of this study was to analyze the influence on head kinematics and injury metrics during the isolated time of head impact by comparing a pedestrian with relaxed neck and a pedestrian with increased tonus. The human body Finite Element model THUMS Version 1.4 was connected to head and neck models developed at KTH and used in pedestrian-to-vehicle impact simulations with a generalized hood, so that the head would impact a surface with an identical impact response in all simulations. In order to isolate the influence of muscle tonus, the model was activated shortly before head impact so the head would have the same initial position prior to impact among different tonus. A symmetric and asymmetric muscle activation scheme that used high level of activation was used in order to create two extremes to investigate. It was found that for the muscle tones used in this study, the influence on the strain in the brain was very minor, in general about 1-14% change. A relatively large increase was observed in a secondary peak in maximum strains in only one of the simulated cases. 

  • 3.
    Alvarez, Victor S
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Kleiven, Svein
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Importance of Windscreen Modelling Approach for Head Injury Prediction2016In: 2016 IRCOBI Conference Proceedings - International Research Council on the Biomechanics of Injury, 2016Conference paper (Refereed)
    Abstract [en]

    The objective of this study is to evaluate the capability of two modelling approaches in capturing  both accelerations and deformations from head impacts, and to evaluate the effect of modelling approach on  brain injury prediction. The first approach is a so‐called smeared technique, in which the properties of the two  glass  sheets and  the intermediate  polyvinyl  butyral  (PVB) are  combined and  divided into  two  coinciding  shell layers, of which one can fracture. The second approach consists of three shell layers, representing the glass and  PVB,  separated by  the  distance of  their  thickness, using a non‐local  failure criterion  to initiate  fracture in  the  glass.  The  two  modelling  approaches  are  compared  to  impact  experiments  of  flat  circular  windscreens,  measuring  deformations  and  accelerations  as  well  as  accelerations  from  impacts  against  full  vehicle  windscreens.  They  are  also  used  to  study  head‐to‐windscreen  impacts  using  a  detailed  Finite  Element  (FE)  model,  varying  velocity,  impact  direction  and  impact  point.  Only  the  non‐local  failure  model  is  able  to  adequately  capture  both  the accelerations and  deformations  of an  impactor. The FE  head model  simulations  also reveal that the choice of modelling approach has a large effect on the both localisation of the strain in the  brain and the characteristics of the strain‐time curve, with a difference in peak strain between 8% and 40%.  

  • 4.
    Fahlstedt, Madelen
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Halldin, Peter
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    S. Alvarez, Victor
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Kleiven, Svein
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Influence of the Body and Neck on Head Kinematics and Brain Injury Risk in Bicycle Accident Situations2016In: IRCOBI Conference Proceedings - International Research Council on the Biomechanics of Injury, International Research Council on the Biomechanics of Injury , 2016, p. 459-478Conference paper (Refereed)
    Abstract [en]

    Previous studies about the influence of the neck on head kinematics and brain injuries have shown different results. Today bicycle helmets are certified with only a headform in radial experiments but could be improved with oblique impacts. Then the question is how the helmet's performance will be affected by the neck and the rest of the body. Therefore, the objective of this study was to use finite element simulations to investigate the influence of the body on head kinematics and injury prediction in single bicycleaccident situations with and without a helmet. The THUMS-KTH model was used to study the difference between head only and full body. In total, a simulation matrix of 120 simulations was compared by altering initial impact posture, head protection, and muscle activation. The results show that the body in impacts against a hard surface can change the amplitudes and curve shapes of the kinematics and brain tissue strain. The study found an average ratio between head only and full body for peak brain tissue strain to be 1.04 (SD 0.11), for peak linear acceleration 1.06 (SD 0.04), for peak angular acceleration 1.08 (SD 0.09) and for peak angular velocity 1.05 (SD 0.13).

  • 5.
    S. Alvarez, Victor
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Understanding Boundary Conditions for Brain Injury Prediction: Finite Element Analysis of Vulnerable Road Users2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Vulnerable road users (VRUs) are overrepresented in the statistics on severe and deadly injuries in traffic accidents, most commonly involving the head. The finite element (FE) method presents the possibility to model complex interactions between the human body and vehicles in order to better understand the injury mechanisms. While the rapid development of computer capacity has allowed for increasingly detailed FE-models, there is always a benefit of reducing the studied problem. Due to its material properties, the brain is more sensitive to rotational motion than to purely linear, resulting in complex injury causation. When studying brain injuries caused by a direct impact to the head, simulations using an isolated head model significantly increases efficiency compared to using a complete human body model. Also evaluation of head protective systems uses isolated mechanical head representations. It is not, however, established the extent to which the boundary conditions of the head determine the outcome of brain injuries.

    FE models of both the entire human body and the isolated head were used in this thesis to study the effect of the body, as well as active neck muscle tension, on brain injury outcome in VRU accidents. A pediatric neck model was also developed to enable the study of age-specific effects. A vehicle windscreen model was developed to evaluate the necessity of capturing the failure deformation during pedestrian head impacts.

    It was shown that the influence of the neck and body on brain injury prediction is greater in longer duration impacts, such as pedestrian head-to-windscreen impacts with an average difference of 21%. In accidents with shorter duration impacts, such as head-to-ground bicycle accidents, the average influence was between 3-12%. The influence did not consistently increase or limit the severity, and was dependent on the degree of rotation induced by the impact, as well as the mode of deformation induced in the neck. It was also shown that the predicted brain injury severity is dependent on capturing the large deformations of fractured windscreen, with the greatest effect near the windscreen frame. The pediatric neck model showed a large effect of age-dependent anatomical changes on inertial head loading, making it a promising tool to study the age-dependent effects in VRU accidents.

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  • 6.
    S. Alvarez, Victor
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Fahlstedt, Madelen
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Halldin, Peter
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Pipkorn, Bengt
    Autoliv Research.
    Kleiven, Svein
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Influence of Body and Head Angular Velocity on Brain Injury Prediction in Pedestrian AccidentsManuscript (preprint) (Other academic)
    Abstract [en]

    Pedestrian protection has historically not been prioritized in the vehicle safety development, but represents a large portion of the severe and deadly injuries in vehicle accidents. One of the most common severely injured body parts is the head and the focus of many researchers and safety system developers. The Finite Element (FE) method is an increasingly popular approach to better understand the injury biomechanics, but due to the large system needed to be solved in pedestrian simulations a common approach is to reduce the problem to a head only impact. In EuroNCAP rating an isolated head form is impacted towards different regions of the vehicle with only linear velocity components. The aim of this study is to determine the effect of removing the neck and body, as well as rotational velocity components on the brain injury prediction. A pedestrian full body human FE model was impacted against a generalized buck model to simulate pedestrian accidents involving windscreen impacts, at three velocities (30, 40 and 50 or 45 km/h), two pedestrian velocities (0 and 5 km/h) and two standard walking gaits. The head position was extracted from the pedestrian full body simulations at 1 ms before head impact. The isolated head was impacted with the vehicle model using either all velocity components from the full body simulations, or only the linear components. The results show that the body and neck can affect the brain injury prediction in windscreen impacts, reducing the strains by up to 49%. It was also shown that removing the rotational impact velocities, in general, further increased the strain, with up to 138%. However, several cases showed a reduction in brain strains for the head only simulations by up to 40%, and in other cases only very small difference down to 1% were seen, indicating a high sensitivity to impact conditions and highlighting the difficulty in generalizing the effect. It is however generally seen that the body is limiting the severity in impacts close to the windscreen center, and amplifying the severity of those close to the lower frame. It could also be seen that removing the angular velocity, in most cases, further increased the difference between the full body and head only simulations.

  • 7.
    S. Alvarez, Victor
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Kleiven, Svein
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Effect of Pediatric Growth on Cervical Spine Injury Risk in Automotive CrashesManuscript (preprint) (Other academic)
    Abstract [en]

    Finite element (FE) models are a powerful tool that can be used to understand injury mechanisms and develop better safety systems. This study aims to extend the understanding of pediatric spine biomechanics, where there is a paucity of studies available. A newly developed and continuously scalable FE model was validated and scaled to 1.5-, 3-, 6-, 10-, 14- and 18-year-old using a non-linear scaling technique, accounting for local topological changes. The oldest and youngest ages were also scaled using homogeneous geometric scaling. To study the effect of pediatric spinal growth on head kinematics and intervertebral disc strain, the models were exerted to 3.5 g acceleration pulse at the T1 vertebra to simulate frontal, rear and side impacts. It was shown that the head rotation decreases with age, but is over predicted when geometrically scaling down from 18- to 1.5-year-old and under predicted when geometrically scaling up from 1.5- to 18-year-old. The strain in the disc, however, showed a clear decrease with age in side impact and for the upper cervical spine in rear impact, indicating a higher susceptibility for neck injury at younger ages. In the frontal impact, no clear age dependence could be seen, suggesting a large contribution from changed facet joint angles, and lower levels of strain, suggesting a lower risk of injury. The results also highlight the benefit of rearward facing children in a seat limiting head lateral motion.

  • 8.
    Strömbäck Alvarez, Victor
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Halldin, Peter
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Kleiven, Svein
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Influence of Neck Muscle Tone on Brain Tissue Strain during Pedestrian Impacts2014In: 11th World Congress on Computational Mechanics (WCCM XI), 5th European Conference on Computational Mechanics (ECCM V), July 20 - 25, 2014, Barcelona, Spain, 2014Conference paper (Refereed)
    Abstract [en]

    Unprotected pedestrians are an exposed group in rural traffic were the most vulnerable humanbody region is the head and the source of many fatal injuries. Brain tissue strain has been shown to correlate well with brain injuries in a detailed FE model [1]. This study was performed to gain a better understanding of the influence that the neck muscle tone has on brain tissue strain during pedestrian head impacts. The study was carried out using a detailed whole body FE model with a detailed neck [2], [3] and brain model [4]. To determine the influence of the muscle tone, a series of simulations were performed where the vehicle speed,pedestrian posture and muscle tone were varied. A generalized hood was also used to get the same impact surface in the different simulations and isolate the influence on strain due changed head kinematics. The influence of increased muscle stiffness was also isolated by adding the increased stiffnes momentaraly before head impact. Hence, the head kinematics did not have time to change and a change in strain was asumed to only be due to the changed neck stiffness. It has previously been shown that the neck muscle tone has a relatively small influence on head kinematics compared to posture, and hence head impact orientation [5]. The influence on brain tissue strain levels was however highly sensitive to impact point on a detailed vehicle due to the complex impact surface. When impacting a generalized surface the diffrences in strain between all simulations were significantly reduced and the influence due to muscle tone was in the same level as due to posture. The isolated influence of increased neck stiffness due to muscle tone was lower than the influence due to slightly changed head impact orientation. The increased neck stiffnes was therefore considered relatively unsignificant when considering brain injuries due to first impact on a vehicle structure in pedestrain accidents.

  • 9.
    Strömbäck Alvarez, Victor
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Halldin, Peter
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Kleiven, Svein
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    The Influence of Neck Muscle Tonus and Posture on Brain Tissue Strain in Pedestrian Head Impacts2014In: Stapp Car Crash Journal, ISSN 1532-8546, Vol. 58, p. ​63-101Article in journal (Refereed)
    Abstract [en]

    Pedestrians are one of the least protected groups in urban traffic and frequently suffer fatal head injuries. An important boundary condition for the head is the cervical spine, and it has previously been demonstrated that neck muscle activation is important for head kinematics during inertial loading. It has also been shown in a recent numerical study that a tensed neck musculature also has some influence on head kinematics during a pedestrian impact situation. The aim of this study was to analyze the influence on head kinematics and injury metrics during the isolated time of head impact by comparing a pedestrian with relaxed neck and a pedestrian with increased tonus. The human body Finite Element model THUMS Version 1.4 was connected to head and neck models developed at KTH and used in pedestrian-to-vehicle impact simulations with a generalized hood, so that the head would impact a surface with an identical impact response in all simulations. In order to isolate the influence of muscle tonus, the model was activated shortly before head impact so the head would have the same initial position prior to impact among different tonus. A symmetric and asymmetric muscle activation scheme that used high level of activation was used in order to create two extremes to investigate. It was found that for the muscle tones used in this study, the influence on the strain in the brain was very minor, in general about 1-14% change. A relatively large increase was observed in a secondary peak in maximum strains in only one of the simulated cases.

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