Response Corridors Research Articles

Response corridors for the medial–lateral compressive stiffness of the human arm: Implications for side impact protection

The biofidelity of side impact anthropomorphic test devices (ATDs) is crucial in order to accurately predict injury risk of human occupants. Although the arm serves as a load path to the thorax, there are currently no biofidelity response requirements for the arm. The purpose of this study was to characterize the compressive stiffness of male and female arms in medial–lateral loading and develop corresponding stiffness response corridors. This was accomplished by performing a series of pendulum tests on 18 isolated post-mortem human surrogate (PMHS) arms, obtained from four male and five female surrogates, at impact velocities of 2m/s and 4m/s. Matched tests were performed on the arm of the SID-IIs ATD for comparison. The arms were oriented vertically with the medial side placed against a rigid wall to simulate loading during a side impact automotive collision. The force versus deflection response data were normalized to that of a 50th percentile male and a 5th percentile female using a new normalizing technique based on initial arm width, and response corridors were developed for each impact velocity. A correlation analysis showed that all non-normalized dependent variables (initial stiffness, secondary stiffness, peak force, and peak deflection) were highly correlated with the initial arm width and initial arm circumference. For both impact velocities the PMHS arms exhibited a considerable amount of deflection under very low force before any substantial increase in force occurred. The compression at which the force began to increase considerably was consistent with the average tolerable medial–lateral arm compression experienced by volunteers. The initial stiffness (K1), secondary stiffness (K2), peak force, and peak deflection were found to significantly increase (p<0.05) with respect to impact velocity for both the non-normalized and normalized PMHS data. Although the response of the SID-IIs arm was similar to that of the female PMHS arms for both impact velocities, the SID-IIs arm did not exhibit a considerable toe region and therefore did not fall within the response corridors for the 5th percentile female. Overall, the results of the current study could lead to improved biofidelity of side impact ATDs by providing valuable data necessary to validate the compressive response of the ATD arm independent of the global ATD thoracic response.

Further Validation of Age-Dependent FE Models of a Mid-Sized Male Thorax

The objective of this study was to further validate three previously-developed, age-dependent finite element models representing 35, 55, and 75 year old mid-sized males. The validation was based on comparisons with the following published tests involving post mortem human subjects: oblique thoracic and abdominal pendulum impact (4-10 m/s), oblique and lateral thoracic pendulum impact (2.5 m/s), and lateral thoracic pendulum impact (4.3 and 6.7 m/s). The responses of the models were compared to cadaveric response corridors and responses from specific cadavers similar in size and age. When compared to the cadaveric response corridors, the model responses were generally within those corridors. When compared to the responses of specific cadavers, the results were mixed. In some of the cases the model responses predicted the age-dependency of the cadaveric responses. In other cases, the model responses had the opposite trend of those of the cadavers. The comparisons of the model-predicted impact force, normalized chest compression, and viscous criterion to those of the cadavers were deemed acceptable. The model-predicted peak rib stress and strain for the different simulations were also reported and were compared to the injury outcome of the tests. Language: en

Dynamic Kinematic Responses of Female Volunteers in Rear Impacts and Comparison to Previous Male Volunteer Tests

Objectives: The objective was to quantify dynamic responses of 50th percentile females in rear impacts and compare to those from similar tests with males. The results will serve as a basis for future work with models, criteria, and safety systems. Methods: A rear impact sled test series with 8 female volunteers was performed at velocity changes of 5 and 7 km/h. The following dynamic response corridors were generated for the head, T1 (first thoracic vertebra) and head relative to T1: (1) accelerations in posterior–anterior direction, (2) horizontal and vertical displacements, (3) angular displacements for 6 females close to the 50th percentile in size. Additionally, the head-to-head restraint distance and contact time and neck injury criterion (NIC) were extracted from the data set. These data were compared to results from previously performed male volunteer tests, representing the 50th percentile male, in equivalent test conditions. T-tests were performed with the statistical significance level of .05 to quantify the significance of the parameter value differences for the males and females. Results: At 7 km/h, the females showed 29 percent earlier head-to-head restraint contact time (p = .0072); 27 percent shorter horizontal rearward head displacement (p = .0017); 36 percent narrower head extension angle (p = .0281); and 52 percent lower NIC value (p = .0239) than the males in previous tests. This was mainly due to 35 percent shorter initial head-to-head restraint distance for the females (p = .0125). The peak head acceleration in the posterior–anterior direction was higher and occurred earlier for the females. Conclusions: The overall result indicated differences in the dynamic response for the female and male volunteers. The results could be used in developing and evaluating a mechanical and/or mathematical average-sized female dummy model for rear impact safety assessment. These models can be used as a tool in the design of protective systems and for further development and evaluation of injury criteria.

Understanding whiplash injury and prevention mechanisms using a human model of the neck

Objectives Various models for rear crash simulation exist and each has unique advantages and limitations. Our goals were to: determine the neck load and motion responses of a human model of the neck (HUMON) during simulated rear crashes; evaluate HUMON's biofidelity via comparisons with in vivo data; and investigate mechanisms of whiplash injury and prevention. Methods HUMON, consisting of a neck specimen ( n = 6) mounted to the torso of BioRID II and carrying a surrogate head and stabilized with muscle force replication, was subjected to simulated rear crashes in an energy-absorbing seat with fixed head restraint (HR) at peak sled accelerations of 9.9 g (Δ V 9.2 kph), 12.0 g (Δ V 11.4 kph), and 13.3 g (Δ V 13.4 kph). Physiologic spinal rotation ranges were determined from intact flexibility tests. Average time–history response corridors (±1 standard deviation) were computed for spinal motions, loads, and injury criteria. Results Neck loads generally increased caudally and consisted of shear, compression, and flexion moment caused by straightening of the kyphotic thoracic and lordotic lumbar curvatures, upward torso ramping, and head inertial and head/HR contact loads. Nonphysiologic rotation occurred in flexion at C7/T1 prior to head/HR contact and in extension at C6/7 and C7/T1 during head/HR contact. Conclusions HUMON's neck load and motion responses compared favorably with in vivo data. Lower cervical spine flexion–compression injuries prior to head/HR contact and extension–compression injuries during head/HR contact may be reduced by refinement of existing seatback, lapbelt, and HR designs and/or development of new injury prevention systems.

A Stochastic Visco-hyperelastic Model of Human Placenta Tissue for Finite Element Crash Simulations

Placental abruption is the most common cause of fetal deaths in motor-vehicle crashes, but studies on the mechanical properties of human placenta are rare. This study presents a new method of developing a stochastic visco-hyperelastic material model of human placenta tissue using a combination of uniaxial tensile testing, specimen-specific finite element (FE) modeling, and stochastic optimization techniques. In our previous study, uniaxial tensile tests of 21 placenta specimens have been performed using a strain rate of 12/s. In this study, additional uniaxial tensile tests were performed using strain rates of 1/s and 0.1/s on 25 placenta specimens. Response corridors for the three loading rates were developed based on the normalized data achieved by test reconstructions of each specimen using specimen-specific FE models. Material parameters of a visco-hyperelastic model and their associated standard deviations were tuned to match both the means and standard deviations of all three response corridors using a stochastic optimization method. The results show a very good agreement between the tested and simulated response corridors, indicating that stochastic analysis can improve estimation of variability in material model parameters. The proposed method can be applied to develop stochastic material models of other biological soft tissues.

Comparison of Hybrid III Child Test Dummies to Pediatric PMHS in Blunt Thoracic Impact Response

The limited availability of pediatric biomechanical impact response data presents a significant challenge to the development of child dummies. In the absence of these data, the development of the current generation of child dummies has been driven by scaling of the biomechanical response requirements of the existing adult test dummies. Recently published pediatric blunt thoracic impact response data provide a unique opportunity to evaluate the efficacy of these scaling methodologies. However, the published data include several processing anomalies and nonphysical features. These features are corrected by minimizing instrumentation and processing error to improve the fidelity of the individual force-deflection responses. Using these data, biomechanical impact response corridors are calculated for a 3-year-old child and a 6-year-old child. These calculated corridors differ from both the originally published postmortem human subject (PMHS) corridors and the impact response requirements of the current child dummies. Furthermore, the response of the Hybrid III 3-year-old test dummy in the same impact condition shows a similar deflection but a significantly higher force than the 3-year-old corridor. The response of the Hybrid III 6-year-old dummy, on the other hand, correlates well with the calculated 6-year-old corridor. The newly developed 3-year-old and 6-year-old blunt thoracic impact response corridors can be used to define data-driven impact response requirements as an alternative to scaling-driven requirements.

Development of Biomechanical Response Corridors of the Head to Blunt Ballistic Temporo-Parietal Impact

There is a need to study the biomechanical response of the head to blunt ballistic impact. While the frequency of less-lethal munition impacts to the head may be less than other vital body regions, more serious injuries have been attributed to these impacts. This study aims to establish biomechanical response corridors for the temporo-parietal region for future development of biomechanical surrogate devices. Seven unembalmed post-mortem human subject specimens were exposed to blunt ballistic temporo-parietal head impact (103 g, 38 mm diameter impactor) to determine the force-time, deformation-time, and force-deformation responses. Comparisons were made to responses from prior blunt ballistic head impact studies, as well as automotive-related impact studies. Peak forces for impact condition A (19.5+/-2.6 m/s) were 3659+/-1248 N with deformations at peak force of 7.3+/-2.1 mm. Peak forces for impact condition B (33.6+/-1.4 m/s) were 5809+/-1874 N with deformations at peak force of 9.9+/-2.6 mm. Seven fractures were produced in the seven specimens. Depressed comminuted fracture types were documented in six of the seven cases. The average stiffness of the temporo-parietal region under blunt ballistic impact was 0.46+/-0.14 kN/mm. Stiffness results indicate that the response of the temporo-parietal region is similar to the forehead under blunt ballistic loading conditions. In addition, the response is significantly less stiff when compared with temporo-parietal impacts performed in automotive-related studies. These data provide the foundation for future research in the area of blunt ballistic head impact research including the development of biomechanical surrogates and computational models.

Tension and Combined Tension-Extension Structural Response and Tolerance Properties of the Human Male Ligamentous Cervical Spine

Tensile loading of the human cervical spine results from noncontact inertial loading of the head as well as mandibular and craniofacial impacts. Current vehicle safety standards include a neck injury criterion based on beam theory that uses a linear combination of the normalized upper cervical axial force and sagittal plane moment. This study examines this criterion by imposing combined axial tension and bending to postmortem human subject (PMHS) ligamentous cervical spines. Tests were conducted on 20 unembalmed PMHSs. Nondestructive whole cervical spine tensile tests with varying cranial end condition and anteroposterior loading location were used to generate response corridors for computational model development and validation. The cervical spines were sectioned into three functional spinal segments (Occiput-C2, C4-C5, and C6-C7) for measurement of tensile structural response and failure testing. The upper cervical spine (Occiput-C2) was found to be significantly less stiff, absorb less strain energy, and fail at higher loads than the lower cervical spine (C4-C5 and C6-C7). Increasing the moment arm of the applied tensile load resulted in larger head rotations, larger moments, and significantly higher tensile ultimate strengths in the upper cervical spine. The strength of the upper cervical spine when loaded through the head center of gravity (2417+/-215 N) was greater than when loaded over the occipital condyles (2032+/-250 N), which is not predicted by beam theory. Beam theory predicts that increased tensile loading eccentricity results in decreased axial failure loads. Analyses of the force-deflection histories suggest that ligament loading in the upper cervical spine depends on the amount of head rotation orientation, which may explain why the neck is stronger in combined tension and extension.

C4–C5 segment finite element model development, validation, and load-sharing investigation

Detailed cervical spine models are necessary to better understand cervical spine response to loading, improve our understanding of injury mechanisms, and specifically for predicting occupant response and injury in auto crash scenarios. The focus of this study was to develop a C4–C5 finite element model with accurate representations of each tissue within the segment. This model incorporates more than double the number of elements of existing models, required for accurate prediction of response. The most advanced material data available were then incorporated using appropriate nonlinear constitutive models to provide accurate predictions of response at physiological levels of loading. This tissue-scale segment model was validated against a wide variety of experimental data including different modes of loading (axial rotation, flexion, extension, lateral bending, and translation), and different load levels. In general, the predicted response of the model was within the single standard deviation response corridors for both low and high load levels. Importantly, this model demonstrates that appropriate refinement of the finite element mesh, representation at the tissue level, and sufficiently detailed material properties and constitutive models provide excellent response predictions without calibration of the model to experimental data. Load sharing between the disc, ligaments, and facet joints was investigated for various modes of loading, and the dominant load-bearing structure was found to correlate with typical anatomical injury sites for these modes of loading. The C4–C5 model forms the basis for the development of a full cervical spine model. Future studies will focus on tissue-level injury prediction and dynamic response.

Dynamic Responses of Female and Male Volunteers in Rear Impacts

Objectives: Whiplash injuries from vehicle collisions are common and costly. These injuries most frequently occur as a result of a rear impact and, compared to males, females have up to twice the risk of whiplash-associated disorders (WAD) resulting from vehicle crashes. The present study focuses on the differences in the dynamic response corridors of males and females in low-severity rear impacts. Methods: In this study, analysis of data from volunteer tests of females from previously published data has been performed. Corridors for the average female response were generated based on 12 volunteers exposed to a change of velocity of 4 km/h and 9 volunteers exposed to a change of velocity of 8 km/h. These corridors were compared to corridors for the average male response that were previously generated based on 11 male volunteers exposed to the same test conditions. Results: Comparison between the male and female data showed that the maximum x-acceleration of the head for the females occurred on average 10 ms earlier and was 29% higher during the 4 km/h test and 12 ms earlier and 9% higher during the 8 km/h test. Head-to-head restraint contact for the females occurred 14 ms earlier at 4 km/h and 11 ms earlier at 8 km/h compared to the males. For the same initial head-to-head restraint distance, head restraint contact occurred 11 and 7 ms earlier for the females than the males at 4 and 8 km/h, respectively. Furthermore, the calculated Neck Injury Criteria (NIC) values were similar for males and females at 4 km/h, whereas they were lower for females compared to the males at 8 km/h (3.2 and 4.0 m2/s2, respectively). Conclusions: The results of this study highlight the need to further investigate the differences in dynamic responses between males and females at low-severity impacts. Such data are fundamental for the development of future computer models and dummies for crash safety assessment. These models can be used not only as a tool in the design and development process of protective systems but also in the process of further evaluation and development of injury criteria.

Biomechanical response of the human mandible to impacts of the chin

The purpose of this study was to determine the force–time and force–displacement response of the human mandible under direct loading at the chin. Sub-fracture response of the mandible and temporomandibular joint (TMJ) were analyzed from 10 cadavers that were impacted at the chin with a 2.8-kg mass at drop heights of 300, 400 and 500 mm and a 5.2-kg mass at 500 mm. Motion of radio-opaque markers adhered to the surface of the bone was recorded at 1000 Hz by a bi-planar X-ray and converted to three-dimensional coordinates. Peak force ranged from 0.90 to 4.54 kN causing chin displacement of 1.2–4.4 mm. A bi-linear response was observed with stiffness of 475.1±199.8 kN/m for chin displacement resulting from loading up to 0.6 kN and 2381.6±495.7 kN/m for loads from 0.6 to 3.25 kN. This defines the biomechanical response of the mandible for chin motion under impact loading. The response of different segments of the mandible and TMJ are also documented. Force–time and force–displacement response corridors for the mandible can be used for finite element model and/or the development and validation of a biomechanical surrogate.

Validation of a Finite Element Model of the Young Normal Lower Cervical Spine

A Finite Element Model (FEM) of the young adult human cervical spine has been developed as a first step in studying the process of spondylotic degeneration. The model was developed using normal geometry and material properties for the lower cervical spine. The model used a three-zone composite disc annulus to reflect the different material properties of the anterior, posterior, and lateral regions of the annulus. Nonlinear ligaments were implemented with a toe region to help the model achieve greater flexibility at low loads. The model was validated against experimental data for normal, nondegenerated cervical spines tested in flexion and extension, right and left lateral bending, and right and left axial rotation at loads of 0.33, 0.5, 1.0, 1.5, and 2.0 Nm. The model was within in vitro experimental standard deviation corridors 100% of the load range for right and left lateral bending. The model was within 80% of the load response corridors for extension and flexion with a deviation <0.3 degrees from the SD corridors. For axial rotation, the model was within 70% of the SD corridors for left axial rotation within 83% of right axial rotation responses. The deviation from SD corridors for axial rotation was generally <0.2 degrees.

Three-dimensional finite element simulation of pelvic fracture during side impact with pelvis–femur–soft tissue complex

Pelvic injuries and fractures (abbreviated injury scale > 2) are caused by intrusion of car door structure during motor vehicle side impacts. The objective was to simulate automotive side impact situation for the occupant. This was performed with a computed tomography scan-based three-dimensional finite element (FE) model of human pelvis–femur–soft tissue complex with spring–dashpot–mass representation of whole body, using wide range of mechanical property variations. It was impacted with a 12-kg rigid impactor at 13.54 m s–1 to impart impact energy of 1100 J. The pelvic fracture condition was well-examined by the peak impact force (16.98 kN), peak strain (23.4%) and peak stress (189 MPa). It was concluded that this FE model could reconstruct clinically observed pelvic injury. Moreover, this FE model could reduce the need for modelling the complex whole body with a huge number of elements and nodes when pelvic fracture is the area of focus. Furthermore, this detailed FE model may be used for finding out different response corridors for automobile side impact, and the detailed responses may be used for developing occupant protective systems, better vehicle door structures and side impact dummies.

Upper Neck Forces and Moments and Cranial Angular Accelerations in Lateral Impact

Biomechanical studies using postmortem human subjects (PMHS) in lateral impact have focused primarily on chest and pelvis injuries, mechanisms, tolerances, and comparison with side impact dummies. A paucity of data exists on the head-neck junction, i.e., forces and moments, and cranial angular accelerations. The objective of this study was to determine lateral impact-induced three-dimensional temporal forces and moments at the head-neck junction and cranial linear and angular accelerations from sled tests using PMHS and compare with responses obtained from an anthropomorphic test device (dummy) designed for lateral impact. Following initial evaluations, PMHS were seated on a sled, restrained using belts, and lateral acceleration was applied. Specimens were instrumented with a pyramid-shaped nine-accelerometer package to record cranial accelerations. A sled accelerometer was used to record the input acceleration. Radiographs and computed tomography scans were obtained to identify pathology. A similar testing protocol was adopted for dummy tests. Results indicated that profiles of forces and moments at the head-neck junction and cranial accelerations were similar between the two models. However, peak forces and moments at the head-neck junction were lower in the dummy than PMHS. Peak cranial linear and angular accelerations were also lower in the dummy than in the PMHS. Fractures to the head-neck complex were not identified in PMHS tests. Peak cranial angular accelerations were suggestive of mild traumatic brain injury with potential for loss of consciousness. Findings from this study with a limited dataset are valuable in establishing response corridors for side impacts and evaluating side impact dummies used in crashworthiness and safety-engineering studies.

Dynamic verification of a multi-body computational model of human head and neck for frontal, lateral, and rear impacts

A multi-body computational model of the human head and neck was previously shown to be in good agreement with experimental findings from actual human cervical spine specimens. The model segments were tested in three directions of loading showing main and coupled motions to be accurate and realistic. The model's ability to predict the dynamic response of the head and neck, when subjected to acceleration pulses representing frontal, lateral, and rear-end impacts, is verified using experimental data derived from sled acceleration tests with human volunteers for 15 g frontal and 7 g lateral impacts and from isolated cervical spine specimen tests for rear-end impacts. Response corridors based on sled acceleration tests with human volunteers for frontal and lateral impacts are used to evaluate the model and investigate the effect of muscle activation on the head-neck motion. Firstly, the impacts are simulated with both passive and active muscle behaviour. Secondly, the local loads in the soft-tissue elements during the frontal impact are analysed. For rear-end impact simulation experiments using ligamentous isolated cervical spine specimens are used to evaluate the model performance before investigating the effects of muscle tensioning. Good agreement with human volunteer response corridors resulting from frontal and lateral impacts, and isolated cervical spine specimen sled test rear-end impact experiments is demonstrated for the model, highlighting the important role the muscles of the neck play in the head-neck response to acceleration impacts. The model is shown to be able to predict the loads and deformations of the cervical spine components making it suitable for injury analysis.

Multi-scale human body model to predict side impact thoracic trauma

This study presents a multi-scale human body model that is capable of predicting full-body response in side impact, including detailed thoracic response and injury. The model is based on a detailed finite element model of a 50th percentile human thorax that has been enhanced with improved finite element mesh geometry and refinement, and incorporation of appropriate high-rate material properties. The prediction of local injury requires modelling at an appropriate scale, typically at the organ level, as included in the human body model. We have introduced representative but simple deformable models at the component level to represent the entire human body for modelling side impact events. This includes simplified coarse-scale deformable models of the abdomen, pelvis, legs and head to achieve the appropriate mass distributions and load transmission paths. This total body model was then validated against post-mortem human subject data for simple pendulum impacts, and more complicated side sled tests that engaged the entire body model. The predicted model response is in excellent agreement with the post-mortem human subject response corridors, providing an important tool for the prediction of thoracic trauma and future evaluation of new safety systems in side impact crash.

Evaluation of frontal occupant protection system responses to crash pulse variations

Occupant Protection System (OPS) is a key component in today's vehicle systems to reduce occupant's injuries during collisions. To assess OPS performance under a wide variety of crash conditions, vehicle barrier tests, laboratory dynamic sled tests and computer simulations are the major tools for the systems development. As the main input to the system, vehicle crash pulse controls the OPS design. Its variations have significant impact on the overall system responses. A well-correlated MADYMO model is employed to evaluate the effects on a frontal OPS. A set of crash pulses are quantitatively analyzed to assess the sled repeatability. Statistical analyses are then performed to define the corresponding dummy dynamic response corridors. The results show that small crash pulse discrepancy could result in significant variations of dummy dynamic responses and injury measurements. This study has the potential to help design a robust occupant restraint system less sensitive to crash pulse variations.

Development of a Three-Year-Old Child FE Model

Objective. Child crash dummies are conventionally used for safety performance evaluations of the child restraint system (CRS) in vehicle crash tests. To investigate injuries to various body regions of a child in detail, mathematical models are useful, and provide information that cannot be analyzed by crash dummies. Therefore, in the present research, a finite element (FE) model of a 3-year-old child has been developed by model-based scaling from the AM50 human FE model, THUMS (Total HUman body for Safety). Methods. The dimensions of each body region were based on the anthropometry data of United States children, and material properties of child bone were estimated from data reported in the literature. Neck flexion, thorax impact responses, and torso flexion were validated against the response corridor of the 3-year-old Hybrid III dummy in calibration tests. A test of lap belt loading to the abdomen was also conducted. FE models of two different types of CRS, a 5-point harness and a tray shield CRS, were also made, and ECE R44 sled impact test simulations were conducted using the child FE model. Results. The characteristics of the child FE model proved to be close to the Hybrid III and child volunteer corridor. In the ECE R44 sled test simulations using the child FE model, the head movement down and head rotation were large in the 5-point harness CRS, and chest deflection was large in the tray shield CRS. In both CRS types, the whole spine flexed in the child FE model. This behavior is different from that of the Hybrid III, where the thorax spine is stiff and only the cervical spine and lumbar spine flex. Conclusions. Although this child FE model has several limitations in areas such as the anatomical shapes and material properties of a child, this model can be a useful tool to examine the behavior of a child in impacts, which may be difficult to predict by using the Hybrid III dummy with its stiff thorax spine box.

New Biofidelic Corridor and Biofidelity Test Procedure for Pedestrian Legform Impactors

The objectives of this research are to propose a new impact response corridor for the ISO legform impactor and to determine the biofidelity of the current legform impactor with rigid leg and thigh developed by the Transport Research Laboratory (TRL). The latest data obtained from Post Mortem Human Subject (PMHS) knee impact tests were analyzed in connection with the proposal, and biofidelity legform impact tests were conducted using the current rigid legform impactor. New normalized biofidelic corridors of impact force corresponding to adult male 50th percentile (AM50) are proposed. The impact test results indicate the current rigid legform impactor does not have sufficient human knee biofidelity. The present results suggest that human tolerance can not be used directly for the injury reference value of the legform impactor. A conversion method is needed to interpret the data measured by current legform impactors as the injury reference value.

The effects of uterine ligaments on fetal injury risk in frontal automobile crashes

A finite element model of a seven-month pregnant uterus was created and integrated into a multibody human model. The pregnant occupant model was validated using known abdominal response corridors. Tests of occupants with no restraint, a three-point belt and a three-point belt plus airbag at 35 km/h were simulated to evaluate the effects of four support ligaments connected to the uterus. The primary injury predictor, the peak uterine strain on the anterior uterus, was not found to change appreciably by the absence of the support ligaments; however, high local stress concentrations were found at the ligament attachment sites. Based on these results, it is recommended that computational models of the pregnant occupant contain the supporting ligaments since the stress concentrations may result in increased injury risk in other test configurations or loading conditions.