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1 This article was downloaded by: [Lessley, David Jonathan] On: 13 August 2010 Access details: Access Details: [subscription number ] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK International Journal of Crashworthiness Publication details, including instructions for authors and subscription information: Kinematics of the thorax under dynamic belt loading conditions D. J. Lessley a ; R. Salzar a ; J. Crandall a ; R. Kent a ; J. R. Bolton a ; C. R. Bass a ; H. Guillemot a ; J. L. Forman a a Center for Applied Biomechanics, University of Virginia, Charlottesville, VA, USA Online publication date: 15 July 2010 To cite this Article Lessley, D. J., Salzar, R., Crandall, J., Kent, R., Bolton, J. R., Bass, C. R., Guillemot, H. and Forman, J. L.(2010) 'Kinematics of the thorax under dynamic belt loading conditions', International Journal of Crashworthiness, 15: 2, To link to this Article: DOI: / URL: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 International Journal of Crashworthiness Vol. 15, No. 2, April 2010, Kinematics of the thorax under dynamic belt loading conditions D.J. Lessley, R. Salzar, J. Crandall, R. Kent, J.R. Bolton, C.R. Bass, H. Guillemot and J.L. Forman Center for Applied Biomechanics, University of Virginia, Charlottesville, VA, USA (Received 29 May 2009; final version received 4 June 2009) This research was completed as part of an ongoing effort to characterise human thoracic response to belt loading in a well controlled and repeatable laboratory environment. This paper presents the results of eight tests conducted on three post-mortem human subjects. The sled test environment provides realistic occupant kinematics and restraint interaction in the inertial environment of a vehicle collision, but is too complex for detailed analysis of thoracic deformation under belt loading. To study in more detail the kinematics of the chest when loaded anteriorly by a seat belt, three male post-mortem human surrogates (31-62 years of age) were mounted on a stationary apparatus that supported the spine and shoulder in a configuration comparable to that achieved in a 48 km/h sled test at the time of maximum chest deformation. The belt was positioned across the anterior torso with attachments at D-ring and buckle locations based on the geometry of a mid-sized sedan. The belt was attached to a trolley driven by a hydraulic ram linked to a universal test machine. Ramp experiments were conducted at rates of 0.5 m/s, 0.9 m/s and 1.2 m/s. Average peak sternal displacements ranged from 13% to 23% of chest depth measured at the central sternum. Belt loads and spinal reaction loads were measured along with six degree-of-freedom (DOF) displacement data of the sternum, 4th and 8th ribs anteriorly, 8th and 10th ribs posteriorly, and acromia bilaterally. Three DOF targets were mounted to the distal clavicles, 8th ribs laterally and along the path of the belt. The targets were tracked optically by a high speed 16-camera motion capture system (VICON MX TM ) in a calibrated space around the torso. Post-test analysis of the target motion included decomposition of the trajectories into Cartesian coordinate displacements with respect to a spine fixed coordinate system. The results showed that the chest deformation closely followed the belt loading regionally with a trough developing where the belt contacted the chest. The anterior rib targets exhibited three-dimensional (3D) translational motion. Displacements in the X direction (anterior-posterior) were the largest, however the Z (vertical) and Y (lateral) displacements comprised nearly 35% and 10% respectively of the total resultant deflection measured at the sternum. Peak posterior deformations were significantly (p <.001) lower than those observed at anterior locations and were below 7 mm except for the final injurious tests in which the peak posterior deformation averaged 13.4 mm (approximately 22% of the average peak anterior ribcage deformation in the injurious tests). Overall, the results provide a detailed 3D mapping of the chest deformation under belt loading, which should be considered in the future development of physical and computational models of the thorax. Keywords: thorax; kinematics; 3D; VICON; deformation 1. Introduction Thoracic trauma is the principle causative factor in 30% of road traffic deaths [23]. The mortality associated with thoracic trauma reflects the variety of critical physiological processes that occur there. The chest contains the primary elements of the respiratory and circulatory systems, and the thoracic cage protects several abdominal organs, including the liver, spleen, kidney and stomach. The mechanisms of injury to the thorax vary, but it is well established that many types of blunt thoracic traumas are causally related to chest deformation [4]. The design of vehicle components and restraint systems therefore depends upon a detailed understanding of how the thorax deforms when a force is applied Blunt Hub Tests Several studies, using a range of loading conditions, rates and test protocols, have characterised the force deformation response of the thorax. Some of these tests involved blunt hub impacts to the mid-sternum of postmortem human subjects (PMHS) using a range of impacting masses and speeds, and fixed-back and free-back posterior support conditions [20, 24]. Patrick [28] performed live human volunteer tests using a similar protocol, and more recent research has involved thoracic and abdominal response under steering wheel rim or rigid bar loading [10, 25 27, 35]. These hub and wheel impact responses remain a convenient, but insufficient, requirement for a biofidelic model. Corresponding author. lessley@virginia.edu ISSN: print / ISSN: online C 2010 Taylor & Francis DOI: /

3 176 D.J. Lessley et al. The current automotive environment is substantially different from the environment when the hub impact tests were being conducted. In particular, the permeation of air bags into the vehicle fleet and the rise in seat belt usage since the 1970s have created a fundamental change in the types of loading experienced by the thorax in frontal crashes [18]. Thoracic loading from the wheel is different in several respects from seat belt loading in that there are differences in the magnitude of the area engaged, the specific anatomical structures that bear the load and the direction of the resultant force vector may differ. Finally, with proper belt fit and use, seat belt loading never involves an impact (i.e. the relative velocity between the belt and the anterior chest is always zero), so the thoracic force deformation is not dominated by inertial and viscous responses Restraint loading Recognising that the thoracic response to a blunt hub impact is not indicative of the response to restraint-type loading, researchers have performed studies of both PMHS and human volunteers loaded by belts and by air bags. Many of these tests have been performed in a vehicle or sled test environment [7 9, 12 14, 16, 29, 30, 32, 37, 38], where it is difficult to measure chest deformation, and the force vector changes direction throughout the event. These studies are therefore useful for studying the occupant kinematics and injury potential in a frontal crash, but have less utility for quantifying the kinetic thoracic force deformation response. Quasi-static tests of belt loading on human volunteers have also been performed [8], but force-deflection corridors have not been developed from these data. Simplified and more controllable dynamic representations of the vehicle environment have been used to evaluate the thoracic kinetic response to dynamic restraint-like loading. L Abbe et al. [22] and Backaitis and St-Laurent [2] presented a series of tests on 10 human volunteers positioned supine on a loading table with a belt passing diagonally over the chest in a layout similar to a seat belt in a passenger car. A pendulum impact was used to load the chest through the belt while belt tension and anterior posterior displacements of the anterior chest wall at several locations were measured. Cesari and Bouquet [5,6] used a similar test setup to test PMHS and the Hybrid III dummy. The thoracic responses measured in these tests have been used for biofidelity assessments [33], but to our knowledge force-deflection biofidelity corridors have never been developed from the results of these tests Load distribution In 2004 and 2008, Kent et al. [15, 18] published a series of dynamic belt loading tests performed on PMHS with four different loading conditions (hub, single and double diagonal belts, and distributed load). This work showed that the loading condition substantially influences the effective stiffness of the heterogeneous chest and the deformation patterns of the various structures. The effect of the anatomical structures being engaged, in particular the shoulder complex, was shown to dominate the effect of differences in the area of engagement between the chest and the loader. Shaw et al. [36] expanded on this work by loading different regions of the anterior thorax using a small rigid indentor. This study did not load the shoulder complex, but did identify regional variation in the effective stiffness and in the deformation patterns of the chest. The deformation patterns were described in terms of the thoracic coupling (the degree to which remote sites displaced in response to the applied input displacements), which was found to depend strongly on the particular site on the chest being loaded. These previous studies have used limited measurements of thoracic kinematics, including relatively few measurement points, and frequent use of only one component (anterior posterior) of the displacement vector. Sled tests of PMHS with two-dimensional (2D) thoracic deformation measurement (via chest bands) and with dummies containing multi-dimensional chest deflection measurement systems have revealed substantial deformation in all three anatomical planes [17, 19, 21, 34]. Thoracic anatomy and deformation patterns also suggest that individual ribs undergo axial rotation during restraint loading. The degree to which these displacements and rotations affect injury risk and thoracic biomechanics under restraint loading remain unknown, but it is presumed that accurate 3D characterisation of human ribcage kinematics is required for continued refinement and reliable application of thoracic models intended for restraint design. The purpose of the current paper is to expand on these previous thoracic characterisation studies by quantifying the detailed 3D kinematics of multiple points on the thorax when loaded by a diagonal belt. The test fixture is based on the concept developed by Kent et al. [18], with an improved spinal mounting scheme that maintains a prescribed kyphotic curvature representative of an occupant in a frontal crash, while allowing for unconstrained motion of the costo-transverse and costo-vertebral joints. Optical 3D measurements were taken of the motion of multiple points on the anterior, lateral and posterior thorax. 2. Methods This study evaluated the 3D kinematic responses of three male PMHS (Table 1) to loading from a two-point shoulder belt. Each subject was mounted on a stationary test fixture (Figure 1) that applied the belt loading to the anterior thorax and also supported the spine and shoulder in a configuration comparable to that achieved in a 48 km/h sled test at the time of maximum chest deformation. The belt was positioned across the anterior torso with attachments at D-ring and

4 International Journal of Crashworthiness 177 Table 1. Description of test subjects. Subject 1 Subject 2 Subject 3 PMHS ID Age Height (cm) Weight (kg) Chest Depth, 4th Rib (mm) Cause of Death Tongue Brain Diabetes, Myocardial Cancer Cancer Infarction buckle locations based on the geometry of a contemporary mid-sized sedan. sail cloth to prevent appreciable belt elongation under load [18]. Upper and lower belt pulley positions were representative of belt anchor locations found in a contemporary mid-sized sedan. A six-axis load cell was used to record the subject posterior reaction while inline cable load cells were used to measure upper and lower belt tensions. Three-dimensional thoracic deformation was recorded at 16 locations (anterior, lateral and posterior) using a 16-camera 3D motion capture system (VICON MX TM ) to track the motion of retroreflective targets attached to each anatomical measurement location. Upper and lower extremities were removed from each subject at the mid humerus and mid femur to facilitate handling Test fixture The test apparatus (Figure 1) was fabricated to reproduce torso configuration with minimal restraints over a range of belt loading rates representative of those observed for a restrained occupant in a 48 km/h frontal sled test [18]. The hydraulically driven test fixture utilised the crosshead of a high-speed material testing machine (Instron model 8500) to control the motion of a linear vertical trolley (see E in Figure 1) located beneath the subject which was accomplished through a master-slave cylinder arrangement. Each end of the two-point belt was secured to the trolley via a single 3.2 mm steel braided cable (wire rope) which was routed around a single fixed pulley. Subject chest deflection was achieved when the trolley was displaced downwards. The two-point belt was constructed of spectra fibre-reinforced 2.2. Test Strategy Spine, shoulder posture, and belt positioning targets were selected as those observed at the time of peak chest deflection in a 48 km/h frontal sled test with a belted 50th percentile male PMHS (Figure 2). The spine of each subject was rigidly secured to an extruded aluminium spinal support bar (Figure 2). Spinal mounts were screwed bilaterally into 10 individual vertebrae from T1 - S3 using 4" stainless steel wood screws (Figure 2). The height and orientation of each spine mount could be adjusted and then securely fixed with respect to the bar to achieve the target spinal posture. Vertebrae from S3 up to T8 were positioned along a line nearly parallel to the support bar, while vertebrae T8 up to T1 were positioned along another line approximately at a Figure 1. Test fixture operation and instrumentation. Hydraulically driven trolley beneath subject displaces belt cable ends, thus loading anterior thorax with two-point belt. Subject spine fixed.

5 178 D.J. Lessley et al. Figure 2. Target and achieved spine posture. Spine posture target (left) (spine posture at time of peak chest deflection in a 48 km/h PMHS frontal sled test). CT image (right) illustrates the achieved spine posture with subject mounted on spinal support bar. Spinal mounts were screwed into individual vertebrae from T1 - S3. The mounted posture was rigidly secured and remained constant throughout the tests for a given subject. 30 o with respect to the line through the lower vertebrae (T8 - S3) (Figure 2). A tensile load was applied to the distal end of the remaining humerus length via a loaded cable attached to a (6.35 mm) bolt through the bone at mid shaft amputation site. The orientation of the tension vector was selected to approximate the shoulder position achieved in 48 km/h frontal sled tests at the time of peak chest deflection due to inertial loading from the upper extremity. This resulted in outstretched bilateral humeri at angles of approximately 25 with respect to the lower spine. Prior to belt loading, each arm cable was pre-tensioned to approximately 30 N (250 N load limiters were used to prevent dislocation of the shoulder joint during loading). Of the 13 total conducted tests, eight ramp tests using the two-point belt configuration were selected for kinematic analysis (Table 2). The ramp functions were conducted over the range of m/s which correlates well with ob- Table 2. Matrix of reported ramp tests. Target Test # Subject Test Type Loading Rate Trolley Displacement Ramp m/s 10% of Chest Depth Ramp m/s 15% of Chest Depth Ramp m/s 10% of Chest Depth Ramp m/s 15% of Chest Depth Ramp m/s 40% of Chest Depth Ramp m/s 15% of Chest Depth Ramp m/s 20% of Chest Depth Ramp m/s 40% of Chest Depth served belt loading rates of 1.0 m/s from PMHS frontal sled tests at 48 km/h. Preceding the ramp tests, a 1 Hz, 10 cycle preconditioning loading function was conducted on each subject to obtain a consistent thoracic force deformation response for which results of previous studies support the necessity of subject pre-conditioning [18, 36]. Following the pre-conditioning, the loading sequence consisted of two ramps (0.5 m/s and 0.9 m/s), a dual-frequency (1 Hz and 5 Hz) 10-cycle validation function (not reported here) and a final injurious ramp at 1.2 m/s. The data from the validation Table 3. Conditions at final point of resultant vector used for kinematic analysis. Rig Subject Skeletal External Time Displacement Reaction Deflection Deflection Test a t peak b D Fixture c F Reaction d D Sternum e D Belt f f a Time of peak trolley displacement. b Peak trolley displacement (this is the vertical displacement of the belt ends). c Peak posterior subject reaction load in the positive X -axis direction. d Skeletal deflection measured at the sternum. e External deflection measured of the belt at a mid-line location. f Values selected at the time of peak reaction load since no local trolley displacement peak is observed.

6 International Journal of Crashworthiness 179 Figure 3. Subject motion capture target locations. Anterior, lateral and posterior measurement locations for kinematic analysis. Each location was tracked with either a single marker or four-marker cluster. Clusters were used for 6 DOF motion, while single markers were used for 3 DOF motion. function was collected to be used in conjunction with the development of a quasi-linear viscoelastic (QLV) model of the thorax [31]. It should be noted that the provided values for loading rate and compression level are based on the vertical displacement of the trolley beneath the table to which the belt cables were attached, actual inputs into the anterior Figure 4. 3D motion capture target hardware.

7 180 D.J. Lessley et al. Figure 5. Measurement locations, symbols and nomenclature. thorax varied slightly and were dependent on the specific belt fit geometry for each subject. Actual displacements of trolley, belt and sternum are provided in Table Instrumentation Thoracic kinematic data was provided using a 16-camera 1000 Hz VICON MX TM 3D motion capture system to track Table 4. Peak resultant vector values by location, subject and test. the motion of retroreflective spherical targets through a calibrated 3D space lying within the cameras collective field of view. The calibration procedure, performed prior to the testing of each subject, established the position and orientation of the cameras with respect to one another which was used to reconstruct the 3D target locations from multiple 2D camera images via a triangulation algorithm within the VICON Nexus software package. Four target clusters Ramp 1 Ramp 2 Ramp 3 Subj. 1 Subj. 2 Subj. 3 Subj. 1 Subj. 2 Subj. 3 Subj. 2 Subj. 3 Location Res. (mm) Res. (mm) Res. (mm) Res. (mm) Res. (mm) Res. (mm) Res. (mm) Res. (mm) ST RA LA NA RA LA RP LP NA 4.4 NA NA NA NA 10RP NA LP NA RA RL RDC RMC NA NA 8RL NA 16.6 NA NA 23.7 NA LDC LL NA - Marker visibility reduced to one camera during loading sequence, preventing 3D reconstruction requiring a minimum of two cameras.

8 International Journal of Crashworthiness 181 Figure 6. Thoracic kinematics for Ramp 1. were secured to selected anatomical locations to facilitate the use of rigid body mechanics to determine the actual motion of the corresponding bone at each time step using a mathematical coordinate transformation. Each cluster was tracked initially relative to a lab-fixed coordinate system; however, relative motion between these local cluster coordinate systems (i.e. sternum and spine) was used to quantify deformation of the thorax. Sixteen (not related to number of cameras) anterior, lateral and posterior locations on the thorax were tracked for kinematic analysis using either a single-marker or fourmarker cluster at each location (Figure 3). Translational (i.e. three degrees of freedom (3 DOF)) data was provided at five locations using single markers, while translational and rotational (6 DOF) data was provided at 11 locations using four-marker clusters.

9 182 D.J. Lessley et al. Figure 7. Thoracic kinematics for Ramp 2. At each measurement location (via a local incision) an aluminium mount was secured to the bone to which the visible single-marker or four-marker cluster was attached (Figure 4). The mounts were attached to the ribs via high-strength nylon straps, while for non-rib locations the mounts were screwed directly into the bone surface using #6 steel screws. Strapping (Figure 4) was utilised at all rib locations to prevent artifactual rib fractures due to stress concentration likely induced by the screws. Single markers were affixed directly to the mount while the fourmarker clusters were attached to the mounts using vertical standoffs of various lengths to accommodate the superficial tissue depth (Figure 4). The far right image in Figure 4 illustrates the anterior marker plates installed in the as tested positions for the sternum and bilateral 4th and 8th ribs.

10 International Journal of Crashworthiness 183 Figure 8. Thoracic kinematics for Ramp Data processing Using VICON software (Nexus 1.2), 1000 Hz 2D video data from the 16 camera system was reconstructed to yield 3D trajectories of each tracked marker with respect to a global coordinate system. From this global data set, a local bonebased coordinate system was developed for each marker cluster. The trajectory of each marker cluster was smoothed through a rigidity constraint using the least squares pose (LSP) estimator as performed by Cappozzo [3]. All marker hardware (Figure 4) was rigidly fixed to the underlying bone surface at each measurement location, thus satisfying a rigid body assumption. Six degree-of-freedom (translational and rotational) motion of the bone at each measurement location was obtained from its corresponding marker cluster using a local coordinate transformation accomplished using hardware digitisation and CT data where applicable. While analysis of the rotational data was performed, these rotational results are beyond the scope of this paper and are not reported. However, this rotational information is an integral part of the coordinate transformation process used to obtain the presented translational data at all 6 DOF measurement locations (Figure 3). For measurement sites using a single marker only 3 DOF (translational) data were obtained. 3. Results A total of eight ramp loading tests with a two-point belt were successfully conducted on three male PMHS. Maximum

11 184 D.J. Lessley et al. Figure 9. Deflection magnitude indicated by color. Greatest deflections observed along the belt path. resultant belt input at the sternum ranged from 24.7 mm to 76.1 mm, producing maximum sternal resultant deflections ranging from 20.7 mm to 55.8 mm resulting from the three loading rates which averaged 0.5 m/s, 0.9 m/s and 1.2 m/s for ramps 1, 2 and 3 respectively. In all tests, kinematic data was obtained at each measurement location with a few compromises due to insufficient target visibility (Table 4). For subjects 2 and 3, the left 8th posterior rib location was compromised due to interference with the scapula and target hardware during loading. Three ramp tests of increasing deflection magnitude and loading rates were conducted on each subject with the exception of subject 1 for which the third ramp test was omitted due to palpatable rib fractures following the second ramp test. All

12 International Journal of Crashworthiness 185 Table 5. Normalised (to Sternum) resultant vector values by location, subject and test. Ramp 1 Ramp 2 Ramp 3 Subj. 1 Subj. 2 Subj. 3 Subj. 1 Subj. 2 Subj. 3 Subj. 2 Subj. 3 Location Res. (mm) Res. (mm) Res. (mm) Res. (mm) Res. (mm) Res. (mm) Res. (mm) Res. (mm) Ave S.D. ST RA LA NA RA LA RP LP NA NA NA NA RP NA LP NA RA RL RDC RMC NA NA RL NA 0.81 NA NA 0.75 NA LDC LL NA - Marker visibility reduced to one camera during loading sequence, preventing 3D reconstruction requiring a minimum of two cameras. results are reported in the SAE occupant coordinate system. Figure 5 illustrates bone-based measurement locations with corresponding symbols and nomenclature for all reported kinematic results. Figures 6 8 illustrate the thoracic kinematics of the three subjects for each of the three ramps in the SAE occupant coordinate system. The 3D deflection of each measurement location is illustrated using the resultant deflection vector between initial and final positions corresponding to zero and peak deflections respectively. Table 3 provides the time, subject reaction load and sternal deformation at the time of peak trolley displacement. For the injurious tests (test 2.8 and test 3.9), values are calculated at the time of peak reaction load since a local maximum trolley displacement was not observed. This method is justified since the time of peak force and peak trolley displacement was nearly coincident for the non-injurious tests. Referring to Figures 6 8, the resultant deflection vectors for each location were plotted in 3D allowing 2D projections to be taken as viewed from the anterior, left side (lateral) and pelvis perspectives. These views are shown in the top row of each figure. The plots in subsequent rows are the 2D projections of the resultant vectors for a given subject corresponding to the view in the top row of same column. A watermark of an outlined thorax for each view is provided with each plot to orient the reader. The initial position of each vector is represented as an open marker (circle for non-posterior locations, diamond for posterior locations) with the resultant vector depicted as a solid line. A colour scale was used to specify the colour of each local resultant based on its proportion to the overall maximum resultant magnitude for a given subject and test. While the actual anatomical measurement locations for each subject were consistent with those illustrated using the H-model [11] in Figure 5, the relative positions of these locations differed slightly from subject to subject due to typical anatomical variation. Internal deflections measured at the sternum averaged 13%, 18% and 23% of chest depth (4th rib) for ramps 1, 2 and 3 respectively. The test condition produced consistently similar deformation patterns across both tests and subjects. Anterior deflections were significantly (p <.001) greater than maximum posterior deflections which averaged 3%, 4% and 7% of chest depth for ramps 1, 2 and 3 respectively, a finding consistent with tests conducted by Ali and Kent [1]. Table 4 provides a summary of peak resultant magnitudes corresponding to the measurement locations in Figure 5. Components of sternal deflection along the X, Y and Z axes averaged approximately 90%, 10% and 35% of resultant magnitude. Peak deflections were consistently observed at locations in closest proximity to the belt. Figure 9 provides an illustration of deflection magnitude indicated by colour corresponding to a proportion of peak resultant deflection. Peak deflection was observed along the belt path in all tests as indicated by the provided colour bar. 4. Discussion The data presented here characterise the 3D kinematic response of three male PMHS to belt loading over a range of deformation levels and loading rates representative of those observed for a restrained occupant in 48 km/h frontal sled test. While the data set is limited to three subjects, to our knowledge this is the first study to quantify the 3D

13 186 D.J. Lessley et al. Figure 10. Resultant magnitude and component contributions by location. The upper chart illustrates the magnitude of resultant deflection normalised to the sternal deflection. The lower chart illustrates the apportioning of the resultant vector along the X-axis, Y-axis, and Z-axis directions. The error bars represent one standard deviation in the normalised data (refer to Equations 1-3). kinematic response of the anterior, lateral and posterior thorax to belt loading representative of contemporary restraints. The consistent nature of the findings across subjects and deformation levels suggests that the presented trends would likely hold with future data from additional subjects. As presented, the data provide valuable preliminary kinematic design targets for the thorax under belt loading for Anthropomorphic Testing Device (ATD) and computational model development as well as a biofidelity assessment of existing models for the utilised loading condition. Generally, the subject responses demonstrate significantly (p <.001) greater anterior resultant magnitude deflections when compared with those of the posterior locations. Among the anterior locations, the greatest deformations were intuitively observed along the belt path. While such trends provide a qualitative kinematic assessment they are of limited value for design purposes Resultant deflection: magnitudes and component contributions To quantify the deflection distribution across the thorax, the resultant deflection magnitudes at each anatomical measurement location were normalised to the sternum resultant magnitude. These data are provided for each subject and test in Table 5. While on average the sternum experienced the greatest resultant deflection, the location of the maximum resultant deflection for a given test varied among belt path locations and averaged 116% of the sternal resultant magnitude. Given typical variations in subject geometry and belt fit coupled with the range of belt loading inputs some variation in the location of maximum deflection is to be expected. Despite this variation in the maximum location, however, the average resultant data clearly illustrate the apportioning of deflection magnitude throughout the thorax, again showing greatest deflection magnitudes along the belt

14 International Journal of Crashworthiness 187 Figure 11. Belt and anatomical deflection component contributions. The upper chart illustrates the normalised anatomical deflection components. The lower chart represents the normalised belt deflection components. with correspondingly lesser resultant magnitudes at locations away from the belt (Figure 10, top bar chart). The resultant deflection (Equation (1)) at each measurement location was decomposed into its X -axis, Y-axis and Z-axis deflection components for all tests. Each component value was normalised by the resultant deflection magnitude occurring at that particular location for the given test (Equation (2)). From these data for the eight (N = 8) tests, the average and standard deviation (σ ), for each component were calculated for each measurement location (Equation (3) demonstrates using the X -axis component (Y and Z axes similar)) and are provided in Figure 10. Equations (1) (3) demonstrate the process to obtain average component contributions with corresponding standard deviations (σ )for a single location. It should be noted that while typically the standard deviation, σ, is used to quantify the variation within a non-normalised data set, here, σ is used to quantify the variation among already normalised values to establish the stability of resultant deflection vector components for each measurement location. res i = X 2 i + Y 2 i + Z 2 i (1)

15 188 D.J. Lessley et al. X Normi = X i res i, Y Normi = Y i res i, Z Normi = Z i res i (2) X Norm = 1 N X Normi, N i=1 σ NormX = 1 N (X Normi X Norm ) N 2 (3) i=1 Figure 10 provides a comprehensive summary of the component contributions of the resultant deflection at each measurement location (excluding posterior, omitted due to minimal resultant magnitudes) along with the normalised deflection magnitude plotted on independent axes. The relative resultant magnitudes plotted on the upper axis provide a reference for the level of deflection achieved at each location, while the corresponding components provide the apportioning of these deflections along the X, Y and Z axes. Interestingly, consistent component contributions are observed across both subjects and loading levels for most locations. Exceptions are the 8LL and 8LA locations which were far from the loading path and experienced the lowest (non-posterior) resultant deflection magnitudes. Also notable are the RA and RDC locations on the unloaded shoulder which were actually displaced upwards as illustrated in Figures 6 8. The RMC location, while still on the unloaded clavicle, is sufficiently influenced by its proximity and coupling with the sternum to exhibit a similar component contribution as points along the loading path. In general, Figure 10 provides the component contributions per unit of resultant deflection for all measured anterior and lateral locations and serves as a clear design target for computational model development as well as a biofidelity assessment. Belt input into the subject was predominately along the X -axis (as illustrated in Figure 11, bottom chart) and exhibited very repeatable behaviour in terms of component contribution into the chest. The normalised X -axis deflection component was nearly constant regardless of position on the belt; however, both the Y-axis and Z-axis component deflections were proportional to the Z-axis position of the measurement location along the belt path. While the belt deflection component contributions were nearly Figure 12. Anterior and lateral views of tests subjects from CT images.

16 International Journal of Crashworthiness 189 uniform along the entire belt length, comparison with nearby anatomical locations illustrated variations between the belt and anatomical component contributions, especially regarding the Y-axis and Z-axis components. These results are not surprising given that the internal and external deflections are coupled though a variable and complex soft tissue interaction along the belt path Limitations While the results and findings appear to be reasonably consistent across subjects and deformation levels, they are obtained from a limited number of subjects for a single loading condition. In addition the ramp 3 injurious test was not conducted on the first subject due to sustained rib fractures on the ramp 2 test. Although retro-reflective target data were reliably obtained there were various instances of insufficient target visibility resulting in isolated loses of kinematic data. The results and findings presented in this study are based upon subjects with similar anatomical geometry as illustrated from anterior and lateral CT views of the individual subjects (Figure 12). How differences in subject geometry could affect the results is not yet known. Finally, while the table top environment provides a controlled and repeatable test condition, the inertial contributions present in the sled test environment are neglected. 5. Conclusion This study characterises the 3D thoracic kinematic response of three male PMHS to belt loading representative of that observed from contemporary restraints in a frontal collision. For the subjects tested, the findings appear consistent across both subjects and deformation levels. Anterior thoracic deflections were found to be significantly (p <.001) greater than posterior deflections and among the anterior deflections the most substantial were observed along the belt path. The components of the resultant deflections were found to have reasonably stable normalised magnitudes over the tested deflection range for each (non-posterior) measurement site. These provide a clear design target for computational model development as well as a biofidelity assessment of existing models. While the findings appear promising for this limited study, additional testing should be conducted to both confirm and expand upon these initial findings. In the interim, the results presented here provide reasonable design targets for a specific loading condition in a non-inertial environment. References [1] T. Ali, R. Kent, D. Murakami, and S. Kobayaashi, Tracking rib deformation under anterior loads using Computed Tomography Imaging, SAE Trans.: J. Passenger Cars 114 (2005), pp Based on SAE Paper [2] S. Backaitis, and A. St-Laurent, Chest deflection characteristics of volunteers and Hybrid III dummies, Paper , 30th Stapp Car Crash Conference, San Diego, CA, [3] A. Cappozzo and A. 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