Thoracic Response to Shoulder Belt Loading: Comparison of Table Top and Frontal Sled Tests with PMHS

Transcription

1 Thoracic Response to Shoulder Loading: Comparison of Table Top and Frontal Sled Tests with PMHS Robert S. Salzar, David J. Lessley, Mark Sochor, Greg Shaw, Richard W. Kent, Jeff R. Crandall Abstract The recent refinement of high rate optical tracking allows dramatically detailed thoracic deformation measurements to be taken during PMHS sled tests. These data allows analysis of restraint belt geometry and the three dimensional thoracic deformations generated by belt impingement. One consequence of this new capability is a better understanding of complementary thoracic characterization experiments such as table top tests and how the thoracic response can be interpreted for applications involving more complex loading mechanisms. This paper reports a detailed evaluation of the timing, magnitude, and direction of the applied belt forces, and the resulting thoracic deformations in two previously performed tests series involving frontal sled tests and table top belt loading tests. In the sled tests, the posteriorly directed component (SAE x) of the belt tension (F B ) is F Bx =.7 F B at the shoulder, but only F Bx =.14 F B where the belt engages the anterolateral torso inferiorly. The corresponding components on the table top are F Bx =.6 F B (shoulder) and F Bx =.48 F B (lower). When these components are cross plotted with chest deflection, pronounced consequences of thoracic anterior wall deformation patterns due to flexion of the thoracic spine and the internal viscera s inertia can be seen in the effective thoracic stiffness. Keywords deformation, geometry, models, safety belts, thorax I. INTRODUCTION Blunt chest trauma from motor vehicle crashes continues to be a major cause of death worldwide, and although significant improvements in restraint designs have reduced the overall rate of mortality from these injuries, continued improvement of the existing injury models is needed. Recent advances in measuring multiple, detailed three dimensional (3D) displacements in the PMHS tests allows for a more precise characterization of dynamic restraint belt geometries and thoracic skeletal motion that arises from belt loading tests [1]. Additionally, quantitative comparisons can now be made between the thoracic responses of PMHS in dynamic sled tests and those obtained from table top tests. It is important to note that while spine fixed tabletop test conditions deliver realistic belt loads, it does not incorporate the kinematic changes or inertial effects experienced by the thorax in the full scale frontal sled tests. Thoracic deformation from anterior loading is generally accepted as the best available predictor of chest injury in frontal crashes [][3]. The National Highway Traffic Safety Administration (NHTSA) specifies a maximum allowable value of chest deformation as recorded by the Hybrid III in frontal crash tests (CFR 49, Part 571, Federal Motor Vehicle Safety Standard (FMVSS) 8). The relevancy of these compression values become even more important when considering the rapidly aging population of drivers in developed countries who may tolerate lower, but not well validated, levels of compression. The forces associated with specific levels of chest compression have been the subject of numerous thoracic characterization studies, each having its own set of boundary and loading conditions. Specifically, thoracic characterization tests on either volunteers or PMHS can be divided into two subsets: specimens in a vertical or occupant like orientation [4][5], or subjects tested in a horizontal posture [1][6][7][8][9][11]. The differences between the two orientations can include the position of the internal viscera with respect to the sternum, the inertial effects of these internal viscera on the posterior side of the anterior chest wall, and the corresponding geometric effects on the shape and curvature of the R. S. Salzar is the Principal Scientist at the Center for Applied Biomechanics (CAB) at the University of Virginia (phone: (434) x135; FAX: (434) ; Salzar@virginia.edu). D. J. Lessley is a Research Scientist at the University of Virginia. Dr. M. Sochor is an Associate Professor of Emergency Medicine at the University of Virginia. G. Shaw is a Senior Scientist at the University of Virginia. R. W. Kent is a Professor of Mechanical and Aerospace Engineering at the University of Virginia. J. R. Crandall is the Nancy and Neal Wade Professor of Engineering and Applied Science at the University of Virginia

2 restraint belt. These differences in testing orientation and boundary conditions will lead, admittedly, to different force deflection profiles resulting in different characterization models, and could change the interpretation of any existing thoracic injury criteria. L Abbe et al. [1] performed diagonal belt tests on supine volunteers at sub failure levels with both muscle tensing and relaxed. Here, while volunteer testing provides the most biofidelic models, the internal viscera are not in the same position as that of a vehicle occupant. Cesari et al. [11] used diagonal belt loading on both PMHS and volunteers, and found with the use of chestbands that the location of maximum chest deflection occurred in areas other than the mid sternum. Kent et al. [6] performed both diagonal belt and hub tests on fifteen supine PMHS, and found distinct differences in thoracic stiffness between the two loading regimes. In this study, Kent also pointed out the differences between dynamic hub loading and belt loading that involve an impact between a moving hub and a stationary chest, and the relative velocity of the belt and the chest at the start of loading. While the inertial load generated by the hub impact significant changes the perceived stiffness of the thorax over that of the belt load, the effect of organ position and initial velocity at the start of load is not considered. Kent [7] performed detailed thoracic compression tests on three supine PMHS on a flat surface using diagonal belt loads. To study the effects of posterior boundary conditions on a supine PMHS, Salzar et al. [1] performed thoracic characterization tests on three supine PMHS with rigid spine fixation, thus removing any external dynamic response that a thorax may experience during belt loading. This study reanalyzes two previously performed test series [5][1] in an attempt to quantify the observed differences in thoracic responses between restrained PMHS in full scale frontal sled tests and PMHS in quasistatic spine fixed thoracic characterization tests on a horizontal table top device. Both the table top and sled test configurations load the anterior thorax with a diagonal shoulder belt restraint, but do so in different inertial reference frames. In the sled tests, the occupant is decelerated from a predetermined velocity using a deceleration pulse representative of a real world frontal impact. While this configuration is more realistic, it is also more complex since significant, but not fully quantified, inertial components from the anterior thoracic wall and the internal viscera contribute to the restraint loads and affect the thoracic deformation pattern during the deceleration. In addition, flexion of the spine during the impact generates dynamic changes in the torso pitch that influence the interaction of the shoulder belt with the body. Meanwhile, the table top test configuration is less complex by design and uses idealized but well defined boundary conditions and tailored belt load timehistories. The table top experiments allow viscoelastic characterization to the chest to be performed, but do not permit determination of all inertial components or the changing interactions that occur with forward pitch of the thorax. This study provides an initial investigation of the effects of belt geometry and the relationship between applied compressive loads to the thorax and stiffness, as well as the inertial effects of the anterior chest wall and internal viscera on thoracic deformations and resulting rib fracture patterns. II. METHODS Specimen and Specimen Preparation Eight male PMHS with approximately 5 th percentile stature and mass were subjected to the same simulated 4 km/h (11.1 m/s) frontal crash on a deceleration test sled. In comparison, three male PMHS with approximately 5 th percentile stature and mass (subject 419/Table top 3 was larger than 5 th percentile in both stature and weight but used in this study due to his relatively young age) were subjected to table top thoracic characterization tests with a rigid spine boundary condition and diagonal belt loading in the range of.5 to 1.5 m/s. Specimen anthropometry is shown in Table 1. All PMHS specimens were screened for Hepatitis A, B, C, and HIV and for pre existing pathology with the potential to influence thoracic properties. Pre test radiographs and CT scans were taken to verify that specimens with acute or healed rib fractures or other pre existing thoracic trauma were excluded from this study. All test procedures were approved by the University of Virginia cadaver institutional review board

3 Table 1. Specimen description for sled and table top tests. (±1 standard deviation is included). Test Number (Specimen ID) Sex Age (yr) Ave. Age (yr) Weight (kg) Ave. Weight (kg) Stature (mm) Ave. Stature (mm) Chest Depth (mm) Ave. Chest Depth (mm) T194 M (411) T195 M (43) T1358 M 54 54± ± ± ±18 (45) T1359 M (46) T136 M (48) T1378 M (443) T1379 M (433) T138 (441) M Table Top 1 M (41) Table Top M 54 49± ± ±14 (413) ±1 Table Top 3 (419) M Test Fixtures The sled test fixture was designed to provide a reasonable approximation of real world frontal impact crash loading of a belted occupant while providing repeatable test conditions that yielded thoracic response data suitable for analysis [13]. Specifically, the design objectives for the test fixture included approximating the shoulder and anterior ribcage loading in a manner representative of a frontal crash of a typical mid sized passenger car (Figure 1a). The restraint webbing, replaced for each test, was manufactured by Narricut (International twill pattern 13195, 6 8% elongation, 6.7 kn minimum tensile strength. Note that the vehicle occupant represented in the sled tests is a right side vehicle passenger. The table top device employed in this test series involves rigidly fixing alternating vertebral bodies of the test specimens using aluminum mounts that allowed free motion of the posterior costo vertebral and costotransverse joints [1]. These mounts were attached to the test device through a series of threaded rods that allowed specifying the desired degree of kyphosis or lordosis at locations in the spine. For these tests, the spine curvature and the torso position were set to the observed spinal posture at the time of peak chest deflection observed in the corresponding sled tests (Figure 1b). The belt used in the table top tests was made of polyethylene fiber reinforced composite (Spectra, E = 97 GPa), and was not replaced between tests. This belt material was used rather than actual seatbelt webbing to isolate the thoracic response from a combined effect that included belt stretch. The technique to position the belt on each sled test is outlined in the previous work [5], and this protocol was repeated for the table top tests and outlined in Kent et al. [6] and Salzar et al. [1]. Because each specimen was loaded multiple times during the test series, the thorax was thoroughly palpated after each test to check for any observable injury or loss of stability. If a fracture was identified, the testing on that specimen was stopped. This procedure was performed previously by Kent et al. [6], where no significant loss of stiffness was noted in subsequent testing of the same PMHS thorax as long as minimal rib fractures have occurred. Note that the vehicle occupant represented in the table top tests is a left side vehicle driver

4 (a) (b) B D C A E Figure 1. (a) Sled test configuration simulating a frontal collision; (b) Table top configuration with fixed spine boundary condition and no initial velocity of the thorax. A is the belt loading cable and location of the belt load cells, B is the arm force limiting cable to maintain arm posture, C and D show posterior load cell locations, and E is the vertical displacement carriage that loads the belt. Instrumentation For both test series, kinematic data, including torso deformations, were collected at Hz using an optoelectric stereophotogrammetric system (OSS) consisting of 16 Vicon MX TM cameras that track the position of retro reflective spherical markers in a calibrated 3D space. This system had been calibrated to have accuracy to within ±1 mm [5]. Additional instrumentation on both test series included multiple strain gages, sternum accelerometers, and seat belt load cells. A matrix of the test device input velocities is shown in Table. For the table top tests, this input velocity is the speed at which the loading carriage displaces, and not the rate of compression of the sternum. Similarly, for the sled tests, the input velocity is the sled velocity at impact and not the chest compression rate; however, the compression rates were comparable between the two conditions and are detailed later in this paper. Table. Matrix of Table top and Sled Test Conditions Test Number Waveform Input Device Velocity (m/s) Target Chest Compression Table top 1. Ramp and hold.5 1% of chest depth Table top 1.3 Ramp and hold 1. 15% of chest depth Table top. Ramp and hold.5 1% of chest depth Table top.3 Ramp and hold 1. 15% of chest depth Table top.8 Ramp and hold 1.5 4% of chest depth Table top 3.3 Ramp and hold.5 15% of chest depth Table top 3.4 Ramp and hold 1. % of chest depth Table top 3.9 Ramp and hold 1.5 4% of chest depth Sled 1 (T194) Sled 8 (T138) 11.1 NA Initial pre test shoulder belt centerline positions relative to the sternum and ribs were maintained across both test series for consistency, and are detailed in the original studies found in [5] and [1]. Local Determination The optical displacement data can be post processed to reference any local or global coordinate system, depending on what motion is being analyzed. For example, motion of the occupant in relation to the vehicle can be obtained by fixing the reference coordinate system to the rigid vehicle. In this study, the motion of the sternum with respect to the spine is obtained by using a local coordinate system fixed to the body of the eighth thoracic vertebra (T8). For the fixed spine table top tests, the local T8 coordinate system is the same as the global coordinate system for the table. In the case of the sled tests, the local coordinate system of T8 is

5 dynamic and follows the motion of the occupant during the test, resulting in the belt force components and the relative motion of the sternum with respect to T8 in local SAE x, y, and z components. For this study, all displacement results are therefore in the occupant SAE coordinate system, with the local coordinate system origin centered in the T8 vertebral body. This optical displacement data is also used in this study to separate the in line upper and lower diagonal belt forces into local x, y, and z force components. For the table top tests, loads cells were placed between the loading carriage and the cables that engage the diagonal belt. However, as the belt wraps around the torso, the local force applied in the x direction changes with the relative angle of the belt (Figure a,b). That is, as the belt wraps around the torso, much of the in line force (tension) recorded by the load cells acts in tangential directions to the sternum (y and z directions) rather than normal to the sternum (x direction). Using the optical markers placed at intervals along the belt, it is possible to estimate the compressive x component of force being applied to the sternum, as it changes with time during the test. (a) (b) Figure. Schematics of diagonal belt and optical markers with assigned coordinates. (a) Anterior view of belted thorax; (b) Inferior view of thorax. Using the position data from the optical belt markers, the coordinate time histories are recorded. For each time increment, a vector defining a chord between markers on the upper diagonal belt and markers on the lower diagonal belt can be defined: U = x x ) i + ( y y ) j + ( z z ) k (1) 1 ( This can be further defined as a unit vector of the connecting chord: u 1 ( x1 x) i + ( y1 y) j + ( z1 z) k = ( x x ) + ( y y ) + ( z z ) () Multiplying the in line belt force tension (F B ) measured in the upper shoulder belt and lower anterolateral belt load cells by the posteriorly directed component (SAE x) of the unit vectors in the upper shoulder belt and the lower anterolateral belt yields an approximate resultant compressive force being applied across these two regions of the thorax. Summing these two x component forces approximates the total compressive force being applied to the thorax: B Bx Bx F = F + F (3)

6 III. RESULTS Using this geometry of the belt as it wraps across the thorax, the rate and magnitude of compressive loading applied to the thorax can be calculated and compared to the rate and magnitude of loading recorded by the inline belt load cells. The geometric effect of the belt curvature as it wraps around the torso and anchors to the vehicle during the sled tests is shown in Table 3 and Figure 3. Peak in line belt forces for the upper diagonal belt were 6751 N, and 5595 N for the lower diagonal belt. When separated into local x, y, and z components acting on the thorax, the average peak x component force in the upper diagonal belt was 4786 N, and 85 N on the lower diagonal belt. The average onset rate of in line force acting on the upper diagonal belt was 15.5 N/ms, and 13. N/ms on the lower diagonal belt. Using the geometric information obtained from the optical markers placed along the belt, the x component load rate of the upper diagonal belt force is estimated at 15.4 N/ms, and 7.7 N/ms for the lower belt load rate. Again, the y and z component belt forces act in tangent to the sternum and do not contribute directly to any thoracic compression. Test Number Table 3. Local applied compressive load rates on sternum vs. load rates as applied to belts. Sled Velocity (m/s) Peak upper Peak x comp upper Peak lower Peak x comp lower Diagonal Load Rate Diagonal Load Rate Thorax x Diagonal Load Rate Diagonal Load Rate Thorax x T T T T T T T T Average St. Dev (a) (b) T194 T1358 T136 T1379 T195 T1359 T1378 T

7 (c) (d) Figure 3. Comparison between the upper (a) and lower (c) belt force time histories and the upper (b) and lower (d) x component belt forces as applied to the thorax. In contrast, the geometric effect of the belt curvature as it wraps around the torso and anchors to the carriage of the table top test device is shown in Table 4. During the fastest nominal loading tests (1.5 m/s), peak in line belt forces for the upper diagonal belt were 5514 N, and 5394 N for the lower diagonal belt. When separated into local x, y, and z components acting on the thorax, the average peak x component force in the upper diagonal belt was 33 N, and 1739 N on the lower diagonal belt. Table 4. Peak in line belt forces and peak compressive x component forces for the table top tests. Test Number Peak Average Peak Average Peak Average Peak Average (Tabletop) x comp x comp ± ± ± ± ± ± ± ± ± ± ± ± The average onset rate of in line force acting on the upper diagonal belt was 79.7 N/ms, and 51.8 N/ms on the lower diagonal belt (Table 5, Figure 4). Note that though the specimen used in Table top 3 was significantly younger and larger than the others (5 th percentile males), the largest force response was obtained during the ramp and hold test of Test.8. Using the geometric information obtained from the optical markers placed along the belt, the x component load rate of the upper belt force is estimated at 77. N/ms, and 6.3 N/ms for the lower belt load rate

8 Test No. (Table top) Table 5. Local applied compressive load rates on sternum vs. load rates as applied to belts. Measured Velocity (m/s) Load Ave. Load Rate Load Rate x comp. Ave. Load Rate Load Ave. Load Rate Rate x comp. Rate In line In line ± ± ± 4.6 Load Rate x comp ± ± ± ± ± ± Ave. Load Rate x comp. 1.± ± ± 4.7 (a) (b) Table-top 1. Table-top 1.3 Table-top. Table-top.3 Table-top.8 Table-top 3.3 Table-top 3.4 Table-top (c) (d) Figure 4. Comparison between the upper (a) and lower (c) belt force time histories and the upper (b) and lower (d) x component belt forces as applied to the thorax. The effective stiffness of the thorax from a diagonal belt load is shown in Figure 5 for both the sled tests as well as the table top tests. These graphs compare the deflection of the sternum in the local x direction with the x component of the belt forces as calculated from the belt load cells and the known geometry of the belt around the thorax. - -

9 (a) T194 T1358 T136 T195 T1359 T1378 T1379 T138 (b) Displacement (mm) Figure 5. Thoracic stiffness relating sternum displacement (x direction) to thoracic forces applied in the x direction in (a) the sled tests, and (b) the table top tests. Note the small inertial spike in force at the start of the loading. The thoracic injuries induced during testing were limited to the skeletal structures. The soft tissues (heart, aorta, lungs and mediastinal structures) were uninjured during this test series. The individual rib fractures were plotted using the University of Michigan rib fracture location method (Figure 6) [14]. Table 6 overlays the rib fractures sustained in the sled tests versus the fractures sustained in the table top tests

10 Figure 6. University of Michigan rib fracture location method [14]. Table 6. Sled test and Table top rib fracture locations. Δ = rib fracture in table top test. X = rib fracture in sled test. * AL = anterolateral, PL = posterolateral Rib # Right Posterior Right PL Right Lateral Right AL Right Anterior Left Anterior Left AL Left Lateral Left PL Left Posterior 1 Δ X Δ X X X X X X X Δ ΔX X 3 X Δ X X X X X ΔΔΔ X X X X 4 ΔΔ X X X X X X ΔΔΔ X X X X X X X X 5 Δ ΔΔX X X X X X X X X X X Δ X X X 6 X X X X Δ X X X X ΔΔΔ X X 7 ΔΔΔX Δ X X X X X X X 8 ΔΔΔ X Δ X X 9 Δ Δ X Δ Δ X X 1 Δ Δ X 11 Δ 1 X There were a total of 7 rib fractures which fell into the 5 anatomical regions for the combined sled tests. Shaw et al. [5] had previously reported 84 individual rib fractures for the 8 sled test subjects. However, if there were two fractures of the same rib and both fractures fell into the same region these fractures were considered to be from the same insult. The total number of individual rib fractures for the table top tests was 33 [1]. For comparison between test configurations, it is important to remember that the sled tests model the right passenger position and the table top tests model the left driver position. The percentage of rib fractures observed in the right anterolateral region of the table top subjects rib cage were 5% (17/33) and in the left anterolateral region of the sled tests were 17% (1/7). The percentage of left anterolateral rib fractures observed for the table top and right anterolateral rib fractures observed for the sled tests were 33% and 4%, respectively. The sled tests produced 13 percent of the right anterior region rib fractures whereas the table top tests produced 3% of the left anterior region rib fractures. The left anterior region observed rib fractures were 7% for the sled tests and 3% for the right anterior region observed in the table top tests. The table top tests did not produce any rib fractures in the left lateral, posterolateral, or posterior regions, whereas the sled tests - -

11 produced 19% of the rib fractures in the left lateral and left posterior regions. Note that the sled tests produced sternal fractures in 7/8 tests (subject 441 did not have a sternal fracture). The table top tests did not produce any sternal fractures. IV. DISCUSSION In the past, knowledge of belt forces during either a sled or component test was limited to load cell sensor data at a point along the belt. Even a rough estimation of applied forces from the belt to a thorax was difficult since the curvature of the belt changes with displacement, which made characterizing this curvature very challenging. New, optical based methods of measuring displacements have dramatically increased the amount of useful information obtained during testing, including detailed motions of the skeletal structures that were previously not possible to observe. For this investigation, detailed displacement measures along the belts allow an estimation of in line and normal forces being applied to the torso by the belts. Knowledge of these component forces reveals different distributions of forces between the upper and lower diagonal belts, changes in the rate of loadings from previously reported load cell load rates, and local stiffness values at the sternum that have been previously estimated from sternum deflections and (at best) posterior reaction loads. By examining only the belt tensions, one may have concluded that the upper and lower shoulder belt forces are rather well balanced. However, by examining the change in belt angles with respect to the thorax, it is seen that the upper diagonal belt has a much large contribution to the compressive forces on the thorax than the lower diagonal belt. For the table top tests, this difference is less pronounced. The upper and lower belt tensions in the table top configuration is well balanced from the standpoint of both the applied forces measured at the load cells, as well as when compensating for the geometric effects of the belt curvature. While the primary purpose of the table top test configuration was for the characterization of thoracic response from belt loadings, certain obvious limitations of the table top test have been long debated. The differences in occupant position between a full scale sled and a table top test can result in differences in internal viscera location and motion. A PMHS in a prone position, as in the table top tests, will have its internal viscera away from the internal anterior chest wall from the effects of gravity. During chest compression, the mechanical effects of organ compression may only become significant after chest compression is sufficient enough to engage the organs directly. Initial mechanical response of the thorax during the first few millimeters of compression may be more compliant due to this lack of viscera contact. This effect has been acknowledged by researchers using the table top device, with attempts to separate out the thoracic stiffness components of skin and external flesh, rib cage, and viscera being performed [7]. The second difference between the sled and table top tests is the presence and direction of inertial forces acting on the anterior thoracic wall from the internal viscera. In a frontal sled test, these inertial forces acting on the anterior chest wall and the viscera may act to stiffen the response of the thorax to belt loading. Indeed, initial results from this study are consistent with [5], and show an outward bulging of the thorax with respect to the spine which may be due to inertial loading. The fact is that the internal viscera and the entire anterior thoracic wall is accelerated on the sled resulting in significantly more inertial based loads on the thorax than is produced by the limited portion of the anterior thorax in the area of the belt that is accelerated on the table top. With the internal viscera not against the internal anterior chest wall in the table top test, the loading of the thorax by the diagonal belt only loads the internal viscera after the initial few mm of chest compression. Of course, the flexion of the whole torso during sled tests may also contribute to this observed bulging of the thorax [5]. As the torso pitches forward initially, the thorax is compressed in the superior inferior direction. This compression would work to reduce the volume in the thoracic cavity for the internal viscera, resulting in forces that would deflect the thorax out in the direction of least resistance

12 The differences in thoracic stiffness observed between the table top and sled tests may be the result of the occupant orientation and the difference in body forces acting in the two test conditions. Thoracic stiffness from the table top involves the intact torso or skin/flesh/skeleton, and, at some point, internal viscera. Upon loading, sternum deflection is immediate and results in a positive stiffness being recorded. Thoracic stiffness recorded during the sled tests does not display the same trends. Upon impact, the inertial forces acting on the anterior thorax and the viscera work to increase the distance between the sternum and the spine, at the same time substantial forces are achieved in the belt. Only once the external belt forces exceed the internal inertial forces does the sternum begin to compress towards the spine. The effect of this tendency for the chest to expand initially upon the thoracic stiffness is profound. During the first milliseconds of a forward collision, the effective initial stiffness of the thorax is negative. Since the initial chest compression is negative (outward displacement), restraint designers would gain a few additional millimeters of travel before actual chest compression begins, potentially altering current restraint designs. The differences in fracture locations between the sled and table top tests were qualitatively minimal, with a large number of fractures occurring in ribs, 3, and 4 on the right anterolateral thorax on the sled tests (right passenger position), and on the left anterolateral thorax on the table top tests (left driver position). It is important to note that these locations of fracture are on the unloaded side of the thorax, and may be due to excessive deflection on the unloaded side noted by chest band data collected by Shaw et al. [5]. A limitation of this study can be traced to the limitations of the original sled test and table top test series; that is, the results of only eight PMHS sled test specimens and three table top specimens were analyzed here. Additional table top tests should be performed to raise the confidence level of the resulting data. However, while variations between individual tests exist in each test configuration, each test configuration produced a characteristic response that was unique to it. For example, in the sled tests, the posteriorly directed component (SAE x) of the belt tension (F B ) is F Bx =.7 F B at the shoulder, but only F Bx =.14 F B where the belt engages the anterolateral torso inferiorly. The corresponding components on the table top are F Bx =.6 F B (shoulder) and F Bx =.48 F B (lower). When these components are cross plotted with chest deflection, pronounced consequences of thoracic anterior wall deformation patterns and of the internal viscera s inertia can be seen in the effective thoracic stiffness. In all table top tests, F Bx builds monotonically with chest deflection. In a sled test the relationship is more complex, with an initial period of chest expansion as F Bx builds. The chest regains its initial shape after approximately 1 3 kn of upper F Bx and exhibits chest deflection thereafter. These findings in no way diminish the importance of using both sled and table top test fixtures in PMHS testing. Of course, the controlled, well defined boundary conditions of the table top device is ideal for thoracic characterization, as the input loading to the thorax can be easily tailored to yield the information required for viscoelastic characterization, such as step hold waveforms where both the instantaneous and long term response can be carefully assessed. V. CONCLUSIONS It has been assumed for some time that the inertial forces acting on the posterior aspect of the anterior chest wall from the internal viscera affects the stiffness of the thorax. However, before the ability to measure detailed geometric interactions of the thorax and belt, the effect from these inertial forces could not be quantified. Initial indications show a profound stiffening effect from these inertial forces, especially during the onset of shoulder belt loading in a frontal collision. This thoracic response is quite different from the thoracic response characterized by this author [1] and others [9][1][11] using data obtained from a table top test device. VI. REFERENCES [1] Lessley, DJ, Salzar, RS, Crandall, JR, Kent, RW, Bolton, JR, Bass, CR, Guillemot, H, Forman, JL. (1) Kinematics of the thorax under dynamic belt loading conditions. International Journal of Crashworthiness, 15():

13 [] Lobdell, T., Kroell, C., Schneider, D., Hering, W., Nahum, A. (1973) Impact Response of the Human Thorax in Proceedings of the Symposium Human Impact Response Measurement and Simulation. Edited by King W and Mertz H. General Motors Research Laboratories, Warren Michigan. Plenum Press New York, London, 1973, pp [3] Kent, RW, Crandall, JR, Bolton, JR, Prasad, P, Nusholtz, G, Mertz, H. (1) The influence of superficial soft tissues and restraint condition on thoracic skeletal injury prediction. Stapp Car Crash Journal, 45: [4] Forman, J, Lessley, DJ, Shaw, CG, Evans, J, Kent, RW, Rouhana, S, Prasad, P. (6) Thoracic response of belted PMHS, the Hybrid III, and the THOR NT mid sized male surrogates in low speed, frontal crashes. Stapp Car Crash Journal, 5: [5] Shaw, CG, Parent, DP, Purtsezov, S, Lessley, DJ, Crandall, JR, Kent, RW, Guillemot, H, Ridella, SA, Takhounts, E, Martin, P. (9) Impact Response of Restrained PMHS in Frontal Sled Tests: Skeletal Deformation Patterns Under Seat Loading. Stapp Car Crash Journal, 53, [6] Kent, RW, Lessley, DJ, Sherwood, C. (4) Thoracic response to Dynamic, Non Impact Loading from a Hub, Distributed, Diagonal, and Double Diagonal s. Stapp Car Crash Journal, 48: [7] Kent, RW. (8) Frontal thoracic response to dynamic loading: the role of superficial tissues, viscera and the rib cage. International Journal of Crashworthiness, 13(3): [8] Shaw, CG, Lessley, DJ, Evans, J, Crandall, JR, Shin, J, Portier, P, Paoloni, G. (7) Quasi static and dynamic thoracic loading tests: cadaveric torsos. IRCOBI Conference on the Biomechanics of Impact. [9] Duma S; Stitzel J; Kemper A; McNally C; Kennedy E; Brolinson G; Matsuoka F. (6) Acquiring Non Censored Rib Fracture Data During Dynamic Loading. Biomedical Sciences Instrumentation, 4: [1] L Abbe, R., Dainty, D., Newman, J., An experimental analysis of thoracic deflection response to belt loading, IRCOBI Conference on the Biomechanics of Impact, Bron, France, 198, [11] Cesari D, Bouquet R (1994) Comparison of hybrid III and human cadaver thorax deformations loaded by a thoracic belt. Stapp Car Crash Conference, 38: [1] Salzar, RS, Bass, CR, Lessley, DJ, Crandall, JR, Kent, RW, Bolton, JR. (9) Viscoelastic response of the thorax under dynamic belt loading. Traffic Injury Prevention, 1(3): [13] Shaw, CG, Parent, DP, Purtsezov, S, Lessley, DJ, Kerrigan, JR, Shin, J, Crandall, JR, Zama, Y, Ejima, S, Kamiji, K, Yasuki, T. (9) Frontal Impact PMHS Sled Tests for FE TORSO Model Development. IRCOBI Conference on the Biomechanics of Impact. [14] Ritchie N, Wang S, Sochor MR, Schneider LW: A Method for Documenting Locations of Rib Fractures for Occupants in Real World Crashes Using Medical Computed Tomography (CT) Scans SAE