The Influence of Seat-Back and Head-Restraint Properties on the Head-Neck Motion During Rear-lmpact

Mats Y. Svenssonl , Per Lövsundl , Yngve Häland2, Stefan Larsson3 l nept. of Injury Prevention, Chalmers University of Technology, S-412 96 Göteborg, Sweden

2Electrolux Autoliv AB, P.O. Box 104, S-447 00 Värgärda, Sweden 3sAAB Automobile AB, A5-2TKAL, S-461 80 Trollhättan, Sweden

ABSTRACT

One type of production-car front-seat with a head-restraint was tested in simulated low-velocity rear-impacts using a Hybrid III-dummy fitted with a modified neck (RID-neck). The seat was modified in various ways to test the influence of different seat properties on the head-neck motion during the impact.

Tue results show that it is possible to influence the head-neck kinematics to a great extent by modifying the properties of the seat-back and the head-restraint. lt was possible to virtually eliminate the neck extension motion during the rear-impact and this will hopefully resull in a significant decrease of the neck injury risk in real world rear­impacts.

INTRODUCTION

Neck injuries often occur during rear-impacts at low impact-velocities, typically less than 20 km/h (12.5 mph) (e.g. Olsson et al., 1990). The accidents usually result in AIS=l neck injuries (sometimes called "whiplash injuries"). In spite of the low AIS rating, about 10% of the injuries lead to permanent disability with a disability degree of�10% (Nygren, 1984). In other AIS= l injuries, the risk of permanent disabilily is only 0.1 % on average (Nygren et al., 1985).

Overall, the injury risk in rear-impacts decrease<l when head restraints were introduced in the front seats. The reduction was higher for fixed restraints than for adjustable ones (24% and 14% respectively) (Nygren et al„ 1985). Field study results indicated that 70-90% of the adjustable head-restraints are maladjusted, most of them in the lowest position (States et al„ 1972). In the USA, O'Neill et al. (1972) found that after the introduction of head­restraints, insurance claims concerning neck injuries to drivers bad decreased by 18%. The protective effect, however, of the headrests varied a great deal between different car models, and in some cases the risks of injury were even increased. Similar findings were presented by Huelke and O'Day ( 1 975). Nygren et al. (1985) found that the risk of neck injury in rear-impacts was not reduced in newer cars. Their study disclosed large differences in protective performance between different car models.

Lövsund et al. (1988) found that the risk of neck injury was twice as high for front-seat occupants

compared to rear-seat occupants in rear-end collisions in Sweden. lt should be noted that most cars in this survey were equipped with head­restraints in the front-seats but not in the rear-seats. These results include compensation for differences in sex and age distribution between front-seat and rear-seat passengers. Similar results have been found in other studies (Kihlberg, 1969; States et al., 1972; Carlsson et al., 1985; Otremski et al., 1989).

Generally there is a difference in design between front-seats and rear-seats. The seat-back of the rear­seat is usually finnly attached to the sides of the car­

.body and yields very little when loaded during a rear-impact. In contrast the front-seat seat-back is relatively loosely attached at its bottom joints. This difference in seat design could explain the difference in injury risk between the front-seat and the rear-seat.

States et al. (1 969) suggested that the elastic rebound of the seat back could be an aggravating faclOr for the whiplash extension motion. The rebound of the seat-back can push the torso forward relative to the vehicle at an early stage of the whiplash extension motion when the head begins rotating rearward. This in turn increases the relative linear and angular velocity of the head relative to the upper torso at the same time as it delays contact between the head and head-restraint, thus causing a larger maximum extension angle. Later studies support this theory (McKenzie and Williams, 1971; Prasad et al., 1975; Romilly et al., 1989; Foret­Bruno et al., 1991; Svensson et al„ 1993a). If the seat-back of the front-seat collapses or yields plastically during a rear-impact., the elastic seat-back rebound is likely to be reduced. Foret-Bruno et al. (1991) reported that seat-back collapse decreased the risk of neck injury in rear-impacts.

At present there is no adequate tool for testing the performance of car seats and head-restraints in rear-impacts. The best available dummy is the Hybrid III. The neck and spinal structure of this dummy is stiff and unlikely to interact with the seat­back in the same compliant way as would the human spine.

Seemann et al. (1986) found the Hybrid III-neck far too stiff to respond in a human-like manner in the sagittal plane and similar findings were reported by Deng (1989) and Foret-Bruno et al. (1991). In volunteer tests, McConnell et al. (1993) found that during the acceleration phase of a rear-impact, when the occupants body was pressed against the seat­back, the spinal curvature straightened. This in turn

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caused an upward motion of tbe bead and tbus an elevated bead contact point on tbe bead-restraint. In a comparative study using volunteers and a Hybrid III-dummy, Scott et al. (1993) found that the dummy was less prone to ramp up along the seat­back tban were tbe volunteers.

Tbe relation between different kinematic and kinetic parameters of tbe bead-neck motion and tbe risk of sustaining an AIS=l neck-injury in a rear­impact is unknown.

McConnell et al. ( 1993) undertook staged rear­end collisions at low impact-velocities. In these tests, volunteers were seated in car-seats with head­restraints. The maxirnum extension angle of the neck never exceeded 45° during the tests. The volunteers were thus not exposed to hyperextension of tbe complete cervical spine and yet symptoms of minor neck injuries in the form of pain in the neck region were experienced.

Menz and Patrick (1967) carried out rear-irnpact sled-tests using a volunteer in a seat with a high rigid seat-back. In this study the volunteers head was always in contact with the seat-back during the irnpact. Tests were undenaken at velocity changes (Av:s) of up to 30 km/h without any symptoms of injury occurring.

The result of tbese two studies indicate that in rear-end impacts it is not enough to avoid hyperextension of the complete cervical spine to prevent neck injury but injuries can be prevented assuming that no head-neck motion occurs during the impact.

As a first step in the construction of a new rear­impact dummy, Svensson and Lövsund ( 1992) developed a Rear Impact Dummy-neck (RIO-neck) for the Hybrid III-dummy. Tue neck was especially designed for low-velocity rear-impact testing. F.quipped with this new neck, the Hybrid III-dummy attained a significantly improved bio-fidelity in low­velocity rear-impact testing.

The aim of the present study was to investigate tbe influence of different seat-back and head­restraint parameters on the head-neck kinematics in low-velocity rear-end collisions by using the RID­neck on the Hybrid III-dummy. The results of this study and those of an ongoing parallel study (Svensson et al., 1993b) determining the sites as well as the mechanisms causing the injuries and the relation of tbe injury risk to the kinematic and kinetic parameters will form a basis for the development of criteria for the improvement of future car seats which will lead to less risk of neck injuries in rear-irnpacts.

MA lERIALS AND METHODS

The present study was preceded by a study where various production-car seats were tested in staged rear-impacts (Svensson et al., 1993a). A

bucket-type front-seat (denoted F l in that study) was chosen for the present study. The seat-back frame of this seat is a sheet-metal construction. lt consists of two side members welded at the top to the two cross members of the seat-back (Fig. 1). A steel sheet is welded to the two cross members and the side members. The head-restraint mounting is fixed to both the cross members and the sheet. Below the lower cross member a steel-thread netting is attached to each of the side members by four coil­springs.

The results from Svensson et al. ( 1993)a show that this design allows a relatively large displacement rearward of the lower torso between the side members during a rear-impact. Since tbe seat-back frame also yielded rearward during tbe rear-irnpact, the dummy torso did not undergo much angular displacement in this seat-type (Svensson et al., 1993a)

For the present study the seat was modified in several ways and tested according to Table 1 . The first modification was to the head-restraint. The head-restraint consisted of a wood block and a 10 mm thick polymer foam padding (hardness: Shore 00 =60) layer on the front surface. lt was covered with the same fabric as the seat. The modified head­restraint was fixed at a certain height and was given a flat and vertical front surface to minimise the vertical forces irnposed on the head during contact with the head-restraint.The top of the head-restraint was placed weil above the level of the head centre of gravity with 0.05 m of vertical distance between the top of the head and the top of the head-restraint. Tue horizontal head to head-restraint distance was adjusted by altering the thickness of the wood block. Two different horizontal distances, 0.04 m and 0.10 m, between the head and the head-restraint were tested.

In some of the tests a rod was mounted between each seat-back side-member and the sides of the lower seat frame (Fig. 1) to increase the stiffness to rearward seat-back deflection.

In some of the tests, three belts (Standard seat­belt webbing, 100 mm wide) were stretched across the seat-back frame from side member to side member, at three levels, 0.10 m, 0.20 m and 0.35 m above the seat-back pivot joint. This was done in order to restrain the rearward displacement of the lower torso during rear-irnpacL

In two of the tests. tbe thickness of the seat-back padding was increased by adding a wedge of padding (poly-ether foam of the same type as the original padding) on top of the original seat-back cushion. This wedge increased the padding depth by 50 mm. from 25 mm to 75 mm, at the top of the seat-back but left the padding thickness of the lower third of the seat-back unchanged (Fig. 2).

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Padding

aj �

Lower cross mambar

Figura 1 : a) Schamatic viaw of the saat-frama saan from tha laft sida showing tha contours of tha saatad dummy. In soma tests a rod was connactad batwean tha lowar saat-frama and tha sida mambar of tha saat-back frama. b) A schamatic frontal viaw of tha saat-back frama with sida mambars and cross­mambars. Between the side members, the steel-thread net is attached to the side members by coil-springs and for soma tasts three belts wara stratchad batwaan tha sida mambars bahind tha nat.

Original padding

Additional padding wadga

Figura 2: A schamatic cross-saction of tha saat, tha original padding layar, and tha additional padding wadga which was usad in two of tha modifications, M7 and MS. Tha contours of tha dummy ara also shown.

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Table 1 : The eight different combinations of seat modifications used in the present study.

Modification Horizontal Seat-back code distance lml stiffened bv rods

M1 0.1 0 M2 0.1 0 X M3 0.04 M4 0.04 X M5 0.04 M6 0.04 X M7 0.04 X MB 0.04 X

The reason for adding the padding-wedge was to allow the upper torso to be displaced rearward relative to the seat-back frame and the head­restraint, thus decreasing the horizontal gap between the head and the head-restraint early on in the crasb event, before any significant relative displacement between the head and the torso bad begun.

The tests were done on a crash-sled at the department of Injury Prevention, Chalmers University of Technology. Two pre-impact sled­velocities (Av) were used, 5.0±0.1 km/h (3.1 mph, 1.4 m/s) and 12.5±0.2 km/h (7.8 mph, 3.5 m/s).

The sied deceleration was set to approximately a square pulse with an amplitude of about 45 mJs2 at Av=5 km/h, and about 70 m/s2 at Av=12.5 km/h (Fig. 3). The seats were mounted rearward facing on the sied. They were mounted to the sled-floor in the Standard attachment holes with their standard angular position of the lower seat-frame.

A 50th percentile Hybrid III-dummy equipped with a RID-neck was used. The dummy was unbelted and its arms were folded in front of the ehest and fixed with adhesive tape to improve repeatability of the tests and prevent the arms from obstructing the vision of the high-speed camera. The dummy was equipped with accelerometers in the head, ehest and pelvis, and with force-moment transducers at the upper neck (R.A. Denton, type: 1716) and at the lower neck (R.A. Denton, type: 1794). The sied acceleration was measured, and all tests were filmed with a high-speed camera.

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RESULTS

The results of the present study show that by making the changes described above to the seat design it is possible to considerably change the motion of tbe body, and particularly that of the bead and the neck, during a rear-end collision.

The rearward horizontal translational and rearward angular displacements between the head and the torso are shown for all tests in Figure 4 for Av=5 km /h and in Figure 5 for Av=12.5 km /h. Tue maximum values for the accelerations and the neck forces and neck moments are listed in Table 2.

The complete data from the transducers and from the film analysis are shown in Figures 6 (Av=5 km/h) and 7 (Av=12.5 km/h). Figures 6a and 7a show the rearward horizontal displacements of the pelvis, shoulder, head and the upper seat-back. Tue rearward horizontal displacement of the head relative to the upper torso is also shown. The rearward angular displacements of the head and the torso as well as the rearward angular displacemem of the head relative to the torso are displayed in Figures 6b and 7b. The X-accelerations of the head, ehest and pelvis (Figures 6c and 7c), the Y-torque measured in the upper and lower neck transducers (Figures 6d and 7d), and the X-shear-force and the Z-axial-force

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- 398 -

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measured in the upper neck transducer (Figures 6e and 7e) are also shown. The X and Z measurements in the lower neck-transducer failed due to defective measurement equipment. For similar reasons the X measuremenl in the upper neck transducer failed for M 1 and al !!.. v= 12.5 km/h also for M3.

Tbe bead-torso angular displacement sharply decreased with decreased horizontal head to head­restraint gap. This is seen when MI and M2 are compared to M3 and M4 respectively in Figures 4 and 5.

The mounting of the rods to the seat-back frame increased the angular head-torso displacement, which can be seen when M I , M3, and MS are compared to M2, M4, and M6 respectively (Figs. 4 and 5). From Figures 6b and 7b it can be seen that the rearward angular displacement of the torso decreased or even changed to forward angular displacement with the rigidified seat-back which explains the !arger angular head-torso displacemenl.

Stretching belts between the side members of the seat-back frame resulted in a decreased angular

head-torso displacement which is seen when M3, M4, and M7 are compared to M5, M6, and M8 respectively (Figs. 4 and 5).

The additional padding wedge decreased the rearward angular head-torso displacement which is seen when M4 and M6 are compared to M7 and M8 respectively (Figs 4 and 5). One exceplion is M6 compared to M8 at Av=5 km/h where a slight increase of the angular displacement occurs late (maximum at about 150 ms) in the crash event.

DISCUSSION

In order to assess the repeatability of the test set­up, further analysis of data from pairs of identical

tests in the test series by Svensson et al. (1993)a was undertaken. Tue deviation of the maximum angular head-torso displacement between two idcntical tests typically was <l .5° at Av=5 km/h and <3.0° at ll.v=l2,5 km/h (from 0 ms to the time of maximum extension angle).

Tue horizontal distance hetween the head and the head-restraint prior to rear-impact bad the largest

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influence on the head-neck motion. The maximum rearward angular displacements during impacts were generally much smaller with 0.04 m distance than with 0.10 m. With a smaller head to head-restraint gap, the rearward head motion was stopped earlier by the bead-restraint, which resulted in a smaller maximum rearward head displacement. Comparisons M 1 relative to M3 and M2 relative to M4 show a reduction from 38° to 9° of the average maximum extension angle.

The stiffness of the seat-back frame intluenced the motion of the torso during the rear-impact. Stiffening of the seat-back frame by adding rods (Fig. 1) resulted in a decrease of the maximum rearward displacement of the upper seat-back, and comparisons of the results of M 1 to M2, M3 to M4 and MS to M6 show a decrease from on average 0.07 m to on average 0.04 m at Liv=12.5 km/h and from 0.04 m to 0.03 m at Liv=5 km/h.

With the rods in place, the rearward displacement of the dummy during rcar-impact decreased at ehest level and increased at the pelvic level. This occurred because the stiffening of the seat-back primarily decreased the rearward displacement at shoulder level. As a result, the maximum angular displacement between the head and the torso increased. The largest increase, from 33° to 43°, occurred at Liv=12.5 km/h with 0.10 m initial head-restraint gap (from M l to M2).

Stretching belts between the side-members of the seat-back frame resulted in an increased rearward horizontal translational displacement of the upper torso relative to the pelvis during rear-impact. This resulted in a decreased rearward angular displacement of the head relative to the torso. The largest decrease of the maximum angular displacement, from 1 1 ° down to 2°, occurred between M7 and M8 at Liv=l2.5 km/h. When the belts were loaded during impact, the seat-back side members probably yielded somewhat inward and this meant that the lower torso could be displaced rearward relatively much in spite of the belts.

The additional padding wedge caused an increase of the rearward displacement of the upper torso relative to the upper seat-back while the displacements of the seat-back and the head­restraint remained minor. This is seen when tests M4 and M6 are compared to M 7 and M8 respectively. The largest increase of the relative rearward displacement of the upper torso, from 0.05 m to 0.08 m, was found when comparing M6 to M8 at Liv=l2.5 km/h. In lest M8 the initial 0.04 m gap between the head and the head-restraint diminished early on in the impact before any significant displacement between the head and torso had started.

Table 2: The maximum values for head acceleration; ehest acceleration; pelvic acceleration; the z and x forces in the upper neck-transducer; the y-moments in the upper and the lower neck transducers; and the rearward angular displacement of the head relative to the torso.

�v=5 km/h Modifica· He ad Chest Pelvic Upper Upper Upper Lower Head-tion code acc. (g) acc. (g) acc. (g) neck neck neck neck torso

x·force (N) z-force (N) y-moment y-moment extension (Nm) (Nm) ang. (deq)

M1 1 1 3.9 3.0 . 142 1 1 36 36 M2 4.9 4.0 2.5 1 40 74 1 2 37 38 M3 1 4.6 4.5 2.8 64 1 69 ·6 1 8 3 M4 1 5.6 4.3 2.7 78 1 93 ·5 22 1 1 M5 1 3.3 4.3 2.7 67 198 ·8 21 4 M6 1 6.2 4.5 2.8 59 1 50 .7 1 8 7 M7 1 9.5 4.0 2.3 41 226 -5 1 9 8 MS 1 7.5 4.0 2.3 ·36 231 ·6 1 5 9

M=12.5 km/h Modifica· Head acc. Chest Pelvic Upper Upper Upper Lower Head· tion code (g) acc. (g) acc. (g) neck x- neck z- neck neck torso

force (N) force (N) y-moment y-moment extension (Nm) (Nm) ang. (deq)

M 1 30.9 9.4 7.2 . 594 -9 84 33 M2 59.5 1 5.6 9.1 31 1 794 27 1 1 2 43 M3 1 8.6 1 0.4 6.9 . 424 -1 5 53 1 2 M4 3 1 . 1 1 2.8 8.2 203 570 · 17 75 1 1 M5 1 7.9 8.8 8.7 2 1 0 396 ·21 45 7 M6 29.8 1 3.7 8.9 230 446 ·20 68 1 0 M7 38.2 1 1 .7 7.0 1 63 465 · 1 9 52 1 1 MS 30.0 1 1 .4 8.2 ·67 451 ·12 28 2

- 404 -

Comparisons of M4 relative to M7 and of M6 relative to MS show that the rearward displacement of the head relative to the torso was delayed due to the additional padding and occurred mainly as a result of the torso rebounding off from the seat­back.

Stiffening of lhe seat-back frarne by adding rods seemed to somewhat aggravate lhe head-neck motion, but when this modification was combined with belts stretched across the frarne from side member to side member, and a padding wedge was added on the upper seat-back, the result was a significantly reduced head-neck motion.

The magnitudes of the forces and moments measured in the neck transducers generally decreased wilh decreased head-torso displacements. Proposed non-injurious maximum levels for neck loads at lhe occipital condyles in rearward bending (neck extension) for volunteers in dynamic tests have been presented by SAE ( 1 986). The proposed levels were: 30.5 Nm bending moment; 231 N shear force and 249 N axial force. For cadavers the bending-moment limit for ligarnentous damage is specified to be in the interval 47-57 Nm. In the present study the rearward bending moments and sbear forces did not significantly exceed the proposed volunteer limits but the axial force in all the tests at Av=l2.5 km/h did. The largest neck loads at lhe occipital joint occurred for M2 at Av=l2.5 km/h. The maximum rearward bending moment was 27 Nm, the maximum shear load was 3 1 1 N and the maximum axial load bad a peak of 794 N. This axial load is more than twice the proposed non-injury limit for volunteers of 249 N.

A head acceleration peak of 59.5 g occurred simultaneously to the axial load peak for M2 at A v= 12.5 km/h. The two peaks appeared whcn the head struck the upper seat-back cross member. In contrast lhe head acceleration for M2 at Av=5 km/h was very smooth since no head contact occurred. The head-restraint mass increased wilh increased lhickness of lhe wood block and this influenced lhe profile of the bead acceleration. This is particularly evident for the tests at Av=5 km/h where the acceleration peaks becarne higher with decreased head to head-restraint gap due to the increased head­restraint mass that followed with a thicker wood block.

Tue exact relation between lhe head-neck motion and the risk of neck injury has not yet been fully established. Based on the findings of Mertz and Patrick ( 1967) and of McConnell et al. (1993) it can be assumed, lhough, that the risk of neck injury in a rear-end collision is related to the linear and angular rearward motion of the bead relative to lhe torso and that these injuries can be prevented by preventing lhis motion from occurring.

On the basis of this assumption lhe resultc; of the present study indicate that it should be possible to radically increase the protective performance of modern car-seats. There i s probably no incompatibility between making the seat-back

strong enough to prevent seat-back collapse during high-speed rear-impacts and improving the neck protection at low-speed rear-impacts, provided lhat the head-restraint is placed close to the head and lhat the stiffness of the seat-back frame and the seat­back cushion are properly chosen .

The experiments in this study do not take into account occupants seated in positions that differ from the Standard position of the dummy.

An improved dummy torso for rear-impact testing is desirable. The torso of the H ybrid III­dummy is very stiff and incapable of interacting with the seat-back in the sarne compliant way as would the human torso. l t can be expected that the motion of the human torso in impact situations corresponding to those of the present study would be somewhat different. The straightening of the spinal curvature and lhe corresponding lenglhening of the seated height of the occupant during the rear­impact reportcd by McConnell et al. ( 1 993) is another factor that cannot be reproduced wilh the Hybrid III-dummy but should be taken into consideration when designing future car-seats.

SUMMARY AND CONCLUSIONS

A production car seat was modified in several ways. An adequately high head-restraint with a flat vertical front surface was attached to the seat-back. Two different head to head-restraint gaps were tested combined wilh different stiffnesses of the seat-back frame and lhe lower seat-back cushion as weil as different depths of the upper seat-back cushion. Rear-impactc; on a sied were staged at Av:s of 5 km/h and 12.5 km/h using a Hybrid IIl-dummy equipped with a RID-neck.

Of the parameters tested in this study, the horizontal head to head-restraint gap proved to have the largest inOuence on the head-neck motion during rear-impacl. The maximum head-torso clisplacement increased wilh increased head to head­restraint gap. Increased stiffness of the seat-back frame resulted in slightly increased maximum head­torso displacement but when this was combined wilh a stiffening of lhe lower seat-back cushion and a deeper upper seat-back cushion, the resuh was a clear reduction of lhe head-torso clisplacement since these two changes resulted in lhe elimination of lhe head to head-restraint gap early in the crash event before any head-torso displacement was initiated.

The results indicate that minor changes to existing car-seats might radically improve the protection against neck injuries in rear impacts. A close fit between the head and the head-restraint in combination with weil chosen stiffnesses of the different seat-back components would almost exclude all extension motion of the cervical spine and thus minimise the neck-injury risk during a rear-impact.

- 405 -

ACKNOWLEDGEMENTS

This study was supported by the Swedish Transport Research Board (1FB), the Swedish National Road Administration, FOLKSAM, Sweden, and Elec1rolux Autoliv AB, Sweden. We thank Fredrik Lindh, M.Sc. and John Lindhe, M.Sc. for their help in arranging the test set-up and running the tests.

REFERENCES

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Deng, Y.-C. (1989): Anthropomorphic Dummy Neck Modelling and lnjury Considerations. Accid. Anal. & Prev. Vol. 2 1 . No 1 . pp. 85-100

Foret-Bruno. J.Y.; Dauvilliers, F.; Tarriere, C. (1991): Influence of the Seal and Head Rest Stiffness on the Risk of Cervical lnjuries in Rear Impact. Proc. 13th ESV Conf. in Paris. France, paper 9 1 -S8-W-19, NHTSA, USA;

Huelke, D . F . ; O ' Day, J. ( 1 975): The Federal M o to r v e h i c l e S a fe t y S t a n d a rd s : Recommendations for Increased Occupant Safety. Proc. Fourth Int. Cong. on Automotive Safety. NHTSA. USA, 275-292;

Kihlberg. J.K. ( 1 969): Flexion-Torsion Neck Injury in Rear Impacts. Proc. 1 3th AAAM Ann. Conf., Tue Univ. of Minnesota. Minneapolis. USA, 1-17;

Lövsund, P. ; Nygren, A.; Salen. B.; Tingvall, C. ( 1 988): Neck Injuries in Rear End Collisions among Front and Rear Seal Occupants. Proc. Int. IRCOBI Conference Biomech. of Impacts. Bergisch­Gladbach, F.R.G., 3 19-325;

McConnell. W. E.; Howard, R. P.; Guzman. H. M . ; Bomar. J. B . ; Raddin. J H.; Benedict, J . V . ; Smith, L. H.; Hatsell, C. P. (1993): Analysis of Human Test Subject Responses to Low Velocity Rear End Impacts. SAE paper no. 930889, SAE/SP-93/975, SAE lnc., Warrendale. Philadelphia, USA. ISBN 1 -56091-360-6. LC 92-63173

McKenzie, J.A.; Williams. J.F. ( 1 9 7 1 ) : The Dynarnic Behaviour of the Head and Cervical Spine during Whiplash. J. Biomech. 4:477-490

Mertz, H.J „ Patrick, L.M. ( 1967): lnvestigation of the Kinematics and Kinetics of Whiplash. Proc. 1 1 th STAPP Car Crash Conf., Anaheim, California. USA, pp. 267-317, SAE Inc., New York, USA. LC 67-22372

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