Dynamic Responses of Female Volunteers in Rear Impact Sled Tests at Two Head Restraint Distances

Abstract

The objective of this study was to assess the biomechanical and kinematic responses of female volunteers with two different head restraint (HR) configurations when exposed to a low-speed rear loading environment. A series of rear impact sled tests comprising eight belted, near 50th percentile female volunteers, seated on a simplified laboratory seat, was performed with a mean sled acceleration of 2.1 g and a velocity change of 6.8 km/h. Each volunteer underwent two tests; the first test configuration, HR10, was performed at the initial HR distance ∼10 cm and the second test configuration, HR15, was performed at ∼15 cm. Time histories, peak values and their timing were derived from accelerometer data and video analysis, and response corridors were also generated. The results were separated into three different categories, HR10 C ( N = 8), HR15 C ( N = 6), and HR15 N C ( N = 2), based on: (1) the targeted initial HR distance [10 cm or 15 cm] and (2) whether the volunteers’ head had made contact with the HR [Contact (C) or No Contact (NC)] during the test event. The results in the three categories deviated significantly. The greatest differences were found for the average peak head angular displacements, ranging from 10° to 64°. Furthermore, the average neck injury criteria (NIC) value was 22% lower in HR10 C (3.9 m ² /s ² ), and 49% greater in HR15 N C (7.4 m ² /s ² ) in comparison to HR15 C (5.0 m ² /s ² ). This study supplies new data suitable for validation of mechanical or mathematical models of a 50th percentile female. A model of a 50th percentile female remains to be developed and is urgently required to complement the average male models to enhance equality in safety assessments. Hence, it is important that future protection systems are developed and evaluated with female properties taken into consideration too. It is likely that the HR15 test configuration is close to the limit for avoiding HR contact for this specific seat setup. Using both datasets (HR15 C and HR15 N C ), each with its corresponding HR contact condition, will be possible in future dummy or model evaluation.

Full text

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ORIGINAL RESEARCH
published: 08 June 2021
doi: 10.3389/fbioe.2021.684003
Edited by:
Michael Kleinberger,
United States Army Research
Laboratory, United States
Reviewed by:
John Bolte,
The Ohio State University,
United States
Chantal Parenteau,
Exponent Inc., United States
Shannon Kroeker,
MEA Forensic Engineers & Scientists,
Canada
*Correspondence:
Anna Carlsson
anna.carlsson@chalmers.se
Specialty section:
This article was submitted to
Biomechanics,
a section of the journal
Frontiers in Bioengineering and
Biotechnology
Received: 22 March 2021
Accepted: 14 May 2021
Published: 08 June 2021
Citation:
Carlsson A, Horion S,
Davidsson J, Schick S, Linder A,
Hell W and Svensson MY (2021)
Dynamic Responses of Female
Volunteers in Rear Impact Sled Tests
at Two Head Restraint Distances.
Front. Bioeng. Biotechnol. 9:684003.
doi: 10.3389/fbioe.2021.684003
Dynamic Responses of Female
Volunteers in Rear Impact Sled Tests
at Two Head Restraint Distances
Anna Carlsson1*, Stefan Horion2, Johan Davidsson3, Sylvia Schick2, Astrid Linder3,4 ,
Wolfram Hell2and Mats Y. Svensson3
1Chalmers Industrial Technology (Chalmers Industriteknik), Gothenburg, Sweden, 2Institute for Legal Medicine,
Ludwig-Maximilians-Universitaet (LMU), Munich, Germany, 3Vehicle Safety Division, Chalmers University of Technology,
Gothenburg, Sweden, 4Swedish National Road and Transport Research Institute (VTI), Gothenburg, Sweden
The objective of this study was to assess the biomechanical and kinematic responses
of female volunteers with two different head restraint (HR) configurations when exposed
to a low-speed rear loading environment. A series of rear impact sled tests comprising
eight belted, near 50th percentile female volunteers, seated on a simplified laboratory
seat, was performed with a mean sled acceleration of 2.1 g and a velocity change
of 6.8 km/h. Each volunteer underwent two tests; the first test configuration, HR10,
was performed at the initial HR distance ∼10 cm and the second test configuration,
HR15, was performed at ∼15 cm. Time histories, peak values and their timing were
derived from accelerometer data and video analysis, and response corridors were also
generated. The results were separated into three different categories, HR10C(N= 8),
HR15C(N= 6), and HR15NC (N= 2), based on: (1) the targeted initial HR distance
[10 cm or 15 cm] and (2) whether the volunteers’ head had made contact with the
HR [Contact (C) or No Contact (NC)] during the test event. The results in the three
categories deviated significantly. The greatest differences were found for the average
peak head angular displacements, ranging from 10◦to 64◦. Furthermore, the average
neck injury criteria (NIC) value was 22% lower in HR10C(3.9 m2/s2), and 49% greater in
HR15NC (7.4 m2/s2) in comparison to HR15C(5.0 m2/s2). This study supplies new data
suitable for validation of mechanical or mathematical models of a 50th percentile female.
A model of a 50th percentile female remains to be developed and is urgently required
to complement the average male models to enhance equality in safety assessments.
Hence, it is important that future protection systems are developed and evaluated with
female properties taken into consideration too. It is likely that the HR15 test configuration
is close to the limit for avoiding HR contact for this specific seat setup. Using both
datasets (HR15Cand HR15NC ), each with its corresponding HR contact condition, will
be possible in future dummy or model evaluation.
Keywords: crash testing, females, soft tissue neck injury, rear impact, sled testing, vehicle safety, volunteers,
whiplash
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INTRODUCTION
Today, low-to-moderate speed rear impact testing is performed
with 50th percentile male dummies, mainly with the BioRID
II, which limits the assessment and development of whiplash
protection systems with regard to female occupant protection
(Linder and Svensson, 2019). In terms of stature and mass, the
50th percentile male crash test dummy roughly corresponds
to the 90th–95th percentile female (Welsh and Lenard, 2001),
resulting in females not being well represented by the existing
low velocity rear impact male dummy; BioRID II. Accident
data shows that females have a greater risk of sustaining
whiplash injuries than males, even under similar crash conditions
(Kihlberg, 1969;O’Neill et al., 1972;Otremski et al., 1989;Morris
and Thomas, 1996;Temming and Zobel, 1998;Chapline et al.,
2000;Krafft et al., 2003;Storvik et al., 2009;Carstensen et al.,
2011). According to these studies, the whiplash injury risk is up
to three times higher for females compared to males.
Passenger vehicles equipped with advanced whiplash
protection systems posed on average a ∼50% lower risk of
long-term whiplash injuries for occupants in rear impacts,
than for occupants in passenger vehicles manufactured after
1997, without whiplash protection systems installed (Kullgren
et al., 2007). Nevertheless, insurance data show that whiplash
injuries account for 63% of all injuries leading to permanent
medical impairment sustained in passenger vehicles on the
Swedish market (Gustafsson et al., 2015). In rear impacts, the risk
reduction for permanent medical impairment is approximately
30% greater for males than for females according to insurance
claims records (Kullgren and Krafft, 2010), which effectively
means that the difference between female and male whiplash
injury risk has increased, although the general whiplash injury
risk has reduced. In recent years, injury statistics show that
whiplash injuries still present a major problem, and that the
whiplash injury risk females are exposed to is substantially higher
(Kullgren et al., 2020).
Low-speed rear impact volunteer tests have shown that
females have greater horizontal head accelerations, greater (or
similar) horizontal T1 accelerations, lesser head and T1 rearward
displacements, lesser (or similar) Neck Injury Criterion (NIC)
values, and more pronounced rebound motions in comparison
to males (Siegmund et al., 1997;Mordaka and Gentle, 2003;
Viano, 2003;Ono et al., 2006;Linder et al., 2008;Schick et al.,
2008;Carlsson et al., 2011, 2012). The results show that there are
characteristic differences in the dynamic response between males
and females in rear impacts.
Based on mathematical simulations, Mordaka and Gentle
(2003) concluded that a “scaled down male model is not
adequate to simulate female responses even though the scaling
constitutes a good height and mass match” (p. 52). Additionally,
Vasavada et al. (2008) found that “male and female necks are not
geometrically similar and indicate that a female-specific model
will be necessary to study gender differences in neck-related
disorders” (p. 114). That is, a female model must be based on data
from tests with females.
The greatest whiplash injury frequencies are associated
with females and males of average statures (Kihlberg, 1969;
Carlsson et al., 2014). Based on US injury statistics, the highest
whiplash injury frequency was recorded for the statures 162.6–
165.1 cm (64–65 in) for the females and 175.3–177.8 cm (69–70
in) for the males, both close to the average statures of the US
population (females: 161.8 cm (63.7 in), males: 175.3 cm (69.0
in); Schneider et al., 1983). Based on Swiss and Swedish insurance
records, Carlsson et al. (2014) concluded that the stature and
mass of the females most frequently injured correspond well
with the average stature and mass of the female populations in
these countries.
Hence, there is a need for 50th percentile female models,
physical and/or computational crash test dummies and human
body models (HBMs), to further improve the vehicle safety for
both females and males (Carlsson, 2012;Carlsson et al., 2017).
Human dynamic response data is important when developing
and evaluating such occupant models. Thus, the objective
of this study was to generate dynamic response data and
investigate differences in seat interaction for near 50th percentile
females in a laboratory seat at two different head restraint
(HR) configurations.
MATERIALS AND METHODS
A series of rear impact sled tests comprising female volunteers
was performed at a velocity change of ∼7 km/h with two
nominal HR distances. The test series was approved by
the ethical committee at the Ludwig-Maximilian University
in Munich, Germany, Approval Reference Number 319-07
(Address: Ethikkommission der Medizinischen Fakultät der
LMU, Pettenkoferstr. 8a, 80336 Munich, Germany).
Volunteers
Female volunteers were recruited by advertisements at the
Ludwig-Maximilian University in Munich, Germany. Potential
subjects were preselected by telephone interviews. Exclusion
criteria included any known histories of spinal symptoms;
former whiplash associated disorders (WADs); former fractures
and/or surgical interventions to the vertebral column; familial
or hereditary spinal disorders, disc protrusion or herniations,
rheumatism and rheumatoid diseases, further orthopaedic
diseases, syndromes and symptoms such as, arthrosis, arthritis,
multiple cartilaginous exostoses, scoliosis, spondylolisthesis;
having been under treatment (massage/non-steroidal anti-
inflammatory drugs/exercises/chiropractic or other therapies) for
the back/neck during the 6 months preceding the tests. The
volunteers were examined by a physician prior to the tests and
further exclusions were made based on these objective findings
or if subjective discomfort in the head/neck/shoulders/back
existed on the test day. Anthropometric data were obtained on
the same occasion.
Eight female volunteers participated in the test series. Their
age ranged between 22 and 29 years at an average of 26 years,
their stature ranged between 161 and 166 cm at an average of
163 cm, and their mass ranged between 55 and 67 kg at an average
of 60 kg (Table 1). According to the University of Michigan
Transportation Research Institute (UMTRI), the stature and mass
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of the 50th percentile female is 162 cm and 62 kg, respectively,
(Schneider et al., 1983). In comparison to the UMTRI data, the
female volunteers were on average 1% taller and 4% lighter than
the 50th percentile female.
Sled and Seat System
A stationary target sled (1,005 kg) equipped with a laboratory seat
was impacted from the rear by a bullet sled (570 kg). The ram-
shaped front structure of the bullet sled activated an iron band,
mounted inside a band-brake on the target sled, dimensioned to
create a predefined acceleration and velocity change of the target
sled. The laboratory seat had the same seatback construction as in
previous tests series (Davidsson et al., 1998;Carlsson et al., 2011).
The seatback was designed to resemble the shape and deflection
properties of a Volvo 850 car seat and consisted of four stiff
panels covered with 20 mm medium quality Tempur foam. The
panels were independently mounted to a rigid seatback frame by
coil springs to allow easy implementation into a computational
model. The seatback was adjusted to 24.1◦. The seat specifications
can be found in the Davidsson et al. (1999) publication. In the
present study, the HR was modified and consisted of a plywood
panel (dimensions: 350 ×230 ×20 mm) covered by firm padding
(polyethylene 220-E) and supported by a rigid steel frame, i.e., it
was not coupled to the deflecting parts of the seatback. This HR
design was chosen to achieve improved reproducibility, based on
experience gained in the earlier test series (Carlsson et al., 2011).
The HR angle was 12.4◦from the vertical plane. The targeted
initial head-to-HR distance was set by adjusting the thickness of
the padding on the HR for each individual (Figure 1). The HR
surface stiffness was not affected by the change in thickness of the
padding, typically from 13 to 8 cm. The seat base was rigid and
TABLE 1 | The age, stature, mass, 1v and head-to-HR distance of the individual
female volunteers (A–H), as well as their average values and standard deviations
(SD).
Test
subject
HR10 HR15
Initial HR Initial HR
distance 10 cm distance 15 cm
Age
[years]
Stature
[cm]
Mass
[kg]
1vb
[km/h]
HR
distanced
[cm]
1vb
[km/h]
HR
distanced
[cm]
Aa27 161.0 54.5 6.95 12.0 6.89 16.3
B 26 163.8 56.8 6.61 7.8 6.75 14.4
C 27 162.8 66.8 6.73 11.5 6.86 15.3
D 23 166.0 56.8 6.72 9.1 6.87 13.5
E 25 165.3 61.2 6.94 9.2 6.69 14.1
F 29 161.4 62.2 6.89 7.3 6.85 14.2
G 22 161.9 60.4 6.73 7.6 6.88 14.9
Ha27 164.4 58.0 6.87 11.4 6.87 16.5
Average 26 163.3 59.6 6.81 9.5 6.83 14.4
SDc2 1.8 3.9 0.12 1.9 0.07 1.1
aNo HR contact at HR15.
bChange of velocity.
cStandard Deviation (SD).
dAt impact.
the flat seat surface (dimensions: 500 ×500 ×20 mm) was angled
16.9◦from the horizontal plane. A plate was mounted on the sled
to resemble a passenger floor pan surface of a car (Figure 1).
The seatback and seat base were covered with double layers of
knitted lycra fabric.
Test Procedures and Test Configurations
The volunteers were seated on the laboratory seat, restrained
by a 3-point seatbelt and instructed to obtain a natural seated
posture, position their feet on the angled plate, place their hands
on their lap, face forward and relax prior to the impact. Then,
prior to the test, the head-to-HR distance was checked, and the
volunteers were reminded to remain seated in a relaxed manner.
There was no countdown, and the volunteers were aware of the
impending impact since the bullet sled created sound as well as
vibrations that could be sensed. Each volunteer underwent two
tests; the first test configuration, labelled HR10, was performed
at the targeted initial HR distance ∼10 cm, and the second
test configuration labelled HR15, was performed at ∼15 cm.
These two HR placements were chosen to provide additional
distance, in 5 cm increments, compared to the 5 ±2 cm in the
earlier test series of Carlsson et al. (2011). The chosen head-
to-HR distances are greater than what is typically found in
recent passenger vehicle seats when seated in neutral, upright
position (Park et al., 2018). The volunteers were asked to leave
the seat for approximately 10 min between the tests. At this
point the volunteers were asked if they wanted to proceed with
the second test (HR15). This non-randomised order of tests was
chosen to allow the volunteers to experience the smaller head-to-
HR distance before continuing. A randomised order of the two
tests would likely only have had a marginal effect on the head
kinematics results. Siegmund et al. (2003) carried out repeated
rear impact volunteer tests to study the influence of habituation
on the neck response. Their results suggests that the habituation
from the first to the second test had negligible influence of the
onset phase of the head motion, although at a much lower rear
impact severity. The same sled pulse was applied in all tests,
with an average mean target sled acceleration of 2.1 g and a
velocity change of 6.8 km/h. The volunteers were wearing their
own clothes, a pair of shorts and a vest/T-shirt during the tests.
Instrumentation and Data Acquisition
The head of each volunteer was equipped with a harness
which was fixed tightly to the head (Figure 1). Linear tri-axial
accelerometers (MSC 322C/AM-100) were mounted on the left
side of the harness and an angular accelerometer (Endevco
7302B) on the right side, approximately at the head centre of
gravity. Linear accelerometers (Endevco 7264–200) in x- and
z-direction were placed on a holder above the T1 vertebra.
The holder was attached to the skin at four points; one above
each of the proximal ends of the clavicles, and two bilateral
and close to the spinal process of the T1 vertebra. The HR
contact was measured by a tape switch (Barger 121 BP). Linear
accelerometers (Endevco 2262A–200) recorded bullet sled and
target sled accelerations in the x-direction. The start of the impact
(T= 0) was defined by a tape switch (Barger 101 B) attached to the
steel bar on the target sled. Video tracking targets were secured on
the volunteers prior to the tests (Figure 1).
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FIGURE 1 | Volunteer test setup; in this case for the head-to-HR distance 15 cm (HR15). Video tracking targets (1) and (2) for determining head displacements and
targets (3) and (4) for T1 displacements. The position of the trochanter major was palpated and measured prior to the test, and its linear displacement was obtained
from targets (5) and (6). The thickness of the dark head restraint padding was adjusted to adapt the head-to-head restraint distance for each volunteer, to either
10 cm or 15 cm.
Two high-speed digital video cameras (Redlake HG100K,
1,504 ×1,128 pixels, 1,000 f/s) monitored the tests from the
left side and perpendicular to the direction of the sled tracks;
one providing a close-up view and one providing an overview.
Both were placed approximately 6.8 m from the midplane of
the volunteers. Sensor data was registered by a Kayser-Trede
MiniDau acquisition unit at 10 kHz sampling rate and anti-alias
filtered at 4 kHz.
Data Analysis
The sled, head and T1 accelerations were filtered at CFC60,
CFC1000, and CFC60, respectively, as defined by SAE J211. Two
different accelerometer coordinate systems were defined; their
centres were located at respective accelerometer positions and
the two systems moved as the position of the volunteer changed
during impact. The coordinate systems were defined according
to SAE J211 (orthogonal right-handed), with the positive x-, y-,
and z-axis forward, rightward and downward, respectively, at the
beginning of the impact.
Videos were digitised in Tema 3.5 software. None of the
displacement data was filtered. The linear displacements of the
head and T1 were obtained from video tracking targets (2) and
(4), respectively (Figure 1). The angular displacement of the
head was derived from targets (1) and (2) and the T1 from
targets (3) and (4). In addition, the position of the trochanter
major was palpated and measured prior to testing, and its linear
displacement was obtained from targets (5) and (6). The actual
head-to-HR distance at the time T= 0 was obtained from video
analysis, and this distance deviated somewhat from the targeted
distance (Table 1). The displacement data was set to zero at
the time of impact (T= 0) and was expressed in a sled fixed
coordinate system.
Peak values and their timing were derived from the data,
and response corridors were generated. A Shapiro-Wilks test
for statistical normality was performed on the data set. For
each dynamic response parameter, we investigated whether the
observed differences in parameter values between HR10Cand
HR15Cwere statistically significant. HR15N C was excluded from
this analysis since this category only involved two samples.
T-tests were performed with the statistical significance level
of .05 with no corrections for multiple comparisons. Response
corridors for the volunteers were defined as the average ±1
standard deviation (SD). The peak values of the head and
T1 x-accelerations, x- and angular displacements as well as
their occurrence in time were determined for each volunteer.
The HR distance was (1) adjusted (pre-test) to 10 and
15 cm, respectively, and (2) estimated from video analysis
at impact (T= 0). The HR contact time was documented.
The NIC value (Boström et al., 1996, 2000) was calculated
from SAE J211/1 (2003) standard CFC60 filtered head and
T1 accelerations.
RESULTS
The results were separated into three different categories, HR10C,
HR15C, and HR15NC, based on
(1) the targeted initial HR distance (10 or 15 cm) and (2)
whether the volunteers’ head had made contact with the HR
during the test event [Contact (C) or No Contact (NC)]:
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HR10C: - 8 tests
- Initial HR distance 10 cm
- HR contact
- Represented by dark grey corridors
HR15C:
- 6 tests
- Initial HR distance 15 cm
- HR contact
- Represented by light grey corridors
HR15NC:
- 2 tests
- Initial HR distance 15 cm
- No HR contact
- Represented by solid black lines
At the time T= 0, the two volunteers (A and H, Table 1) with
no HR contact (HR15N C) were placed in a separate group since
they had somewhat greater actual head-to-HR distance (16.3 and
16.5 cm) in comparison to the six volunteers with HR contact
(ranging from 13.5 to 15.3 cm). The greater distance may be the
reason why no contact occurred. No symptoms from the neck
were reported by the volunteers after the tests.
Response corridors were defined as the average ±1 SD from
the average response for the eight female volunteers, except for
two cases where no HR contact had occurred. In Supplementary
Appendix 1, in the online supplement, each individual response
curve is presented together with the corridors.
The average speed change applied was
6.8 ±0.1 km/h (Figure 2).
Initial HR Distance and HR Contact
Estimated from video analysis (at T= 0), the HR distance was
on average 9.5 cm in HR10C, 14.4 cm in HR15C, and 16.4 cm
in HR15NC (Table 2). The HR contact started 23% (P= 0.000)
and ended 16% (P= 0.000) earlier, respectively, in HR10Cin
comparison to HR15C, however, the length of the HR contact was
approximately the same (40 and 37 ms, respectively).
Linear Displacements
Linear displacements are presented in Figure 3 and Table 2,
as well as in Supplementary Figures A1.1–3 in the online
supplement. On average, HR10Cresulted in 18% less (P= 0.001)
and 19% earlier (P= 0.000) peak rearward x-displacement of
the head (negative values in Figure 3A) compared to HR15C.
In T1, the peak rearward x-displacement (negative values in
Figure 3B) was on average 7% less [not statistically significant
(NS)] and 6% earlier (P= 0.017) compared to HR15C. This
TABLE 2 | Summary of results from the tests comprising near 50th percentile female volunteers.
HR10CHR15CHR15NC
Initial HR distance 10 cm Initial HR distance 15 cm Initial HR distance 15 cm No
HR contact (N= 8) HR contact (N= 6) HR contact (N = 2)
Variable Peak Peak Peak
Average (SD) Time Average (SD) Time Average Time
X-Displacementa[mm] [ms] [mm] [ms] [mm] [ms]
- Head –113 (12) 121 (11) –138 (9) 149 (7) –133 156
- T1 –96 (9) 127 (5) –104 (8) 135 (6) –92 126
- Head relative to T1 –26 (15) 142 (47) –50 (13) 188 (33) –100 211
- Trochanter Major –96 (6) 123 (5) –94 (3) 122 (5) –96 124
Angular displacement [◦] [ms] [◦] [ms] [◦] [ms]
- Head 10 (9) 140 (44) 28 (9) 202 (13) 64 237
- T1 18 (2) 144 (6) 24 (3) 159 (8) 20 149
- Head relative to T1b–12 (6) 131 (18) –7 (2) 126 (27) –5 100
- Head relative to T1c5 (11) 263 (67) 15 (9) 235 (15) 47 242
X-Acceleration [m/s2] [ms] [m/s2] [ms] [m/s2] [ms]
- Head 193 (35) 116 (12) 106 (40) 147 (8) 32 115
- T1 62 (10) 130 (8) 47 (6) 135 (13) 49 132
NIC [m2/s2] [ms] [m2/s2] [ms] [m2/s2] [ms]
3.9 (1.1) 91 (19) 5.0 (2.1) 123 (23) 7.4 134
Head restraint (HR) [mm] [ms] [mm] [ms] [mm] [ms]
- Head-to-HR distanced95 (19) – 144 (6) – 164 –
- Contact (start) – 99 (12) – 129 (8) – None
- Contact (end) – 139 (11) – 166 (7) – None
aRelative to the sled.
bFirst peak.
cSecond peak.
dAt T = 0 ms (based on video analysis).
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FIGURE 2 | The sled pulse for tests comprising 50th percentile female
volunteers.
resulted in substantial differences between the configurations in
the rearward x-displacement of the head relative to T1 (negative
values in Figure 3C); HR10Cwas on average 48% less (P= 0.009)
and 24% earlier (NS) in comparison to HR15C, while HR15N C
was 102% greater and 12% later, in comparison to HR15C
(Table 2). In the trochanter major, the rearward x-displacement
was similar for the two configurations, HR10 and HR15 (Figure 4
and Table 2).
The rebound motion was most pronounced in HR10C, with an
earlier return to the initial position (= 0 cm) and a greater forward
x-displacement after 500 ms (positive values in Figures 3A,B). In
HR10C, the head returned to the initial position on average 39%
earlier (P= 0.006) in comparison to HR15C(217 and 356 ms,
respectively). For the two volunteers in HR15N C the head did
not return to its original position. The entire rebound motion
was not captured by the cameras. After 500 ms, the average
forward x-displacement of the head was 90% greater (NS) in
HR10C(76 mm) compared to HR15C(40 mm) (Figure 3A).
Correspondingly for the T1, the average forward displacement
after 500 ms was 61% greater (NS) for HR10C(48 mm) than
HR15C(30 mm), while the T1 lagged behind (–19 mm) for
HR15NC (Figure 3B).
Angular Displacements
Angular displacements are presented in Figure 5 and Table 2,
as well as in Supplementary Figures A1.4–6 in the online
supplement. The rearward angular displacements showed
substantial differences in the two configurations (positive angles
in Figure 5). In comparison to HR15C, the peak rearward head
angular displacement was 64% less (P= 0.003) and 31% earlier
(P= 0.006) in HR10C(Figure 5A). For the two volunteers that
never made head-to-HR contact in HR15N C, the peak rearward
head angular displacement was 128% greater and 17% later than
the other six volunteers in HR15C. The corresponding numbers
for T1 were 25% less (P= 0.001) and 9% earlier (P= 0.002) in
HR10C(Figure 5B). Because the rearward angular displacement
FIGURE 3 | X-displacement of the (A) head, (B) T1, and (C) head relative to T1 for near 50th percentile female volunteers.
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FIGURE 4 | X-displacements of the trochanter major for near 50th percentile
female volunteers.
of T1 started earlier in comparison to the head, the volunteers
exhibited a small forward angulation (flexion) of the head relative
to T1 during the first ∼100 ms for all HR conditions (negative
angles in Figure 5C). In HR10C, this forward peak head relative
to T1 angular displacement was on average 75% greater (NS)
in comparison to HR15C. Furthermore, the early HR contact in
HR10Cresulted in less rearward angulation (extension) of the
head relative to T1, whereas in HR15C, the extension of the head
relative to T1 was more prominent. In comparison to HR15C, the
peak rearward head relative to T1 angular displacement was 70%
less (NS) in HR10C(Table 2).
During the rebound, HR10Cshowed an earlier return of
the head and T1 angles to their initial positions (= 0◦), and
a more pronounced forward flexion after 500 ms (negative
angles in Figures 5A,B) compared to HR15. The head returned
to the initial position on average 23% earlier in HR10Cin
comparison to HR15C(249 and 324 ms, respectively, based on
the average curves of the corridors in Figure 5A). In the T1,
the corresponding values were 24% earlier in HR10Ccompared
to HR15C(236 and 312 ms, respectively, based on the average
curves of the corridors in Figure 5B). The average curves of the
corridors were used due to some of the volunteers not returning
to their initial position within the time frame, 500 ms (thus
calculating the significance was not meaningful). In HR15N C,
neither the head nor the T1 returned to the initial position in
any of the two tests. After 500 ms, HR10Cshowed a 102% larger
forward flexion of the head in comparison to HR15C(–13.6◦
and –6.7,◦respectively, NS), while in HR15N C the head remained
in extension (22◦). Correspondingly, after 500 ms the T1 angular
displacement was on average 179% greater in HR10Cthan in
HR15C(–8,1◦and 2,9◦, respectively, NS), while in HR15N C the
T1 remained in extension (10◦).
Sensor Data
Linear head and T1 accelerations are presented in Figure 6 and
Table 2, as well as in Supplementary Figures A1.5–12 in the
online supplement. The peak head forward x-acceleration was on
average 82% greater (P= 0.001) and 22% earlier (P= 0.000) in
HR10C, and 69% less and 22% earlier in HR15NC, as compared to
HR15C(positive values in Figure 6A). In the T1, the peak forward
acceleration was on average 34% greater (P= 0.004) in HR10C
compared to HR15C(positive values in Figure 6B).
In comparison to HR15C(5.0 m2/s2at 123 ms), the NIC value
was on average 22% lower (NS) and 26% earlier (P= 0.015) in
HR10C(3.9 m2/s2at 91 ms), and 49% greater and 9% later in
HR15NC (7.4 m2/s2at 134 ms) (Table 2).
DISCUSSION
To further improve vehicle safety for both females and males,
50th percentile female models, physical and/or computational
crash test dummies and human-body models (HBMs), are
required (Carlsson, 2012, 2017). Human dynamic response data
is important when developing and evaluating these occupant
models. Thus, the objective of this study was to generate response
corridors and investigate differences in seat interaction for near
50th percentile females in a laboratory seat at two different
HR configurations.
The eight female volunteers participating in the tests were
closely matched in size (163.3 ±1.8 cm/59.6 ±3.9 kg, Table 1)
to the 50th percentile female according to the UMTRI study
(162 cm/62 kg, Schneider et al., 1983). It is important to note that
the average anthropometry varies between different regions of
the world. However, we aimed for an anthropometric definition
representative for the world population. The anthropometry
study of the WorldSID project (Moss et al., 2000) concluded that
the size of a world-harmonised 50th percentile adult male would
correspond well with the size of the 50th percentile adult male
as defined by the UMTRI project (Robbins, 1983a,b;Schneider
et al., 1983). We found it reasonable to make the same assumption
regarding the 50th percentile adult female (Carlsson et al., 2014).
The present test setup is based on an earlier setup with an
average head-to-HR distance of 5.5 cm for the female volunteers
(Davidsson et al., 1999;Carlsson et al., 2011). The new setup
was designed to provide a greater initial HR distance in 5 cm
increments (10 and 15 cm, respectively). The increased HR
distance was introduced to enable larger relative motions between
the head and the upper torso. Since the initial HR distance
was greater compared to previous test series, the mean sled
acceleration was reduced from ∼3 to ∼2 g to ensure the
volunteers’ safety. The selection of the reduced mean acceleration
was based on a previous study (Krafft et al., 2002), reporting
that long-term whiplash injury risks approached 0% for mean
vehicle accelerations below 3 g. Krafft et al. presented mean
accelerations ranging from 1 to 7 g indicating that the current
sled pulse is representative of the lower range of real-world
rear impacts. Furthermore, in comparison to previous test series
the design of the laboratory seat was simplified to facilitate
computational modelling and reproducibility. The earlier Volvo
850 seat base was replaced with a rigid, flat surface. In addition,
the earlier spring-mounted HR panel was replaced by a rigid,
adjustable construction to obtain a more precise and reproducible
position of the HR during impact (Figure 1). The initial HR
distance was adjusted by adding padding to the HR, i.e., the
geometry was similar for all volunteers in HR10Cand HR15C,
respectively. Consequently, seatback panels were flexing like a
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FIGURE 5 | Angular displacement of the (A) head, (B) T1, and (C) head relative to T1 for near 50th percentile female volunteers.
FIGURE 6 | X-accelerations of the (A) head and (B) T1 for near 50th percentile female volunteers.
standard car seat, while the HR stayed still relative to the sled
during the dynamic event. This seatback and HR design deviates
from a typical vehicle front seat, however, in this study, the
reproducibility was given priority.
The results from the HR15 test configuration were separated
into two groups. In the first group, HR15C, the head did contact
the HR, while in the second group, HR15N C, HR contact did
not occur. It is likely that the HR15 test configuration is close
to the limit of avoiding HR contact for this specific seat setup.
At the time T= 0, the HR15N C volunteers had a greater head-
to-HR distance (16.3 and 16.5 cm) in comparison to the HR15C
volunteers (ranging from 13.5 to 15.3 cm). Furthermore, HR15NC
also had lesser x-displacements of the head (13.3 and 13.8 cm)
and T1 (9.2 and 10.4 cm) (Table 2). The greater distance likely
explains why no HR contact occurred. The results deviated
significantly between the two groups after the time of HR contact
in the HR15Cgroup (on average 129 ms, Table 2 and Figures 3–
6). It will be possible to use both datasets in future dummy
and model evaluations, each with its corresponding HR contact
condition. The grey corridors of HR15Ccan be used in case the
dummy or model contacts the HR (targeting 129 ms), while the
black lines of the HR15NC can be used in non-contact cases.
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FIGURE 7 | The relative HR distance and contact time; peak x-displacements (head, T1, head relative to T1, trochanter major); angular displacements (head, T1,
head relative to T1); x-accelerations (head, T1) and NIC value for HR10C, HR15C(normalised to 1) and HR15NC .
Together, the two datasets represent parts of the mid-sized female
population. When evaluating a dummy or a model, it is desirable
to also evaluate it against other volunteer datasets to obtain a
more robust representation of the population.
The relative peak values from accelerometer signals and data
from video analysis for the two configurations, HR10 and HR15,
are summarised in Figure 7. The HR15Ctest was used as
a reference, normalised to 1 (represented by blue bars). The
greatest differences between the three categories, HR10C, HR15C
and HR15NC, were found for the head and head relative to T1
angular displacements. There is also a considerable difference
in the T1 angular displacement for the two test configurations,
HR10 and HR15, however, not between the two categories
HR15Cand HR15NC. Thus, the results indicate that the T1
angular displacement for the HR15 can be regarded as an upper
limit for this test setup. Similar results can be seen for the
head x-displacement, with a difference between the two test
configurations, HR10 and HR15, this has not, however, been
observed between the two categories HR15Cand HR15N C. This
result supports the idea that the HR15 test configuration is close
to the limit of whether HR contact will occur, for this setup.
The data also indicate that the initial HR distance was somewhat
greater for the two volunteers in the HR15N C category, which
might explain why their heads did not reach the HR. A significant
increase was observed in the head relative to T1 x-displacements
for increasing HR distance, HR10C, HR15Cand HR15N C. In
contrast, the x-displacement of the trochanter major (pelvic
region) seems unaffected by the different HR configurations. The
head x-acceleration decrease for increasing HR distances, HR10C,
HR15C, and HR15NC, may (partly) be explained by increasing
head angular displacements. Furthermore, an increase of the
NIC-values for increasing HR distances, HR10C, HR15C, and
HR15NC, was also recorded.
This study has several limitations. Due to financial constraints,
the test series was limited to eight volunteers in two HR
configurations. Although additional volunteers would have been
valuable, this sample size is in line with other similar studies.
The volunteers were young (22–29 years); an older sample might
have had a somewhat different response. However, the age of
the volunteers in the present study corresponds quite well to
the age group with the highest whiplash injury risk (Jakobsson
et al., 2000). Moreover, the outcome might have been affected by
the volunteers not being exposed to the two HR configurations
in a randomised order. It was decided to make the tests non-
randomised to give the volunteers the option of discontinuing
their participation once they had been exposed to the shorter
head-to-HR distance. Furthermore, the outcome might also have
been affected by the volunteers being aware of the impending
impact. An “unexpected” impact was not possible to achieve,
since the noise and vibrations caused by the bullet sled could
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Carlsson et al. Dynamic Responses of Female Volunteers
be sensed. In addition, electromyographic (EMG) activity was
not measured in this study. This type of measurement would
potentially have given information about to what extent the
volunteers were relaxed or tense at the time of impact (T= 0).
Philippens et al. (2002) compared the dynamic response of
the 50th percentile male BioRID to volunteer and post-mortem
human subject (PMHS) data and observed similar responses in
low-speed rear impact tests. The dynamic response of the BioRID
dummy was validated with regard to male volunteer tests in
Davidsson et al. (1999), the same tests that the female volunteers
in Carlsson et al. (2011) were compared to. However, the results
from the latter study show that the female volunteers had a
somewhat different dynamic response than the male volunteers.
Similar findings have been reported in other studies (Siegmund
et al., 1997;Mordaka and Gentle, 2003;Viano, 2003;Ono et al.,
2006;Linder et al., 2008;Schick et al., 2008;Carlsson et al.,
2012). There does not seem to be a simple way to “reinterpret”
or “scale” data obtained with the BioRID II to address the female
dynamic response (Carlsson, 2012). Thus, it is important that
future whiplash protection systems are developed and evaluated
with consideration of the female properties as well. With this
study we have been able to supply new data that can be used for
validation of a 50th percentile low speed rear impact female crash
test dummy and/or computational models.
DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be
made available by the authors, without undue reservation.
ETHICS STATEMENT
The studies involving human participants were reviewed
and approved by the Ludwig-Maximilian University in
Munich, Germany Approval Reference Number 319–07
Address: Ethikkommission der Medizinischen Fakultät der
LMU, Pettenkoferstr. 8a, 80336 Munich, Germany. The
patients/participants provided their written informed consent
to participate in this study.
AUTHOR CONTRIBUTIONS
AC: preparation, execution, documentation, and analysis of the
test series and main author. SH: preparation and execution of the
test series, internal review of the manuscript. JD: preparation of
the test series, advice based on earlier experience in experimental
whiplash injury research including volunteer testing and crash
test dummy development, and internal review of the manuscript.
SS: medical responsibility, preparation of the test series, advice
based on earlier experience in experimental whiplash injury
research including volunteer testing, and internal review of the
manuscript. AL: EU project coordinator, contributed to the
planning of the test series, advice based on earlier experience
in experimental whiplash injury research including volunteer
testing, and internal review of the manuscript. WH: WP-leader
in the ADSEAT project, contributed to the planning of the test
series, and internal review of the manuscript. MS: principal
investigator, WP-leader in the two involved EU-projects, initiated
the work in the present study, contributed with advice based
on earlier experience in experimental whiplash injury research
including volunteer testing and crash test dummy development,
and contributed to the writing and internal review of the
manuscript. All authors contributed to the article and approved
the submitted version.
FUNDING
This study was part of the ADSEAT (Adaptive Seat to Reduce
Neck Injuries for Female and Male Occupants) project funded
by the European Commission, Project No. 233904. The data
was adapted for evaluation tasks in the project VIRTUAL
(Open Access Virtual Testing Protocols for Enhanced Road User
Safety) that has received funding from the European Union
Horizon 2020 Research and Innovation Programme under Grant
Agreement No. 768960. The writing of this paper was funded by
Folksams Forskningsstiftelse, Sweden.
ACKNOWLEDGMENTS
We thank to everyone involved in the sled test series, Carsten
Reinkemeyer and the staff at Allianz Test Centre, Ismaning,
Germany, and Claudia Helmreich, and Claudia Oehme at
Ludwig-Maximilians-Universitaet (LMU), Munich, Germany,
and Elisabet Agar who performed the language review.
SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found
online at: https://www.frontiersin.org/articles/10.3389/fbioe.
2021.684003/full#supplementary-material
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Conflict of Interest: The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could be construed as a
potential conflict of interest.
Copyright © 2021 Carlsson, Horion, Davidsson, Schick, Linder, Hell and Svensson.
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