Documentation:FIB book/The Effects of Out-of-Position Reclined Seat Postures on Vehicle Crash Injuries

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Introduction

Out-of-position (OOP) is defined as passengers not sitting upright in a forward-facing position, which includes reclined seat angles, phone-related neck strain, improper seat belt use, etc. While most drivers are aware of the importance of sitting in a standard position, various distractions can occur, putting the occupant in an out-of-position posture. For example, the desire to adjust the radio, answer an important phone call, or modify the air conditioning temperature can momentarily alter a driver's position. Reaching for a cell phone can lead to neck strain and flexion, adding severity of injuries on top of potential crashes. This is just one aspect of OOP driving. OOP has been identified as a significant contributing factor in many accidents, and it appears frequently in our daily lives, with or without our awareness. According to statistics from a 2020 survey, at least 37 percent of US drivers reported picking up a hand-held cell phone in the past 30 days, with 5% admitting to doing so frequently. Additionally, 23% of the respondents acknowledged trying to send text messages while driving[1].

Apart from actions during driving that can result in OOP, other occupants in the vehicle, especially passengers, tend to adjust their seats to the reclined positions to reduce fatigue during extended travel. According to recent research involving 40 participating drivers, 21% of drivers prefer different degrees of reclined seats rather than sitting in the standard way[2].  For kid seats, it is usually recommended 30-45 recline angles from upright, and for adults, the safe angle during a moving vehicle varies according to the specific seat manufacturer and is usually between 10-20 degrees[3]. While some flexibility in seating positions is acceptable, the drivers should not compromise the ability to maintain control of the vehicle. In addition, passengers, especially children, should be properly secured with seat belts in seats of proper angles, or in appropriate child safety seats as required by law. Prioritizing driving safety is paramount.

Research shows OOPs can potentially increase the likelihood and severity of car crash injuries[4]. The following sections will focus on the reclined seat as a typical representation of OOP. By doing a literature review on various aspects, the goal is to gain a better understanding of the potential injuries to the head, neck, and thorax resulting from reclined seat postures.

Previous Work

Previous work has provided a comprehensive and multifaceted approach to understanding the severity of injuries associated with reclined seat position in vehicle crashes, as well as a foundation for the biomechanics and injury mechanism involved. The following sections contain procedures used for reclined seat testing, examples and results of physical and theoretical testing systems, and an overview of common types of injuries associated with reclined OOP.

Some examples of the testing methods include a variety of methodologies ranging from physical testing systems such as the Hybrid III 50cc ATD[5] and the Post Mortem Human Subject (PMHS)[6] to advanced theoretical models such as the MADYMO Human Body Model[7] and Finite Element Analysis[8]. The use of multiple methods not only enhances the depth and scope of the studies, but also ensures a balance between the controlled reproducibility and biofidelity.  

Methods and Procedure

Physical Testing Systems 

Hybrid III 50cc ATD

A 2022 study aims to investigate the impact of various reclined seat positions on passenger safety during vehicle crashes (Figure 1)[5]. The experiments were conducted using a sled test rig and a standard Hybrid III 50cc ATD. The ATD was seated on a standard seat fixed on rails, with the seatback position being the variable under examination. The study addressed the limitations of current safety testing, which often does not account for non-standard seatback positions such as fully-reclined seats. The crash tests were documented using a Phantom Veo 410L high-speed camera recording at 5000 FPS, and an Endevco piezoresistive accelerometer measured head acceleration in the X, Y, and Z directions to calculate the head injury criteria (HIC).

Figure 1. The test station prepared for the tests using Hybrid III[5].
Post-Mortem Human Subjects (PMHS)

In another study, to assess thoracolumbar spine kinematics and injuries, 7 PMHS (3 females and 4 males) were used to mirror sex-based differences in injury risk in reclined frontal impacts[6]. To represent mid-sized individuals, female subjects averaged 61 years, 168 cm, and 60 kg, while male subjects averaged 55 years, 177.8 cm, and 74 kg. The PMHS underwent frontal impact sled tests at 50 km/h using a reverse acceleration sled system designed to replicate the dynamics of a real vehicle impact. In each test, the PMHS was reclined to 50° and restrained by a prototype three-point seat belt system to mitigate submarining. Vertebral and pelvic 3-D motions were captured using optical motion tracking, and lumbar spine injuries were monitored using pressure transducers.

The 2022 study using the Hybrid III 50cc ATD contributes significantly by addressing the limitations of conventional safety testing, which often neglects non-standard seatback positions. The study utilizing PMHS stands out for its focus on the biomechanics of the thoracolumbar spine during frontal impacts in a reclined position.

Theoretical Testing System

MADYMO and Crash Models

A recently published paper examines the effect of a braking maneuver on an occupant who is in a highly reclined seating position[7]. The study used the MADYMO human body model of a 50th percentile average-size male dummy, who was seated at the front passenger seat with a fully reclined seatback angle (53°). The MADYMO human body model is a mathematical dynamics software model that can be used to predict the human response in a fully reclined position. The initial human body posture was at a position where the cervical joint was the most relaxed. The study aims to study the occupant responses from two scenarios: with or without the braking maneuver before the frontal crash. Two simulation models were used to facilitate testing. The precrash model (VaAM-I) was implemented to measure the initial position of the occupant after the vehicle had a 1s deceleration of braking before a 200 ms crash. During the braking, the predetermined braking pulse and the muscle activation were applied to the model. The dynamic state of the vehicle after braking, the seatbelt force and the seating position of the occupant were used as inputs for the following crash test. A predetermined crash pulse was applied, and the seatbelt pre-tensioner and airbag were activated. The non-precrash model (VaAM-O) was a similar setup as the VaAM-O model for the 200 ms of a crash, but it simulates the case without the braking maneuver in place.

In both models, the occupant was seated in a fully-reclined position. Figure.3 in the paper[7] showed that a great range of motion of the occupant can be observed, in particular, the head, neck and chest showed significant forward displacement. Only the head was in contact with the airbag. This could lead to a lack of protection for other body parts. The submarining effect was also found to occur during the crash, and its severity continued to build up with time. The finding from this paper suggests that regardless of whether or not the pre-crash braking is applied, the OOP position can lead to potential injuries in several parts of the body.

Finite Element Analysis and THUMS Models

Another study used finite element analysis to assess the occupant response in a reclined seat position during a full frontal impact[8]. The reclined seat FE model was set at 45°, and its seatback rotational stiffness was also studied. The occupant was represented with a computer model, THUMS. It was positioned in a relaxed posture in the reclined sea with its head contacting the headrest. The spine angles of the model were examined. During the simulation of a frontal crash, data of the head rotational velocities and center of gravity displacement were measured using database historical nodes. Three impact speeds were used to simulate the frontal collision with a predetermined acceleration pulse from existing cadaver testing. The human-response outputs from the simulation were assessed with corresponding indices, such as the head longitudinal and vertical displacement, which were examined with the Neck Injury Criteria.

Figure.7[8] in the paper demonstrate the occupant kinematics in an upright and a reclined seat under various impact velocities. During a 40 km/h impact, in both positions, the occupant experiences slipping upward. In the upright position, the lap belt maintains contact with the lower body. However, in the reclined seat position, the lap belt slips off the occupant’s thighs due to a higher seatback rotation angle. This suggests that in the reclined seat position, the protection of the seatbelt could be compromised. In addition, it was also found that the occupant’s head experiences higher vertical displacement, which could potentially lead to more traumas.

Types of Injuries

Below are some of the common injuries related to OOP which are summarized and categorized into two groups of injuries. The first group covers the injuries from the crash when the occupant is in the reclined seat position. An example would be looking at head and neck injuries. The second group covers injuries due to the vehicle restraint systems. Vehicle restraint systems, such as seat belts, are designed mainly for occupants in the proper upright position. Thus, being in a reclined seat position can not only reduce the effectiveness of the seat belt but also introduce other injuries due to the incorrect seat belt restraint.

Injuries from Direct Crash Impact

Head/Neck

A study examining the influence of seatback reclination indicates that the reclined seat position can cause neck injuries[5]. It was found that with the higher angle of reclination, the velocity of motion experienced by the dummy body increases. The changes in head and neck motion were significant, in particular, the head accelerations caused by head rotation were measured. At the most significant reclined angle, the head rotation is great enough to cause neck injuries, such as whiplash.

Thoracolumbar Spine

In a study using PMHS to investigate the relationship between a reclined seating position and thoracolumbar spine kinematics and injuries[6], lumbar spine fractures were common (especially at the L1 level) among the primary types of injuries observed. They typically present as burst fractures, which are the result of high axial compression combined with clavicular bending loads present on the spine. These L1 fractures tend to occur within a very short time frame (61-65 ms after impact), indicating a strong correlation with peak axial forces transmitted through the spine. Clavicle fractures, particularly on the right side, usually occur in conjunction with lumbar spine fractures in the reclined frontal impact tests and are attributed to the shoulder girdle loading. Other injuries observed included sacral and iliac wing fractures. In addition, the number of rib fractures was typically higher with L1 fractures, which may imply a correlation between stronger ribs and a reduced likelihood of lumbar injuries.

Injuries Due to Restraint Systems

The submarine effect describes the scenario in which the lap belt of a seat belt restraint system slips above the occupants' hips and iliac spines, thus restraining the passenger around the abdomen and distributing force over the internal abdominal organs. During frontal crashes, injuries most commonly induced by submarining include abdominal injuries of AIS 3+, dorso-lumbar fractures of AIS 2+, and lower member fractures of AIS 2+ as identified through various analyses of real-world crash data[9][10].

Figure 2. Submarining effect illustrated by a finite element THOR model in OOP[9].


When an occupant is in a reclined seat, their pelvis is positioned further forward on the seat in comparison to sitting upright, and the lap belt is positioned horizontally across their hips rather than vertically[8]. This describes how reclined positions differ from upright positions in terms of seat belt positioning and how the deviation from optimal restraint positioning can lead to submarining, as this position increases the likelihood of the seatbelt failing to keep the occupant secure in the event of a crash.

Other restraints affected by occupants being OOP are airbags. When an occupant reclines, they are no longer in the optimal position to interact with airbags when deployed during a crash. Most notably, when an occupant submarines, the distance between their head and the front airbags increases as they fall down their seat. As noted through a review study, there is an increase in head velocity due to the interaction of the head and airbag when reclined, compared to upright[11].

Problems

An issue with many of the previous studies is the need for more data. Reclined frontal crash data found on Crash Injury Research (CIREN) and NASS-CDS is often limited and incomplete. From 90,412 frontal crash cases reported from 1995 to 2000 about 50% were partially upright, 0.3% fully reclined, 24% seat angle not recorded, 8% not adjustable, and 17.6% fully reclined[12]. In addition, seat belt use for those fully reclined was 57.8% compared to 77.6% for those fully upright and 75.6% for those partially reclined[12]. The exact seat angle for those partially upright is also not listed. These limitations and the potential for inaccuracies in crash data, make case data analysis more difficult, and the results less concrete[12].

When recreating OOP scenarios for crash testing, there are many variables to consider, especially when accounting for injuries caused by restraint systems. There are no standard seatbelt and airbag restraint systems which lends to each crash test study having slightly different variables. The effectiveness of these restraint systems is also subject to occupant factors such as weight, height, and position. To perform a reclined occupant frontal crash test that considers the effect of restraint systems, one must consider the following: degree of recline, type of seat belt restraint - number of pretensioners and number of load limiters, type of airbag - curtain, roof, or frontal, occupant weight and height, and positioning of the occupant relative to the seat and restraint system. Each crash test study thus has its own unique set of parameters, and while conclusions can be drawn from individual studies, it is often difficult to form a consensus based on various crash test studies, as it is difficult to make direct comparisons given different parameters.

Aside from issues due to the variables in the factors of crash testing, there are also other limitations due to the testing models and test subjects themselves. Biofidelity plays a large role in the accuracy of test results, and unfortunately, using current ATDs and PMHS to conduct reclined crash testing can not fully represent humans in the simulations conducted. The primary limitation of using the Hybrid III ATD is its inability to accurately replicate reclined postures, as it has a fixed angle between the thighs and torso. Validating its results necessitates low-speed volunteer tests or PMHS tests, but there is a scarcity of data about these methods[13][14].

There are several potential limitations to using computer models and simulations for studying the impacts of reclined seats on occupants. For most computing software or virtual testing environments, the amount of input variables that can be applied for testing is often limited due to limited processing power[7]. For example, when having the braking maneuver first and then the crash, these two scenarios were measured separately instead of continuously. Having two separate tests might result in changes in testing variables and does not fully represent real-life scenarios. Another potential limitation of computer simulation is that the interior parts inside the human model are often neglected or simplified[8]. This means the human body is likely a passive model, with no or little muscle activities. This can cause changes and inaccuracies in the simulation outcomes, and potentially miss out injuries for interior body components. This issue is often present when simulating the submarine phenomena. The lack of biofidelity in superficial tissue and its interaction with the skeleton within the model often results in the seatbelt improperly engaging with the flesh and often sliding under the skeleton rather than sliding along the human body [9][14].

Controversies

Most of the reviewed studies compared the different occupant kinematics during vehicle crashes for fully-upright and fully-reclined seat positions. However, the angle of the seatback has not been standardized, meaning the definition of a fully upright or a fully-reclined position can vary depending on the type and design of the chair. This can lead to controversial claims surrounding the results of reclined seat position studies as such terms may not be used and defined accurately.

Future Work

Test Improvements

Current Anthropomorphic Test Devices (ATD) have been shown to predict the potential injury risks on occupants associated with different reclined seat angles. However, future works are needed. While the present tests employed the Hybrid III dummy for sled testing, future improvements could involve more sophisticated and biofidelic ATDs, such as THOR. Compared to the Hybrid III, THOR offers enhanced multidirectional load cells, providing a more accurate representation of human anatomy and biomechanics. Having more biofidelic neck-head attachments of THOR can also visualize deformations following car crash tests[15].

There are also possible improvements to the computer programs for data collection. One would be implementing continuous simulations rather than separate ones. Model validation should also be done regularly based on real-world experiments, such as cadaver testing and injury databases[7]. This can potentially help match the simulation results with the physical injuries observed from tests. Another potential improvement could be further enhancing the ability to represent various test conditions. For example, the NHTSA THOR FE model was only able to achieve a maximum recline angle of 40°[15]. However, in reality, a larger recline angle can be expected.

Potential Prevention Methods

As previously noted, the hazards associated with improper seat belt usage, which can exert excessive pressure on a passenger's abdominal organs, may necessitate the development of improved seat belt designs. One of the latest innovations in this regard is the inflatable seat belt, capable of reducing injuries and providing enhanced passenger protection. In contrast to traditional seat belts, inflatable seat belts inflate during a car collision, distributing crash forces over a larger area, and enhancing safety, especially for rear-seat passengers. According to a sled test using Hybrid III 50th percentile male ATDs, the inflatable seat belt can significantly decrease neck flexion movement in the lower neck, even in low-speed collisions[16]. Another possible prevention method is to educate drivers and passengers on the potential risk of OOP when travelling in vehicles. This could help raise people's awareness of OOP and potentially increase their safety.

It is important to recognize that reclined seats are only one aspect of out-of-position (OOP) driving. Other common forms of OOP include cell phone usage and facing other passengers during conversations. Research in this area is ongoing, and addressing the various OOP scenarios is a complex but necessary undertaking for enhancing road safety.

References

  1. ↑ “2020 Traffic Safety Culture Index,” AAA Foundation for Traffic Safety. Accessed: Nov. 06, 2023. [Online]. Available: https://aaafoundation.org/2020-traffic-safety-culture-index/
  2. ↑ J. Park, Y. Choi, B. Lee, S. Sah, K. Jung, and H. You, “Sitting Strategy Analysis based on Driving Postures and Seating Pressure Distributions,” Proc. Hum. Factors Ergon. Soc. Annu. Meet., vol. 57, no. 1, pp. 1983–1986, Sep. 2013, doi: 10.1177/1541931213571443.
  3. ↑ “Physiomed_Sitting_Guide_-_Driving_Digital.pdf.” Accessed: Dec. 03, 2023. [Online]. Available: https://www.physiomed.co.uk/uploads/guide/file/21/Physiomed_Sitting_Guide_-_Driving_Digital.pdf
  4. ↑ T. L. McMurry, G. S. Poplin, G. Shaw, and M. B. Panzer, “Crash safety concerns for out-of-position occupant postures: A look toward safety in highly automated vehicles,” Traffic Inj. Prev., vol. 19, no. 6, pp. 582–587, 2018, doi: 10.1080/15389588.2018.1458306.
  5. ↑ 5.0 5.1 5.2 5.3 A. Górniak et al., “Influence of a Passenger Position Seating on Recline Seat on a Head Injury during a Frontal Crash,” Sensors, vol. 22, no. 5, Art. no. 5, Jan. 2022, doi: 10.3390/s22052003.
  6. ↑ 6.0 6.1 6.2 J. Shin et al., “Comparison of thoracolumbar spine kinematics and injuries in reclined frontal impact sled tests between mid-size adult female and male PMHS,” Accid. Anal. Prev., vol. 193, p. 107334, Dec. 2023, doi: 10.1016/j.aap.2023.107334.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 D. Tran, G. Müller, and S. Müller, “The effect of a braking maneuver on the occupant’s kinematics of a highly reclined seating position in a frontal crash,” Traffic Inj. Prev., vol. 24, no. 4, pp. 299–306, May 2023, doi: 10.1080/15389588.2023.2177508.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 A. V. Ngo, J. Becker, D. Thirunavukkarasu, P. Urban, S. Koetniyom, and J. Carmai, “Investigation of occupant kinematics and injury risk in a reclined and rearward-facing seat under various frontal crash velocities,” J. Safety Res., vol. 79, pp. 26–37, Dec. 2021, doi: 10.1016/j.jsr.2021.08.001.
  9. ↑ 9.0 9.1 9.2 O. Richard, J. Uriot, X. Trosseille, and M. Sokołowski, “Occupant Restraint Optimisation in Frontal Crash to Mitigate the Risk of Submarining in Out‐of‐Position Situation,” 2015. Accessed: Nov. 06, 2023. [Online]. Available: https://www.semanticscholar.org/paper/Occupant-Restraint-Optimisation-in-Frontal-Crash-to-Richard-Uriot/efd67ffca6bcf8bfda98e9edf191cfdd869b9c5f
  10. ↑ Y. C. Leung et al., “Submarining Injuries of 3 Pt. Belted Occupants in Frontal Collisions – Description, Mechanisms and Protection,” SAE International, Warrendale, PA, SAE Technical Paper 821158, Feb. 1982. doi: 10.4271/821158.
  11. ↑ J. Östh and K. Bohman, “Evaluation of Kinematics and Restraint Interaction when Repositioning a Driver from a Reclined to an Upright Position Prior to Frontal Impact using Active Human Body Model Simulations,” 2020. Accessed: Nov. 06, 2023. [Online]. Available: https://www.semanticscholar.org/paper/Evaluation-of-Kinematics-and-Restraint-Interaction-%C3%96sth-Bohman/e1d73376fa05d4f14d633c8911f04c39cb09014b
  12. ↑ 12.0 12.1 12.2 S. Dissanaike, R. Kaufman, C. D. Mack, C. Mock, and E. Bulger, “The effect of reclined seats on mortality in motor vehicle collisions,” J. Trauma, vol. 64, no. 3, pp. 614–619, Mar. 2008, doi: 10.1097/TA.0b013e318164d071.
  13. ↑ P. Vezin, K. Bruyere-Garnier, F. Bermond, and J. P. Verriest, “Comparison of Hybrid III, Thor-alpha and PMHS Response in Frontal Sled Tests,” Stapp Car Crash J., vol. 46, pp. 1–26, Nov. 2002, doi: 10.4271/2002-22-0001.
  14. ↑ 14.0 14.1 R. Richardson et al., “Kinematic and Injury Response of Reclined PMHS in Frontal Impacts,” Stapp Car Crash J., vol. 64, pp. 83–153, Nov. 2020.
  15. ↑ 15.0 15.1 J. Forman, H. Lin, B. Gepner, T. Wu, and M. Panzer, “Occupant Safety in Automated Vehicles: - Effect of Seatback Recline on Occupant Restraint -,” Int. J. Automot. Eng., vol. 10, no. 2, pp. 139–143, 2019, doi: 10.20485/jsaeijae.10.2_139.
  16. ↑ Krayterman, Dmitriy, “Comparative Analysis of THOR-NT ATD vs. Hybrid III ATD in Laboratory Vertical Shock Testing.” Accessed: Nov. 06, 2023. [Online]. Available: https://apps.dtic.mil/sti/citations/ADA586077
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