Documentation:FIB book/Factors in Rear Row Seated Child Injury Biomechanics

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1.0 Introduction

Anthropometric Test Devices (ATDs) are used to assess vehicle safety. Although continuous development and evaluation has been done for front-seat occupants, less focus has been placed on rear-seat safety, with more limited studies and restraint systems focusing on implementation in front seats[1]. There is a growing need to improve the critical risk associated with rear passenger safety, both due to the frequency of injury and the dominant youth demographic of occupants. However, there are limitations in the availability of biofidelic child ATD models that can accurately simulate the injury profile and body reaction in vehicle crashes, especially for those aged 9-15 years old that are most frequently observed in rear passenger seats[2].

Figure 1. Proportion of front and rear seat passengers sustaining injury to various body regions.

Rear seat passengers most frequently experience injuries due to interaction with the seat back, head restraint and the vertical support B-pillar of the car's interior, located between the front and rear doors. Seatbelt interaction is also a source of injury due to the high compression against the body[3]. In rear seat occupants, injuries are commonly observed in the thorax, with the chest and abdomen experiencing AIS 2+ ratings, indicating moderate to severe injuries[4]. The head is also more likely to experience injury due to impact with interior surfaces and lack of rear airbags[5]. A comparison of injuries for front and rear seat occupants can be seen in Figure 1[5].  

Although less emphasis is placed on rear seat safety development, some advances have been made. Lap and shoulder belts are used in all rear seating, and they have evolved from previous single lap belts, proving 25% more effective[2]. Despite this, the prevalence of serious injury was approximately the same for lap belts when compared to lap and shoulder belts. Injuries also differ depending on whether occupants are wearing seat belts. This remains an issue as individuals seated in the back seat are 11% more likely to be unbelted than those in the front seat[6].

The use of booster seats is also an important consideration for child occupancy crash testing. Booster seats aid in correcting lap and shoulder belt placement, reducing the likelihood of injury for those aged 4-8. Although compliance for this age group is reported to be 90%[7], only 30% of children are actually using booster seats[8]. This highlights the importance of implementing proper child restraint systems to reduce injury, especially in critical risk areas such as rear vehicle seating.

This literature review will focus on child rear seat safety and the factors that influence degrees of injury experienced, such as body positioning, booster seat presence and whether or not a seatbelt is worn. It will also explore the different methods for simulating child occupants in testing scenarios and the current limitations of these findings. Investigation into future developments will also be conducted, including next-generation kinematic booster seats and improvements to the Large Omnidirectional Child (LODC) Anthropometric test device design.

2.0 Importance of Simulating Biofidelic Child Responses

In order to understand the kinematics of rear seat passenger it is important to consider the demographic and the means by which safety standards and regulations for this seat are set. In order to set these requirements an ATD must simulate kinematics and kinetics of pediatric passengers in the rear row. The study by Stammen et al.[9] presents a comprehensive evaluation of the biofidelity of the LODC ATD, alongside a parallel assessment of the Hybrid III 10C ATD, by comparing each device’s mechanical response to human cadaver data. This assessment is crucial for improving child safety in vehicular crashes, as accurate biofidelity ensures that crash test dummies closely replicate human responses, leading to improved injury prevention strategies. This section focuses on key aspects of the study, including head drop test biofidelity, rotational stiffness, and crash kinematic response, and contextualizes these findings within the broader field of child injury biomechanics.

2.1 Biofidelity Evaluation and Crash Kinematics Key Test Scenarios

2.1.1 Head Drop Test Biofidelity

Table 1: Drop test biofidelity scores for two different ATDs.

The head drop test, commonly used to assess the biofidelity of ATDs in simulating human head impacts, was used by Stammen et al.[9] to compare the LODC and HIII-10C models. One of the key metrics used to quantify this comparison was the BioRank score, which measures how closely an ATD’s response matches the mean response of post-mortem human subjects (PMHS), with lower values indicating greater biofidelity[9].

As shown in Table 1[9], the average BioRank head score dropped from 1.81 with the HIII-10C ATD to a score of 0.61 with the LODC ATD. This improvement indicates that the LODC ATD exhibits a more accurate impact response than the HII-10C ATD, more closely replicating the kinematics of real-world pediatric biomechanics data.

The biggest difference was observed in the head resultant acceleration, which decreased from 3.21 g to 0.12 g. This significant reduction suggests that the LODC ATD more accurately replicates pediatric head impact responses, potentially contributing to improved head injury risk prediction in lab-based assessments[9]. Also, the HIC 36 scores for the LODC were considerably lower than those for HIII-10C, further supporting its improved biofidelity in this test[9].

2.1.2 Rotational Stiffness Path

Rotational stiffness plays a pivotal role in comprehending the mechanical behavior of the neck and head under impact conditions. As presented in Table 2 [9], the LODC ATD achieves a BioRank score of 1.15 for rotational stiffness, in contrast to the  HIII-10C ATD, which has a score of 2.46. This can also be visualized in Figure 2[9] where it can be observed that LODC ATD(red line) more closely follows the corridor for human responses (grey shaded area) for the force relaxation response.

Figure 2: Head kinetics for LODC versus HIII.

This substantial improvement shows the LODC ATD’s superior rotational stiffness characteristics, more closely resembling PMHS data (shaded in grey in Table 2[9]). However, it is worth acknowledging that the LODC is not without limitations, exhibiting an average BioRank score of 1.65 for the lower neck compared to 1.47 for the HIII-10C, indicating that the HIII-10C provides a slightly more biofidelic response in the lower neck region.

Table 2: Sled test BioRank scores for head accelerations.

2.1.3 Crash Kinematic Response

Figure 3. Kinematics of 2 ATD's heads compared to a PMHS.

In a 40 km/h crash testing, the kinematic response of the LODC ATD was found to be more consistent with human-like motion trajectories than the Hybrid III, particularly in replicating head movement patterns[9]. Figure 3[9] shows that the LODC head trajectory more closely resembles that of pediatric PMHS in both peak forward translation and downward motion[9]. This improvement is primarily due to the LODC’s more flexible thoracic spine and softer thorax, which allow for greater forward excursion (i.e., how far the head or torso moves forward during a crash)[9]. The Hybrid III, by contrast, frequently exhibited excessive stiffness and unnatural rebound motions.

Figure 4. Evaluation of LODC and HIII of differing belt positions and their impact on biofidelic kinematics.

This evidence further demonstrates the LODC head trajectory’s closer resemblance to that of the PMHS, indicating improved biofidelity in the replicating pediatric head motion under impact, which is critical for accurately assessing injury mechanisms.

Figure 4[9] demonstrates the testing setup used to obtain the kinematic results for the evaluation of head kinematics using a sled test[9]. The "stick figure" end positioning of the head and rotational points on the ATD further demonstrates the biofidelic response of the ATD's and the slight improved performance of the LODC ATD.

While the LODC demonstrates improved head and neck biofidelity compared to the HIII-10C in standardized tests, these findings must be interpreted within the broader context of pediatric crash biomechanics. Studies like Seacrist et al.[10] and Lopez-Valdes et al.[11] have shown that even with similar forward excursions, pediatric ATDs often fail to match the angular velocity, torso kinematics, and spine flexibility of real children and post-mortem subjects. These discrepancies suggest that mechanical improvements alone, such as lower BioRank scores or better head trajectories, may not fully capture how children actually respond in crashes.

Therefore, the following section explores how these biomechanical limitations are further complicated by real-world factors like posture variation and restraint misuse.

3.0 Trouble Areas: Factors in Injury Arising from Improper Posture

As we saw in the previous section, the influence of belt positioning in even minor ways has large influence over the kinematics and kinetics felt by an ordinary child seated in the rear seat. These changes is forces are even more apparent for a change in the posture of the child seated in the rear row. While belt positioning is one factor in the equation, belt slack and posture prior to the incident are equally important. This increase in kinetics for poorly positioned ATDs, even with existing testing equipment, creates the need to have these ATDs represent accurate out-of-position kinetics and kinematics in order to accurately capture the forces felt. Contentious ATD posture and scoring of injury severity for rear-row seated pediatric occupants will be evaluated in the following sections.

3.1 Biofidelity of Child ATDs for Improper Posture/Securement

Figure 6. Example of a childlike ATD (Q6).

Although ATDs have come a long way in evaluating childlike biofidelic responses, these tests rely heavily on the  “nominal position”[12]. As seen in Figure 5[13], the relative positioning of ATDs on various booster seats is not all the same, and various designs incorporate different joint geometries influencing the final positioning of mimicked anatomical landmarks. Most notably, the slouched positioning of the child is not adequately represented by the ATD and differs between ATD’s as well, as evident by the head locations of the child (black x) being in a significantly different position than all ATD’s heads.

Since safety standards are designed based on ATD testing in “nominal positions”, it is important to understand the influence of posture on biomechanics. As shown in Figure 5[13], one of the ATDs that matched closely to real-life observed placements was the Q6 ATD (see Figure 6[14]).

A study on posture involving the Q6 ATD found that for forward and inboard leaning positions (leaning into the center of the vehicle away from the windows), there is a significant increase in injurious criteria such as HIC 15 and Abdominal pressure[15].

In addition to improper posture, while various papers are contentious about the exact amount, anywhere from as little as ⅓ to ½ of the children riding in the rear seat are restrained properly[12][16]. The significance of this is apparent when evaluating the increase in likelihood of injury to be obtained.

Figure 5. Childlike ATD's biofidelity of slouched posture commonly seen in child occupants.

In comparing HIC36 values for the test in Figure 7[12] below, failure to tightly secure the booster seat, even with nominal posture results in unacceptable head accelerations via the FMVSS standard[12] for a HIII.

Figure 7. Test for the influence that a loose car seat tightening can have.

Three different belt types with varying levels of attachment security underwent the sled test pictured above[12]. This was done for both front-facing and rear-facing booster seat setups. For either direction of setup, the HIC 36 reached unacceptable levels, with the most correlated variable being the tether tightness of the booster seat[12]. The studies involving an examination of posture and proper restraint reveal safety testing considerations for vehicles to be improved for the future. Since improper belt securement and improper posture both contribute to an increase in forces felt by the child seated in the rear row and making them more susceptible to severe injury when going by the ISS =15 standard.

Additionally, there is trouble determining the exact injury mechanisms of out-of-position children and misused safety features. Contention of creating the most biofidelic ATD must encompass the ability to be tested for injuries caused by abnormal postures in order to capture the injurious potential of these forces.

3.2 Injury Criteria for Rear Seat Pediatric Occupants

Another area of contention is the injurious values for pediatric patients. As seen in the last section, slight factors of positioning have a drastic influence on the force felt by the rear-row seated occupant. In the past ten years, researchers looking at epidemiological data have suggested increasing the definition of severe injury for pediatric occupants due to their ability to withstand and recover. Using binary logistic regression for a data set of over 400,000 cases, they found that an ISS of 25 for pediatric patients correlated to the same level of risk as an ISS of 15 for adult patients[17] (see Figure 8[17]).

Figure 8. Binary logistic regression of ISS score for adult and pediatric occupants.

This regression suggests that the standard of injurious factors felt by child ATDs for evaluation of rear-seat child injury may need to change its threshold to be proportionally different from a uniformly scaled-down version of the HIII 50% male used currently. Alternatively, different injury criteria could be utilized to assess the potential seriousness of a crash for pediatric occupants utilizing ATDs with good thoracic instrumentation. The thoracic trauma index could also be a good overall predictor of mortality for pediatric occupants, as the study found that head and chest injuries with AIS 3+ severity were the best indicators of mortality in pediatric occupants[17].

Further, additional epidemiological studies demonstrated that 42% of car seat age pediatric occupants seated in the back were improperly restrained making them more likely to experience these higher injurious values[16]. With the standard of injury unclear for pediatric occupants, and factors such as posture and positioning playing such a large role in rear-seat injury, it is becoming increasingly crucial to develop either safer standards or better protective equipment.

4.0 Future Work

Future improvements in booster seat design, restraint systems, and crash testing methods play a crucial role in enhancing protection for children in rear seats to reduce injury risks.

4.1 Next-Generation Child Safety Seats

Figure 9. Belt fitting on bony anatomy.

Child safety seats are essential for child protection in a collision. Injuries can be reduced by up to 59% for those between ages four and seven by simply using a booster seat[18]. One reason for this is the positioning of the lap belt[19] (see Figure 9[19]).

However, not all child safety seats are created equal, and designs have changed significantly over the years. Different types include infant, convertible, combination, and booster seats. Because the response of the safety seat depends on the child’s body dimensions, an adjustable seat based on the child's growth could be safer[19].

Next, the design of the booster seat affects children’s natural seating positions, potentially rendering design choices less effective. For example, one study found that a design with large side head supports found that during normal riding, the child’s head and shoulders were not in contact with the booster’s back most of the time[20]. This highlights a potential gap in ergonomics and long-term comfort, as well as a need to understand how the design of a seat influences seating position and behaviour.

4.2 LDOC Future Improvements

Figure 10. Way to bias positioning for childlike ATDs at the abdominal level.

The Large Omni-Directional Child Anthropomorphic Test Device (LODC) represents a significant advancement in child crash testing technology. Even though it outclasses the HIII-10C[9], an evaluation suggests improvements to increase biofidelic response. Below are examples of three issues and proposed solutions[21]:

  1. Improper forward translation of the abdomen which can lead to inaccurate injury predictions.
    • Solution: Adding brackets to fix the abdomen and lumbar spine to prevent excessive displacement (see Figure 10[21]).
  2. Shoulder belt entrapment between the clavicle and upper arm.
    • Solution: Requiring geometry reworks to prevent catching on angular discontinuities[21].
  3. High chest deflection in belted tests compared to pediatric responses and the HIII-10C. An earlier study[9] suggested it may be a combination of potentiometer location and the flexible spine design.
    • Solution: The thorax needs higher stiffness to achieve biofidelic response. This could be done by thickening geometry[21].

5.0 Conclusion

This review examined how well current ATDs predict child injury risks in rear seats, with a focus on the LODC ATD compared to the Hybrid III 10-year-old model. While the Hybrid III 10-year-old model has been widely used in crash testing, it lacks biofidelity in head movement, neck stiffness, and kinematic response. The LODC ATD was introduced to address these limitations, demonstrating improvements in head impact response, rotational stiffness, and overall human-like motion. However, even with these advancements, ATD testing still assumes that children sit in perfect, upright positions, which is rarely the case in real-world crashes.

As discussed, the Q6 ATD (Figure 6[14]) demonstrated that forward and inboard leaning postures increase injury criteria such as HIC15 and abdominal pressure. As shown by epidemiological data, a large portion of severely injured pediatric crash victims were improperly restrained, which emphasizes the discrepancy between standardized ATD testing conditions and actual child seating behaviour. Therefore, these findings raise concerns about whether current ATD-based crash tests accurately represent the way injuries occur in real crashes.

To improve child safety, researchers could introduce posture variability in crash tests to ensure dummies are assessed in more realistic seating positions. Refining LODC’s flexibility and biofidelity may also improve its ability to better mimic how children move and interact with restraints. Lastly, updating crash test regulations to account for out-of-position seating scenarios and restraint misuse may enhance injury risk assessments. Therefore, ATD-based crash testing can become more representative of real-world injury risks by addressing these gaps, which can lead to improved child safety regulations, better restraint system designs, and thus a reduction in rear-seat injuries.

6.0 References

  1. ↑ J. Hu, M. P. Reed, J. D. Rupp, K. Fischer, P. Lange, and A. Adler, "Optimizing seat belt and airbag designs for rear seat occupant protection in frontal crashes," Stapp Car Crash Journal, vol. 61, pp. 67-100, Nov. 2017. [Online]. Available: https://mreed.umtri.umich.edu/mreed/pubs/Hu_2017_Stapp_Rear_Seat.pdf​:contentReference[oaicite:1]{index=1}
  2. ↑ Jump up to: 2.0 2.1 S. Kuppa, “Rear seat occupant protection in frontal crashes,” National Highway Traffic Safety Administration (NHTSA), 2005. [Online]. Available: https://www.researchgate.net/profile/Shashi-Kuppa/publication/239606067_REAR_SEAT_OCCUPANT_PROTECTION_IN_FRONTAL_CRASHES/links/5475baf60cf29afed612b269/REAR-SEAT-OCCUPANT-PROTECTION-IN-FRONTAL-CRASHES.pdf
  3. ↑ M. J. Trowbridge and R. Kent, “Rear-Seat Motor Vehicle Travel in the U.S.,” American Journal of Preventive Medicine, vol. 37, no. 4, pp. 321–323, Oct. 2009, doi: https://doi.org/10.1016/j.amepre.2009.05.021.
  4. ↑ Beck B, Bilston LE, Brown J. Injury patterns of rear seat occupants in frontal impact: an in-depth crash investigation study. Inj Prev. 2016 Jun;22(3):165-70. doi: 10.1136/injuryprev-2015-041715. Epub 2015 Dec 9. PMID: 26658341.
  5. ↑ Jump up to: 5.0 5.1 Brown J, Bilston LE The scope and nature of injuries to rear seat passengers in NSW using linked hospital admission and police data. Traffic Inj Prev 2014;15:462–9. doi:10.1080/15389588.2013.833662, UBC eLinkCrossRefPubMedGoogle Scholar
  6. ↑ D. R. Durbin et al., “Rear seat safety: Variation in protection by occupant, crash and vehicle characteristics,” Accident Analysis & Prevention, vol. 80, pp. 185–192, Jul. 2015, doi: https://doi.org/10.1016/j.aap.2015.04.006.
  7. ↑ M. J. Trowbridge and R. Kent, “Rear-Seat Motor Vehicle Travel in the U.S.,” American Journal of Preventive Medicine, vol. 37, no. 4, pp. 321–323, Oct. 2009, doi: https://doi.org/10.1016/j.amepre.2009.05.021.
  8. ↑ K. B. Arbogast, J. S. Jermakian, M. J. Kallan, and D. R. Durbin, “Effectiveness of Belt Positioning Booster Seats: An Updated Assessment,” PEDIATRICS, vol. 124, no. 5, pp. 1281–1286, Oct. 2009, doi: https://doi.org/10.1542/peds.2009-0908.
  9. ↑ Jump up to: 9.00 9.01 9.02 9.03 9.04 9.05 9.06 9.07 9.08 9.09 9.10 9.11 9.12 9.13 9.14 9.15 9.16 Stammen, Jason & Suntay, Brian & Moorhouse, Kevin & Carlson, Michael & Kang, Yun-Seok. (2016). The Large Omnidirectional Child (LODC) ATD: Biofidelity Comparison with the Hybrid III 10 Year Old. Stapp car crash journal. 60.
  10. ↑ T. Seacrist et al., “Kinematic Comparison of Pediatric Human Volunteers and the Hybrid III 6-Year-Old Anthropomorphic Test Device,” Annals of advances in automotive medicine. Association for the Advancement of Automotive Medicine. Annual Scientific Conference, vol. 54, pp. 97–108, 2010, Available: https://pubmed.ncbi.nlm.nih.gov/21050595/
  11. ↑ F. J. Lopez-Valdes, J. Forman, R. Kent, O. Bostrom, and M. Segui-Gomez, “A comparison between a child-size PMHS and the Hybrid III 6 YO in a sled frontal impact,” Annals of advances in automotive medicine. Association for the Advancement of Automotive Medicine. Annual Scientific Conference, vol. 53, pp. 237–46, Oct. 2009, Available: https://pubmed.ncbi.nlm.nih.gov/20184847/
  12. ↑ Jump up to: 12.0 12.1 12.2 12.3 12.4 12.5 M. A. Manary, K. D. Klinich, M. Reed, C. A. C. Flannagan, and N. R. Orton, Investigation of crash consequences for common child restraint misuse, Rep. No. DOT HS 813 100, Nat. Highway Traffic Safety Admin., July 2021, https://doi.org/10.21949/1530223
  13. ↑ Jump up to: 13.0 13.1 Gretchen H. Baker, Katarina Bohman, Julie A. Mansfield, Lotta Jakobsson, John H. Bolte, Comparison of child and ATD belt fit and posture on belt-positioning boosters during self-selected, holding device, and nominal conditions, Accident Analysis & Prevention, Volume 192, 2023, 107280, ISSN 0001-4575, https://doi.org/10.1016/j.aap.2023.107280.
  14. ↑ Jump up to: 14.0 14.1 Cellbond, "Q6 Child ATD," [Online]. Available: https://www.cellbond.com/products/atds/child/q6/. [Accessed: Apr. 14, 2025].​
  15. ↑ Jalaj Maheshwari, Madeline Griffith, Gretchen Baker, Declan Patton, Julie Mansfield, Effect of naturalistic seating postures and seatbelt routing on booster-seated Q6 ATD kinematics and kinetics in frontal impacts, Accident Analysis & Prevention, Volume 189, 2023, 107140, ISSN 0001-4575, https://doi.org/10.1016/j.aap.2023.107140.
  16. ↑ Jump up to: 16.0 16.1 Eva M. Urrechaga, Alessia C. Cioci, Megan K. Allen, Rebecca A. Saberi, Gareth P. Gilna, Alexa G. Turpin, Eduardo A. Perez, Henri R. Ford, Juan E. Sola, Chad M. Thorson, Improper Restraint Use in Pediatric Patients Involved in Motor Vehicle Collisions, Journal of Surgical Research, Volume 273, 2022, Pages 57-63, ISSN 0022-4804, https://doi.org/10.1016/j.jss.2021.12.015.
  17. ↑ Jump up to: 17.0 17.1 17.2 Brown, Joshua B. MD, MSc; Gestring, Mark L. MD; Leeper, Christine M. MD; Sperry, Jason L. MD, MPH; Peitzman, Andrew B. MD; Billiar, Timothy R. MD; Gaines, Barbara A. MD. The value of the injury severity score in pediatric trauma: Time for a new definition of severe injury?. Journal of Trauma and Acute Care Surgery 82(6):p 995-1001, June 2017. | DOI: 10.1097/TA.0000000000001440
  18. ↑ Ehiri JE, Ejere HO, Magnussen L, Emusu D, King W, Osberg JS. Interventions for promoting booster seat use in four to eight year olds traveling in motor vehicles. Cochrane Database Syst Rev. 2006 Jan 25;2006(1):CD004334. doi: 10.1002/14651858.CD004334.pub2. PMID: 16437484; PMCID: PMC8805601.
  19. ↑ Jump up to: 19.0 19.1 19.2 Nuraresya I, Nirmal U, Ng PK. A Comprehensive Review on the Development of Car Booster Seats for Children. Current Journal of Applied Science and Technology. 2019 Oct 10;1–21.
  20. ↑ Andersson M, Bohman K, Osvalder AL. Effect of Booster Seat Design on Children's Choice of Seating Positions During Naturalistic Riding. Ann Adv Automot Med. 2010;54:171-80. PMID: 21050601; PMCID: PMC3242564.
  21. ↑ Jump up to: 21.0 21.1 21.2 21.3 Suntay B, Carlson M, Stammen J. Evaluation of the Large Omni-Directional Child Anthropomorphic Test Device [Internet]. Washington, DC: National Highway Traffic Safety Administration; 2019 Jul. Available from: https://rosap.ntl.bts.gov/view/dot/41843/dot_41843_DS1.pdf


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