Investigation of the driver injury risks in small overlap impact with automatic emergency braking

Abstract

The forward displacement caused by precrash braking may influence occupant injury risks in frontal impacts. However, the occupant injury mechanisms in the small overlap impact (SOI) with autonomous emergency braking (AEB) systems are not yet fully understood. This study aimed to investigate the driver injury mechanisms in SOI with AEB via finite element modelling using the THUMS occupant model and a validated vehicle model. Simulations were conducted under the braking-crash loadings which combined the simplified braking curves with representative SOI accelerations. Simulation results indicated a considerable increase in occupant thoracic compression owing to forward displacement induced by AEB, though the risk of brain injury remained uncertain. The presence of AEB in SOI changed occupant injury patterns, showing varied injury risk trends across different body regions. The main injury mechanisms under AEB crash load conditions were the reduced buffering effect of the restraint system and increased ride-down efficiency caused by AEB, contributing to greater uncertainty in overall occupant injury risk.

1. Introduction

Combining active and passive safety systems is a key strategy for enhancing vehicle safety and has emerged as a significant industry trend in recent years [1]. Autonomous emergency braking (AEB) is a critical technology for enhancing safety in the precrash phase, and is increasingly being installed in new vehicles. Studies have demonstrated that AEB could reduce vehicle speed prior to a collision, thus significantly decreasing injury severity [2], which is the primary function of the AEB system. However, precrash braking by AEB may cause occupants to produce forward displacements owing to inertial forces with the intervention of active safety [3, 4], which was uniformly referred to as ‘displacement’ in subsequent articles. Changes in occupant posture and position resulting from these manoeuvres may impact the effectiveness of crash protection systems [5]. Even without AEB, driver-initiated evasive manoeuvres (braking, steering) also caused a change in the occupant's posture before the crash [6]. Previous modelling studies have demonstrated that occupant precrash posture significantly impacts injury risks in frontal crashes [7, 8].

To study the injury characteristics caused by the crash after the occupant's displacements, Zhu et al. [9] used the finite element method to analyse out-of-position occupant injury risks under varying AEB braking strengths, finding that injury risks increased with occupant displacement. Wang et al. [10] examined the effect of AEB on occupant injury risks after frontal impacts. While the AEB system can help prevent collisions, it also causes a forward displacement of the occupant's body, moving it away from the optimal protective position in the restraint system and thereby increasing injury risk. Previous studies have shown that the intervention of the AEB system in frontal crashes leads to an increase in the risk of neck injury [11] and rib fracture [12]. However, when AEB was activated before the frontal crash, the risks of concussion were reduced [13]. Accordingly, forward displacements could cause positive or negative effects on the occupant injury risks during a crash. On the one hand, the occupant loses space in the passenger compartment for energy dissipation, resulting in higher loads. On the other hand, AEB considerably reduces the overall crash energy [14]. This indicated that the occupant injury mechanisms in the frontal crashes with AEB have not been clearly understood.

In frontal crashes, a small overlap impact (SOI) is defined as a collision where the primary load path occurs outside all major longitudinal structures, typically the two frame rails. Notably, SOIs account for 24% of injury or fatal frontal impacts overall [15]; this figure increases to 27% in the United Kingdom [16], 48% in Sweden [17], 22% in the United States [18] and 26% in Germany [19]. Thus, SOIs are associated with high morbidity and mortality rates. In-depth studies of SOIs have revealed that vehicle energy management is often insufficient, leading to numerous injuries owing to occupant compartment intrusion [20, 21]. However, the impact of occupant forward displacement owing to the AEB on occupant injury risk has not been well quantified. It is essential to consider the combined effects of active and passive safety technologies on occupant injury risk to further enhance vehicle safety performance.

The current design process for vehicle restraint systems relies heavily on crash tests using a limited number of anthropomorphic test devices (ATDs) in standard driving postures. However, studies have shown that braking displacement and injury risks are generally higher for real occupants than for dummies [6, 12], a result of dummy stiffness, sitting position and belt slack. Dummies were built for crash tests, not for pre-crash phases [14]. From an injury biomechanics perspective, these increased injury risks are partly attributed to differences in the geometry, composition and material properties of human bones and soft tissues [22]. From an infidelity perspective, the displacement of key parts of ATDs was much smaller than that of volunteers or human models when the braking strength was consistent [4]. ATDs did not exhibit fidelity in low inertial load scenarios. The displacement and injury risk assessment tools used for integrated simulation of braking and crash could incorporate these characteristic differences.

Therefore, the aim of the current study was to investigate the injury mechanisms of occupants in SOIs resulting from the varying characteristics of AEB. First, a finite element (FE) model of the driver-vehicle subsystem was developed using the Total Human Model for Safety Version 6 (THUMS V6) [23] along with a validated Accord vehicle model featuring a restraint system [24]. Simulations were then performed under AEB crash loadings, incorporating simplified braking curves and representative SOI acceleration. Finally, the kinematics and injury risks from the THUMS model were analysed.

2. Methods

Firstly, 139 AEB curves from 24 automobiles were collected from the China Insurance Automotive Safety Index (C-IASI) tests. These curves were simplified for feature analysis using a novel method proposed by this study. Meanwhile, two crash accelerations were selected from 126 C-IASI SOI tests to serve as boundaries, representing typical ‘energy absorption’ [25] and ‘sideswipe’ [26] cases.

2.1. The AEB brake curve for selection

The analysed AEB curves were derived from two test conditions. The first is the car-to-car rear moving (CCRm) scenario, where the target vehicle travels at low speed and the frontal structure of the subject vehicle impacts the rear structure of the target vehicle [27]. The other is CPNSOC-50 (car to pedestrian nearside single obstruction child 50% condition) scenario [28]. This study refers to the relevant provisions of AEB function evaluation in the C-IASI 2020 Test Protocol [29]. The AEB curves were processed using a 2-pole phase-less Butterworth filter with a cut-off frequency of 6 Hz to eliminate interference, including spikes and noise. AEB curves with retained correlation greater than 0.8 were analysed for consistency. Following these preprocessing steps, the AEB curves were simplified using a theoretical model, so as to extract the characteristic parameters.

Various curve simplification methods are currently widely used. In this study, the area equivalent method in time domain is adopted, where speed changes are assumed to be equal over the same time interval. Ref. [30] categorizes the acceleration curve into three main stages: the ‘a–b’ stage (braking action stage), the ‘b–c’ stage (braking maintenance stage) and the ‘c–d’ stage (braking release stage) (Fig. 1). The key demarcation points are the start time of the steady-state braking phase |${t_1}$| (the moment when acceleration first exceeds 85% of the peak [29]), the end time of the steady state braking phase |${t_2}$| (when acceleration last exceeds 85% of the peak) and the time when acceleration returns to zero at the end of braking. The equivalent waveform must meet the following criteria: the times of the demarcation points |${t_1}$|⁠, |${t_2}$| and |${t_3}$| must be identical, and the velocity variation during the ‘b–c’ segment must be equal.

The original and simplified curves are shown in Fig. 1. The curves are simplified according to the boundary conditions, where the vehicle velocity change |$\Delta {{\rm{V}}_{b - c}}$| in the ‘b–c’ segment is the area of the rectangular |$bc{t_2}{t_1}$|⁠. The formula is as follows:

According to Formula (1):

where |$\Delta {V_{b - c}}$| is the velocity variation of the ‘b–c’ stage, |${V_{{t_1}}}$| is the speed at time |${t_1}$|⁠, |${V_{{t_2}}}$| is the speed at time |${t_2}$| and |$a$| is the ‘b–c’ segment acceleration. |$k$| is the slope of ‘b–c’ acceleration.

According to the proposed simplification method, the braking curves of 13 vehicles with high consistency in the CPNSOC-50 and CCRm scenarios are simplified respectively. The simplified results are shown in Table 1, and the distribution is shown in Figs. 2 and 3. Based on the simplified results of curves and the analysis of characteristic parameters in the two above-mentioned scenarios, it can be observed that AEB strength ranged from −0.9 g to −0.6 g during the braking maintenance stage, with the majority of AEB slopes falling between −5 g/s and −1 g/s. Additionally, in the CCRm scenarios, the AEB strength remained above 0.85 g, compared to the emergency CPNSOC-50 scenarios, where AEB slopes were generally lower.

2.2. SOI curve selection

Original equipment manufacturers typically implement energy absorption strategies such as ‘energy absorption’ and ‘sideswipe’ in the design of auto body structure to enhance occupant safety during SOIs [31]. The terms ‘energy absorption’ and ‘sideswipe’ represent two distinct and extreme vehicle response cases observed in the SOI tests. This classification is based on the vehicle design strategies outlined in the relevant literature [32], in the SOI test, the X-direction crash residual velocity below the left B-pillar of the vehicle is higher in the ‘sideswipe’ strategy and lower in the ‘energy absorption’ strategy. Consequently, curves representing a typical ‘energy absorption’ case were selected for further crash simulations (Fig. 4) and a ‘sideswipe’ case (Fig. 5).

2.3. Restraint models for crash simulations

In this study, the restraint system model is extracted from the Accord vehicle model developed and verified by the National Highway Traffic Safety Administration [24]. This FE model includes the THUMS V6 (with normal muscle activation levels), the seat, the belt, the steering wheel (which is rigidly connected to the vehicle's motion and airbag) and the curtain airbag. The simulation matrix is presented in Table 2, which consists of two SOI curves, one AEB curve with braking amplitudes of 0.9 g and a slop of 5 g/s. The crash curves of AEB and SOI (Fig. 6) are obtained by combining the braking curve with the crash curve. All simulations were subjected to inertial loads at three points in the X, Y and Z directions, while the effects of the intrusion of the vehicle restraint system were ignored. The complete simulation model is shown in Fig. 7.

2.4. Ride-down efficiency

Several studies have been performed on occupant–vehicle interactions using ride-down and restraint energy component analysis [33–35]. The kinematic energy of the occupant at the moment of the frontal crash, denoted as |${E_0}$|⁠, is absorbed by both the vehicle and the restraint system. The energy absorbed by the vehicle is referred to as ride-down energy, |${E_{\mathrm{ rd}}}$|⁠, while the energy absorbed by the restraint system is termed restraint energy, |${E_{\mathrm{ rs}}}$|⁠. The relationship for the frontal crash, |${E_0}$|⁠, can be expressed as follows:

where F represents contact force between the occupant and the restraint system, |${x_0}$| denotes the occupant's forward displacement in the global coordinate system, |${x_\mathrm{ v}}$| indicates the vehicle's forward displacement in the global coordinate system and |${x_{\mathrm{ ov}}}$| refers to the occupant's forward displacement in the vehicle coordinate system.

To facilitate a unified analysis of dimensions after curve integration, the concept of energy density is introduced. Its calculation formula is as follows:

where |${e_0}$| is the energy density of |${E_0}$|⁠, |${e_{\mathrm{ rd}}}$| is the energy density of |${E_{\mathrm{ rd}}}$| and |${e_{\mathrm{ rs}}}$| is the energy density of |${E_{\mathrm{ rs}}}$|⁠. All units are in J/kg. The ride-down efficiency is calculated as the ratio of ride-down energy |${E_{\mathrm{ rd}}}$| to the occupant's kinetic energy |${E_0}$|⁠, expressed as follows:

In this study, the method for calculating the ride-down efficiency is as follows: Firstly, the curves of acceleration and body displacement at T4 of the THUMS chest, as well as the curves of chest acceleration and chest–vehicle relative displacement, are constructed. Next, the maximum energy density (⁠|${e_{\mathrm{ rd}}}$|⁠) absorbed by the vehicle and the maximum energy density (⁠|${e_{\mathrm{ rs}}}$|⁠) absorbed by the restraint system during the crash are calculated through curve integration. Finally, ride-down efficiency is determined by dividing |${E_{\mathrm{ rd}}}$| by |${E_0}$|⁠.

3. Results

3.1. Occupant kinematics

Figs. 8(a) and 8(b) show the displacement of the upper torso of the THUMS during the AEB phase, from 200 ms to 1000 ms. As the AEB strength increases, the upper torso experiences considerable displacement. At 650 ms, the upper body reaches its maximum displacement, with the head, chest (T4) and pelvis displacing 188.2 mm, 106.3 mm and 12.1 mm, respectively. Subsequently, the body begins to rebound owing to the restraint provided by the seat belts.

The occupant forward displacements in SOIs with and without AEB are compared in Fig. 9. The timing of maximum displacement for key parts of the occupant's upper torso is similar in both AEB and non-AEB scenarios, although the peak displacement occurs later in the ‘energy absorption’ case compared to the ‘sideswipe’ case. In the ‘sideswipe’ scenario, the maximum forward displacement of the head and chest of the THUMS during the crash phase is only slightly influenced by AEB, while the pelvis experiences a smaller forward displacement with AEB compared to without it. Conversely, in the ‘energy absorption’ case, the head displacement with AEB is less than that without AEB. In conjunction with the THUMS motion, the displacement between the two scenarios is equal, approximately 1.08 s, and the airbag volume change with AEB is less than that without AEB. The airbags constrain head displacement, leading to varying performance results. Furthermore, the presence or absence of AEB has minimal impact on pelvic displacement.

In Fig. 10(a), there is minimal occupant forward and lateral displacement during the first 30 ms. The airbag fully deploys before making contact with the body. In Fig. 10(b), at the onset of the crash, the occupant experiences an initial movement away from the airbag, but this distance decreases owing to the action of the AEB. At 50 ms, when the airbag is not fully deployed, the occupant's head begins to contact the airbag, approximately 15 ms earlier than in the scenario without AEB. This early contact results in a difference in occupant position by the end of the crash.

In Fig. 11(a), the lateral displacement of THUMS in the ‘sideswipe’ case is higher than that in the ‘energy absorption’ case in Fig. 10(a), while the forward displacement is opposite. In Fig. 11(b), the displacements of the upper torso are evident from the onset of the crash with the AEB. The face and chest make contact with the airbag before it is fully deployed. By 1060 ms, the entire upper torso moves closer to the airbag as the crash begins. Although the airbag continues to inflate and wraps around the upper chest–neck–head complex, the body still experiences forward displacement. At 1070 ms, the hip and knees reach their maximum travel distance, primarily restricted by the lap belt and seat cushion. Meanwhile, the upper torso continues to maintain forward displacement until 690 ms. Between 1070 ms and 1150 ms, the head and chest reach their maximum forward displacement before bouncing back; however, the upper torso is considerably distorted owing to the restraint of the seatbelt.

3.2. Occupant injury risks

The occupant injury risk indicators in SOIs with and without AEB are illustrated by Table 3. For each simulation, the injury risk indicators for the head (including intracranial pressure, intracranial equivalent stress, brain injury criterion (BrIC), cumulative strain damage measure 15% (CSDM15) and maximum principal strain (MPS), cervical vertebrae (cancellous bone stress, cortical bone stress), cervical ligaments (anterior longitudinal ligament, posterior longitudinal ligament, capsular ligament, flavum ligament and interspinous ligament), chest (rib strain rate, heart pressure, liver pressure, lung pressure and thoracic compression) are output. The maximum intracranial, heart and lung pressures for the occupants in each case are also displayed in Fig. 12.

Without AEB, the cerebral tissue injury risks for THUMS under SOI are all below the human injury threshold. However, from the perspective of BrIC, the BrIC indicator in the ‘energy absorption’ case is 0.942, which is close to 1, suggesting that the injury risk for AIS4+ is nearly 50% [38]. By selecting a strain threshold of 0.15 for CSDM calculations, the CSDM15 indicators for both white and grey matter in the cerebrum are approximately 90%. This results in an approximately 85% risk of concussion [39]. The head injury risks in the ‘sideswipe’ case are relatively low. In addition, it can be concluded from Table 3 that the injury risks for cervical ligaments are higher in both response cases, suggesting a potential risk of tearing injuries in the occupant's neck. The simulated pressure values for the heart, lungs and other visceral soft tissues exceeded the tolerance limits for these areas, particularly the lung pressure, which surpassed the threshold by nearly 18 times. This indicates a considerable likelihood of severe damage to the internal soft tissues. In the ‘sideswipe’ case, the neck injury indicators were generally higher than those in the ‘energy absorption’ case, while the injury indicators for the head and chest showed the opposite trend.

With the AEB system in place, the head does not make contact with any rigid parts of the vehicle owing to the presence of the airbag. Overall, occupant injuries are lower compared to scenarios without AEB; however, many injuries, including those to the heart and lungs, still exceed the human injury threshold. In the ‘sideswipe’ case, head injury indicators excluding intracranial pressure were higher than in scenarios without AEB. The BrIC value rose to 0.91, an increase of nearly 0.3 compared to cases without AEB, leading to a nearly 35% increase in the probability of brain injury risk classified as AIS 4+ [38]. The CSDM15 value of white and grey matter increased by nearly 30%, MPS increased by nearly 0.2 and the probability of concussion is close to 80% [39]. AEB considerably increased head injury risk in the ‘sideswipe’ case with AEB. Conversely, in the ‘energy absorption’ case, the injury risks at each site decreased with the inclusion of AEB, primarily owing to greater foundational damage in the absence of AEB. Notably, thoracic compression with AEB is higher than without AEB when combining both response cases.

3.3. Ride-down efficiency

The vehicle absorbs energy density |${e_{\mathrm{ rd}}}$|⁠, and the restraint system absorbs energy density |${e_{\mathrm{ rs}}}$| in the X-direction for each scenario showed in Fig. 13. The ride-down efficiency in the X-direction for all scenarios is compared in Fig. 14. Without AEB, the energy densities |${e_{\mathrm{ rd}}}$| for the ‘energy absorption’ and ‘sideswipe’ cases are 360.24 and 88.54 J/kg, respectively, resulting in ride-down efficiencies of 87% and 82%. Both of these values are lower than those observed with AEB.

As shown in Figs. 15 and 16, the overall energy density in the Y-direction is lower than in the X-direction; however, the energy densities |${e_{\mathrm{ rd}}}$| and |${e_{\mathrm{ rs}}}$| for the ‘sideswipe’ case are higher than for the ‘energy absorption’ case, both with and without AEB.

4. Discussion

In this study, FE models were created using the THUMS model alongside a validated vehicle model. Simulations were performed under braking–crash loadings, which consisted of combinations of simplified braking curves and representative SOI acceleration. In summary, emergency braking leads to changes in posture and body velocity, resulting in considerable variations in occupant kinematics and injury risks during a crash.

4.1. Kinematics difference

In the scenario without AEB, the THUMS exhibited distinct kinematics in the two different SOI response cases. Based on the kinematic trajectory of the THUMS chest, the upper torso of the THUMS is pressed against the seat at the moment of the crash. However, the X-direction displacement of the THUMS is greater in the ‘energy absorption’ case of the SOI in Fig. 9. In contrast, the Y-direction displacement is the opposite. From an energy perspective, the ratio of Y-direction energy density |${e_{\mathrm{ rd}}}$| to total energy density (the sum of X-direction energy density |${e_{\mathrm{ rd}}}$| and Y-direction energy density |${e_{\mathrm{ rd}}}$|⁠) for the ‘sideswipe’ case is 22.8%, considerably higher than the 3.8% observed in the ‘energy absorption’ case. Thus, the THUMS crash response in the Y-direction for the ‘sideswipe’ case is more intense, whereas the X-direction response in the ‘energy absorption’ case is more severe. This aligns with the definitions of the two crash responses in the literature [25, 26].

In the case of AEB, the occupants experience forward head movement and deviation from the standard seating position owing to the effects of AEB [44], This results in premature chest contact with the airbag, with first contact times of 52 ms for SOI(E) and 44 ms for AEB+SOI(E), leaving insufficient time and space for the airbag to fully deploy. Consequently, the occupants’ face and chest make contact with the airbag earlier compared to scenarios without AEB, resulting in a difference in X-direction kinematics during the crash phase. In the ‘sideswipe’ case, the THUMS experiences oblique motion relative to the vehicle at the onset of the crash. Constrained in the X-direction by the restraint system, it rebounds after reaching maximum forward displacements, as illustrated in Fig. 11. Under the influence of AEB, the displacements of the upper torso are considerably greater than those of the pelvis owing to the belt restraint. Additionally, the maximum displacements of the head and chest are limited by the belt, resulting in both experiencing the same displacement. When comparing scenarios with and without AEB, there is only a small difference in the X-direction kinematics of the occupant's torso; the only variation is in the timing of when they reach their maximum displacement.

4.2. Injury risks difference

In the case without AEB, an analysis of injury risks and energy management reveals that the total energy density of the ‘energy absorption’ case is considerably higher than that of the ‘sideswipe’ case. Consequently, the total crash energy in the ‘energy absorption’ case is also much greater than in the ‘sideswipe’ case. With a 5% difference in ride-down efficiency, the energy absorbed by the restraint system in the ‘energy absorption’ case of SOI, along with the energy transferred to the occupant, is greater, leading to an overall higher risk of occupant injury compared to the ‘sideswipe’ case of SOI. This aligns with the injury values presented in Table 3.

In the case of AEB, the occupant's sitting position changes under its influence, which may increase the uncertainty of injury risks [45, 46]. At the onset of the crash, the occupant's head was leaning forward by approximately 188 mm. This initial forward-leaning position, combined with the driver's initial kinetic energy, considerably influences the interaction between the upper body and the airbag. Additionally, the driver's initial out-of-position status affects the energy absorption mechanism of the torso during the crash.

In the ‘energy absorption’ case with AEB, the energy density |${e_{\mathrm{ rd}}}$| in the X-direction is greater than that without AEB. However, the energy density |${e_{\mathrm{ rs}}}$| in the X-direction decreases, leading to an increase in ride-down efficiency. Consequently, the energy transmitted by the restraint system to the occupant is minimized, resulting in a reduction of head and partial chest injuries [33]. Similarly, Mishra et al. [13] performed FE simulations using the same loading method and found that AEB resulted in considerably lower risks of rib fractures and concussions. These findings indicate that AEB, when applied before SOI, lowers the injury risk to the head. However, as the pre-collision AEB increases, the vehicle's motion intensifies owing to the increase in energy density |${e_{\mathrm{ rd}}}$|⁠, causing the occupant to make contact with the airbag sooner. This early contact ultimately results in increased thoracic compression. As noted in the literature [47, 48], under frontal collision conditions, AEB results in considerable forward displacement of the occupant's upper torso, causing the chest to make premature contact with the airbag. When the airbag deploys, it exerts a certain level of impact on the chest. Combined with the restraint from the seatbelt, these two factors contribute to increased thoracic compression for the occupant. However, the energy transferred to the occupant by the restraint system is reduced owing to the lower energy density |${e_{\mathrm{ rs}}}$| in the X-direction, leading to a decrease in overall chest injury indicators, such as lung pressure and heart pressure, which is still above the injury threshold and has a high injury risk. According to the literature [49], under the SOI, the thoracic structure exhibits no considerable distortion, and the distribution of stress and strain in the ribcage remains relatively uniform, resulting in considerable foundation damage to the occupants. With AEB, the distribution of stress and strain in the ribcage changes, leading to a change in the injury mechanism. Consequently, several injury indicators related to the chest are reduced.

In the ‘sideswipe’ case with AEB, the energy density |${e_{\mathrm{ rd}}}$| in the X-direction is higher than that without AEB. The increase in the ride-down efficiency destroyed the original optimal energy absorption distribution ratio, increased the amount of chest compression [50] and increased the uncertainty of protection. With the reduction of |${e_{\mathrm{ rs}}}$| in both the X- and Y-directions, the energy transmitted to the occupant decreases, leading to a lower risk of injury to the neck, chest, ribs and visceral soft tissues. At the same time, the BrIC with AEB is higher than without AEB because BrIC is used to assess the risk of brain injury resulting from head rotation [38]. This indicates that the rotational load on the occupant's head increased with the addition of AEB, ultimately leading to an increase in the CSDM value and MPS at a strain threshold of 0.15, thereby increasing the risk of head concussions. Interestingly, some studies have noted that the posture changes induced by AEB considerably reduce head injury risks. A possible explanation for these lower risks is that initial head injury risks are relatively low in scenarios where there is no contact between the head and the interior of the vehicle [45, 51, 52].

In summary, the AEB changed the injury patterns of the occupants before the crash. Distinct trends in injury risk were observed across different body regions, including an increase in thoracic compression, while the injury risk to the craniocerebral region remained uncertain. These findings align with those of previous studies.

4.3. Limitations

Only two crash curves are used in this study to represent a limited range of accidents. However, these selected crash curves exhibit significant differences in vehicle motion during the crash phase, as well as substantial variations in occupant kinematics and injury responses. The foundation damage calculated using the loading curve of the ‘energy absorption’ case is greater. Additionally, this study uses a single-vehicle model and does not consider changes in crash speeds after braking; therefore, the findings may not be generalizable to all vehicle conditions and crash scenarios. The injury trends identified in this study are generally consistent with those from other research based on field crash data. The muscular activity of individuals involved in car crashes plays a crucial role; however, this study only considers the THUMS muscle activation levels in a relaxed state and does not fully account for other muscle activation levels.

5. Conclusions

The emergency braking-induced changes in posture and body velocity led to significant variations in occupant kinematics and injury risks during SOIs. The specific influences are outlined as follows:

1. With the AEB, the occupant's torso shifted forward during the braking phase, changing their posture during the collision and causing the head and chest to make contact with the airbag sooner.

2. In SOIs both with and without AEB, the energy absorption and distribution for the driver during the vehicle crash phase varied owing to the precrash forward displacements caused by the AEB. This change resulted from a weakened buffering effect of the restraint system, increased ride-down efficiency and changed injury patterns among occupants, leading to differing trends in injury risks across various body parts and ultimately creating greater uncertainty regarding occupant injury risks.

Acknowledgements

All the authors of this study would like to thank the colleagues who provided the AEB curve, especially the expert James Cheng and Ford Company for their help in the crash analysis. The authors gratefully acknowledge the financial support from the Natural Science Foundation of Chongqing (Grant No. CSTB2023NSCQ-BSX0011).

Author contributions

Tiqiang Fan (Formal analysis, Methodology), Zhenlong Nie (Data curation, Visualization, Writing – original draft, Writing – review & editing), Xinming Wan (Resources, Software), Yu Liu (Investigation), Peilong Yang (Project administration), Jianzhuo Chen (Resources, Software) and Lihai Ren (Supervision, Writing – review & editing)

Conflict of interest statement. None declared.

References