## 4 Discussion

Table 2: Average maximum head, chest, and pelvis accelerations, as well as neck flexion, extension moments, chest deflections, and seatbelt loads, using a correctly positioned conventional seatbelt (Figure 1), equipped with neither pretensioner nor force limiter (Test D), a pretensioner only (Test E), a force limiter only (Test F), or both a pretensioner and force limiter (Test G). limiter improved the safety of rear seats [32]. We, therefore, evaluated the efficiency of pretensioner and force limiter systems for pregnant passengers. A previous study evaluated these systems for the pregnant drivers using the MAMA-2B dummy [33] and concluded that activating the pretensioners and force limiters in a rear-end collision could improve safety. However, to the best of our knowledge, our study is the first to examine the use of pretensioners and force limiters during frontal collisions involving pregnant rear-seat passengers. Using a pretensioner, which quickly retracts some of the seatbelt webbing during a collision, the maximum chest displacement, maximum resultant acceleration of the head, chest, and pelvis, neck extension, and seatbelt loads were smaller than those resulting from conventional seatbelt use. These results indicate that the pretensioner improves restraint and, thus, the safety of the passenger. However, the chest deflections and neck flexion were slightly higher than those resulting from conventional seatbelt use because of the early restraint of the pretensioner. Using a force limiter, which releases the webbing gradually, while maintaining a constant force to restrict the force on the chest, the chest displacement was larger than that resulting from conventional seatbelt use. When both the pretensioner and force limiter were applied, the magnitudes of almost all measured parameters (except for the chest displacement) were smaller than those resulting from conventional seatbelt use. Furthermore, the combined use of the pretensioner and force limiter produced a resultant head and chest acceleration, neck flexion, and extension, chest deflections, and seatbelt loads lower than those resulting from use of one or neither of them. According to [PERSON] et al., both the skull fracture and neck injury risks with an abbreviated injury scale (AIS) score > 3 are less than 1% [34]. Based on the findings of [PERSON] et al., the chest resultant acceleration of 284.4 m/s\({}^{2}\) obtained in the present study presents only a 10.9% risk of injuries, with an AIS > 3 [34; 35]. [PERSON] et al. reported that a chest injury with an AIS \(\geq\) 3 did not occur when the shoulder belt load was less than 4.0 kN [36]. Additionally, according to vehicle safety regulations, the chest deflection of 19.8 mm obtained in the present study presents only a 7.2% risk of injuries with an AIS > 3 [37]. Therefore, we conclude that the risk of chest injury to pregnant rear-seat passengers might be reduced by using pretensioner and force limiter systems. Furthermore, as previously suggested, because the uterine fundus extends into the upper abdomen in late-term pregnancy, reductions of both the shoulder belt load and chest deflection would minimize the direct forces on the uterus [38]. When both pretensioner and force limiter were applied, the average lap belt load was 7.2 kN. On the basis of swine experiments, [PERSON] reported that abdominal injuries with AIS \(\geq\) 3 occurred in 50% of cases, when the maximum lap belt load was 3.96 kN [39]. Therefore, we concluded that the risk of abdominal injury to pregnant women might be also reduced by using a seatbelt equipped with a pretensioner and force limiter. There were some limitations to the present study. First, the safety of each method of seatbelt use was assessed using only a single test. However, the MAMA-2B dummy used in this study was developed for frontal MVC tests and resulted in highly reproducible results. Indeed, the MVC test for the accreditation of the dummy to the international standard was performed only once.