Characterizing aware and unaware vehicle occupant responses during sled-simulated evasive swerving

Previous studies showed that awareness reduced adult occupants' out-of-position posture, but it is unclear if this finding can be extended to child occupants and time extended pre-crash maneuvers. This is important to be examined as, in highly automated vehicles, reduced awareness of impending maneuvers in all populations of occupants may influence restraint effectiveness. Therefore, we investigated the effect of awareness, age, and pre-crash maneuver duration on the motion and muscle activity of vehicle occupants during sled-simulated pre-crash evasive swerving maneuvers. Forty seatbelt restrained subjects (9-40 years old) experienced a sled oscillatory perturbation (4 cycles, peak acceleration 0.7g) mimicking a series of swerving maneuvers under 2 conditions: an aware condition where subjects were instructed to brace before the maneuver onset, and an unaware condition, with no bracing instructions given. A 3D motion capture system, electromyography (EMG), and seatbelt load cells captured head and trunk kinematics, muscle activation, and seat belt reaction forces, respectively. Data were normalized by the subject seated height (Kinematics), the subject maximum voluntary isometric contraction (MVIC) (Muscle Activation), and the subject body weight (BW) (Seat Belt Reaction Forces). The effects of awareness, age, cycle, and interactions of those variables on peak lateral head and trunk displacement into and out-of-the belt were examined with a Mixed Effects Model and Tukey's post-hoc tests (p≤0.05). In the aware condition, all subjects had reduced normalized peak head (0.09 ± 0.05) and trunk (0.11 ± 0.05) displacements compared to the unaware condition (Head: 0.14 ± 0.06, Trunk: 0.17 ± 0.07) (p<0.05). For the into the belt direction in the unaware condition, children showed the smallest peak head and trunk displacement of all age groups (p<0.01) while teens had the greatest peak head and trunk displacement compared to all other age groups (p<0.02). Similar trends were found in the out of the belt direction in the unaware condition, where children showed the smallest peak head and trunk displacement compared to adolescents and teens (p<0.01) but not adults (p > 0.05). Peak lateral head and trunk displacement was greatest in the first swerve out of the belt (p<0.01) and the smallest in the second swerve, regardless of age and awareness. In the unaware condition, all occupants showed greater shoulder seat belt reaction loads than the aware condition (unaware 1.6-2.7 N/kg, aware 1.0-1.4 N/kg). Children also showed greater neck muscle activation (50-64% MVIC) compared to other muscles and older occupants regardless of awareness condition or direction of motion. Awareness reduced out-of-position postures in pre-crash vehicle maneuvers across all ages, including pediatric occupants. In the unaware condition, young children move less (normalized by seated height) than older teens and adults. This may be due to children using a different bracing strategy than older occupants as shown by the greater neck muscle activation. On the other hand, in the unaware condition, teens showed the greatest motion, potentially due to their developing neuromotor control transitioning to a more mature stage. Occupant responses changed over the maneuver duration, likely due to becoming more familiar with oscillatory motion throughout the cycles. Across all age groups in the unaware condition, occupants relied more heavily on the seat belt to maintain their position rather than relying on an active bracing strategy, particularly when moving out of the belt, suggesting the importance of good seatbelt-torso interaction in unaware occupants. Further investigation examining occupant awareness in various pre-crash maneuvers and seating configurations is warranted to understand how to protect unaware occupants in future vehicle and crash dynamics.