The effect of age and bracing on vehicle occupant responses during sled-simulated evasive swerving.

Objective: Bracing can reduce adult occupants' out-of-position postures, but it is unclear if this finding can be extended to child occupants. We investigated the effect of bracing, and age on the motion of vehicle occupants of different ages during sled-simulated pre-crash maneuvers.

Methods: Forty seatbelt restrained subjects (9-40 y.o.) experienced sled simulated evasive swerving maneuvers during a brace condition, where subjects actively brace before the maneuver onset, and an unbraced condition. A 3D motion capture system, electromyography (EMG), and seatbelt load cells captured head and trunk kinematics, muscle activation, and seat belt reaction forces, respectively. The effects of age and bracing on peak lateral head and trunk displacement were examined with Mixed Effects Model ( ≤ 0.05).

Results: In the braced condition, all subjects had reduced normalized peak head (0.09 ± 0.05) and trunk (0.11 ± 0.05) displacements compared to the unbraced condition (Head: 0.14 ± 0.06, Trunk: 0.17 ± 0.07) ( < 0.05). For the into-the-belt direction in the unbraced condition, children showed the smallest peak head and trunk displacement of all age groups ( < 0.01) while teens had the greatest peak head and trunk displacement compared to all other age groups ( < 0.02). Similar trends were found in the out-of-the-belt direction in the unbraced condition ( < 0.01). In the unbraced condition, all occupants showed greater shoulder seat belt reaction loads than the braced condition (unbraced 1.6-2.7 N/kg, braced 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 the bracing condition or direction of motion.

Conclusion: Bracing reduced out-of-position postures across all ages. In the unbraced condition, children move less than teens and adults due to greater activation in the neck muscles. In the unbraced condition, teens showed the greatest motion, potentially due to their developing neuromotor control transitioning to a more mature stage. Across all age groups in the unbraced condition, occupants relied more heavily on the seat belt to maintain their position rather than their muscles to brace, suggesting the importance of good seatbelt-torso interaction in unbraced occupants.