The effect of time-extended evasive swerving maneuvers on occupants' bracing strategies.

Objective: Occupant bracing behavior in pre-crash maneuvers has been previously investigated but the effect of the duration of the pre-crash maneuver on bracing is unknown. This is critical to understand as time-extended pre-crash maneuvers may emerge in cases where drivers lose control of a vehicle and in autonomous vehicles as they may take different approaches to avoid crashes than the current vehicles. Therefore, the aim of this study was to understand the effect of pre-crash maneuver duration on child and adult occupants' bracing behavior and resulting kinematics.

Methods: Forty seatbelt restrained subjects (9-40 years old) experienced sled-simulated time-extended lateral swerving maneuvers (8 s, 4 cycles, peak acceleration 0.7 g) producing an alternating motion initially out-of-the-belt, followed by into-the-belt for each cycle. In a braced condition, subjects were instructed to hold on to a laterally placed handle with their right hand before the maneuver onset, while in an unbraced condition no instructions were given. A 3D-motion capture system, electromyography (EMG), and seatbelt load cells captured head and trunk kinematics (normalized to seated height), muscle activation (normalized to maximum voluntary isometric contraction, MVIC), and seatbelt reaction forces (normalized to body weight), respectively. The effects of cycles and interaction with bracing and age on peak lateral head and trunk displacement into- and out-of-the belt were examined with Mixed-Effects Models and Tukey's post-hoc tests ( ≤ 0.05).

Results: Out-of-the-belt peak lateral head and trunk displacements were the greatest in the first cycle and the smallest in the second cycle ( < 0.01). The third and four cycles were not significantly different from one another ( > 0.8). Into-the-belt peak lateral head and trunk displacements were smaller in the first cycle than the remaining cycles ( < 0.001) and were not significantly different across the remaining cycles ( > 0.8). No interactions between cycle, age and bracing were found ( > 0.3). Right bicep, trapezius and rectus femoris activations slightly increased with increasing cycles in the unbraced condition and in the into-the-belt direction for the 9-11 year-old group. Out-of-belt seat belt loads increased with increasing cycles in the unbraced condition for all age groups.

Conclusions: Occupant kinematics as a result of their bracing behavior changed across cycles of swerving maneuvers from an exaggerated displacement in cycle 1 to an overcompensation due to bracing in cycle 2, ending with a plateau of a moderate displacement in cycle 3 and 4. Younger children (age 9-11) took longer to adapt to the oscillatory motion as they increased their muscle activation over time unlike the other age groups. These findings suggest that it may take time for occupants to find the optimal bracing strategy in time-extended maneuvers. Furthermore, children may find challenging to calibrate their bracing response overtime from a neuromotor perspective.