Kinematics of a Standing Passenger Subjected to an Emergency Braking Deceleration Pulse

Standing passengers in public transports can experience movements of large amplitude if they are subjected to longitudinal deceleration pulses and not properly restrained, possibly leading to injuries due to impacts on interior fittings. The objective of this work was to better know the passenger kinematics in order to optimize the interior layout and minimize the consequences of these impacts. Ten healthy young male subjects (age 23 to 34, mean stature 1.78m, mean weight 74kg) participated in an experiment where, without prior notice, the platform on which they were standing was subjected to an acceleration pulse of large duration, typical of those encountered in public transports. High speed video camera and surface markers were used to recorded and reconstruct in 2D the motion of the main body segments. The results reported here concern 3 postural configurations : free standing forward facing, free standing rearward facing and forward facing leaning against a buttock rest. In all cases, the subjects could not maintain their balance and had to execute several steps to avoid falling. The performance of the subjects in resisting to the forced motion was characterized by the velocity and forward excursion of the head, which condition the injury risk. The posture leaning against the buttock rest led to the smallest head velocity. A performance index was defined as the ratio between the head velocity and the platform velocity. It had a negative correlation with the step length, the feet velocity and the forward inclination of the torso during the forward steps, and a positive correlation with the averaged forward projection of the lower limbs. Overall, the duration of the pulse proved to be a challenge even for healthy subjects, with only two subjects stopping before the mattress and a fast motion of the lower extremities that may not be possible for elderly subjects with lower muscular performance. This suggests that studies based on short perturbations may not be sufficient to fully understand reactions to emergency braking situations.