The aims of the present investigation were to delineate the internal biomechanics of the spine under vertical impact vector and assess the probability of injury. Male and female whole-body human finite element models were used. The restrained occupants were positioned on the seat, and caudo-cephalad impacts were applied to the base. Different acceleration-time profiles (50-200 ms pulse durations, 11-46 g peak accelerations) were used as inputs in both models. The resulting stress-strain profiles in the cortical and cancellous bones were evaluated at different vertebral levels. Using the peak transmitted forces at the thoracolumbar disc level as the response variable, the probability of injury for the male spine was obtained from experimental risk curves for the various pulses. Results showed that the shorter pulse durations and rise times impart greater loading on the thoracolumbar spine. The analysis of von Mises stress and strain distributions showed that the compression-related fractures are multifaceted with contributions from both the cortical and cancellous bony components of the body. Profiles are provided in the paper. The intervertebral disc may be involved in the fracture mechanism, because it acts as a medium of load transfer between adjacent vertebrae. Injury risks for the shortest pulse was sixty-three percent, and for the widest pulse it was close to zero, and injury probabilities for other pulses are given. The present study provides insights into the mechanisms of internal load transfer and describe injury risk levels from caudal to cephalad impacts.

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