Violent shaking versus a bicycle trailer ride: experimental comparison of kinematics in infant injury scenarios

Fatal traumatic head injury in infants is regularly inflicted by a perpetrator, with head injury from shaking trauma (IHI-ST) being a major type. In IHI-ST, perpetrators violently shake an infant back-and-forth, whipping its head back-and-forth. Courts often debate whether shaking caused injuries or whether an alternative accidental scenario raised by the defendant did, like a bumpy bicycle-trailer ride. Head and torso kinematics of an infant dummy (six-week-old) during bicycle-trailer rides across bumps with different heights and slopes were recorded at different speeds. The validity of this alternative scenario was investigated by comparing it with violent shaking kinematics. Trailer ride conditions substantially affected the magnitude of dummy accelerations, velocities and neck flexion. Even for the highest speeds and steepest/highest bumps, head motion was less rotational than during violent shaking. While sagittal peak head linear accelerations were of similar magnitude, head angular accelerations in the trailer were less than half those during violent shaking. The head's median instantaneous radius of rotation was around 12 times shorter during shaking, indicative of greater rotational motion. Because of differences in kinematics, differences in the exposure duration, and the lack of injury thresholds between both scenarios, determining the resulting differences in injury risks requires further research.