Brain-skull displacement in case of head trauma has frequently been quoted in the literature. The common lesions linked with this relative movement mechanism are focal contusions and subdural hematomas (SDH). Specialists also agree that an angular acceleration of the head can cause intra-cerebral shearing and constitutes a lesion mechanism of the diffuse axonal injuries (DAI). The specific type of shock which releases one or the other of these mechanisms has not been studied in depth and has often been attributed to the angular or linear preponderance of the acceleration. In our earlier work we tried to describe the dynamic behaviour of the head. In vivo mechanical impedance recordings of the head revealed a de-coupling of the cerebral mass at about 100 Hz. In this study we have consolidated this result by low amplitude frequency by frequency vibration analysis of four voluntary subjects. The proposed analysis makes it possible to distinguish between the two lesion mechanisms, based on the temporal or frequency characteristics of the impact force. This position is illustrated by interpreting various observations given in the literature, which until now could not be explained.